tria.cc

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// -----------------------------------------------------------------------------
//
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception OR LGPL-2.1-or-later
// Copyright (C) 1999 - 2026 by the deal.II authors
//
// This file is part of the deal.II library.
//
// Detailed license information governing the source code and contributions
// can be found in LICENSE.md and CONTRIBUTING.md at the top level directory.
//
// -----------------------------------------------------------------------------
 
 
#include <deal.II/base/array_view.h>
#include <deal.II/base/geometry_info.h>
#include <deal.II/base/memory_consumption.h>
#include <deal.II/base/mpi.templates.h>
#include <deal.II/base/mpi_large_count.h>
#include <deal.II/base/mpi_stub.h>
#include <deal.II/base/ndarray.h>
#include <deal.II/base/signaling_nan.h>
#include <deal.II/base/std_cxx26/inplace_vector.h>
#include <deal.II/base/thread_management.h>
#include <deal.II/base/utilities.h>
 
#include <deal.II/distributed/tria.h>
#include <deal.II/distributed/tria_base.h>
 
#include <deal.II/grid/grid_tools.h>
#include <deal.II/grid/magic_numbers.h>
#include <deal.II/grid/manifold.h>
#include <deal.II/grid/reference_cell.h>
#include <deal.II/grid/tria.h>
#include <deal.II/grid/tria_accessor.h>
#include <deal.II/grid/tria_description.h>
#include <deal.II/grid/tria_faces.h>
#include <deal.II/grid/tria_iterator.h>
#include <deal.II/grid/tria_levels.h>
#include <deal.II/grid/tria_objects_orientations.h>
 
#include <boost/archive/text_iarchive.hpp>
#include <boost/archive/text_oarchive.hpp>
 
#include <algorithm>
#include <array>
#include <cmath>
#include <cstdint>
#include <fstream>
#include <functional>
#include <limits>
#include <list>
#include <map>
#include <memory>
#include <numeric>
#include <vector>
 
 
DEAL_II_NAMESPACE_OPEN
 
 
namespace internal
{
  namespace TriangulationImplementation
  {
    NumberCache<1>::NumberCache()
      : n_levels(0)
      , n_lines(0)
      , n_active_lines(0)
    // all other fields are
    // default constructed
    {}
 
 
 
    std::size_t
    NumberCache<1>::memory_consumption() const
    {
      std::size_t mem =
        MemoryConsumption::memory_consumption(n_levels) +
        MemoryConsumption::memory_consumption(n_lines) +
        MemoryConsumption::memory_consumption(n_lines_level) +
        MemoryConsumption::memory_consumption(n_active_lines) +
        MemoryConsumption::memory_consumption(n_active_lines_level);
 
      if (active_cell_index_partitioner)
        mem += active_cell_index_partitioner->memory_consumption();
 
      for (const auto &partitioner : level_cell_index_partitioners)
        if (partitioner)
          mem += partitioner->memory_consumption();
 
      return mem;
    }
 
 
    NumberCache<2>::NumberCache()
      : n_quads(0)
      , n_active_quads(0)
    // all other fields are
    // default constructed
    {}
 
 
 
    std::size_t
    NumberCache<2>::memory_consumption() const
    {
      return (NumberCache<1>::memory_consumption() +
              MemoryConsumption::memory_consumption(n_quads) +
              MemoryConsumption::memory_consumption(n_quads_level) +
              MemoryConsumption::memory_consumption(n_active_quads) +
              MemoryConsumption::memory_consumption(n_active_quads_level));
    }
 
 
 
    NumberCache<3>::NumberCache()
      : n_hexes(0)
      , n_active_hexes(0)
    // all other fields are
    // default constructed
    {}
 
 
 
    std::size_t
    NumberCache<3>::memory_consumption() const
    {
      return (NumberCache<2>::memory_consumption() +
              MemoryConsumption::memory_consumption(n_hexes) +
              MemoryConsumption::memory_consumption(n_hexes_level) +
              MemoryConsumption::memory_consumption(n_active_hexes) +
              MemoryConsumption::memory_consumption(n_active_hexes_level));
    }
 
 
 
    template <int dim>
    Strides<dim>::Strides()
      : max_children_per_cell(numbers::invalid_unsigned_int)
      , max_faces_per_cell(numbers::invalid_unsigned_int)
      , max_lines_per_cell(numbers::invalid_unsigned_int)
      , max_vertices_per_cell(numbers::invalid_unsigned_int)
      , max_children_per_face(numbers::invalid_unsigned_int)
      , max_lines_per_face(numbers::invalid_unsigned_int)
    {}
 
 
 
    template <int dim>
    Strides<dim>::Strides(
      const std::vector<ReferenceCell<dim>> &reference_cells)
      : max_children_per_cell(0u)
      , max_faces_per_cell(0u)
      , max_lines_per_cell(0u)
      , max_vertices_per_cell(0u)
      , max_children_per_face(0u)
      , max_lines_per_face(0u)
    {
      for (const ReferenceCell<dim> &reference_cell : reference_cells)
        {
          max_children_per_cell =
            std::max(max_children_per_cell,
                     reference_cell.n_isotropic_children());
          max_faces_per_cell =
            std::max(max_faces_per_cell, reference_cell.n_faces());
          max_lines_per_cell =
            std::max(max_lines_per_cell, reference_cell.n_lines());
          max_vertices_per_cell =
            std::max(max_vertices_per_cell, reference_cell.n_vertices());
 
          for (unsigned int face_no : reference_cell.face_indices())
            {
              const auto face_reference_cell =
                reference_cell.face_reference_cell(face_no);
              max_children_per_face =
                std::max(max_children_per_face,
                         face_reference_cell.n_isotropic_children());
              max_lines_per_face =
                std::max(max_lines_per_face, face_reference_cell.n_lines());
            }
        }
    }
  } // namespace TriangulationImplementation
 
 
  template <int dim, int spacedim>
  DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
  CellAttachedDataSerializer<dim, spacedim>::CellAttachedDataSerializer()
    : variable_size_data_stored(false)
  {}
 
 
  template <int dim, int spacedim>
  DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
  void CellAttachedDataSerializer<dim, spacedim>::pack_data(
    const std::vector<cell_relation_t> &cell_relations,
    const std::vector<
      typename internal::CellAttachedData<dim, spacedim>::pack_callback_t>
      &pack_callbacks_fixed,
    const std::vector<
      typename internal::CellAttachedData<dim, spacedim>::pack_callback_t>
                   &pack_callbacks_variable,
    const MPI_Comm &mpi_communicator)
  {
    Assert(src_data_fixed.empty(),
           ExcMessage("Previously packed data has not been released yet!"));
    Assert(src_sizes_variable.empty(), ExcInternalError());
 
    const unsigned int n_callbacks_fixed    = pack_callbacks_fixed.size();
    const unsigned int n_callbacks_variable = pack_callbacks_variable.size();
 
    // Store information that we packed variable size data in
    // a member variable for later.
    variable_size_data_stored = (n_callbacks_variable > 0);
 
    // If variable transfer is scheduled, we will store the data size that
    // each variable size callback function writes in this auxiliary
    // container. The information will be stored by each cell in this vector
    // temporarily.
    std::vector<unsigned int> cell_sizes_variable_cumulative(
      n_callbacks_variable);
 
    // Prepare the buffer structure, in which each callback function will
    // store its data for each active cell.
    // The outmost shell in this container construct corresponds to the
    // data packed per cell. The next layer resembles the data that
    // each callback function packs on the corresponding cell. These
    // buffers are chains of chars stored in an std::vector<char>.
    // A visualisation of the data structure:
    /* clang-format off */
      // |             cell_1                | |             cell_2                | ...
      // ||  callback_1  ||  callback_2  |...| ||  callback_1  ||  callback_2  |...| ...
      // |||char|char|...|||char|char|...|...| |||char|char|...|||char|char|...|...| ...
    /* clang-format on */
    std::vector<std::vector<std::vector<char>>> packed_fixed_size_data(
      cell_relations.size());
    std::vector<std::vector<std::vector<char>>> packed_variable_size_data(
      variable_size_data_stored ? cell_relations.size() : 0);
 
    //
    // --------- Pack data for fixed and variable size transfer ---------
    //
    // Iterate over all cells, call all callback functions on each cell,
    // and store their data in the corresponding buffer scope.
    {
      auto cell_rel_it           = cell_relations.cbegin();
      auto data_cell_fixed_it    = packed_fixed_size_data.begin();
      auto data_cell_variable_it = packed_variable_size_data.begin();
      for (; cell_rel_it != cell_relations.cend(); ++cell_rel_it)
        {
          const auto &dealii_cell = cell_rel_it->first;
          const auto &cell_status = cell_rel_it->second;
 
          // Assertions about the tree structure.
          switch (cell_status)
            {
              case CellStatus::cell_will_persist:
              case CellStatus::cell_will_be_refined:
                // double check the condition that we will only ever attach
                // data to active cells when we get here
                Assert(dealii_cell->is_active(), ExcInternalError());
                break;
 
              case CellStatus::children_will_be_coarsened:
                // double check the condition that we will only ever attach
                // data to cells with children when we get here. however, we
                // can only tolerate one level of coarsening at a time, so
                // check that the children are all active
                Assert(dealii_cell->is_active() == false, ExcInternalError());
                for (unsigned int c = 0; c < dealii_cell->n_children(); ++c)
                  Assert(dealii_cell->child(c)->is_active(),
                         ExcInternalError());
                break;
 
              case CellStatus::cell_invalid:
                // do nothing on invalid cells
                break;
 
              default:
                DEAL_II_ASSERT_UNREACHABLE();
                break;
            }
 
          // Reserve memory corresponding to the number of callback
          // functions that will be called.
          // If variable size transfer is scheduled, we need to leave
          // room for an array that holds information about how many
          // bytes each of the variable size callback functions will
          // write.
          // On cells flagged with CellStatus::cell_invalid, only its CellStatus
          // will be stored.
          const unsigned int n_fixed_size_data_sets_on_cell =
            1 + ((cell_status == CellStatus::cell_invalid) ?
                   0 :
                   ((variable_size_data_stored ? 1 : 0) + n_callbacks_fixed));
          data_cell_fixed_it->resize(n_fixed_size_data_sets_on_cell);
 
          // We continue with packing all data on this specific cell.
          auto data_fixed_it = data_cell_fixed_it->begin();
 
          // First, we pack the CellStatus information.
          // to get consistent data sizes on each cell for the fixed size
          // transfer, we won't allow compression
          *data_fixed_it =
            Utilities::pack(cell_status, /*allow_compression=*/false);
          ++data_fixed_it;
 
          // Proceed with all registered callback functions.
          // Skip cells with the CellStatus::cell_invalid flag.
          if (cell_status != CellStatus::cell_invalid)
            {
              // Pack fixed size data.
              for (auto callback_it = pack_callbacks_fixed.cbegin();
                   callback_it != pack_callbacks_fixed.cend();
                   ++callback_it, ++data_fixed_it)
                {
                  *data_fixed_it = (*callback_it)(dealii_cell, cell_status);
                }
 
              // Pack variable size data.
              // If we store variable size data, we need to transfer
              // the sizes of each corresponding callback function
              // via fixed size transfer as well.
              if (variable_size_data_stored)
                {
                  const unsigned int n_variable_size_data_sets_on_cell =
                    ((cell_status == CellStatus::cell_invalid) ?
                       0 :
                       n_callbacks_variable);
                  data_cell_variable_it->resize(
                    n_variable_size_data_sets_on_cell);
 
                  auto callback_it      = pack_callbacks_variable.cbegin();
                  auto data_variable_it = data_cell_variable_it->begin();
                  auto sizes_variable_it =
                    cell_sizes_variable_cumulative.begin();
                  for (; callback_it != pack_callbacks_variable.cend();
                       ++callback_it, ++data_variable_it, ++sizes_variable_it)
                    {
                      *data_variable_it =
                        (*callback_it)(dealii_cell, cell_status);
 
                      // Store data sizes for each callback function first.
                      // Make it cumulative below.
                      *sizes_variable_it = data_variable_it->size();
                    }
 
                  // Turn size vector into its cumulative representation.
                  std::partial_sum(cell_sizes_variable_cumulative.begin(),
                                   cell_sizes_variable_cumulative.end(),
                                   cell_sizes_variable_cumulative.begin());
 
                  // Serialize cumulative variable size vector
                  // value-by-value. This way we can circumvent the overhead
                  // of storing the container object as a whole, since we
                  // know its size by the number of registered callback
                  // functions.
                  data_fixed_it->resize(n_callbacks_variable *
                                        sizeof(unsigned int));
                  for (unsigned int i = 0; i < n_callbacks_variable; ++i)
                    std::memcpy(&(data_fixed_it->at(i * sizeof(unsigned int))),
                                &(cell_sizes_variable_cumulative.at(i)),
                                sizeof(unsigned int));
 
                  ++data_fixed_it;
                }
 
              // Double check that we packed everything we wanted
              // in the fixed size buffers.
              Assert(data_fixed_it == data_cell_fixed_it->end(),
                     ExcInternalError());
            }
 
          ++data_cell_fixed_it;
 
          // Increment the variable size data iterator
          // only if we actually pack this kind of data
          // to avoid getting out of bounds.
          if (variable_size_data_stored)
            ++data_cell_variable_it;
        } // loop over cell_relations
    }
 
    //
    // ----------- Gather data sizes for fixed size transfer ------------
    //
    // Generate a vector which stores the sizes of each callback function,
    // including the packed CellStatus transfer.
    // Find the very first cell that we wrote to with all callback
    // functions (i.e. a cell that was not flagged with
    // CellStatus::cell_invalid) and store the sizes of each buffer.
    //
    // To deal with the case that at least one of the processors does not
    // own any cell at all, we will exchange the information about the data
    // sizes among them later. The code in between is still well-defined,
    // since the following loops will be skipped.
    std::vector<unsigned int> local_sizes_fixed(
      1 + n_callbacks_fixed + (variable_size_data_stored ? 1 : 0));
    for (const auto &data_cell : packed_fixed_size_data)
      {
        if (data_cell.size() == local_sizes_fixed.size())
          {
            auto sizes_fixed_it = local_sizes_fixed.begin();
            auto data_fixed_it  = data_cell.cbegin();
            for (; data_fixed_it != data_cell.cend();
                 ++data_fixed_it, ++sizes_fixed_it)
              {
                *sizes_fixed_it = data_fixed_it->size();
              }
 
            break;
          }
      }
 
    // Check if all cells have valid sizes.
    for (auto data_cell_fixed_it = packed_fixed_size_data.cbegin();
         data_cell_fixed_it != packed_fixed_size_data.cend();
         ++data_cell_fixed_it)
      {
        Assert((data_cell_fixed_it->size() == 1) ||
                 (data_cell_fixed_it->size() == local_sizes_fixed.size()),
               ExcInternalError());
      }
 
    // Share information about the packed data sizes
    // of all callback functions across all processors, in case one
    // of them does not own any cells at all.
    std::vector<unsigned int> global_sizes_fixed(local_sizes_fixed.size());
    Utilities::MPI::max(local_sizes_fixed,
                        mpi_communicator,
                        global_sizes_fixed);
 
    // Construct cumulative sizes, since this is the only information
    // we need from now on.
    sizes_fixed_cumulative.resize(global_sizes_fixed.size());
    std::partial_sum(global_sizes_fixed.begin(),
                     global_sizes_fixed.end(),
                     sizes_fixed_cumulative.begin());
 
    //
    // ---------- Gather data sizes for variable size transfer ----------
    //
    if (variable_size_data_stored)
      {
        src_sizes_variable.reserve(packed_variable_size_data.size());
        for (const auto &data_cell : packed_variable_size_data)
          {
            int variable_data_size_on_cell = 0;
 
            for (const auto &data : data_cell)
              variable_data_size_on_cell += data.size();
 
            src_sizes_variable.push_back(variable_data_size_on_cell);
          }
      }
 
    //
    // ------------------------ Build buffers ---------------------------
    //
    const unsigned int expected_size_fixed =
      cell_relations.size() * sizes_fixed_cumulative.back();
    const unsigned int expected_size_variable =
      std::accumulate(src_sizes_variable.begin(),
                      src_sizes_variable.end(),
                      std::vector<int>::size_type(0));
 
    // Move every piece of packed fixed size data into the consecutive
    // buffer.
    src_data_fixed.reserve(expected_size_fixed);
    for (const auto &data_cell_fixed : packed_fixed_size_data)
      {
        // Move every fraction of packed data into the buffer
        // reserved for this particular cell.
        for (const auto &data_fixed : data_cell_fixed)
          std::move(data_fixed.begin(),
                    data_fixed.end(),
                    std::back_inserter(src_data_fixed));
 
        // If we only packed the CellStatus information
        // (i.e. encountered a cell flagged CellStatus::cell_invalid),
        // fill the remaining space with invalid entries.
        // We can skip this if there is nothing else to pack.
        if ((data_cell_fixed.size() == 1) &&
            (sizes_fixed_cumulative.size() > 1))
          {
            const std::size_t bytes_skipped =
              sizes_fixed_cumulative.back() - sizes_fixed_cumulative.front();
 
            src_data_fixed.insert(src_data_fixed.end(),
                                  bytes_skipped,
                                  static_cast<char>(-1)); // invalid_char
          }
      }
 
    // Move every piece of packed variable size data into the consecutive
    // buffer.
    if (variable_size_data_stored)
      {
        src_data_variable.reserve(expected_size_variable);
        for (const auto &data_cell : packed_variable_size_data)
          {
            // Move every fraction of packed data into the buffer
            // reserved for this particular cell.
            for (const auto &data : data_cell)
              std::move(data.begin(),
                        data.end(),
                        std::back_inserter(src_data_variable));
          }
      }
 
    // Double check that we packed everything correctly.
    Assert(src_data_fixed.size() == expected_size_fixed, ExcInternalError());
    Assert(src_data_variable.size() == expected_size_variable,
           ExcInternalError());
  }
 
 
 
  template <int dim, int spacedim>
  DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
  void CellAttachedDataSerializer<dim, spacedim>::unpack_cell_status(
    std::vector<
      typename CellAttachedDataSerializer<dim, spacedim>::cell_relation_t>
      &cell_relations) const
  {
    Assert(sizes_fixed_cumulative.size() > 0,
           ExcMessage("No data has been packed!"));
    if (cell_relations.size() > 0)
      {
        Assert(dest_data_fixed.size() > 0,
               ExcMessage("No data has been received!"));
      }
 
    // Size of CellStatus object that will be unpacked on each cell.
    const unsigned int size = sizes_fixed_cumulative.front();
 
    // Iterate over all cells and overwrite the CellStatus
    // information from the transferred data.
    // Proceed buffer iterator position to next cell after
    // each iteration.
    auto cell_rel_it   = cell_relations.begin();
    auto dest_fixed_it = dest_data_fixed.cbegin();
    for (; cell_rel_it != cell_relations.end();
         ++cell_rel_it, dest_fixed_it += sizes_fixed_cumulative.back())
      {
        cell_rel_it->second = // cell_status
          Utilities::unpack<CellStatus>(dest_fixed_it,
                                        dest_fixed_it + size,
                                        /*allow_compression=*/false);
      }
  }
 
 
 
  template <int dim, int spacedim>
  DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
  void CellAttachedDataSerializer<dim, spacedim>::unpack_data(
    const std::vector<
      typename CellAttachedDataSerializer<dim, spacedim>::cell_relation_t>
                      &cell_relations,
    const unsigned int handle,
    const std::function<
      void(const cell_iterator &,
           const CellStatus &,
           const boost::iterator_range<std::vector<char>::const_iterator> &)>
      &unpack_callback) const
  {
    // We decode the handle returned by register_data_attach() back into
    // a format we can use. All even handles belong to those callback
    // functions which write/read variable size data, all odd handles
    // interact with fixed size buffers.
    const bool         callback_variable_transfer = (handle % 2 == 0);
    const unsigned int callback_index             = handle / 2;
 
    // Cells will always receive fixed size data (i.e., CellStatus
    // information), but not necessarily variable size data (e.g., with a
    // ParticleHandler a cell might not contain any particle at all).
    // Thus it is sufficient to check if fixed size data has been received.
    Assert(sizes_fixed_cumulative.size() > 0,
           ExcMessage("No data has been packed!"));
    if (cell_relations.size() > 0)
      {
        Assert(dest_data_fixed.size() > 0,
               ExcMessage("No data has been received!"));
      }
 
    std::vector<char>::const_iterator dest_data_it;
    std::vector<char>::const_iterator dest_sizes_cell_it;
 
    // Depending on whether our callback function unpacks fixed or
    // variable size data, we have to pursue different approaches
    // to localize the correct fraction of the buffer from which
    // we are allowed to read.
    unsigned int offset         = numbers::invalid_unsigned_int;
    unsigned int size           = numbers::invalid_unsigned_int;
    unsigned int data_increment = numbers::invalid_unsigned_int;
 
    if (callback_variable_transfer)
      {
        // For the variable size data, we need to extract the
        // data size from the fixed size buffer on each cell.
        //
        // We packed this information last, so the last packed
        // object in the fixed size buffer corresponds to the
        // variable data sizes.
        //
        // The last entry of sizes_fixed_cumulative corresponds
        // to the size of all fixed size data packed on the cell.
        // To get the offset for the last packed object, we need
        // to get the next-to-last entry.
        const unsigned int offset_variable_data_sizes =
          sizes_fixed_cumulative[sizes_fixed_cumulative.size() - 2];
 
        // This iterator points to the data size that the
        // callback_function packed for each specific cell.
        // Adjust buffer iterator to the offset of the callback
        // function so that we only have to advance its position
        // to the next cell after each iteration.
        dest_sizes_cell_it = dest_data_fixed.cbegin() +
                             offset_variable_data_sizes +
                             callback_index * sizeof(unsigned int);
 
        // Let the data iterator point to the correct buffer.
        dest_data_it = dest_data_variable.cbegin();
      }
    else
      {
        // For the fixed size data, we can get the information about
        // the buffer location on each cell directly from the
        // sizes_fixed_cumulative vector.
        offset         = sizes_fixed_cumulative[callback_index];
        size           = sizes_fixed_cumulative[callback_index + 1] - offset;
        data_increment = sizes_fixed_cumulative.back();
 
        // Let the data iterator point to the correct buffer.
        // Adjust buffer iterator to the offset of the callback
        // function so that we only have to advance its position
        // to the next cell after each iteration.
        if (cell_relations.begin() != cell_relations.end())
          dest_data_it = dest_data_fixed.cbegin() + offset;
      }
 
    // Iterate over all cells and unpack the transferred data.
    auto cell_rel_it   = cell_relations.begin();
    auto dest_sizes_it = dest_sizes_variable.cbegin();
    for (; cell_rel_it != cell_relations.end(); ++cell_rel_it)
      {
        const auto &dealii_cell = cell_rel_it->first;
        const auto &cell_status = cell_rel_it->second;
 
        if (callback_variable_transfer)
          {
            // Update the increment according to the whole data size
            // of the current cell.
            data_increment = *dest_sizes_it;
 
            if (cell_status != CellStatus::cell_invalid)
              {
                // Extract the corresponding values for offset and size from
                // the cumulative sizes array stored in the fixed size
                // buffer.
                if (callback_index == 0)
                  offset = 0;
                else
                  std::memcpy(&offset,
                              &(*(dest_sizes_cell_it - sizeof(unsigned int))),
                              sizeof(unsigned int));
 
                std::memcpy(&size,
                            &(*dest_sizes_cell_it),
                            sizeof(unsigned int));
 
                size -= offset;
 
                // Move the data iterator to the corresponding position
                // of the callback function and adjust the increment
                // accordingly.
                dest_data_it += offset;
                data_increment -= offset;
              }
 
            // Advance data size iterators to the next cell, avoid iterating
            // past the end of dest_sizes_cell_it
            if (cell_rel_it != cell_relations.end() - 1)
              dest_sizes_cell_it += sizes_fixed_cumulative.back();
            ++dest_sizes_it;
          }
 
        switch (cell_status)
          {
            case CellStatus::cell_will_persist:
            case CellStatus::children_will_be_coarsened:
              unpack_callback(dealii_cell,
                              cell_status,
                              boost::make_iterator_range(dest_data_it,
                                                         dest_data_it + size));
              break;
 
            case CellStatus::cell_will_be_refined:
              unpack_callback(dealii_cell->parent(),
                              cell_status,
                              boost::make_iterator_range(dest_data_it,
                                                         dest_data_it + size));
              break;
 
            case CellStatus::cell_invalid:
              // Skip this cell.
              break;
 
            default:
              DEAL_II_ASSERT_UNREACHABLE();
              break;
          }
 
        if (cell_rel_it != cell_relations.end() - 1)
          dest_data_it += data_increment;
      }
  }
 
 
 
  template <int dim, int spacedim>
  DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
  void CellAttachedDataSerializer<dim, spacedim>::save(
    const unsigned int global_first_cell,
    const unsigned int global_num_cells,
    const std::string &file_basename,
    const MPI_Comm    &mpi_communicator) const
  {
    Assert(sizes_fixed_cumulative.size() > 0,
           ExcMessage("No data has been packed!"));
 
#ifdef DEAL_II_WITH_MPI
    // Large fractions of this function have been copied from
    // DataOutInterface::write_vtu_in_parallel.
    // TODO: Write general MPIIO interface.
 
    const unsigned int myrank =
      Utilities::MPI::this_mpi_process(mpi_communicator);
    const unsigned int mpisize =
      Utilities::MPI::n_mpi_processes(mpi_communicator);
 
    if (mpisize > 1)
      {
        const unsigned int bytes_per_cell = sizes_fixed_cumulative.back();
 
        //
        // ---------- Fixed size data ----------
        //
        {
          const std::string fname_fixed =
            std::string(file_basename) + "_fixed.data";
 
          MPI_Info info;
          int      ierr = MPI_Info_create(&info);
          AssertThrowMPI(ierr);
 
          MPI_File fh;
          ierr = MPI_File_open(mpi_communicator,
                               fname_fixed.c_str(),
                               MPI_MODE_CREATE | MPI_MODE_WRONLY,
                               info,
                               &fh);
          AssertThrowMPI(ierr);
 
          ierr = MPI_File_set_size(fh, 0); // delete the file contents
          AssertThrowMPI(ierr);
          // this barrier is necessary, because otherwise others might already
          // write while one core is still setting the size to zero.
          ierr = MPI_Barrier(mpi_communicator);
          AssertThrowMPI(ierr);
          ierr = MPI_Info_free(&info);
          AssertThrowMPI(ierr);
          // ------------------
 
          // Write cumulative sizes to file.
          // Since each processor owns the same information about the data
          // sizes, it is sufficient to let only the first processor perform
          // this task.
          if (myrank == 0)
            {
              ierr = Utilities::MPI::LargeCount::File_write_at_c(
                fh,
                0,
                sizes_fixed_cumulative.data(),
                sizes_fixed_cumulative.size(),
                MPI_UNSIGNED,
                MPI_STATUS_IGNORE);
              AssertThrowMPI(ierr);
            }
 
          // Write packed data to file simultaneously.
          const MPI_Offset size_header =
            sizes_fixed_cumulative.size() * sizeof(unsigned int);
 
          // Make sure we do the following computation in 64bit integers to be
          // able to handle 4GB+ files:
          const MPI_Offset my_global_file_position =
            size_header +
            static_cast<MPI_Offset>(global_first_cell) * bytes_per_cell;
 
          ierr =
            Utilities::MPI::LargeCount::File_write_at_c(fh,
                                                        my_global_file_position,
                                                        src_data_fixed.data(),
                                                        src_data_fixed.size(),
                                                        MPI_BYTE,
                                                        MPI_STATUS_IGNORE);
          AssertThrowMPI(ierr);
 
          ierr = MPI_File_close(&fh);
          AssertThrowMPI(ierr);
        }
 
 
 
        //
        // ---------- Variable size data ----------
        //
        if (variable_size_data_stored)
          {
            const std::string fname_variable =
              std::string(file_basename) + "_variable.data";
 
            MPI_Info info;
            int      ierr = MPI_Info_create(&info);
            AssertThrowMPI(ierr);
 
            MPI_File fh;
            ierr = MPI_File_open(mpi_communicator,
                                 fname_variable.c_str(),
                                 MPI_MODE_CREATE | MPI_MODE_WRONLY,
                                 info,
                                 &fh);
            AssertThrowMPI(ierr);
 
            ierr = MPI_File_set_size(fh, 0); // delete the file contents
            AssertThrowMPI(ierr);
            // this barrier is necessary, because otherwise others might already
            // write while one core is still setting the size to zero.
            ierr = MPI_Barrier(mpi_communicator);
            AssertThrowMPI(ierr);
            ierr = MPI_Info_free(&info);
            AssertThrowMPI(ierr);
 
            // Write sizes of each cell into file simultaneously.
            {
              const MPI_Offset my_global_file_position =
                static_cast<MPI_Offset>(global_first_cell) *
                sizeof(unsigned int);
 
              // It is very unlikely that a single process has more than
              // 2 billion cells, but we might as well check.
              AssertThrow(src_sizes_variable.size() <
                            static_cast<std::size_t>(
                              std::numeric_limits<int>::max()),
                          ExcNotImplemented());
 
              ierr = Utilities::MPI::LargeCount::File_write_at_c(
                fh,
                my_global_file_position,
                src_sizes_variable.data(),
                src_sizes_variable.size(),
                MPI_INT,
                MPI_STATUS_IGNORE);
              AssertThrowMPI(ierr);
            }
 
            // Gather size of data in bytes we want to store from this
            // processor and compute the prefix sum. We do this in 64 bit
            // to avoid overflow for files larger than 4GB:
            const std::uint64_t size_on_proc = src_data_variable.size();
            std::uint64_t       prefix_sum   = 0;
            ierr                             = MPI_Exscan(&size_on_proc,
                              &prefix_sum,
                              1,
                              MPI_UINT64_T,
                              MPI_SUM,
                              mpi_communicator);
            AssertThrowMPI(ierr);
 
            const MPI_Offset my_global_file_position =
              static_cast<MPI_Offset>(global_num_cells) * sizeof(unsigned int) +
              prefix_sum;
 
            // Write data consecutively into file.
            ierr = Utilities::MPI::LargeCount::File_write_at_c(
              fh,
              my_global_file_position,
              src_data_variable.data(),
              src_data_variable.size(),
              MPI_BYTE,
              MPI_STATUS_IGNORE);
            AssertThrowMPI(ierr);
 
 
            ierr = MPI_File_close(&fh);
            AssertThrowMPI(ierr);
          }
      } // if (mpisize > 1)
    else
#endif
      {
        (void)global_first_cell;
        (void)global_num_cells;
        (void)mpi_communicator;
 
        //
        // ---------- Fixed size data ----------
        //
        {
          const std::string fname_fixed =
            std::string(file_basename) + "_fixed.data";
 
          std::ofstream file(fname_fixed, std::ios::binary | std::ios::out);
          AssertThrow(file.fail() == false, ExcIO());
 
          // Write header data.
          file.write(reinterpret_cast<const char *>(
                       sizes_fixed_cumulative.data()),
                     sizes_fixed_cumulative.size() * sizeof(unsigned int));
 
          // Write packed data.
          file.write(reinterpret_cast<const char *>(src_data_fixed.data()),
                     src_data_fixed.size() * sizeof(char));
        }
 
        //
        // ---------- Variable size data ----------
        //
        if (variable_size_data_stored)
          {
            const std::string fname_variable =
              std::string(file_basename) + "_variable.data";
 
            std::ofstream file(fname_variable,
                               std::ios::binary | std::ios::out);
            AssertThrow(file.fail() == false, ExcIO());
 
            // Write header data.
            file.write(reinterpret_cast<const char *>(
                         src_sizes_variable.data()),
                       src_sizes_variable.size() * sizeof(int));
 
            // Write packed data.
            file.write(reinterpret_cast<const char *>(src_data_variable.data()),
                       src_data_variable.size() * sizeof(char));
          }
      }
  }
 
 
  template <int dim, int spacedim>
  DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
  void CellAttachedDataSerializer<dim, spacedim>::load(
    const unsigned int global_first_cell,
    const unsigned int global_num_cells,
    const unsigned int local_num_cells,
    const std::string &file_basename,
    const unsigned int n_attached_deserialize_fixed,
    const unsigned int n_attached_deserialize_variable,
    const MPI_Comm    &mpi_communicator)
  {
    Assert(dest_data_fixed.empty(),
           ExcMessage("Previously loaded data has not been released yet!"));
 
    variable_size_data_stored = (n_attached_deserialize_variable > 0);
 
#ifdef DEAL_II_WITH_MPI
    // Large fractions of this function have been copied from
    // DataOutInterface::write_vtu_in_parallel.
    // TODO: Write general MPIIO interface.
 
    const unsigned int mpisize =
      Utilities::MPI::n_mpi_processes(mpi_communicator);
 
    if (mpisize > 1)
      {
        //
        // ---------- Fixed size data ----------
        //
        {
          const std::string fname_fixed =
            std::string(file_basename) + "_fixed.data";
 
          MPI_Info info;
          int      ierr = MPI_Info_create(&info);
          AssertThrowMPI(ierr);
 
          MPI_File fh;
          ierr = MPI_File_open(
            mpi_communicator, fname_fixed.c_str(), MPI_MODE_RDONLY, info, &fh);
          AssertThrowMPI(ierr);
 
          ierr = MPI_Info_free(&info);
          AssertThrowMPI(ierr);
 
          // Read cumulative sizes from file.
          // Since all processors need the same information about the data
          // sizes, let each of them retrieve it by reading from the same
          // location in the file.
          sizes_fixed_cumulative.resize(1 + n_attached_deserialize_fixed +
                                        (variable_size_data_stored ? 1 : 0));
          ierr = Utilities::MPI::LargeCount::File_read_at_c(
            fh,
            0,
            sizes_fixed_cumulative.data(),
            sizes_fixed_cumulative.size(),
            MPI_UNSIGNED,
            MPI_STATUS_IGNORE);
          AssertThrowMPI(ierr);
 
          // Allocate sufficient memory.
          const unsigned int bytes_per_cell = sizes_fixed_cumulative.back();
          dest_data_fixed.resize(static_cast<std::size_t>(local_num_cells) *
                                 bytes_per_cell);
 
          // Read packed data from file simultaneously.
          const MPI_Offset size_header =
            sizes_fixed_cumulative.size() * sizeof(unsigned int);
 
          // Make sure we do the following computation in 64bit integers to be
          // able to handle 4GB+ files:
          const MPI_Offset my_global_file_position =
            size_header +
            static_cast<MPI_Offset>(global_first_cell) * bytes_per_cell;
 
          ierr =
            Utilities::MPI::LargeCount::File_read_at_c(fh,
                                                       my_global_file_position,
                                                       dest_data_fixed.data(),
                                                       dest_data_fixed.size(),
                                                       MPI_BYTE,
                                                       MPI_STATUS_IGNORE);
          AssertThrowMPI(ierr);
 
 
          ierr = MPI_File_close(&fh);
          AssertThrowMPI(ierr);
        }
 
        //
        // ---------- Variable size data ----------
        //
        if (variable_size_data_stored)
          {
            const std::string fname_variable =
              std::string(file_basename) + "_variable.data";
 
            MPI_Info info;
            int      ierr = MPI_Info_create(&info);
            AssertThrowMPI(ierr);
 
            MPI_File fh;
            ierr = MPI_File_open(mpi_communicator,
                                 fname_variable.c_str(),
                                 MPI_MODE_RDONLY,
                                 info,
                                 &fh);
            AssertThrowMPI(ierr);
 
            ierr = MPI_Info_free(&info);
            AssertThrowMPI(ierr);
 
            // Read sizes of all locally owned cells.
            dest_sizes_variable.resize(local_num_cells);
 
            const MPI_Offset my_global_file_position_sizes =
              static_cast<MPI_Offset>(global_first_cell) * sizeof(unsigned int);
 
            ierr = Utilities::MPI::LargeCount::File_read_at_c(
              fh,
              my_global_file_position_sizes,
              dest_sizes_variable.data(),
              dest_sizes_variable.size(),
              MPI_INT,
              MPI_STATUS_IGNORE);
            AssertThrowMPI(ierr);
 
 
            // Compute my data size in bytes and compute prefix sum. We do this
            // in 64 bit to avoid overflow for files larger than 4 GB:
            const std::uint64_t size_on_proc =
              std::accumulate(dest_sizes_variable.begin(),
                              dest_sizes_variable.end(),
                              0ULL);
 
            std::uint64_t prefix_sum = 0;
            ierr                     = MPI_Exscan(&size_on_proc,
                              &prefix_sum,
                              1,
                              MPI_UINT64_T,
                              MPI_SUM,
                              mpi_communicator);
            AssertThrowMPI(ierr);
 
            const MPI_Offset my_global_file_position =
              static_cast<MPI_Offset>(global_num_cells) * sizeof(unsigned int) +
              prefix_sum;
 
            dest_data_variable.resize(size_on_proc);
 
            ierr = Utilities::MPI::LargeCount::File_read_at_c(
              fh,
              my_global_file_position,
              dest_data_variable.data(),
              dest_data_variable.size(),
              MPI_BYTE,
              MPI_STATUS_IGNORE);
            AssertThrowMPI(ierr);
 
            ierr = MPI_File_close(&fh);
            AssertThrowMPI(ierr);
          }
      }
    else // if (mpisize > 1)
#endif
      {
        (void)mpi_communicator;
        (void)global_first_cell;
        (void)global_num_cells;
 
        //
        // ---------- Fixed size data ----------
        //
        {
          const std::string fname_fixed =
            std::string(file_basename) + "_fixed.data";
 
          std::ifstream file(fname_fixed, std::ios::binary | std::ios::in);
          AssertThrow(file.fail() == false, ExcIO());
 
          sizes_fixed_cumulative.resize(1 + n_attached_deserialize_fixed +
                                        (variable_size_data_stored ? 1 : 0));
          // Read header data.
          file.read(reinterpret_cast<char *>(sizes_fixed_cumulative.data()),
                    sizes_fixed_cumulative.size() * sizeof(unsigned int));
 
          const unsigned int bytes_per_cell = sizes_fixed_cumulative.back();
          dest_data_fixed.resize(static_cast<std::size_t>(local_num_cells) *
                                 bytes_per_cell);
 
          // Read packed data.
          file.read(reinterpret_cast<char *>(dest_data_fixed.data()),
                    dest_data_fixed.size() * sizeof(char));
        }
 
        //
        // ---------- Variable size data ----------
        //
        if (variable_size_data_stored)
          {
            const std::string fname_variable =
              std::string(file_basename) + "_variable.data";
 
            std::ifstream file(fname_variable, std::ios::binary | std::ios::in);
            AssertThrow(file.fail() == false, ExcIO());
 
            // Read header data.
            dest_sizes_variable.resize(local_num_cells);
            file.read(reinterpret_cast<char *>(dest_sizes_variable.data()),
                      dest_sizes_variable.size() * sizeof(int));
 
            // Read packed data.
            const std::uint64_t size =
              std::accumulate(dest_sizes_variable.begin(),
                              dest_sizes_variable.end(),
                              0ULL);
            dest_data_variable.resize(size);
            file.read(reinterpret_cast<char *>(dest_data_variable.data()),
                      dest_data_variable.size() * sizeof(char));
          }
      }
  }
 
 
  template <int dim, int spacedim>
  DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
  void CellAttachedDataSerializer<dim, spacedim>::clear()
  {
    variable_size_data_stored = false;
 
    // free information about data sizes
    sizes_fixed_cumulative.clear();
    sizes_fixed_cumulative.shrink_to_fit();
 
    // free fixed size transfer data
    src_data_fixed.clear();
    src_data_fixed.shrink_to_fit();
 
    dest_data_fixed.clear();
    dest_data_fixed.shrink_to_fit();
 
    // free variable size transfer data
    src_sizes_variable.clear();
    src_sizes_variable.shrink_to_fit();
 
    src_data_variable.clear();
    src_data_variable.shrink_to_fit();
 
    dest_sizes_variable.clear();
    dest_sizes_variable.shrink_to_fit();
 
    dest_data_variable.clear();
    dest_data_variable.shrink_to_fit();
  }
 
} // namespace internal
 
// anonymous namespace for internal helper functions
namespace
{
  // return whether the given cell is
  // patch_level_1, i.e. determine
  // whether either all or none of
  // its children are further
  // refined. this function can only
  // be called for non-active cells.
  template <int dim, int spacedim>
  bool
  cell_is_patch_level_1(
    const TriaIterator<::CellAccessor<dim, spacedim>> &cell)
  {
    Assert(cell->is_active() == false, ExcInternalError());
 
    unsigned int n_active_children = 0;
    for (unsigned int i = 0; i < cell->n_children(); ++i)
      if (cell->child(i)->is_active())
        ++n_active_children;
 
    return (n_active_children == 0) ||
           (n_active_children == cell->n_children());
  }
 
 
 
  // return, whether a given @p cell will be
  // coarsened, which is the case if all
  // children are active and have their coarsen
  // flag set. In case only part of the coarsen
  // flags are set, remove them.
  template <int dim, int spacedim>
  bool
  cell_will_be_coarsened(
    const TriaIterator<::CellAccessor<dim, spacedim>> &cell)
  {
    // only cells with children should be
    // considered for coarsening
 
    if (cell->has_children())
      {
        unsigned int       children_to_coarsen = 0;
        const unsigned int n_children          = cell->n_children();
 
        for (unsigned int c = 0; c < n_children; ++c)
          if (cell->child(c)->is_active() && cell->child(c)->coarsen_flag_set())
            ++children_to_coarsen;
        if (children_to_coarsen == n_children)
          return true;
        else
          for (unsigned int c = 0; c < n_children; ++c)
            if (cell->child(c)->is_active())
              cell->child(c)->clear_coarsen_flag();
      }
    // no children, so no coarsening
    // possible. however, no children also
    // means that this cell will be in the same
    // state as if it had children and was
    // coarsened. So, what should we return -
    // false or true?
    // make sure we do not have to do this at
    // all...
    Assert(cell->has_children(), ExcInternalError());
    // ... and then simply return false
    return false;
  }
 
 
  // return, whether the face @p face_no of the
  // given @p cell will be refined after the
  // current refinement step, considering
  // refine and coarsen flags and considering
  // only those refinemnts that will be caused
  // by the neighboring cell.
 
  // this function is used on both active cells
  // and cells with children. on cells with
  // children it also of interest to know 'how'
  // the face will be refined. thus there is an
  // additional third argument @p
  // expected_face_ref_case returning just
  // that. be aware, that this variable will
  // only contain useful information if this
  // function is called for an active cell.
  //
  // thus, this is an internal function, users
  // should call one of the two alternatives
  // following below.
  template <int dim, int spacedim>
  bool
  face_will_be_refined_by_neighbor_internal(
    const TriaIterator<::CellAccessor<dim, spacedim>> &cell,
    const unsigned int                                       face_no,
    RefinementCase<dim - 1> &expected_face_ref_case)
  {
    // first of all: set the default value for
    // expected_face_ref_case, which is no
    // refinement at all
    expected_face_ref_case = RefinementCase<dim - 1>::no_refinement;
 
    const typename Triangulation<dim, spacedim>::cell_iterator neighbor =
      cell->neighbor(face_no);
 
    // If we are at the boundary, there is no
    // neighbor which could refine the face
    if (neighbor.state() != IteratorState::valid)
      return false;
 
    if (neighbor->has_children())
      {
        // if the neighbor is refined, it may be
        // coarsened. if so, then it won't refine
        // the face, no matter what else happens
        if (cell_will_be_coarsened(neighbor))
          return false;
        else
          // if the neighbor is refined, then it
          // is also refined at our current
          // face. It will stay so without
          // coarsening, so return true in that
          // case.
          {
            expected_face_ref_case = cell->face(face_no)->refinement_case();
            return true;
          }
      }
 
    // now, the neighbor is not refined, but
    // perhaps it will be
    const RefinementCase<dim> nb_ref_flag = neighbor->refine_flag_set();
    if (nb_ref_flag != RefinementCase<dim>::no_refinement)
      {
        // now we need to know, which of the
        // neighbors faces points towards us
        const unsigned int neighbor_neighbor = cell->neighbor_face_no(face_no);
        // check, whether the cell will be
        // refined in a way that refines our
        // face
        const RefinementCase<dim - 1> face_ref_case =
          GeometryInfo<dim>::face_refinement_case(
            nb_ref_flag,
            neighbor_neighbor,
            neighbor->face_orientation(neighbor_neighbor),
            neighbor->face_flip(neighbor_neighbor),
            neighbor->face_rotation(neighbor_neighbor));
        if (face_ref_case != RefinementCase<dim - 1>::no_refinement)
          {
            const typename Triangulation<dim, spacedim>::face_iterator
                      neighbor_face   = neighbor->face(neighbor_neighbor);
            const int this_face_index = cell->face_index(face_no);
 
            // there are still two basic
            // possibilities here: the neighbor
            // might be coarser or as coarse
            // as we are
            if (neighbor_face->index() == this_face_index)
              // the neighbor is as coarse as
              // we are and will be refined at
              // the face of consideration, so
              // return true
              {
                expected_face_ref_case = face_ref_case;
                return true;
              }
            else
              {
                // the neighbor is coarser.
                // this is the most complicated
                // case. It might be, that the
                // neighbor's face will be
                // refined, but that we will
                // not see this, as we are
                // refined in a similar way.
 
                // so, the neighbor's face must
                // have children. check, if our
                // cell's face is one of these
                // (it could also be a
                // grand_child)
                for (unsigned int c = 0; c < neighbor_face->n_children(); ++c)
                  if (neighbor_face->child_index(c) == this_face_index)
                    {
                      // if the flagged refine
                      // case of the face is a
                      // subset or the same as
                      // the current refine case,
                      // then the face, as seen
                      // from our cell, won't be
                      // refined by the neighbor
                      if ((neighbor_face->refinement_case() | face_ref_case) ==
                          neighbor_face->refinement_case())
                        return false;
                      else
                        {
                          // if we are active, we
                          // must be an
                          // anisotropic child
                          // and the coming
                          // face_ref_case is
                          // isotropic. Thus,
                          // from our cell we
                          // will see exactly the
                          // opposite refine case
                          // that the face has
                          // now...
                          Assert(
                            face_ref_case ==
                              RefinementCase<dim - 1>::isotropic_refinement,
                            ExcInternalError());
                          expected_face_ref_case =
                            ~neighbor_face->refinement_case();
                          return true;
                        }
                    }
 
                // so, obviously we were not
                // one of the children, but a
                // grandchild. This is only
                // possible in 3d.
                Assert(dim == 3, ExcInternalError());
                // In that case, however, no
                // matter what the neighbor
                // does, it won't be finer
                // after the next refinement
                // step.
                return false;
              }
          } // if face will be refined
      }     // if neighbor is flagged for refinement
 
    // no cases left, so the neighbor will not
    // refine the face
    return false;
  }
 
  // version of above function for both active
  // and non-active cells
  template <int dim, int spacedim>
  bool
  face_will_be_refined_by_neighbor(
    const TriaIterator<::CellAccessor<dim, spacedim>> &cell,
    const unsigned int                                       face_no)
  {
    RefinementCase<dim - 1> dummy = RefinementCase<dim - 1>::no_refinement;
    return face_will_be_refined_by_neighbor_internal(cell, face_no, dummy);
  }
 
  // version of above function for active cells
  // only. Additionally returning the refine
  // case (to come) of the face under
  // consideration
  template <int dim, int spacedim>
  bool
  face_will_be_refined_by_neighbor(
    const TriaActiveIterator<::CellAccessor<dim, spacedim>> &cell,
    const unsigned int                                             face_no,
    RefinementCase<dim - 1> &expected_face_ref_case)
  {
    return face_will_be_refined_by_neighbor_internal(cell,
                                                     face_no,
                                                     expected_face_ref_case);
  }
 
 
 
  template <int dim, int spacedim>
  bool
  satisfies_level1_at_vertex_rule(
    const Triangulation<dim, spacedim> &triangulation)
  {
    std::vector<unsigned int> min_adjacent_cell_level(
      triangulation.n_vertices(), triangulation.n_levels());
    std::vector<unsigned int> max_adjacent_cell_level(
      triangulation.n_vertices(), 0);
 
    for (const auto &cell : triangulation.active_cell_iterators())
      for (const unsigned int v : cell->vertex_indices())
        {
          min_adjacent_cell_level[cell->vertex_index(v)] =
            std::min<unsigned int>(
              min_adjacent_cell_level[cell->vertex_index(v)], cell->level());
          max_adjacent_cell_level[cell->vertex_index(v)] =
            std::max<unsigned int>(
              min_adjacent_cell_level[cell->vertex_index(v)], cell->level());
        }
 
    for (unsigned int k = 0; k < triangulation.n_vertices(); ++k)
      if (triangulation.vertex_used(k))
        if (max_adjacent_cell_level[k] - min_adjacent_cell_level[k] > 1)
          return false;
    return true;
  }
 
 
 
  template <int dim, int spacedim>
  unsigned int
  middle_vertex_index(
    const typename Triangulation<dim, spacedim>::line_iterator &line)
  {
    if (line->has_children())
      return line->child(0)->vertex_index(1);
    return numbers::invalid_unsigned_int;
  }
 
 
  template <int dim, int spacedim>
  unsigned int
  middle_vertex_index(
    const typename Triangulation<dim, spacedim>::quad_iterator &quad)
  {
    switch (static_cast<unsigned char>(quad->refinement_case()))
      {
        case RefinementCase<2>::cut_x:
          return middle_vertex_index<dim, spacedim>(quad->child(0)->line(1));
          break;
        case RefinementCase<2>::cut_y:
          return middle_vertex_index<dim, spacedim>(quad->child(0)->line(3));
          break;
        case RefinementCase<2>::cut_xy:
          return quad->child(0)->vertex_index(3);
          break;
        default:
          break;
      }
    return numbers::invalid_unsigned_int;
  }
 
 
 
  template <class TRIANGULATION>
  typename TRIANGULATION::DistortedCellList
  collect_distorted_coarse_cells(const TRIANGULATION &)
  {
    return typename TRIANGULATION::DistortedCellList();
  }
 
 
 
  template <int dim>
  typename Triangulation<dim, dim>::DistortedCellList
  collect_distorted_coarse_cells(const Triangulation<dim, dim> &triangulation)
  {
    typename Triangulation<dim, dim>::DistortedCellList distorted_cells;
    for (const auto &cell : triangulation.cell_iterators_on_level(0))
      {
        Point<dim> vertices[GeometryInfo<dim>::vertices_per_cell];
        for (const unsigned int i : GeometryInfo<dim>::vertex_indices())
          vertices[i] = cell->vertex(i);
 
        Tensor<0, dim> determinants[GeometryInfo<dim>::vertices_per_cell];
        GeometryInfo<dim>::alternating_form_at_vertices(vertices, determinants);
 
        for (const unsigned int i : GeometryInfo<dim>::vertex_indices())
          if (determinants[i] <=
              1e-9 * Utilities::fixed_power<dim>(cell->diameter()))
            {
              distorted_cells.distorted_cells.push_back(cell);
              break;
            }
      }
 
    return distorted_cells;
  }
 
 
  template <int dim, int spacedim>
  bool
  has_distorted_children(
    const typename Triangulation<dim, spacedim>::cell_iterator &cell)
  {
    Assert(cell->has_children(), ExcInternalError());
    Assert(cell->reference_cell().is_hyper_cube(), ExcNotImplemented());
 
    if constexpr (dim == spacedim)
      {
        for (unsigned int c = 0; c < cell->n_children(); ++c)
          {
            // We call this function during mesh refinement, at a time
            // when the cell vertex cache has not yet been
            // re-built. As a consequence, we cannot call
            // cell->vertex(), or cell->diameter() (the latter
            // implicitly depending on the former). Instead, we have to
            // use a helper function to extract vertex locations into
            // an array that bypasses the cache.
 
            const unsigned int n_vertices = cell->child(c)->n_vertices();
            std_cxx26::inplace_vector<unsigned int,
                                      ReferenceCells::max_n_vertices<dim>()>
              child_vertex_indices(n_vertices);
            GridTools::internal::extract_vertices_without_cache<dim, spacedim>(
              cell->child(c), child_vertex_indices);
 
            Point<dim> child_vertices[GeometryInfo<dim>::vertices_per_cell];
            for (unsigned int i = 0; i < n_vertices; ++i)
              child_vertices[i] = cell->get_triangulation()
                                    .get_vertices()[child_vertex_indices[i]];
 
            Tensor<0, dim> determinants[GeometryInfo<dim>::vertices_per_cell];
            GeometryInfo<dim>::alternating_form_at_vertices(child_vertices,
                                                            determinants);
 
            double child_diameter = numbers::signaling_nan<double>();
            if constexpr (dim == 1)
              child_diameter = (child_vertices[1] - child_vertices[0]).norm();
            else if constexpr (dim == 2)
              // Take the longer one of the two diagonals of the quadrilateral
              child_diameter =
                std::max({(child_vertices[3] - child_vertices[0]).norm(),
                          (child_vertices[2] - child_vertices[1]).norm()});
            else if constexpr (dim == 3)
              // Take the longest of the four diagonals of the hexahedron
              child_diameter =
                std::max({(child_vertices[7] - child_vertices[0]).norm(),
                          (child_vertices[6] - child_vertices[1]).norm(),
                          (child_vertices[2] - child_vertices[5]).norm(),
                          (child_vertices[3] - child_vertices[4]).norm()});
            else
              DEAL_II_NOT_IMPLEMENTED();
 
            for (const unsigned int i : GeometryInfo<dim>::vertex_indices())
              if (determinants[i] <=
                  1e-9 * Utilities::fixed_power<dim>(child_diameter))
                return true;
          }
 
        return false;
      }
    else
      // Like for collect_distorted_coarse_cells, there is nothing
      // that we can do in this case.
      return false;
  }
 
 
 
  template <int dim, int spacedim>
  void
  update_periodic_face_map_recursively(
    const typename Triangulation<dim, spacedim>::cell_iterator &cell_1,
    const typename Triangulation<dim, spacedim>::cell_iterator &cell_2,
    unsigned int                                                n_face_1,
    unsigned int                                                n_face_2,
    const types::geometric_orientation                          orientation,
    typename std::map<
      std::pair<typename Triangulation<dim, spacedim>::cell_iterator,
                unsigned int>,
      std::pair<std::pair<typename Triangulation<dim, spacedim>::cell_iterator,
                          unsigned int>,
                types::geometric_orientation>> &periodic_face_map)
  {
    using FaceIterator = typename Triangulation<dim, spacedim>::face_iterator;
    const FaceIterator face_1 = cell_1->face(n_face_1);
    const FaceIterator face_2 = cell_2->face(n_face_2);
 
    const auto inverse_orientation =
      face_1->reference_cell().get_inverse_combined_orientation(orientation);
 
    if constexpr (running_in_debug_mode())
      {
        const auto [face_orientation, face_rotation, face_flip] =
          internal::split_face_orientation(orientation);
 
        Assert((dim != 1) || (face_orientation == true && face_flip == false &&
                              face_rotation == false),
               ExcMessage("The supplied orientation "
                          "(face_orientation, face_flip, face_rotation) "
                          "is invalid for 1d"));
 
        Assert((dim != 2) || (face_flip == false && face_rotation == false),
               ExcMessage("The supplied orientation "
                          "(face_orientation, face_flip, face_rotation) "
                          "is invalid for 2d"));
      }
 
    Assert(face_1 != face_2, ExcMessage("face_1 and face_2 are equal!"));
 
    Assert(face_1->at_boundary() && face_2->at_boundary(),
           ExcMessage("Periodic faces must be on the boundary"));
 
    // Check if the requirement that each edge can only have at most one hanging
    // node, and as a consequence neighboring cells can differ by at most
    // one refinement level is enforced. In 1d, there are no hanging nodes and
    // so neighboring cells can differ by more than one refinement level.
    Assert(dim == 1 || std::abs(cell_1->level() - cell_2->level()) < 2,
           ExcInternalError());
 
    // insert periodic face pair for both cells
    using CellFace =
      std::pair<typename Triangulation<dim, spacedim>::cell_iterator,
                unsigned int>;
    const CellFace cell_face_1(cell_1, n_face_1);
    const CellFace cell_face_2(cell_2, n_face_2);
    const std::pair<CellFace, types::geometric_orientation>
      cell_face_orientation_2(cell_face_2, orientation);
 
    const std::pair<CellFace, std::pair<CellFace, types::geometric_orientation>>
      periodic_faces(cell_face_1, cell_face_orientation_2);
 
    // Only one periodic neighbor is allowed
    Assert(periodic_face_map.count(cell_face_1) == 0, ExcInternalError());
    periodic_face_map.insert(periodic_faces);
 
    if (dim == 1)
      {
        if (cell_1->has_children())
          {
            if (cell_2->has_children())
              {
                update_periodic_face_map_recursively<dim, spacedim>(
                  cell_1->child(n_face_1),
                  cell_2->child(n_face_2),
                  n_face_1,
                  n_face_2,
                  orientation,
                  periodic_face_map);
              }
            else // only face_1 has children
              {
                update_periodic_face_map_recursively<dim, spacedim>(
                  cell_1->child(n_face_1),
                  cell_2,
                  n_face_1,
                  n_face_2,
                  orientation,
                  periodic_face_map);
              }
          }
      }
    else // dim == 2 || dim == 3
      {
        if (cell_1->has_children())
          {
            if (cell_2->has_children())
              {
                // In the case that both faces have children, we loop over all
                // children and apply update_periodic_face_map_recursively
                // recursively:
 
                Assert(face_1->n_children() ==
                           GeometryInfo<dim>::max_children_per_face &&
                         face_2->n_children() ==
                           GeometryInfo<dim>::max_children_per_face,
                       ExcNotImplemented());
 
                const auto reference_cell = cell_1->reference_cell();
 
                for (unsigned int i = 0;
                     i < GeometryInfo<dim>::max_children_per_face;
                     ++i)
                  {
                    // Lookup the index for the second face
                    const unsigned int j =
                      reference_cell.standard_to_real_face_vertex(
                        i, n_face_1, inverse_orientation);
 
                    // find subcell ids that belong to the subface indices
                    unsigned int child_cell_1 =
                      GeometryInfo<dim>::child_cell_on_face(
                        cell_1->refinement_case(),
                        n_face_1,
                        i,
                        cell_1->face_orientation(n_face_1),
                        cell_1->face_flip(n_face_1),
                        cell_1->face_rotation(n_face_1),
                        face_1->refinement_case());
                    unsigned int child_cell_2 =
                      GeometryInfo<dim>::child_cell_on_face(
                        cell_2->refinement_case(),
                        n_face_2,
                        j,
                        cell_2->face_orientation(n_face_2),
                        cell_2->face_flip(n_face_2),
                        cell_2->face_rotation(n_face_2),
                        face_2->refinement_case());
 
                    Assert(cell_1->child(child_cell_1)->face(n_face_1) ==
                             face_1->child(i),
                           ExcInternalError());
                    Assert(cell_2->child(child_cell_2)->face(n_face_2) ==
                             face_2->child(j),
                           ExcInternalError());
 
                    // precondition: subcell has the same orientation as cell
                    // (so that the face numbers coincide) recursive call
                    update_periodic_face_map_recursively<dim, spacedim>(
                      cell_1->child(child_cell_1),
                      cell_2->child(child_cell_2),
                      n_face_1,
                      n_face_2,
                      orientation,
                      periodic_face_map);
                  }
              }
            else // only face_1 has children
              {
                for (unsigned int i = 0;
                     i < GeometryInfo<dim>::max_children_per_face;
                     ++i)
                  {
                    // find subcell ids that belong to the subface indices
                    unsigned int child_cell_1 =
                      GeometryInfo<dim>::child_cell_on_face(
                        cell_1->refinement_case(),
                        n_face_1,
                        i,
                        cell_1->face_orientation(n_face_1),
                        cell_1->face_flip(n_face_1),
                        cell_1->face_rotation(n_face_1),
                        face_1->refinement_case());
 
                    // recursive call
                    update_periodic_face_map_recursively<dim, spacedim>(
                      cell_1->child(child_cell_1),
                      cell_2,
                      n_face_1,
                      n_face_2,
                      orientation,
                      periodic_face_map);
                  }
              }
          }
      }
  }
 
  // Given the child number and parent's line orientation, return the child face
  // number.
  unsigned int
  child_line_index(const unsigned int                 child_no,
                   const types::geometric_orientation line_orientation)
  {
    AssertIndexRange(child_no, ReferenceCells::Line.n_children());
    Assert(line_orientation == numbers::default_geometric_orientation ||
             line_orientation == numbers::reverse_line_orientation,
           ExcInternalError());
    constexpr auto D = numbers::default_geometric_orientation;
    if (child_no == 0)
      return line_orientation == D ? 0 : 1;
    else
      return line_orientation == D ? 1 : 0;
  }
} // end of anonymous namespace
 
 
namespace internal
{
  namespace TriangulationImplementation
  {
    // make sure that if in the following we
    // write Triangulation<dim,spacedim>
    // we mean the *class*
    // ::Triangulation, not the
    // enclosing namespace
    // internal::TriangulationImplementation
    using ::Triangulation;
 
    DeclException1(ExcInternalErrorOnCell,
                   int,
                   << "Something went wrong upon construction of cell "
                   << arg1);
    DeclException1(ExcCellHasNegativeMeasure,
                   int,
                   << "Cell " << arg1
                   << " has negative measure. This typically "
                   << "indicates some distortion in the cell, or a mistakenly "
                   << "swapped pair of vertices in the input to "
                   << "Triangulation::create_triangulation().");
    DeclException3(ExcInvalidVertexIndex,
                   int,
                   int,
                   int,
                   << "Error while creating cell " << arg1
                   << ": the vertex index " << arg2 << " must be between 0 and "
                   << arg3 << '.');
    DeclException3(
      ExcInteriorLineCantBeBoundary,
      int,
      int,
      types::boundary_id,
      << "The input data for creating a triangulation contained "
      << "information about a line with indices " << arg1 << " and " << arg2
      << " that is described to have boundary indicator "
      << static_cast<int>(arg3)
      << ". However, this is an internal line not located on the "
      << "boundary. You cannot assign a boundary indicator to it." << std::endl
      << std::endl
      << "If this happened at a place where you call "
      << "Triangulation::create_triangulation() yourself, you need "
      << "to check the SubCellData object you pass to this function."
      << std::endl
      << std::endl
      << "If this happened in a place where you are reading a mesh "
      << "from a file, then you need to investigate why such a line "
      << "ended up in the input file. A typical case is a geometry "
      << "that consisted of multiple parts and for which the mesh "
      << "generator program assumes that the interface between "
      << "two parts is a boundary when that isn't supposed to be "
      << "the case, or where the mesh generator simply assigns "
      << "'geometry indicators' to lines at the perimeter of "
      << "a part that are not supposed to be interpreted as "
      << "'boundary indicators'.");
    DeclException5(
      ExcInteriorQuadCantBeBoundary,
      int,
      int,
      int,
      int,
      types::boundary_id,
      << "The input data for creating a triangulation contained "
      << "information about a quad with indices " << arg1 << ", " << arg2
      << ", " << arg3 << ", and " << arg4
      << " that is described to have boundary indicator "
      << static_cast<int>(arg5)
      << ". However, this is an internal quad not located on the "
      << "boundary. You cannot assign a boundary indicator to it." << std::endl
      << std::endl
      << "If this happened at a place where you call "
      << "Triangulation::create_triangulation() yourself, you need "
      << "to check the SubCellData object you pass to this function."
      << std::endl
      << std::endl
      << "If this happened in a place where you are reading a mesh "
      << "from a file, then you need to investigate why such a quad "
      << "ended up in the input file. A typical case is a geometry "
      << "that consisted of multiple parts and for which the mesh "
      << "generator program assumes that the interface between "
      << "two parts is a boundary when that isn't supposed to be "
      << "the case, or where the mesh generator simply assigns "
      << "'geometry indicators' to quads at the surface of "
      << "a part that are not supposed to be interpreted as "
      << "'boundary indicators'.");
    DeclException2(
      ExcMultiplySetLineInfoOfLine,
      int,
      int,
      << "In SubCellData the line info of the line with vertex indices " << arg1
      << " and " << arg2 << " appears more than once. "
      << "This is not allowed.");
    DeclException3(
      ExcInconsistentLineInfoOfLine,
      int,
      int,
      std::string,
      << "In SubCellData the line info of the line with vertex indices " << arg1
      << " and " << arg2 << " appears multiple times with different (valid) "
      << arg3 << ". This is not allowed.");
    DeclException5(
      ExcInconsistentQuadInfoOfQuad,
      int,
      int,
      int,
      int,
      std::string,
      << "In SubCellData the quad info of the quad with line indices " << arg1
      << ", " << arg2 << ", " << arg3 << " and " << arg4
      << " appears multiple times with different (valid) " << arg5
      << ". This is not allowed.");
 
    DeclException2(ExcMemoryInexact,
                   int,
                   int,
                   << "The containers have sizes " << arg1 << " and " << arg2
                   << ", which is not as expected.");
 
    template <int dim, int spacedim>
    void
    monitor_memory(const TriaLevel<dim, spacedim> &tria_level,
                   const unsigned int              true_dimension)
    {
      Assert(2 * true_dimension * tria_level.refine_flags.size() ==
               tria_level.neighbors.size(),
             ExcMemoryInexact(tria_level.refine_flags.size(),
                              tria_level.neighbors.size()));
      Assert(2 * true_dimension * tria_level.coarsen_flags.size() ==
               tria_level.neighbors.size(),
             ExcMemoryInexact(tria_level.coarsen_flags.size(),
                              tria_level.neighbors.size()));
    }
 
 
 
    void
    monitor_memory(const TriaObjects &tria_object, const unsigned int)
    {
      Assert(tria_object.n_objects() == tria_object.used.size(),
             ExcMemoryInexact(tria_object.n_objects(),
                              tria_object.used.size()));
      Assert(tria_object.n_objects() == tria_object.user_flags.size(),
             ExcMemoryInexact(tria_object.n_objects(),
                              tria_object.user_flags.size()));
      Assert(tria_object.n_objects() ==
               tria_object.boundary_or_material_id.size(),
             ExcMemoryInexact(tria_object.n_objects(),
                              tria_object.boundary_or_material_id.size()));
      Assert(tria_object.n_objects() == tria_object.manifold_id.size(),
             ExcMemoryInexact(tria_object.n_objects(),
                              tria_object.manifold_id.size()));
      Assert(tria_object.n_objects() == tria_object.user_data.size(),
             ExcMemoryInexact(tria_object.n_objects(),
                              tria_object.user_data.size()));
 
      if (tria_object.structdim > 0)
        {
          const unsigned int factor = tria_object.children_per_object / 2;
          Assert(factor * tria_object.n_objects() ==
                   tria_object.children.size(),
                 ExcMemoryInexact(tria_object.n_objects(),
                                  tria_object.children.size()));
        }
    }
  } // namespace TriangulationImplementation
} // namespace internal
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
class Triangulation<dim, spacedim>::Policy
{
public:
  virtual ~Policy() = default;
 
  virtual void
  update_neighbors(Triangulation<dim, spacedim> &tria) = 0;
 
  virtual void
  delete_children(Triangulation<dim, spacedim> &triangulation,
                  typename Triangulation<dim, spacedim>::cell_iterator &cell,
                  std::vector<unsigned int> &line_cell_count,
                  std::vector<unsigned int> &quad_cell_count) = 0;
 
  virtual typename Triangulation<dim, spacedim>::DistortedCellList
  execute_refinement(Triangulation<dim, spacedim> &triangulation,
                     const bool check_for_distorted_cells) = 0;
 
  virtual void
  prevent_distorted_boundary_cells(
    Triangulation<dim, spacedim> &triangulation) = 0;
 
  virtual void
  prepare_refinement_dim_dependent(
    Triangulation<dim, spacedim> &triangulation) = 0;
 
  virtual bool
  coarsening_allowed(
    const typename Triangulation<dim, spacedim>::cell_iterator &cell) = 0;
 
  virtual std::unique_ptr<Policy>
  clone() = 0;
};
 
 
namespace internal
{
  namespace TriangulationImplementation
  {
 
    template <int dim, int spacedim, typename T>
    class PolicyWrapper : public Triangulation<dim, spacedim>::Policy
    {
    public:
      void
      update_neighbors(Triangulation<dim, spacedim> &tria) override
      {
        T::update_neighbors(tria);
      }
 
      void
      delete_children(
        Triangulation<dim, spacedim>                         &tria,
        typename Triangulation<dim, spacedim>::cell_iterator &cell,
        std::vector<unsigned int>                            &line_cell_count,
        std::vector<unsigned int> &quad_cell_count) override
      {
        T::delete_children(tria, cell, line_cell_count, quad_cell_count);
      }
 
      typename Triangulation<dim, spacedim>::DistortedCellList
      execute_refinement(Triangulation<dim, spacedim> &triangulation,
                         const bool check_for_distorted_cells) override
      {
        return T::execute_refinement(triangulation, check_for_distorted_cells);
      }
 
      void
      prevent_distorted_boundary_cells(
        Triangulation<dim, spacedim> &triangulation) override
      {
        T::prevent_distorted_boundary_cells(triangulation);
      }
 
      void
      prepare_refinement_dim_dependent(
        Triangulation<dim, spacedim> &triangulation) override
      {
        T::prepare_refinement_dim_dependent(triangulation);
      }
 
      bool
      coarsening_allowed(
        const typename Triangulation<dim, spacedim>::cell_iterator &cell)
        override
      {
        return T::template coarsening_allowed<dim, spacedim>(cell);
      }
 
      std::unique_ptr<typename Triangulation<dim, spacedim>::Policy>
      clone() override
      {
        return std::make_unique<PolicyWrapper<dim, spacedim, T>>();
      }
    };
 
 
    struct ArrayOfArrays
    {
      ArrayOfArrays()
        : offsets{0} {};
 
      ArrayOfArrays(const ArrayOfArrays &) = default;
 
      ArrayOfArrays(ArrayOfArrays &&) = default;
 
      ArrayOfArrays(const std::vector<std::size_t>  &offsets,
                    const std::vector<unsigned int> &columns)
        : offsets(offsets)
        , columns(columns)
      {}
 
      ArrayOfArrays(std::vector<std::size_t>  &&offsets,
                    std::vector<unsigned int> &&columns)
        : offsets(std::move(offsets))
        , columns(std::move(columns))
      {}
 
      ArrayOfArrays &
      operator=(const ArrayOfArrays &) = default;
 
      ArrayOfArrays &
      operator=(ArrayOfArrays &&) = default;
 
      unsigned int
      size() const
      {
        return offsets.size() - 1;
      }
 
      ArrayView<const unsigned int>
      operator[](const unsigned int i) const
      {
        AssertIndexRange(i, offsets.size() - 1);
        return ArrayView<const unsigned int>(&columns[offsets[i]],
                                             offsets[i + 1] - offsets[i]);
      }
 
    private:
      std::vector<std::size_t> offsets;
 
      std::vector<unsigned int> columns;
    };
 
 
 
    template <int dim>
    struct Connectivity
    {
      Connectivity(const std::vector<ReferenceCell<dim>> &cell_types,
                   const std::set<ReferenceCell<dim>>    &all_reference_cells)
        : cell_types(cell_types)
        , all_reference_cells_(all_reference_cells)
      {
        if constexpr (running_in_debug_mode())
          for (const auto &reference_cell : cell_types)
            Assert(all_reference_cells_.count(reference_cell) == 1u,
                   ExcInternalError());
      }
 
      const std::set<ReferenceCell<dim>> &
      all_reference_cells() const
      {
        return all_reference_cells_;
      }
 
      std::vector<types::geometric_orientation> &
      entity_orientations(const unsigned int structdim)
      {
        if (structdim == 1)
          return line_orientation;
 
        AssertDimension(structdim, 2);
 
        return quad_orientation;
      }
 
      const std::vector<types::geometric_orientation> &
      entity_orientations(const unsigned int structdim) const
      {
        if (structdim == 1)
          return line_orientation;
 
        AssertDimension(structdim, 2);
 
        return quad_orientation;
      }
 
      template <int structdim>
      std::vector<ReferenceCell<structdim>> &
      entity_types()
      {
        if constexpr (structdim == dim)
          return cell_types;
        else if constexpr ((structdim == 2) && (dim == 3))
          {
            return quad_types;
          }
        else
          // for vertices/lines the entity types are clear
          // and we shouldn't need this function
          DEAL_II_ASSERT_UNREACHABLE();
      }
 
      template <int structdim>
      const std::vector<ReferenceCell<structdim>> &
      entity_types() const
      {
        if constexpr (structdim == dim)
          return cell_types;
        else if constexpr ((structdim == 2) && (dim == 3))
          {
            return quad_types;
          }
        else
          // for vertices/lines the entity types are clear
          // and we shouldn't need this function
          DEAL_II_ASSERT_UNREACHABLE();
      }
 
      ArrayOfArrays &
      entity_to_entities(const unsigned int from, const unsigned int to)
      {
        if (from == dim && to == dim)
          return neighbors;
        else if (from == dim && to == dim - 1)
          return cell_entities;
        else if (dim == 3 && from == 2 && to == 0)
          return quad_vertices;
        else if (dim == 3 && from == 2 && to == 1)
          return quad_lines;
        else if (from == 1 && to == 0)
          return line_vertices;
 
        DEAL_II_NOT_IMPLEMENTED();
 
        return cell_entities;
      }
 
      const ArrayOfArrays &
      entity_to_entities(const unsigned int from, const unsigned int to) const
      {
        if (from == dim && to == dim)
          return neighbors;
        else if (from == dim && to == dim - 1)
          return cell_entities;
        else if (dim == 3 && from == 2 && to == 0)
          return quad_vertices;
        else if (dim == 3 && from == 2 && to == 1)
          return quad_lines;
        else if (from == 1 && to == 0)
          return line_vertices;
 
        DEAL_II_NOT_IMPLEMENTED();
 
        return cell_entities;
      }
 
    private:
      std::vector<ReferenceCell<dim>> cell_types;
 
      std::set<ReferenceCell<dim>> all_reference_cells_;
 
      ArrayOfArrays line_vertices;
 
      std::vector<types::geometric_orientation> line_orientation;
 
      ArrayOfArrays quad_vertices;
      ArrayOfArrays quad_lines;
 
      std::vector<types::geometric_orientation> quad_orientation;
 
      ArrayOfArrays cell_entities;
      ArrayOfArrays neighbors;
 
      std::vector<ReferenceCell<2>> quad_types;
    };
 
 
 
    ArrayOfArrays
    determine_neighbors(const ArrayOfArrays &cells_to_faces)
    {
      // First determine the maximal face number used. The number of faces
      // is then one more (faces are counted starting at zero):
      unsigned int n_faces = 0;
      for (unsigned int cell = 0; cell < cells_to_faces.size(); ++cell)
        {
          const ArrayView<const unsigned int> face_indices =
            cells_to_faces[cell];
          n_faces = std::max(n_faces,
                             *std::max_element(face_indices.begin(),
                                               face_indices.end()) +
                               1);
        }
 
      std::vector<std::size_t> cells_to_cells_offsets(cells_to_faces.size() + 1,
                                                      0u);
      for (unsigned int cell = 0; cell < cells_to_faces.size(); ++cell)
        {
          const ArrayView<const unsigned int> face_indices =
            cells_to_faces[cell];
          cells_to_cells_offsets[cell + 1] =
            cells_to_cells_offsets[cell] + face_indices.size();
        }
      std::vector<unsigned int> cells_to_cells_columns(
        cells_to_cells_offsets.back(),
        /* default: cells is at the boundary in this direction */ -1);
 
      std::vector<std::pair<unsigned int, unsigned int>> neighbors(
        n_faces,
        std::make_pair(numbers::invalid_unsigned_int,
                       numbers::invalid_unsigned_int));
 
      // loop over all cells
      unsigned int global_face_index = 0;
      for (unsigned int cell = 0; cell < cells_to_faces.size(); ++cell)
        {
          // ... and all its faces
          const auto faces = cells_to_faces[cell];
          for (unsigned int f = 0; f < faces.size(); ++f, ++global_face_index)
            {
              if (neighbors[faces[f]].first == numbers::invalid_unsigned_int)
                {
                  // face is visited the first time -> save the visiting cell
                  // and the face pointer
                  neighbors[faces[f]] = std::make_pair(cell, global_face_index);
                }
              else
                {
                  // face is visited the second time -> now we know the cells
                  // on both sides of the face and we can determine for both
                  // cells the neighbor
                  cells_to_cells_columns[global_face_index] =
                    neighbors[faces[f]].first;
                  cells_to_cells_columns[neighbors[faces[f]].second] = cell;
                }
            }
        }
 
      return ArrayOfArrays(std::move(cells_to_cells_offsets),
                           std::move(cells_to_cells_columns));
    }
 
 
 
    template <int max_n_vertices, int face_dim, int dim, typename FU>
    void
    build_face_entities_templated(
      const std::vector<ReferenceCell<dim>>     &cell_types,
      const ArrayOfArrays                       &cells_to_vertices,
      ArrayOfArrays                             &cells_to_faces, // result
      ArrayOfArrays                             &crs_0,          // result
      std::vector<types::geometric_orientation> &orientations,   // result
      const FU                                  &second_key_function)
    {
      const bool compatibility_mode = true;
 
      // note: we do not pre-allocate memory for these arrays because it turned
      // out that counting unique entities is more expensive than push_back().
      std::vector<std::size_t>  offsets_0;
      std::vector<unsigned int> columns_0;
 
      unsigned int n_entities = 0;
 
      for (const auto &c : cell_types)
        {
          // Make sure that there are only two possibilities for
          // face_dim that we can cover with the ?: statement below:
          Assert((face_dim == dim - 1) || ((dim == 3) && (face_dim == 1)),
                 ExcInternalError());
          n_entities += (face_dim == dim - 1 ? c.n_faces() : c.n_lines());
        }
 
      // step 1: store each d-dimensional entity of a cell (described by their
      // vertices) into a vector and create a key for them
      //
      // note: it turned out to be more efficient to have a vector of tuples
      // than to have two vectors (sorting becomes inefficient)
      std::vector<
        std::tuple<std::array<unsigned int, max_n_vertices>, unsigned int>>
        keys; // key (sorted vertices), cell-entity index
 
      std::vector<std::array<unsigned int, max_n_vertices>> ad_entity_vertices;
      std::vector<ReferenceCell<face_dim>>                  ad_entity_types;
      std::vector<std::array<unsigned int, max_n_vertices>> ad_compatibility;
 
      keys.reserve(n_entities);
      ad_entity_vertices.reserve(n_entities);
      ad_entity_types.reserve(n_entities);
      ad_compatibility.reserve(n_entities);
 
      std::vector<std::size_t> cells_to_faces_offsets(cell_types.size() + 1);
      cells_to_faces_offsets[0] = 0;
 
      constexpr unsigned int offset = 1;
 
      // loop over all cells
      for (unsigned int c = 0, counter = 0; c < cell_types.size(); ++c)
        {
          const auto &cell_type = cell_types[c];
 
          // Make sure that there are only two possibilities for
          // face_dimensionality that we can cover with the ?: statement below:
          Assert((face_dim == dim - 1) || ((dim == 3) && (face_dim == 1)),
                 ExcInternalError());
          const unsigned int n_face_entities =
            (face_dim == dim - 1 ? cell_type.n_faces() : cell_type.n_lines());
          cells_to_faces_offsets[c + 1] =
            cells_to_faces_offsets[c] + n_face_entities;
 
          // ... loop over all its entities
          const ArrayView<const unsigned int> local_vertices =
            cells_to_vertices[c];
          for (unsigned int e = 0; e < n_face_entities; ++e)
            {
              // ... determine global entity vertices
              std::array<unsigned int, max_n_vertices> entity_vertices;
              std::fill(entity_vertices.begin(), entity_vertices.end(), 0);
 
              // Same as above, make sure that there are only two possibilities
              // for face_dimensionality that we can cover with the ?: statement
              // below:
              Assert((face_dim == dim - 1) || ((dim == 3) && (face_dim == 1)),
                     ExcInternalError());
              for (unsigned int i = 0;
                   i < (face_dim == dim - 1 ?
                          cell_type.face_reference_cell(e).n_vertices() :
                          ReferenceCells::Line.n_vertices());
                   ++i)
                entity_vertices[i] =
                  local_vertices[face_dim == dim - 1 ?
                                   cell_type.face_to_cell_vertices(
                                     e,
                                     i,
                                     numbers::default_geometric_orientation) :
                                   cell_type.line_to_cell_vertices(e, i)] +
                  offset;
 
              // ... create key
              std::array<unsigned int, max_n_vertices> key = entity_vertices;
              std::sort(key.begin(), key.end());
              keys.emplace_back(key, counter++);
 
              ad_entity_vertices.emplace_back(entity_vertices);
 
              if constexpr (face_dim == dim - 1)
                ad_entity_types.emplace_back(cell_type.face_reference_cell(e));
              else if constexpr (face_dim == dim - 2)
                {
                  // Since we only deal with meshes up to dimension 3,
                  // something that has co-dimensionality -2 must either be
                  // a vertex or a line, regardless of what the object we are
                  // working on actually us:
                  if constexpr (face_dim == 0)
                    ad_entity_types.emplace_back(ReferenceCells::Vertex);
                  else if constexpr (face_dim == 1)
                    ad_entity_types.emplace_back(ReferenceCells::Line);
                  else
                    // But it's probably useful to be conservative if someone
                    // ever comes along to implement 4d meshes:
                    DEAL_II_ASSERT_UNREACHABLE();
                }
              else
                DEAL_II_ASSERT_UNREACHABLE();
 
              if (compatibility_mode)
                ad_compatibility.emplace_back(
                  second_key_function(entity_vertices, cell_type, c, e));
            }
        }
 
      std::vector<unsigned int> cells_to_faces_indices(keys.size());
      orientations.assign(keys.size(), numbers::default_geometric_orientation);
 
      // step 2: sort according to key so that entities with same key can be
      // merged
      std::sort(keys.begin(), keys.end());
 
      if (compatibility_mode)
        {
          unsigned int n_unique_entities        = 0;
          unsigned int n_unique_entity_vertices = 0;
 
          std::array<unsigned int, max_n_vertices> ref_key, new_key;
          std::fill(ref_key.begin(), ref_key.end(), 0);
          for (unsigned int i = 0; i < keys.size(); ++i)
            {
              const auto offset_i = std::get<1>(keys[i]);
 
              if (ref_key != std::get<0>(keys[i]))
                {
                  ref_key = std::get<0>(keys[i]);
 
                  ++n_unique_entities;
                  n_unique_entity_vertices +=
                    ad_entity_types[offset_i].n_vertices();
 
                  new_key = ad_compatibility[offset_i];
                }
 
              std::get<0>(keys[i]) = new_key;
            }
 
          std::sort(keys.begin(), keys.end());
 
          offsets_0.reserve(n_unique_entities + 1);
          columns_0.reserve(n_unique_entity_vertices);
        }
 
 
      std::array<unsigned int, max_n_vertices> ref_key;
      std::array<unsigned int, max_n_vertices> ref_indices;
      std::fill(ref_key.begin(), ref_key.end(), 0);
 
      unsigned int counter = ::numbers::invalid_unsigned_int;
      for (unsigned int i = 0; i < keys.size(); i++)
        {
          const auto offset_i = std::get<1>(keys[i]);
 
          if (ref_key != std::get<0>(keys[i]))
            {
              // new key: default orientation is correct
              ++counter;
              ref_key     = std::get<0>(keys[i]);
              ref_indices = ad_entity_vertices[offset_i];
 
              offsets_0.push_back(columns_0.size());
              for (const auto j : ad_entity_vertices[offset_i])
                if (j != 0)
                  columns_0.push_back(j - offset);
            }
          else
            {
              // previously seen key: set orientation relative to the first
              // occurrence
              orientations[offset_i] =
                ad_entity_types[offset_i]
                  .template get_combined_orientation<unsigned int>(
                    make_array_view(ad_entity_vertices[offset_i].begin(),
                                    ad_entity_vertices[offset_i].begin() +
                                      ad_entity_types[offset_i].n_vertices()),
                    make_array_view(ref_indices.begin(),
                                    ref_indices.begin() +
                                      ad_entity_types[offset_i].n_vertices()));
            }
          cells_to_faces_indices[offset_i] = counter;
        }
      offsets_0.push_back(columns_0.size());
 
      // Finally build the output objects:
      crs_0 = ArrayOfArrays(std::move(offsets_0), std::move(columns_0));
      cells_to_faces = ArrayOfArrays(std::move(cells_to_faces_offsets),
                                     std::move(cells_to_faces_indices));
    }
 
 
 
    template <int face_dim, int dim, typename FU>
    void
    build_face_entities(const std::vector<ReferenceCell<dim>> &cell_types,
                        const ArrayOfArrays &cells_to_vertices,
                        ArrayOfArrays       &cells_to_faces,
                        ArrayOfArrays       &crs_0,
                        std::vector<types::geometric_orientation> &orientations,
                        const FU &second_key_function)
    {
      unsigned int max_n_vertices = 0;
 
      // If we are dealing with faces of cells, figure out how many vertices
      // each face may have. Otherwise, we're in 3d and are dealing with
      // lines, for which we know the number of vertices:
      for (const auto &c : cell_types)
        if (face_dim == dim - 1)
          {
            for (unsigned int f = 0; f < c.n_faces(); ++f)
              max_n_vertices =
                std::max(max_n_vertices, c.face_reference_cell(f).n_vertices());
          }
        else if (face_dim == 1)
          max_n_vertices = std::max(max_n_vertices, 2u);
        else
          DEAL_II_ASSERT_UNREACHABLE();
 
      if (max_n_vertices == 2)
        build_face_entities_templated<2, face_dim>(cell_types,
                                                   cells_to_vertices,
                                                   cells_to_faces,
                                                   crs_0,
                                                   orientations,
                                                   second_key_function);
      else if (max_n_vertices == 3)
        build_face_entities_templated<3, face_dim>(cell_types,
                                                   cells_to_vertices,
                                                   cells_to_faces,
                                                   crs_0,
                                                   orientations,
                                                   second_key_function);
      else if (max_n_vertices == 4)
        build_face_entities_templated<4, face_dim>(cell_types,
                                                   cells_to_vertices,
                                                   cells_to_faces,
                                                   crs_0,
                                                   orientations,
                                                   second_key_function);
      else
        AssertThrow(false, ::StandardExceptions::ExcNotImplemented());
    }
 
 
 
    template <int dim>
    void
    build_intersection(
      const std::vector<ReferenceCell<dim>>           &cell_types,
      const ArrayOfArrays                             &cells_to_vertices,
      const ArrayOfArrays                             &cells_to_lines,
      const ArrayOfArrays                             &lines_to_vertices,
      const ArrayOfArrays                             &cells_to_quads,
      const ArrayOfArrays                             &quads_to_vertices,
      const std::vector<types::geometric_orientation> &ori_cq,
      ArrayOfArrays                                   &quads_to_lines, // result
      std::vector<types::geometric_orientation>       &ori_ql,         // result
      std::vector<ReferenceCell<2>>                   &quad_t_id       // result
    )
    {
      quad_t_id.resize(quads_to_vertices.size());
 
      // Build the offsets of the compressed representation of the
      // quads-to-lines array by counting the number of lines of each face.
      // We do this by looping over all cells and querying their faces.
      // This encounters faces in a random order, so we first
      // store the number of lines of the f'th face in the (f+1)st
      // offset and then run a prefix sum to sum up all
      // previous offsets
      std::vector<std::size_t> quads_to_lines_offsets(quads_to_vertices.size() +
                                                      1);
      for (unsigned int c = 0; c < cells_to_quads.size(); ++c)
        {
          // loop over faces
          const ArrayView<const unsigned int> faces = cells_to_quads[c];
          for (unsigned int f = 0; f < faces.size(); ++f)
            {
              quads_to_lines_offsets[faces[f] + 1] =
                cell_types[c].face_reference_cell(f).n_lines();
            }
        }
      quads_to_lines_offsets[0] = 0;
      for (unsigned int i = 0; i < quads_to_lines_offsets.size() - 1; ++i)
        quads_to_lines_offsets[i + 1] += quads_to_lines_offsets[i];
 
      // Now that we have the offsets, allocate memory for the indices:
      std::vector<unsigned int> quads_to_lines_indices(
        quads_to_lines_offsets.back());
      ori_ql.assign(quads_to_lines_offsets.back(),
                    numbers::default_geometric_orientation);
 
      // loop over cells
      unsigned int global_face_index = 0;
      for (unsigned int c = 0; c < cells_to_quads.size(); ++c)
        {
          const ReferenceCell<dim> cell_type = cell_types[c];
 
          // loop over faces
          const ArrayView<const unsigned int> faces = cells_to_quads[c];
          for (unsigned int f_index = 0; f_index < faces.size();
               ++f_index, ++global_face_index)
            {
              const unsigned int f = faces[f_index];
 
              // only faces with default orientation have to do something
              if (ori_cq[global_face_index] !=
                  numbers::default_geometric_orientation)
                continue;
 
              // loop over the lines of this face
              quad_t_id[f] = cell_type.face_reference_cell(f_index);
              for (unsigned int l = 0; l < quad_t_id[f].n_lines(); ++l)
                {
                  // determine global index of line
                  const unsigned int local_line_index =
                    cell_type.face_to_cell_lines(
                      f_index, l, numbers::default_geometric_orientation);
                  const unsigned int global_line_index =
                    cells_to_lines[c][local_line_index];
                  quads_to_lines_indices[quads_to_lines_offsets[f] + l] =
                    global_line_index;
 
                  // determine orientation of line
                  bool same = true;
                  for (unsigned int v = 0; v < 2; ++v)
                    if (cells_to_vertices
                          [c][cell_type.face_and_line_to_cell_vertices(
                            f_index,
                            l,
                            v,
                            numbers::default_geometric_orientation)] !=
                        lines_to_vertices[global_line_index][v])
                      {
                        same = false;
                        break;
                      }
 
                  // ... comparison gives orientation
                  ori_ql[quads_to_lines_offsets[f] + l] =
                    same ? numbers::default_geometric_orientation :
                           numbers::reverse_line_orientation;
                }
            }
        }
 
      // Finally build the output object:
      quads_to_lines = ArrayOfArrays(std::move(quads_to_lines_offsets),
                                     std::move(quads_to_lines_indices));
    }
 
 
 
    template <int dim>
    Connectivity<dim>
    build_connectivity(const std::vector<ReferenceCell<dim>> &cell_types,
                       const std::set<ReferenceCell<dim>> &all_reference_cells,
                       const ArrayOfArrays                &cells_to_vertices)
    {
      Connectivity<dim> connectivity(cell_types, all_reference_cells);
 
      ArrayOfArrays temp1; // needed for 3d
 
      if constexpr (dim == 1)
        connectivity.entity_to_entities(1, 0) = cells_to_vertices;
 
      if constexpr (dim == 2 || dim == 3) // build lines
        {
          std::vector<types::geometric_orientation> dummy;
 
          build_face_entities<1>(
            connectivity.template entity_types<dim>(),
            cells_to_vertices,
            dim == 2 ? connectivity.entity_to_entities(2, 1) : temp1,
            connectivity.entity_to_entities(1, 0),
            dim == 2 ? connectivity.entity_orientations(1) : dummy,
            [](auto key, const auto &, const auto &, const auto &) {
              //  to ensure same enumeration as in deal.II
              return key;
            });
        }
 
      if constexpr (dim == 3) // build quads
        {
          build_face_entities<2>(
            connectivity.template entity_types<3>(),
            cells_to_vertices,
            connectivity.entity_to_entities(3, 2),
            connectivity.entity_to_entities(2, 0),
            connectivity.entity_orientations(2),
            [&](auto key, // of type std::array<unsigned int, max_n_vertices>
                          // but max_n_vertices is not known here
                const ReferenceCell<dim> &cell_type,
                const unsigned int       &c,
                const unsigned int       &f) {
              //  to ensure same enumeration as in deal.II
              AssertIndexRange(cell_type.face_reference_cell(f).n_lines(),
                               key.size() + 1);
 
              unsigned int l = 0;
 
              for (; l < cell_type.face_reference_cell(f).n_lines(); ++l)
                {
                  AssertIndexRange(l, key.size());
                  key[l] = temp1[c][cell_type.face_to_cell_lines(
                             f, l, numbers::default_geometric_orientation)] +
                           1 /*offset!*/;
                }
 
              for (; l < key.size(); ++l)
                key[l] = 0;
 
              return key;
            });
 
          // create connectivity: quad -> line
          build_intersection(connectivity.template entity_types<3>(),
                             cells_to_vertices,
                             temp1,
                             connectivity.entity_to_entities(1, 0),
                             connectivity.entity_to_entities(3, 2),
                             connectivity.entity_to_entities(2, 0),
                             connectivity.entity_orientations(2),
                             connectivity.entity_to_entities(2, 1),
                             connectivity.entity_orientations(1),
                             connectivity.template entity_types<2>());
        }
 
      // determine neighbors
      connectivity.entity_to_entities(dim, dim) =
        determine_neighbors(connectivity.entity_to_entities(dim, dim - 1));
 
      return connectivity;
    }
 
 
 
    template <int dim>
    Connectivity<dim>
    build_connectivity(const std::vector<CellData<dim>> &cells)
    {
      AssertThrow(cells.size() > 0, ExcMessage("No cells have been provided!"));
 
      // determine cell types and process vertices
      std::vector<unsigned int> cell_vertices;
      cell_vertices.reserve(std::accumulate(cells.begin(),
                                            cells.end(),
                                            0u,
                                            [](const unsigned int   accumulator,
                                               const CellData<dim> &cell) {
                                              return accumulator +
                                                     cell.vertices.size();
                                            }));
 
      std::vector<std::size_t> cell_vertices_offsets;
      cell_vertices_offsets.reserve(cells.size() + 1);
      cell_vertices_offsets.push_back(0);
 
      std::vector<ReferenceCell<dim>> cell_types;
      cell_types.reserve(cells.size());
 
      // Looping over all the cells can be expensive, so we mark the kinds of
      // ReferenceCells we have just once
      std::set<ReferenceCell<dim>> all_reference_cells;
      // loop over cells and create CRS
      for (const auto &cell : cells)
        {
          if constexpr (running_in_debug_mode())
            {
              auto vertices_unique = cell.vertices;
              std::sort(vertices_unique.begin(), vertices_unique.end());
              vertices_unique.erase(std::unique(vertices_unique.begin(),
                                                vertices_unique.end()),
                                    vertices_unique.end());
 
              Assert(
                vertices_unique.size() == cell.vertices.size(),
                ExcMessage(
                  "The definition of a cell refers to the same vertex several "
                  "times. This is not possible. A common reason is that "
                  "CellData::vertices has a size that does not match the "
                  "size expected from the reference cell. Please resize "
                  "CellData::vertices or use the appropriate constructor of "
                  "CellData."));
            }
 
          cell_types.push_back(
            ReferenceCells::n_vertices_to_reference_cell<dim>(
              cell.vertices.size()));
          all_reference_cells.insert(cell_types.back());
 
          // create CRS of vertices (to remove template argument dim)
          cell_vertices.insert(cell_vertices.end(),
                               cell.vertices.begin(),
                               cell.vertices.end());
          cell_vertices_offsets.push_back(cell_vertices.size());
        }
 
      // do the actual work
      return build_connectivity(cell_types,
                                all_reference_cells,
                                {std::move(cell_vertices_offsets),
                                 std::move(cell_vertices)});
    }
 
 
    struct Implementation
    {
      template <int dim, int spacedim>
      static void
      compute_number_cache_dim(
        const Triangulation<dim, spacedim>                    &triangulation,
        const unsigned int                                     level_objects,
        internal::TriangulationImplementation::NumberCache<1> &number_cache)
      {
        using line_iterator =
          typename Triangulation<dim, spacedim>::line_iterator;
 
        number_cache.n_levels = 0;
        if (level_objects > 0)
          // find the last level on which there are used cells
          for (unsigned int level = 0; level < level_objects; ++level)
            if (triangulation.begin(level) != triangulation.end(level))
              number_cache.n_levels = level + 1;
 
        // no cells at all?
        Assert(number_cache.n_levels > 0, ExcInternalError());
 
        //---------------------------------
        // update the number of lines on the different levels in the
        // cache
        number_cache.n_lines        = 0;
        number_cache.n_active_lines = 0;
 
        // for 1d, lines have levels so take count the objects per
        // level and globally
        if (dim == 1)
          {
            number_cache.n_lines_level.resize(number_cache.n_levels);
            number_cache.n_active_lines_level.resize(number_cache.n_levels);
 
            for (unsigned int level = 0; level < number_cache.n_levels; ++level)
              {
                // count lines on this level
                number_cache.n_lines_level[level]        = 0;
                number_cache.n_active_lines_level[level] = 0;
 
                line_iterator line = triangulation.begin_line(level),
                              endc =
                                (level == number_cache.n_levels - 1 ?
                                   line_iterator(triangulation.end_line()) :
                                   triangulation.begin_line(level + 1));
                for (; line != endc; ++line)
                  {
                    ++number_cache.n_lines_level[level];
                    if (line->has_children() == false)
                      ++number_cache.n_active_lines_level[level];
                  }
 
                // update total number of lines
                number_cache.n_lines += number_cache.n_lines_level[level];
                number_cache.n_active_lines +=
                  number_cache.n_active_lines_level[level];
              }
          }
        else
          {
            // for dim>1, there are no levels for lines
            number_cache.n_lines_level.clear();
            number_cache.n_active_lines_level.clear();
 
            line_iterator line = triangulation.begin_line(),
                          endc = triangulation.end_line();
            for (; line != endc; ++line)
              {
                ++number_cache.n_lines;
                if (line->has_children() == false)
                  ++number_cache.n_active_lines;
              }
          }
      }
 
      template <int dim, int spacedim>
      static void
      compute_number_cache_dim(
        const Triangulation<dim, spacedim>                    &triangulation,
        const unsigned int                                     level_objects,
        internal::TriangulationImplementation::NumberCache<2> &number_cache)
      {
        // update lines and n_levels in number_cache. since we don't
        // access any of these numbers, we can do this in the
        // background
        Threads::Task<void> update_lines = Threads::new_task([&]() {
          compute_number_cache_dim(
            triangulation,
            level_objects,
            // Select the 1d part of the cache:
            static_cast<internal::TriangulationImplementation::NumberCache<1>
                          &>(number_cache));
        });
 
        using quad_iterator =
          typename Triangulation<dim, spacedim>::quad_iterator;
 
        //---------------------------------
        // update the number of quads on the different levels in the
        // cache
        number_cache.n_quads        = 0;
        number_cache.n_active_quads = 0;
 
        // for 2d, quads have levels so take count the objects per
        // level and globally
        if (dim == 2)
          {
            // count the number of levels; the function we called above
            // on a separate Task for lines also does this and puts it into
            // number_cache.n_levels, but this datum may not yet be
            // available as we call the function on a separate task
            unsigned int n_levels = 0;
            if (level_objects > 0)
              // find the last level on which there are used cells
              for (unsigned int level = 0; level < level_objects; ++level)
                if (triangulation.begin(level) != triangulation.end(level))
                  n_levels = level + 1;
 
            number_cache.n_quads_level.resize(n_levels);
            number_cache.n_active_quads_level.resize(n_levels);
 
            for (unsigned int level = 0; level < n_levels; ++level)
              {
                // count quads on this level
                number_cache.n_quads_level[level]        = 0;
                number_cache.n_active_quads_level[level] = 0;
 
                quad_iterator quad = triangulation.begin_quad(level),
                              endc =
                                (level == n_levels - 1 ?
                                   quad_iterator(triangulation.end_quad()) :
                                   triangulation.begin_quad(level + 1));
                for (; quad != endc; ++quad)
                  {
                    ++number_cache.n_quads_level[level];
                    if (quad->has_children() == false)
                      ++number_cache.n_active_quads_level[level];
                  }
 
                // update total number of quads
                number_cache.n_quads += number_cache.n_quads_level[level];
                number_cache.n_active_quads +=
                  number_cache.n_active_quads_level[level];
              }
          }
        else
          {
            // for dim>2, there are no levels for quads
            number_cache.n_quads_level.clear();
            number_cache.n_active_quads_level.clear();
 
            quad_iterator quad = triangulation.begin_quad(),
                          endc = triangulation.end_quad();
            for (; quad != endc; ++quad)
              {
                ++number_cache.n_quads;
                if (quad->has_children() == false)
                  ++number_cache.n_active_quads;
              }
          }
 
        // wait for the background computation for lines
        update_lines.join();
      }
 
      template <int dim, int spacedim>
      static void
      compute_number_cache_dim(
        const Triangulation<dim, spacedim>                    &triangulation,
        const unsigned int                                     level_objects,
        internal::TriangulationImplementation::NumberCache<3> &number_cache)
      {
        // update quads, lines and n_levels in number_cache. since we
        // don't access any of these numbers, we can do this in the
        // background
        Threads::Task<void> update_quads_and_lines = Threads::new_task([&]() {
          compute_number_cache_dim(
            triangulation,
            level_objects,
            // Select the 1d and 2d part of the cache:
            static_cast<internal::TriangulationImplementation::NumberCache<2>
                          &>(number_cache));
        });
 
        using hex_iterator =
          typename Triangulation<dim, spacedim>::hex_iterator;
 
        //---------------------------------
        // update the number of hexes on the different levels in the
        // cache
        number_cache.n_hexes        = 0;
        number_cache.n_active_hexes = 0;
 
        // for 3d, hexes have levels so take count the objects per
        // level and globally
        if (dim == 3)
          {
            // count the number of levels; the function we called
            // above on a separate Task for quads (recursively, via
            // the lines function) also does this and puts it into
            // number_cache.n_levels, but this datum may not yet be
            // available as we call the function on a separate task
            unsigned int n_levels = 0;
            if (level_objects > 0)
              // find the last level on which there are used cells
              for (unsigned int level = 0; level < level_objects; ++level)
                if (triangulation.begin(level) != triangulation.end(level))
                  n_levels = level + 1;
 
            number_cache.n_hexes_level.resize(n_levels);
            number_cache.n_active_hexes_level.resize(n_levels);
 
            for (unsigned int level = 0; level < n_levels; ++level)
              {
                // count hexes on this level
                number_cache.n_hexes_level[level]        = 0;
                number_cache.n_active_hexes_level[level] = 0;
 
                hex_iterator hex  = triangulation.begin_hex(level),
                             endc = (level == n_levels - 1 ?
                                       hex_iterator(triangulation.end_hex()) :
                                       triangulation.begin_hex(level + 1));
                for (; hex != endc; ++hex)
                  {
                    ++number_cache.n_hexes_level[level];
                    if (hex->has_children() == false)
                      ++number_cache.n_active_hexes_level[level];
                  }
 
                // update total number of hexes
                number_cache.n_hexes += number_cache.n_hexes_level[level];
                number_cache.n_active_hexes +=
                  number_cache.n_active_hexes_level[level];
              }
          }
        else
          {
            // for dim>3, there are no levels for hexes
            number_cache.n_hexes_level.clear();
            number_cache.n_active_hexes_level.clear();
 
            hex_iterator hex  = triangulation.begin_hex(),
                         endc = triangulation.end_hex();
            for (; hex != endc; ++hex)
              {
                ++number_cache.n_hexes;
                if (hex->has_children() == false)
                  ++number_cache.n_active_hexes;
              }
          }
 
        // wait for the background computation for quads
        update_quads_and_lines.join();
      }
 
 
      template <int dim, int spacedim>
      static void
      compute_number_cache(
        const Triangulation<dim, spacedim>                      &triangulation,
        const unsigned int                                       level_objects,
        internal::TriangulationImplementation::NumberCache<dim> &number_cache)
      {
        compute_number_cache_dim(triangulation, level_objects, number_cache);
 
        number_cache.active_cell_index_partitioner =
          std::make_shared<const Utilities::MPI::Partitioner>(
            triangulation.n_active_cells());
 
        number_cache.level_cell_index_partitioners.resize(
          triangulation.n_levels());
        for (unsigned int level = 0; level < triangulation.n_levels(); ++level)
          number_cache.level_cell_index_partitioners[level] =
            std::make_shared<const Utilities::MPI::Partitioner>(
              triangulation.n_cells(level));
      }
 
 
      template <int spacedim>
      static void
      update_neighbors(Triangulation<1, spacedim> &)
      {}
 
 
      template <int dim, int spacedim>
      static void
      update_neighbors(Triangulation<dim, spacedim> &triangulation)
      {
        // each face can be neighbored on two sides by cells. according to the
        // face's intrinsic normal we define the left neighbor as the one for
        // which the face normal points outward, and store that one first; the
        // second one is then the right neighbor for which the face normal
        // points inward. This information depends on the type of cell and
        // local number of face for the 'standard ordering and orientation' of
        // faces and then on the face_orientation information for the real
        // mesh. Set up a table to have fast access to those offsets (0 for
        // left and 1 for right). Some of the values are invalid as they
        // reference too large face numbers, but we just leave them at a zero
        // value.
        //
        // Note, that in 2d for lines as faces the normal direction given in
        // the GeometryInfo class is not consistent. We thus define here that
        // the normal for a line points to the right if the line points
        // upwards.
        //
        // There is one more point to consider, however: if we have
        // dim<spacedim, then we may have cases where cells are inverted. In
        // effect, both cells think they are the left neighbor of an edge, for
        // example, which leads us to forget neighborship information (a case
        // that shows this is codim_one/hanging_nodes_02). We store whether a
        // cell is inverted using the direction_flag, so if a cell has a false
        // direction_flag, then we need to invert our selection whether we are
        // a left or right neighbor in all following computations.
        //
        // first index:  dimension (minus 2)
        // second index: local face index
        // third index:  face_orientation (false and true)
        constexpr unsigned int left_right_offset[2][6][2] = {
          // quadrilateral
          {{0, 1},  // face 0, face_orientation = false and true
           {1, 0},  // face 1, face_orientation = false and true
           {1, 0},  // face 2, face_orientation = false and true
           {0, 1},  // face 3, face_orientation = false and true
           {0, 0},  // face 4, invalid face
           {0, 0}}, // face 5, invalid face
                    // hexahedron
          {{0, 1}, {1, 0}, {0, 1}, {1, 0}, {0, 1}, {1, 0}}};
 
        // now create a vector of the two active neighbors (left and right)
        // for each face and fill it by looping over all cells. For cases with
        // anisotropic refinement and more than one cell neighboring at a
        // given side of the face we will automatically get the active one on
        // the highest level as we loop over cells from lower levels first.
        std::vector<typename Triangulation<dim, spacedim>::cell_iterator>
          adjacent_cells(2 * triangulation.n_raw_faces(), triangulation.end());
 
        for (const auto &cell : triangulation.cell_iterators())
          for (auto f : cell->face_indices())
            {
              const typename Triangulation<dim, spacedim>::face_iterator face =
                cell->face(f);
 
              const unsigned int offset =
                (cell->direction_flag() ?
                   left_right_offset[dim - 2][f][cell->face_orientation(f)] :
                   1 -
                     left_right_offset[dim - 2][f][cell->face_orientation(f)]);
 
              adjacent_cells[2 * face->index() + offset] = cell;
 
              // if this cell is not refined, but the face is, then we'll have
              // to set our cell as neighbor for the child faces as
              // well. Fortunately the normal orientation of children will be
              // just the same.
              if (dim == 2)
                {
                  if (cell->is_active() && face->has_children())
                    {
                      adjacent_cells[2 * face->child(0)->index() + offset] =
                        cell;
                      adjacent_cells[2 * face->child(1)->index() + offset] =
                        cell;
                    }
                }
              else // -> dim == 3
                {
                  // We need the same as in 2d here. Furthermore, if the face
                  // is refined with cut_x or cut_y then those children again
                  // in the other direction, and if this cell is refined
                  // isotropically (along the face) then the neighbor will
                  // (probably) be refined as cut_x or cut_y along the
                  // face. For those neighboring children cells, their
                  // neighbor will be the current, inactive cell, as our
                  // children are too fine to be neighbors. Catch that case by
                  // also acting on inactive cells with isotropic refinement
                  // along the face. If the situation described is not
                  // present, the data will be overwritten later on when we
                  // visit cells on finer levels, so no harm will be done.
                  if (face->has_children() &&
                      (cell->is_active() ||
                       GeometryInfo<dim>::face_refinement_case(
                         cell->refinement_case(), f) ==
                         RefinementCase<dim - 1>::isotropic_refinement))
                    {
                      for (unsigned int c = 0; c < face->n_children(); ++c)
                        adjacent_cells[2 * face->child(c)->index() + offset] =
                          cell;
                      if (face->child(0)->has_children())
                        {
                          adjacent_cells[2 * face->child(0)->child(0)->index() +
                                         offset] = cell;
                          adjacent_cells[2 * face->child(0)->child(1)->index() +
                                         offset] = cell;
                        }
                      if (face->child(1)->has_children())
                        {
                          adjacent_cells[2 * face->child(1)->child(0)->index() +
                                         offset] = cell;
                          adjacent_cells[2 * face->child(1)->child(1)->index() +
                                         offset] = cell;
                        }
                    } // if cell active and face refined
                }     // else -> dim==3
            }         // for all faces of all cells
 
        // now loop again over all cells and set the corresponding neighbor
        // cell. Note, that we have to use the opposite of the
        // left_right_offset in this case as we want the offset of the
        // neighbor, not our own.
        for (const auto &cell : triangulation.cell_iterators())
          for (auto f : cell->face_indices())
            {
              const unsigned int offset =
                (cell->direction_flag() ?
                   left_right_offset[dim - 2][f][cell->face_orientation(f)] :
                   1 -
                     left_right_offset[dim - 2][f][cell->face_orientation(f)]);
              cell->set_neighbor(
                f, adjacent_cells[2 * cell->face(f)->index() + 1 - offset]);
            }
      }
 
 
      template <int dim, int spacedim>
      static void
      create_triangulation(const std::vector<Point<spacedim>> &vertices,
                           const std::vector<CellData<dim>>   &cells,
                           const SubCellData                  &subcelldata,
                           Triangulation<dim, spacedim>       &tria)
      {
        AssertThrow(vertices.size() > 0, ExcMessage("No vertices given"));
        AssertThrow(cells.size() > 0, ExcMessage("No cells given"));
 
        // Check that all cells have positive volume.
#ifndef _MSC_VER
        // TODO: The following code does not compile with MSVC. Find a way
        // around it
        if (dim == spacedim)
          for (unsigned int cell_no = 0; cell_no < cells.size(); ++cell_no)
            {
              // If we should check for distorted cells, then we permit them
              // to exist. If a cell has negative measure, then it must be
              // distorted (the converse is not necessarily true); hence
              // throw an exception if no such cells should exist.
              if (tria.check_for_distorted_cells)
                {
                  const double cell_measure = GridTools::cell_measure<spacedim>(
                    vertices,
                    ArrayView<const unsigned int>(cells[cell_no].vertices));
                  AssertThrow(cell_measure > 0, ExcGridHasInvalidCell(cell_no));
                }
            }
#endif
 
        // copy vertices
        tria.vertices = vertices;
        tria.vertices_used.assign(vertices.size(), true);
 
        // compute connectivity
        const auto connectivity = build_connectivity(cells);
        const auto n_cells      = cells.size();
 
        // set up reference cells for the first time
        const auto all_reference_cells =
          Utilities::MPI::compute_set_union(connectivity.all_reference_cells(),
                                            tria.get_mpi_communicator());
        tria.reference_cells.assign(all_reference_cells.begin(),
                                    all_reference_cells.end());
        tria.strides = internal::TriangulationImplementation::Strides<dim>(
          tria.reference_cells);
 
        // clear old content
        tria.levels.clear();
        tria.levels.push_back(
          std::make_unique<
            internal::TriangulationImplementation::TriaLevel<dim, spacedim>>(
            tria.strides.max_children_per_cell,
            tria.strides.max_faces_per_cell,
            tria.strides.max_vertices_per_cell));
 
        if (dim > 1)
          tria.faces = std::make_unique<
            ::internal::TriangulationImplementation::TriaFaces<dim>>(
            tria.strides.max_children_per_face,
            tria.strides.max_lines_per_face);
 
        const ArrayOfArrays  empty;
        const ArrayOfArrays &lines_to_vertices =
          dim > 1 ? connectivity.entity_to_entities(1, 0) : empty;
        const ArrayOfArrays &quads_to_lines =
          dim == 3 ? connectivity.entity_to_entities(2, 1) : empty;
        const auto n_lines = lines_to_vertices.size();
        AssertThrow(
          n_lines <= std::size_t(std::numeric_limits<int>::max()),
          ExcMessage("The number of lines must be less than the largest int."));
        const auto n_quads = quads_to_lines.size();
        AssertThrow(
          n_quads <= std::size_t(std::numeric_limits<int>::max()),
          ExcMessage("The number of quads must be less than the largest int."));
 
        if (dim > 1)
          tria.faces->allocate(n_lines, n_quads);
 
        // TriaObjects: lines
        if (dim > 1)
          {
            auto &faces = *tria.faces;
            // loop over lines
            for (int line = 0; line < int(n_lines); ++line)
              {
                const auto vertices = lines_to_vertices[line];
                for (unsigned int v = 0; v < vertices.size(); ++v)
                  faces.lines.set_bounding_object(line, v, vertices[v]);
              }
          }
 
        // TriaObjects: quads
        if constexpr (dim == 3)
          {
            auto &faces = *tria.faces;
            // get connectivity between quads and lines
 
            // loop over all quads -> entity type, line indices/orientations
            for (int q = 0, k = 0; q < int(n_quads); ++q)
              {
                // set entity type of quads
                const auto face_reference_cell =
                  connectivity.template entity_types<2>()[q];
                faces.set_quad_type(q, face_reference_cell);
 
                // loop over all its lines
                const auto lines = quads_to_lines[q];
                for (unsigned int l = 0; l < lines.size(); ++l, ++k)
                  {
                    AssertIndexRange(l, face_reference_cell.n_lines());
                    // set line index
                    faces.quads.set_bounding_object(q, l, lines[l]);
 
                    // set line orientations
                    faces.set_line_orientation(
                      q, l, connectivity.entity_orientations(1)[k]);
                  }
              }
          }
 
        // TriaObjects/TriaLevel: cell
        {
          auto &cells_0 = tria.levels[0]->cells; // data structure to be filled
          auto &level   = *tria.levels[0];
 
          // get connectivity between cells/faces and cells/cells
          const auto &cells_to_faces =
            connectivity.entity_to_entities(dim, dim - 1);
          const auto &cells_to_neighbor_cells =
            connectivity.entity_to_entities(dim, dim);
 
          // in 2d optional: since in in pure QUAD meshes same line
          // orientations can be guaranteed
          bool orientation_needed = false;
          if (dim == 3)
            orientation_needed = true;
          else if (dim == 2)
            {
              const auto &orientations = connectivity.entity_orientations(1);
              orientation_needed       = std::any_of(
                orientations.begin(), orientations.end(), [](const auto &a) {
                  return a != numbers::default_geometric_orientation;
                });
            }
 
          level.allocate(n_cells, orientation_needed);
          // loop over all cells
          unsigned int global_face_index = 0;
          for (unsigned int cell = 0; cell < n_cells; ++cell)
            {
              // set material ids
              cells_0.boundary_or_material_id[cell].material_id =
                cells[cell].material_id;
 
              // set manifold ids
              cells_0.manifold_id[cell] = cells[cell].manifold_id;
 
              // set entity types
              level.reference_cell[cell] =
                connectivity.template entity_types<dim>()[cell];
              Assert(all_reference_cells.count(level.reference_cell[cell]) ==
                       1u,
                     ExcInternalError());
 
              // loop over faces
              const auto faces     = cells_to_faces[cell];
              const auto neighbors = cells_to_neighbor_cells[cell];
              for (unsigned int f = 0; f < faces.size();
                   ++f, ++global_face_index)
                {
                  // set neighbor if not at boundary
                  if (neighbors[f] != numbers::invalid_unsigned_int)
                    level.set_neighbor(cell, f, 0, neighbors[f]);
 
                  // set face indices
                  cells_0.set_bounding_object(cell, f, faces[f]);
 
                  // set face orientation if needed
                  if (orientation_needed)
                    {
                      level.face_orientations.set_combined_orientation(
                        cell,
                        f,
                        connectivity.entity_orientations(dim -
                                                         1)[global_face_index]);
                    }
                }
            }
        }
 
        // TriaFaces: boundary id of boundary faces
        if (dim > 1)
          {
            auto &bids_face = dim == 3 ?
                                tria.faces->quads.boundary_or_material_id :
                                tria.faces->lines.boundary_or_material_id;
 
            // count number of cells a face is belonging to
            std::vector<unsigned int> count(bids_face.size(), 0);
 
            // get connectivity between cells/faces
            const auto &cells_to_faces =
              connectivity.entity_to_entities(dim, dim - 1);
 
            // count how many cells are adjacent to the same face
            for (unsigned int cell = 0; cell < cells.size(); ++cell)
              for (const unsigned int face : cells_to_faces[cell])
                ++count[face];
 
            // loop over all faces
            for (unsigned int face = 0; face < count.size(); ++face)
              {
                if (count[face] != 1) // inner face
                  continue;
 
                // boundary faces ...
                bids_face[face].boundary_id = 0;
 
                if (dim != 3)
                  continue;
 
                // ... and the lines of quads in 3d
                const auto &quads_to_lines =
                  connectivity.entity_to_entities(2, 1);
                for (const unsigned int line : quads_to_lines[face])
                  tria.faces->lines.boundary_or_material_id[line].boundary_id =
                    0;
              }
          }
        else // 1d
          {
            constexpr unsigned int t_tba   = static_cast<unsigned int>(-1);
            constexpr unsigned int t_inner = static_cast<unsigned int>(-2);
 
            std::vector<unsigned int> type(vertices.size(), t_tba);
 
            const auto &cells_to_vertices =
              connectivity.entity_to_entities(1, 0);
 
            for (unsigned int cell = 0; cell < cells.size(); ++cell)
              {
                const auto vertices = cells_to_vertices[cell];
                for (unsigned int v = 0; v < vertices.size(); ++v)
                  if (type[vertices[v]] != t_inner)
                    type[vertices[v]] =
                      (type[vertices[v]] == t_tba ? v : t_inner);
              }
 
            for (unsigned int face = 0; face < type.size(); ++face)
              {
                // note: we also treat manifolds here!?
                (*tria.vertex_to_manifold_id_map_1d)[face] =
                  numbers::flat_manifold_id;
                if (type[face] != t_inner && type[face] != t_tba)
                  (*tria.vertex_to_boundary_id_map_1d)[face] = type[face];
              }
          }
 
        // SubCellData: line
        if (dim >= 2)
          process_subcelldata(connectivity.entity_to_entities(1, 0),
                              tria.faces->lines,
                              subcelldata.boundary_lines,
                              vertices);
 
        // SubCellData: quad
        if (dim == 3)
          process_subcelldata(connectivity.entity_to_entities(2, 0),
                              tria.faces->quads,
                              subcelldata.boundary_quads,
                              vertices);
      }
 
 
      template <int structdim, int spacedim>
      static void
      process_subcelldata(
        const ArrayOfArrays                    &crs,
        TriaObjects                            &obj,
        const std::vector<CellData<structdim>> &boundary_objects_in,
        const std::vector<Point<spacedim>>     &vertex_locations)
      {
        AssertDimension(obj.structdim, structdim);
 
        if (boundary_objects_in.empty())
          return; // empty subcelldata -> nothing to do
 
        // pre-sort subcelldata
        auto boundary_objects = boundary_objects_in;
 
        // ... sort vertices
        for (auto &boundary_object : boundary_objects)
          std::sort(boundary_object.vertices.begin(),
                    boundary_object.vertices.end());
 
        // ... sort cells
        std::sort(boundary_objects.begin(),
                  boundary_objects.end(),
                  [](const auto &a, const auto &b) {
                    return a.vertices < b.vertices;
                  });
 
        [[maybe_unused]] unsigned int counter = 0;
 
        std_cxx26::inplace_vector<unsigned int,
                                  ReferenceCells::max_n_vertices<structdim>()>
          key;
 
        for (unsigned int o = 0; o < obj.n_objects(); ++o)
          {
            auto &boundary_id = obj.boundary_or_material_id[o].boundary_id;
            auto &manifold_id = obj.manifold_id[o];
 
            // assert that object has not been visited yet and its value
            // has not been modified yet
            AssertThrow(boundary_id == 0 ||
                          boundary_id == numbers::internal_face_boundary_id,
                        ExcNotImplemented());
            AssertThrow(manifold_id == numbers::flat_manifold_id,
                        ExcNotImplemented());
 
            // create key
            const ArrayView<const unsigned int> indices = crs[o];
            key.assign(indices.begin(), indices.end());
            std::sort(key.begin(), key.end());
 
            // is subcelldata provided? -> binary search
            const auto subcell_object =
              std::lower_bound(boundary_objects.begin(),
                               boundary_objects.end(),
                               key,
                               [&](const auto &cell, const auto &key) {
                                 return cell.vertices < key;
                               });
 
            // no subcelldata provided for this object
            if (subcell_object == boundary_objects.end() ||
                subcell_object->vertices != key)
              continue;
 
            ++counter;
 
            // set manifold id
            manifold_id = subcell_object->manifold_id;
 
            // set boundary id
            if (subcell_object->boundary_id !=
                numbers::internal_face_boundary_id)
              {
                AssertThrow(
                  boundary_id != numbers::internal_face_boundary_id,
                  ExcMessage(
                    "The input arguments for creating a triangulation "
                    "specified a boundary id for an internal face. This "
                    "is not allowed."
                    "\n\n"
                    "The object in question has vertex indices " +
                    [subcell_object]() {
                      std::string s;
                      for (const auto v : subcell_object->vertices)
                        s += std::to_string(v) + ',';
                      return s;
                    }() +
                    " which are located at coordinates " +
                    [vertex_locations, subcell_object]() {
                      std::ostringstream s;
                      for (unsigned int i = 0;
                           i < subcell_object->vertices.size();
                           ++i)
                        s << '('
                          << vertex_locations[subcell_object->vertices[i]]
                          << (i != subcell_object->vertices.size() - 1 ? "), " :
                                                                         ")");
                      return s.str();
                    }() +
                    "."));
                boundary_id = subcell_object->boundary_id;
              }
          }
 
        // make sure that all subcelldata entries have been processed
        // TODO: this is not guaranteed, why?
        // AssertDimension(counter, boundary_objects_in.size());
      }
 
 
 
      template <int spacedim>
      static void
      delete_children(Triangulation<1, spacedim> &triangulation,
                      typename Triangulation<1, spacedim>::cell_iterator &cell,
                      std::vector<unsigned int> &,
                      std::vector<unsigned int> &)
      {
        const unsigned int dim = 1;
 
        // first we need to reset the
        // neighbor pointers of the
        // neighbors of this cell's
        // children to this cell. This is
        // different for one dimension,
        // since there neighbors can have a
        // refinement level differing from
        // that of this cell's children by
        // more than one level.
 
        Assert(!cell->child(0)->has_children() &&
                 !cell->child(1)->has_children(),
               ExcInternalError());
 
        // first do it for the cells to the
        // left
        if (cell->neighbor(0).state() == IteratorState::valid)
          if (cell->neighbor(0)->has_children())
            {
              typename Triangulation<dim, spacedim>::cell_iterator neighbor =
                cell->neighbor(0);
              Assert(neighbor->level() == cell->level(), ExcInternalError());
 
              // right child
              neighbor = neighbor->child(1);
              while (true)
                {
                  Assert(neighbor->neighbor(1) == cell->child(0),
                         ExcInternalError());
                  neighbor->set_neighbor(1, cell);
 
                  // move on to further
                  // children on the
                  // boundary between this
                  // cell and its neighbor
                  if (neighbor->has_children())
                    neighbor = neighbor->child(1);
                  else
                    break;
                }
            }
 
        // now do it for the cells to the
        // left
        if (cell->neighbor(1).state() == IteratorState::valid)
          if (cell->neighbor(1)->has_children())
            {
              typename Triangulation<dim, spacedim>::cell_iterator neighbor =
                cell->neighbor(1);
              Assert(neighbor->level() == cell->level(), ExcInternalError());
 
              // left child
              neighbor = neighbor->child(0);
              while (true)
                {
                  Assert(neighbor->neighbor(0) == cell->child(1),
                         ExcInternalError());
                  neighbor->set_neighbor(0, cell);
 
                  // move on to further
                  // children on the
                  // boundary between this
                  // cell and its neighbor
                  if (neighbor->has_children())
                    neighbor = neighbor->child(0);
                  else
                    break;
                }
            }
 
 
        // delete the vertex which will not
        // be needed anymore. This vertex
        // is the second of the first child
        triangulation.vertices_used[cell->child(0)->vertex_index(1)] = false;
 
        // invalidate children.  clear user
        // pointers, to avoid that they may
        // appear at unwanted places later
        // on...
        for (unsigned int child = 0; child < cell->n_children(); ++child)
          {
            cell->child(child)->clear_user_data();
            cell->child(child)->clear_user_flag();
            cell->child(child)->clear_used_flag();
          }
 
 
        // delete pointer to children
        cell->clear_children();
        cell->clear_user_flag();
      }
 
 
 
      template <int spacedim>
      static void
      delete_children(Triangulation<2, spacedim> &triangulation,
                      typename Triangulation<2, spacedim>::cell_iterator &cell,
                      std::vector<unsigned int> &line_cell_count,
                      std::vector<unsigned int> &)
      {
        const unsigned int        dim      = 2;
        const RefinementCase<dim> ref_case = cell->refinement_case();
 
        Assert(line_cell_count.size() == triangulation.n_raw_lines(),
               ExcInternalError());
 
        // vectors to hold all lines which
        // may be deleted
        std::vector<typename Triangulation<dim, spacedim>::line_iterator>
          lines_to_delete(0);
 
        lines_to_delete.reserve(4 * 2 + 4);
 
        // now we decrease the counters for
        // lines contained in the child
        // cells
        for (unsigned int c = 0; c < cell->n_children(); ++c)
          {
            typename Triangulation<dim, spacedim>::cell_iterator child =
              cell->child(c);
            for (unsigned int l = 0; l < GeometryInfo<dim>::lines_per_cell; ++l)
              --line_cell_count[child->line_index(l)];
          }
 
 
        // delete the vertex which will not
        // be needed anymore. This vertex
        // is the second of the second line
        // of the first child, if the cell
        // is refined with cut_xy, else there
        // is no inner vertex.
        // additionally delete unneeded inner
        // lines
        if (ref_case == RefinementCase<dim>::cut_xy)
          {
            triangulation
              .vertices_used[cell->child(0)->line(1)->vertex_index(1)] = false;
 
            lines_to_delete.push_back(cell->child(0)->line(1));
            lines_to_delete.push_back(cell->child(0)->line(3));
            lines_to_delete.push_back(cell->child(3)->line(0));
            lines_to_delete.push_back(cell->child(3)->line(2));
          }
        else
          {
            unsigned int inner_face_no =
              ref_case == RefinementCase<dim>::cut_x ? 1 : 3;
 
            // the inner line will not be
            // used any more
            lines_to_delete.push_back(cell->child(0)->line(inner_face_no));
          }
 
        // invalidate children
        for (unsigned int child = 0; child < cell->n_children(); ++child)
          {
            cell->child(child)->clear_user_data();
            cell->child(child)->clear_user_flag();
            cell->child(child)->clear_used_flag();
          }
 
 
        // delete pointer to children
        cell->clear_children();
        cell->clear_refinement_case();
        cell->clear_user_flag();
 
        // look at the refinement of outer
        // lines. if nobody needs those
        // anymore we can add them to the
        // list of lines to be deleted.
        for (unsigned int line_no = 0;
             line_no < GeometryInfo<dim>::lines_per_cell;
             ++line_no)
          {
            typename Triangulation<dim, spacedim>::line_iterator line =
              cell->line(line_no);
 
            if (line->has_children())
              {
                // if one of the cell counters is
                // zero, the other has to be as well
 
                Assert((line_cell_count[line->child_index(0)] == 0 &&
                        line_cell_count[line->child_index(1)] == 0) ||
                         (line_cell_count[line->child_index(0)] > 0 &&
                          line_cell_count[line->child_index(1)] > 0),
                       ExcInternalError());
 
                if (line_cell_count[line->child_index(0)] == 0)
                  {
                    for (unsigned int c = 0; c < 2; ++c)
                      Assert(!line->child(c)->has_children(),
                             ExcInternalError());
 
                    // we may delete the line's
                    // children and the middle vertex
                    // as no cell references them
                    // anymore
                    triangulation
                      .vertices_used[line->child(0)->vertex_index(1)] = false;
 
                    lines_to_delete.push_back(line->child(0));
                    lines_to_delete.push_back(line->child(1));
 
                    line->clear_children();
                  }
              }
          }
 
        // finally, delete unneeded lines
 
        // clear user pointers, to avoid that
        // they may appear at unwanted places
        // later on...
        // same for user flags, then finally
        // delete the lines
        typename std::vector<
          typename Triangulation<dim, spacedim>::line_iterator>::iterator
          line    = lines_to_delete.begin(),
          endline = lines_to_delete.end();
        for (; line != endline; ++line)
          {
            (*line)->clear_user_data();
            (*line)->clear_user_flag();
            (*line)->clear_used_flag();
          }
      }
 
 
 
      template <int spacedim>
      static void
      delete_children(Triangulation<3, spacedim> &triangulation,
                      typename Triangulation<3, spacedim>::cell_iterator &cell,
                      std::vector<unsigned int> &line_cell_count,
                      std::vector<unsigned int> &quad_cell_count)
      {
        const unsigned int dim = 3;
 
        Assert(line_cell_count.size() == triangulation.n_raw_lines(),
               ExcInternalError());
        Assert(quad_cell_count.size() == triangulation.n_raw_quads(),
               ExcInternalError());
 
        // first of all, we store the RefineCase of
        // this cell
        const RefinementCase<dim> ref_case = cell->refinement_case();
        // vectors to hold all lines and quads which
        // may be deleted
        std::vector<typename Triangulation<dim, spacedim>::line_iterator>
          lines_to_delete(0);
        std::vector<typename Triangulation<dim, spacedim>::quad_iterator>
          quads_to_delete(0);
 
        lines_to_delete.reserve(12 * 2 + 6 * 4 + 6);
        quads_to_delete.reserve(6 * 4 + 12);
 
        // now we decrease the counters for lines and
        // quads contained in the child cells
        for (unsigned int c = 0; c < cell->n_children(); ++c)
          {
            typename Triangulation<dim, spacedim>::cell_iterator child =
              cell->child(c);
            const auto line_indices = TriaAccessorImplementation::
              Implementation::get_line_indices_of_cell(*child);
            for (const unsigned int l : cell->line_indices())
              --line_cell_count[line_indices[l]];
            for (auto f : GeometryInfo<dim>::face_indices())
              --quad_cell_count[child->quad_index(f)];
          }
 
        //-------------------------------------
        // delete interior quads and lines and the
        // interior vertex, depending on the
        // refinement case of the cell
        //
        // for append quads and lines: only append
        // them to the list of objects to be deleted
 
        switch (ref_case)
          {
            case RefinementCase<dim>::cut_x:
              quads_to_delete.push_back(cell->child(0)->face(1));
              break;
            case RefinementCase<dim>::cut_y:
              quads_to_delete.push_back(cell->child(0)->face(3));
              break;
            case RefinementCase<dim>::cut_z:
              quads_to_delete.push_back(cell->child(0)->face(5));
              break;
            case RefinementCase<dim>::cut_xy:
              quads_to_delete.push_back(cell->child(0)->face(1));
              quads_to_delete.push_back(cell->child(0)->face(3));
              quads_to_delete.push_back(cell->child(3)->face(0));
              quads_to_delete.push_back(cell->child(3)->face(2));
 
              lines_to_delete.push_back(cell->child(0)->line(11));
              break;
            case RefinementCase<dim>::cut_xz:
              quads_to_delete.push_back(cell->child(0)->face(1));
              quads_to_delete.push_back(cell->child(0)->face(5));
              quads_to_delete.push_back(cell->child(3)->face(0));
              quads_to_delete.push_back(cell->child(3)->face(4));
 
              lines_to_delete.push_back(cell->child(0)->line(5));
              break;
            case RefinementCase<dim>::cut_yz:
              quads_to_delete.push_back(cell->child(0)->face(3));
              quads_to_delete.push_back(cell->child(0)->face(5));
              quads_to_delete.push_back(cell->child(3)->face(2));
              quads_to_delete.push_back(cell->child(3)->face(4));
 
              lines_to_delete.push_back(cell->child(0)->line(7));
              break;
            case RefinementCase<dim>::cut_xyz:
              quads_to_delete.push_back(cell->child(0)->face(1));
              quads_to_delete.push_back(cell->child(2)->face(1));
              quads_to_delete.push_back(cell->child(4)->face(1));
              quads_to_delete.push_back(cell->child(6)->face(1));
 
              quads_to_delete.push_back(cell->child(0)->face(3));
              quads_to_delete.push_back(cell->child(1)->face(3));
              quads_to_delete.push_back(cell->child(4)->face(3));
              quads_to_delete.push_back(cell->child(5)->face(3));
 
              quads_to_delete.push_back(cell->child(0)->face(5));
              quads_to_delete.push_back(cell->child(1)->face(5));
              quads_to_delete.push_back(cell->child(2)->face(5));
              quads_to_delete.push_back(cell->child(3)->face(5));
 
              lines_to_delete.push_back(cell->child(0)->line(5));
              lines_to_delete.push_back(cell->child(0)->line(7));
              lines_to_delete.push_back(cell->child(0)->line(11));
              lines_to_delete.push_back(cell->child(7)->line(0));
              lines_to_delete.push_back(cell->child(7)->line(2));
              lines_to_delete.push_back(cell->child(7)->line(8));
              // delete the vertex which will not
              // be needed anymore. This vertex
              // is the vertex at the heart of
              // this cell, which is the sixth of
              // the first child
              triangulation.vertices_used[cell->child(0)->vertex_index(7)] =
                false;
              break;
            default:
              // only remaining case is
              // no_refinement, thus an error
              DEAL_II_ASSERT_UNREACHABLE();
              break;
          }
 
 
        // invalidate children
        for (unsigned int child = 0; child < cell->n_children(); ++child)
          {
            cell->child(child)->clear_user_data();
            cell->child(child)->clear_user_flag();
 
            for (auto f : GeometryInfo<dim>::face_indices())
              // set flags denoting deviations from standard orientation of
              // faces back to initialization values
              cell->child(child)->set_combined_face_orientation(
                f, numbers::default_geometric_orientation);
 
            cell->child(child)->clear_used_flag();
          }
 
 
        // delete pointer to children
        cell->clear_children();
        cell->clear_refinement_case();
        cell->clear_user_flag();
 
        // so far we only looked at inner quads,
        // lines and vertices. Now we have to
        // consider outer ones as well. here, we have
        // to check, whether there are other cells
        // still needing these objects. otherwise we
        // can delete them. first for quads (and
        // their inner lines).
 
        for (const unsigned int quad_no : GeometryInfo<dim>::face_indices())
          {
            typename Triangulation<dim, spacedim>::quad_iterator quad =
              cell->face(quad_no);
 
            Assert(
              (GeometryInfo<dim>::face_refinement_case(ref_case, quad_no) &&
               quad->has_children()) ||
                GeometryInfo<dim>::face_refinement_case(ref_case, quad_no) ==
                  RefinementCase<dim - 1>::no_refinement,
              ExcInternalError());
 
            switch (quad->refinement_case())
              {
                case RefinementCase<dim - 1>::no_refinement:
                  // nothing to do as the quad
                  // is not refined
                  break;
                case RefinementCase<dim - 1>::cut_x:
                case RefinementCase<dim - 1>::cut_y:
                  {
                    // if one of the cell counters is
                    // zero, the other has to be as
                    // well
                    Assert((quad_cell_count[quad->child_index(0)] == 0 &&
                            quad_cell_count[quad->child_index(1)] == 0) ||
                             (quad_cell_count[quad->child_index(0)] > 0 &&
                              quad_cell_count[quad->child_index(1)] > 0),
                           ExcInternalError());
                    // it might be, that the quad is
                    // refined twice anisotropically,
                    // first check, whether we may
                    // delete possible grand_children
                    unsigned int deleted_grandchildren       = 0;
                    unsigned int number_of_child_refinements = 0;
 
                    for (unsigned int c = 0; c < 2; ++c)
                      if (quad->child(c)->has_children())
                        {
                          ++number_of_child_refinements;
                          // if one of the cell counters is
                          // zero, the other has to be as
                          // well
                          Assert(
                            (quad_cell_count[quad->child(c)->child_index(0)] ==
                               0 &&
                             quad_cell_count[quad->child(c)->child_index(1)] ==
                               0) ||
                              (quad_cell_count[quad->child(c)->child_index(0)] >
                                 0 &&
                               quad_cell_count[quad->child(c)->child_index(1)] >
                                 0),
                            ExcInternalError());
                          if (quad_cell_count[quad->child(c)->child_index(0)] ==
                              0)
                            {
                              // Assert, that the two
                              // anisotropic
                              // refinements add up to
                              // isotropic refinement
                              Assert(quad->refinement_case() +
                                         quad->child(c)->refinement_case() ==
                                       RefinementCase<dim>::cut_xy,
                                     ExcInternalError());
                              // we may delete the
                              // quad's children and
                              // the inner line as no
                              // cell references them
                              // anymore
                              quads_to_delete.push_back(
                                quad->child(c)->child(0));
                              quads_to_delete.push_back(
                                quad->child(c)->child(1));
                              if (quad->child(c)->refinement_case() ==
                                  RefinementCase<2>::cut_x)
                                lines_to_delete.push_back(
                                  quad->child(c)->child(0)->line(1));
                              else
                                lines_to_delete.push_back(
                                  quad->child(c)->child(0)->line(3));
                              quad->child(c)->clear_children();
                              quad->child(c)->clear_refinement_case();
                              ++deleted_grandchildren;
                            }
                        }
                    // if no grandchildren are left, we
                    // may as well delete the
                    // refinement of the inner line
                    // between our children and the
                    // corresponding vertex
                    if (number_of_child_refinements > 0 &&
                        deleted_grandchildren == number_of_child_refinements)
                      {
                        typename Triangulation<dim, spacedim>::line_iterator
                          middle_line;
                        if (quad->refinement_case() == RefinementCase<2>::cut_x)
                          middle_line = quad->child(0)->line(1);
                        else
                          middle_line = quad->child(0)->line(3);
 
                        lines_to_delete.push_back(middle_line->child(0));
                        lines_to_delete.push_back(middle_line->child(1));
                        triangulation
                          .vertices_used[middle_vertex_index<dim, spacedim>(
                            middle_line)] = false;
                        middle_line->clear_children();
                      }
 
                    // now consider the direct children
                    // of the given quad
                    if (quad_cell_count[quad->child_index(0)] == 0)
                      {
                        // we may delete the quad's
                        // children and the inner line
                        // as no cell references them
                        // anymore
                        quads_to_delete.push_back(quad->child(0));
                        quads_to_delete.push_back(quad->child(1));
                        if (quad->refinement_case() == RefinementCase<2>::cut_x)
                          lines_to_delete.push_back(quad->child(0)->line(1));
                        else
                          lines_to_delete.push_back(quad->child(0)->line(3));
 
                        // if the counters just dropped
                        // to zero, otherwise the
                        // children would have been
                        // deleted earlier, then this
                        // cell's children must have
                        // contained the anisotropic
                        // quad children. thus, if
                        // those have again anisotropic
                        // children, which are in
                        // effect isotropic children of
                        // the original quad, those are
                        // still needed by a
                        // neighboring cell and we
                        // cannot delete them. instead,
                        // we have to reset this quad's
                        // refine case to isotropic and
                        // set the children
                        // accordingly.
                        if (quad->child(0)->has_children())
                          if (quad->refinement_case() ==
                              RefinementCase<2>::cut_x)
                            {
                              // now evereything is
                              // quite complicated. we
                              // have the children
                              // numbered according to
                              //
                              // *---*---*
                              // |n+1|m+1|
                              // *---*---*
                              // | n | m |
                              // *---*---*
                              //
                              // from the original
                              // anisotropic
                              // refinement. we have to
                              // reorder them as
                              //
                              // *---*---*
                              // | m |m+1|
                              // *---*---*
                              // | n |n+1|
                              // *---*---*
                              //
                              // for isotropic refinement.
                              //
                              // this is a bit ugly, of
                              // course: loop over all
                              // cells on all levels
                              // and look for faces n+1
                              // (switch_1) and m
                              // (switch_2).
                              const typename Triangulation<dim, spacedim>::
                                quad_iterator switch_1 =
                                                quad->child(0)->child(1),
                                              switch_2 =
                                                quad->child(1)->child(0);
 
                              Assert(!switch_1->has_children(),
                                     ExcInternalError());
                              Assert(!switch_2->has_children(),
                                     ExcInternalError());
 
                              const int switch_1_index = switch_1->index();
                              const int switch_2_index = switch_2->index();
                              for (unsigned int l = 0;
                                   l < triangulation.levels.size();
                                   ++l)
                                for (unsigned int h = 0;
                                     h <
                                     triangulation.levels[l]->cells.n_objects();
                                     ++h)
                                  for (const unsigned int q :
                                       GeometryInfo<dim>::face_indices())
                                    {
                                      const int index =
                                        triangulation.levels[l]
                                          ->cells.get_bounding_object_indices(
                                            h)[q];
                                      if (index == switch_1_index)
                                        triangulation.levels[l]
                                          ->cells.get_bounding_object_indices(
                                            h)[q] = switch_2_index;
                                      else if (index == switch_2_index)
                                        triangulation.levels[l]
                                          ->cells.get_bounding_object_indices(
                                            h)[q] = switch_1_index;
                                    }
                              // now we have to copy
                              // all information of the
                              // two quads
                              int switch_1_lines[4], switch_2_lines[4];
                              for (int i = 0; i < 4; ++i)
                                {
                                  switch_1_lines[i] =
                                    switch_1->line(i)->index();
                                  switch_2_lines[i] =
                                    switch_2->line(i)->index();
                                }
                              const types::geometric_orientation
                                switch_1_line_orientations[4] = {
                                  switch_1->line_orientation(0),
                                  switch_1->line_orientation(1),
                                  switch_1->line_orientation(2),
                                  switch_1->line_orientation(3)};
                              const types::boundary_id switch_1_boundary_id =
                                switch_1->boundary_id();
                              const unsigned int switch_1_user_index =
                                switch_1->user_index();
                              const bool switch_1_user_flag =
                                switch_1->user_flag_set();
 
                              switch_1->set_bounding_object_indices(
                                {switch_2_lines[0],
                                 switch_2_lines[1],
                                 switch_2_lines[2],
                                 switch_2_lines[3]});
                              switch_1->set_line_orientation(
                                0, switch_2->line_orientation(0));
                              switch_1->set_line_orientation(
                                1, switch_2->line_orientation(1));
                              switch_1->set_line_orientation(
                                2, switch_2->line_orientation(2));
                              switch_1->set_line_orientation(
                                3, switch_2->line_orientation(3));
                              switch_1->set_boundary_id_internal(
                                switch_2->boundary_id());
                              switch_1->set_manifold_id(
                                switch_2->manifold_id());
                              switch_1->set_user_index(switch_2->user_index());
                              if (switch_2->user_flag_set())
                                switch_1->set_user_flag();
                              else
                                switch_1->clear_user_flag();
 
                              switch_2->set_bounding_object_indices(
                                {switch_1_lines[0],
                                 switch_1_lines[1],
                                 switch_1_lines[2],
                                 switch_1_lines[3]});
                              switch_2->set_line_orientation(
                                0, switch_1_line_orientations[0]);
                              switch_2->set_line_orientation(
                                1, switch_1_line_orientations[1]);
                              switch_2->set_line_orientation(
                                2, switch_1_line_orientations[2]);
                              switch_2->set_line_orientation(
                                3, switch_1_line_orientations[3]);
                              switch_2->set_boundary_id_internal(
                                switch_1_boundary_id);
                              switch_2->set_manifold_id(
                                switch_1->manifold_id());
                              switch_2->set_user_index(switch_1_user_index);
                              if (switch_1_user_flag)
                                switch_2->set_user_flag();
                              else
                                switch_2->clear_user_flag();
 
                              const unsigned int child_0 =
                                quad->child(0)->child_index(0);
                              const unsigned int child_2 =
                                quad->child(1)->child_index(0);
                              quad->clear_children();
                              quad->clear_refinement_case();
                              quad->set_refinement_case(
                                RefinementCase<2>::cut_xy);
                              quad->set_children(0, child_0);
                              quad->set_children(2, child_2);
                              std::swap(quad_cell_count[child_0 + 1],
                                        quad_cell_count[child_2]);
                            }
                          else
                            {
                              // the face was refined
                              // with cut_y, thus the
                              // children are already
                              // in correct order. we
                              // only have to set them
                              // correctly, deleting
                              // the indirection of two
                              // anisotropic refinement
                              // and going directly
                              // from the quad to
                              // isotropic children
                              const unsigned int child_0 =
                                quad->child(0)->child_index(0);
                              const unsigned int child_2 =
                                quad->child(1)->child_index(0);
                              quad->clear_children();
                              quad->clear_refinement_case();
                              quad->set_refinement_case(
                                RefinementCase<2>::cut_xy);
                              quad->set_children(0, child_0);
                              quad->set_children(2, child_2);
                            }
                        else
                          {
                            quad->clear_children();
                            quad->clear_refinement_case();
                          }
                      }
                    break;
                  }
                case RefinementCase<dim - 1>::cut_xy:
                  {
                    // if one of the cell counters is
                    // zero, the others have to be as
                    // well
 
                    Assert((quad_cell_count[quad->child_index(0)] == 0 &&
                            quad_cell_count[quad->child_index(1)] == 0 &&
                            quad_cell_count[quad->child_index(2)] == 0 &&
                            quad_cell_count[quad->child_index(3)] == 0) ||
                             (quad_cell_count[quad->child_index(0)] > 0 &&
                              quad_cell_count[quad->child_index(1)] > 0 &&
                              quad_cell_count[quad->child_index(2)] > 0 &&
                              quad_cell_count[quad->child_index(3)] > 0),
                           ExcInternalError());
 
                    if (quad_cell_count[quad->child_index(0)] == 0)
                      {
                        // we may delete the quad's
                        // children, the inner lines
                        // and the middle vertex as no
                        // cell references them anymore
                        lines_to_delete.push_back(quad->child(0)->line(1));
                        lines_to_delete.push_back(quad->child(3)->line(0));
                        lines_to_delete.push_back(quad->child(0)->line(3));
                        lines_to_delete.push_back(quad->child(3)->line(2));
 
                        for (unsigned int child = 0; child < quad->n_children();
                             ++child)
                          quads_to_delete.push_back(quad->child(child));
 
                        triangulation
                          .vertices_used[quad->child(0)->vertex_index(3)] =
                          false;
 
                        quad->clear_children();
                        quad->clear_refinement_case();
                      }
                  }
                  break;
 
                default:
                  DEAL_II_ASSERT_UNREACHABLE();
                  break;
              }
          }
 
        // now we repeat a similar procedure
        // for the outer lines of this cell.
 
        // if in debug mode: check that each
        // of the lines for which we consider
        // deleting the children in fact has
        // children (the bits/coarsening_3d
        // test tripped over this initially)
        for (unsigned int line_no = 0;
             line_no < GeometryInfo<dim>::lines_per_cell;
             ++line_no)
          {
            typename Triangulation<dim, spacedim>::line_iterator line =
              cell->line(line_no);
 
            Assert(
              (GeometryInfo<dim>::line_refinement_case(ref_case, line_no) &&
               line->has_children()) ||
                GeometryInfo<dim>::line_refinement_case(ref_case, line_no) ==
                  RefinementCase<1>::no_refinement,
              ExcInternalError());
 
            if (line->has_children())
              {
                // if one of the cell counters is
                // zero, the other has to be as well
 
                Assert((line_cell_count[line->child_index(0)] == 0 &&
                        line_cell_count[line->child_index(1)] == 0) ||
                         (line_cell_count[line->child_index(0)] > 0 &&
                          line_cell_count[line->child_index(1)] > 0),
                       ExcInternalError());
 
                if (line_cell_count[line->child_index(0)] == 0)
                  {
                    for (unsigned int c = 0; c < 2; ++c)
                      Assert(!line->child(c)->has_children(),
                             ExcInternalError());
 
                    // we may delete the line's
                    // children and the middle vertex
                    // as no cell references them
                    // anymore
                    triangulation
                      .vertices_used[line->child(0)->vertex_index(1)] = false;
 
                    lines_to_delete.push_back(line->child(0));
                    lines_to_delete.push_back(line->child(1));
 
                    line->clear_children();
                  }
              }
          }
 
        // finally, delete unneeded quads and lines
 
        // clear user pointers, to avoid that
        // they may appear at unwanted places
        // later on...
        // same for user flags, then finally
        // delete the quads and lines
        typename std::vector<
          typename Triangulation<dim, spacedim>::line_iterator>::iterator
          line    = lines_to_delete.begin(),
          endline = lines_to_delete.end();
        for (; line != endline; ++line)
          {
            (*line)->clear_user_data();
            (*line)->clear_user_flag();
            (*line)->clear_used_flag();
          }
 
        typename std::vector<
          typename Triangulation<dim, spacedim>::quad_iterator>::iterator
          quad    = quads_to_delete.begin(),
          endquad = quads_to_delete.end();
        for (; quad != endquad; ++quad)
          {
            (*quad)->clear_user_data();
            (*quad)->clear_children();
            (*quad)->clear_refinement_case();
            (*quad)->clear_user_flag();
            (*quad)->clear_used_flag();
          }
      }
 
 
      template <int spacedim>
      static void
      create_children(
        Triangulation<2, spacedim> &triangulation,
        unsigned int               &next_unused_vertex,
        typename Triangulation<2, spacedim>::raw_line_iterator
          &next_unused_line,
        typename Triangulation<2, spacedim>::raw_cell_iterator
          &next_unused_cell,
        const typename Triangulation<2, spacedim>::cell_iterator &cell)
      {
        const unsigned int dim = 2;
        // clear refinement flag
        const RefinementCase<dim> ref_case = cell->refine_flag_set();
        cell->clear_refine_flag();
 
        /* For the refinement process: since we go the levels up from the
           lowest, there are (unlike above) only two possibilities: a neighbor
           cell is on the same level or one level up (in both cases, it may or
           may not be refined later on, but we don't care here).
 
           First:
           Set up an array of the 3x3 vertices, which are distributed on the
           cell (the array consists of indices into the @p{vertices} std::vector
 
           2--7--3
           |  |  |
           4--8--5
           |  |  |
           0--6--1
 
           note: in case of cut_x or cut_y not all these vertices are needed for
           the new cells
 
           Second:
           Set up an array of the new lines (the array consists of iterator
           pointers into the lines arrays)
 
           .-6-.-7-.         The directions are:  .->-.->-.
           1   9   3                              ^   ^   ^
           .-10.11-.                              .->-.->-.
           0   8   2                              ^   ^   ^
           .-4-.-5-.                              .->-.->-.
 
           cut_x:
           .-4-.-5-.
           |   |   |
           0   6   1
           |   |   |
           .-2-.-3-.
 
           cut_y:
           .---5---.
           1       3
           .---6---.
           0       2
           .---4---.
 
 
           Third:
           Set up an array of neighbors:
 
             6  7
            .--.--.
           1|  |  |3
            .--.--.
           0|  |  |2
            .--.--.
             4   5
 
           We need this array for two reasons: first to get the lines which will
           bound the four subcells (if the neighboring cell is refined, these
           lines already exist), and second to update neighborship information.
           Since if a neighbor is not refined, its neighborship record only
           points to the present, unrefined, cell rather than the children we
           are presently creating, we only need the neighborship information
           if the neighbor cells are refined. In all other cases, we store
           the unrefined neighbor address
 
           We also need for every neighbor (if refined) which number among its
           neighbors the present (unrefined) cell has, since that number is to
           be replaced and because that also is the number of the subline which
           will be the interface between that neighbor and the to be created
           cell. We will store this number (between 0 and 3) in the field
           @p{neighbors_neighbor}.
 
           It would be sufficient to use the children of the common line to the
           neighbor, if we only wanted to get the new sublines and the new
           vertex, but because we need to update the neighborship information of
           the two refined subcells of the neighbor, we need to search these
           anyway.
 
           Convention:
           The created children are numbered like this:
 
           .--.--.
           |2 . 3|
           .--.--.
           |0 | 1|
           .--.--.
        */
        // collect the indices of the eight surrounding vertices
        //   2--7--3
        //   |  |  |
        //   4--8--5
        //   |  |  |
        //   0--6--1
        int new_vertices[9];
        for (unsigned int vertex_no = 0; vertex_no < 4; ++vertex_no)
          new_vertices[vertex_no] = cell->vertex_index(vertex_no);
        for (unsigned int line_no = 0; line_no < 4; ++line_no)
          if (cell->line(line_no)->has_children())
            new_vertices[4 + line_no] =
              cell->line(line_no)->child(0)->vertex_index(1);
 
        if (ref_case == RefinementCase<dim>::cut_xy)
          {
            // find the next
            // unused vertex and
            // allocate it for
            // the new vertex we
            // need here
            while (triangulation.vertices_used[next_unused_vertex] == true)
              ++next_unused_vertex;
            Assert(next_unused_vertex < triangulation.vertices.size(),
                   ExcMessage(
                     "Internal error: During refinement, the triangulation "
                     "wants to access an element of the 'vertices' array "
                     "but it turns out that the array is not large enough."));
            triangulation.vertices_used[next_unused_vertex] = true;
 
            new_vertices[8] = next_unused_vertex;
 
            // determine middle vertex by transfinite interpolation to be
            // consistent with what happens to quads in a
            // Triangulation<3,3> when they are refined
            triangulation.vertices[next_unused_vertex] =
              cell->center(true, true);
          }
 
 
        // Now the lines:
        typename Triangulation<dim, spacedim>::raw_line_iterator new_lines[12];
        unsigned int                                             lmin = 8;
        unsigned int                                             lmax = 12;
        if (ref_case != RefinementCase<dim>::cut_xy)
          {
            lmin = 6;
            lmax = 7;
          }
 
        for (unsigned int l = lmin; l < lmax; ++l)
          {
            while (next_unused_line->used() == true)
              ++next_unused_line;
            new_lines[l] = next_unused_line;
            ++next_unused_line;
 
            AssertIsNotUsed(new_lines[l]);
          }
 
        if (ref_case == RefinementCase<dim>::cut_xy)
          {
            //   .-6-.-7-.
            //   1   9   3
            //   .-10.11-.
            //   0   8   2
            //   .-4-.-5-.
 
            // lines 0-7 already exist, create only the four interior
            // lines 8-11
            unsigned int l = 0;
            for (const unsigned int face_no : GeometryInfo<dim>::face_indices())
              for (unsigned int c = 0; c < 2; ++c, ++l)
                new_lines[l] = cell->line(face_no)->child(c);
            Assert(l == 8, ExcInternalError());
 
            new_lines[8]->set_bounding_object_indices(
              {new_vertices[6], new_vertices[8]});
            new_lines[9]->set_bounding_object_indices(
              {new_vertices[8], new_vertices[7]});
            new_lines[10]->set_bounding_object_indices(
              {new_vertices[4], new_vertices[8]});
            new_lines[11]->set_bounding_object_indices(
              {new_vertices[8], new_vertices[5]});
          }
        else if (ref_case == RefinementCase<dim>::cut_x)
          {
            //   .-4-.-5-.
            //   |   |   |
            //   0   6   1
            //   |   |   |
            //   .-2-.-3-.
            new_lines[0] = cell->line(0);
            new_lines[1] = cell->line(1);
            new_lines[2] = cell->line(2)->child(0);
            new_lines[3] = cell->line(2)->child(1);
            new_lines[4] = cell->line(3)->child(0);
            new_lines[5] = cell->line(3)->child(1);
            new_lines[6]->set_bounding_object_indices(
              {new_vertices[6], new_vertices[7]});
          }
        else
          {
            Assert(ref_case == RefinementCase<dim>::cut_y, ExcInternalError());
            //   .---5---.
            //   1       3
            //   .---6---.
            //   0       2
            //   .---4---.
            new_lines[0] = cell->line(0)->child(0);
            new_lines[1] = cell->line(0)->child(1);
            new_lines[2] = cell->line(1)->child(0);
            new_lines[3] = cell->line(1)->child(1);
            new_lines[4] = cell->line(2);
            new_lines[5] = cell->line(3);
            new_lines[6]->set_bounding_object_indices(
              {new_vertices[4], new_vertices[5]});
          }
 
        for (unsigned int l = lmin; l < lmax; ++l)
          {
            new_lines[l]->set_used_flag();
            new_lines[l]->clear_user_flag();
            new_lines[l]->clear_user_data();
            new_lines[l]->clear_children();
            // interior line
            new_lines[l]->set_boundary_id_internal(
              numbers::internal_face_boundary_id);
            new_lines[l]->set_manifold_id(cell->manifold_id());
          }
 
        // Now add the four (two)
        // new cells!
        typename Triangulation<dim, spacedim>::raw_cell_iterator
          subcells[ReferenceCells::max_n_children<dim>()];
        while (next_unused_cell->used() == true)
          ++next_unused_cell;
 
        const unsigned int n_children = GeometryInfo<dim>::n_children(ref_case);
        for (unsigned int i = 0; i < n_children; ++i)
          {
            AssertIsNotUsed(next_unused_cell);
            subcells[i] = next_unused_cell;
            ++next_unused_cell;
            if (i % 2 == 1 && i < n_children - 1)
              while (next_unused_cell->used() == true)
                ++next_unused_cell;
          }
 
        if (ref_case == RefinementCase<dim>::cut_xy)
          {
            // children:
            //   .--.--.
            //   |2 . 3|
            //   .--.--.
            //   |0 | 1|
            //   .--.--.
            // lines:
            //   .-6-.-7-.
            //   1   9   3
            //   .-10.11-.
            //   0   8   2
            //   .-4-.-5-.
            subcells[0]->set_bounding_object_indices({new_lines[0]->index(),
                                                      new_lines[8]->index(),
                                                      new_lines[4]->index(),
                                                      new_lines[10]->index()});
            subcells[1]->set_bounding_object_indices({new_lines[8]->index(),
                                                      new_lines[2]->index(),
                                                      new_lines[5]->index(),
                                                      new_lines[11]->index()});
            subcells[2]->set_bounding_object_indices({new_lines[1]->index(),
                                                      new_lines[9]->index(),
                                                      new_lines[10]->index(),
                                                      new_lines[6]->index()});
            subcells[3]->set_bounding_object_indices({new_lines[9]->index(),
                                                      new_lines[3]->index(),
                                                      new_lines[11]->index(),
                                                      new_lines[7]->index()});
          }
        else if (ref_case == RefinementCase<dim>::cut_x)
          {
            // children:
            //   .--.--.
            //   |  .  |
            //   .0 . 1.
            //   |  |  |
            //   .--.--.
            // lines:
            //   .-4-.-5-.
            //   |   |   |
            //   0   6   1
            //   |   |   |
            //   .-2-.-3-.
            subcells[0]->set_bounding_object_indices({new_lines[0]->index(),
                                                      new_lines[6]->index(),
                                                      new_lines[2]->index(),
                                                      new_lines[4]->index()});
            subcells[1]->set_bounding_object_indices({new_lines[6]->index(),
                                                      new_lines[1]->index(),
                                                      new_lines[3]->index(),
                                                      new_lines[5]->index()});
          }
        else
          {
            Assert(ref_case == RefinementCase<dim>::cut_y, ExcInternalError());
            // children:
            //   .-----.
            //   |  1  |
            //   .-----.
            //   |  0  |
            //   .-----.
            // lines:
            //   .---5---.
            //   1       3
            //   .---6---.
            //   0       2
            //   .---4---.
            subcells[0]->set_bounding_object_indices({new_lines[0]->index(),
                                                      new_lines[2]->index(),
                                                      new_lines[4]->index(),
                                                      new_lines[6]->index()});
            subcells[1]->set_bounding_object_indices({new_lines[1]->index(),
                                                      new_lines[3]->index(),
                                                      new_lines[6]->index(),
                                                      new_lines[5]->index()});
          }
 
        types::subdomain_id subdomainid = cell->subdomain_id();
 
        for (unsigned int i = 0; i < n_children; ++i)
          {
            subcells[i]->set_used_flag();
            subcells[i]->clear_refine_flag();
            subcells[i]->clear_user_flag();
            subcells[i]->clear_user_data();
            subcells[i]->clear_children();
            // inherit material properties
            subcells[i]->set_material_id(cell->material_id());
            subcells[i]->set_manifold_id(cell->manifold_id());
            subcells[i]->set_subdomain_id(subdomainid);
 
            // We only store the parent for every second cell. That's because
            // cells are created during refinement in multiples of two, and so
            // two successive cells always share the same parent. As a
            // consequence, we can save a bit of work by skipping setting parent
            // indices for the odd children.
            if (i % 2 == 0)
              subcells[i]->set_parent(cell->index());
          }
 
 
 
        // set child index for even children i=0,2 (0)
        for (unsigned int i = 0; i < n_children / 2; ++i)
          cell->set_children(2 * i, subcells[2 * i]->index());
        // set the refine case
        cell->set_refinement_case(ref_case);
 
        // note that the
        // refinement flag was
        // already cleared at the
        // beginning of this function
 
        if (dim == spacedim - 1)
          for (unsigned int c = 0; c < n_children; ++c)
            cell->child(c)->set_direction_flag(cell->direction_flag());
      }
 
 
 
      template <int spacedim>
      static typename Triangulation<1, spacedim>::DistortedCellList
      execute_refinement_isotropic(
        Triangulation<1, spacedim> & /*triangulation*/,
        const bool /*check_for_distorted_cells*/)
      {
        DEAL_II_ASSERT_UNREACHABLE();
      }
 
 
 
      template <int spacedim>
      static typename Triangulation<2, spacedim>::DistortedCellList
      execute_refinement_isotropic(Triangulation<2, spacedim> &triangulation,
                                   const bool check_for_distorted_cells)
      {
        constexpr int dim = 2;
 
        // Check whether a new level is needed. We have to check for
        // this on the highest level only
        for (const auto &cell : triangulation.active_cell_iterators_on_level(
               triangulation.levels.size() - 1))
          if (cell->refine_flag_set())
            {
              triangulation.levels.push_back(
                std::make_unique<internal::TriangulationImplementation::
                                   TriaLevel<dim, spacedim>>(
                  triangulation.strides.max_children_per_cell,
                  triangulation.strides.max_faces_per_cell,
                  triangulation.strides.max_vertices_per_cell));
              break;
            }
 
        for (typename Triangulation<dim, spacedim>::line_iterator line =
               triangulation.begin_line();
             line != triangulation.end_line();
             ++line)
          {
            line->clear_user_flag();
            line->clear_user_data();
          }
 
        unsigned int n_single_lines   = 0;
        unsigned int n_lines_in_pairs = 0;
        unsigned int needed_vertices  = 0;
 
        const bool orientation_needed =
          !triangulation.all_reference_cells_are_hyper_cube();
 
        for (int level_no = triangulation.levels.size() - 2; level_no >= 0;
             --level_no)
          {
            // count number of flagged cells on this level and compute
            // how many new vertices and new lines will be needed
            std::size_t needed_cells = 0;
 
            for (const auto &cell :
                 triangulation.active_cell_iterators_on_level(level_no))
              if (cell->refine_flag_set())
                {
                  if (cell->reference_cell() == ReferenceCells::Triangle)
                    {
                      needed_cells += 4;
                      needed_vertices += 0;
                      n_single_lines += 3;
                    }
                  else if (cell->reference_cell() ==
                           ReferenceCells::Quadrilateral)
                    {
                      needed_cells += 4;
                      needed_vertices += 1;
                      n_single_lines += 4;
                    }
                  else
                    {
                      AssertThrow(false, ExcNotImplemented());
                    }
 
                  for (const auto line_no : cell->face_indices())
                    {
                      auto line = cell->line(line_no);
                      if (line->has_children() == false)
                        line->set_user_flag();
                    }
                }
 
            // count number of used cells on the next higher level
            auto      &next_level = *triangulation.levels[level_no + 1];
            const auto used_cells = std::count(next_level.cells.used.begin(),
                                               next_level.cells.used.end(),
                                               true);
 
            if (used_cells + needed_cells > next_level.size())
              next_level.allocate_end((used_cells + needed_cells) -
                                        next_level.size(),
                                      orientation_needed,
                                      // We are in 2d so there are no tetrahedra
                                      false);
 
            // TODO: perhaps we can merge this with TriaLevel::allocate_end()
            next_level.cells.allocate_end(needed_cells, 0);
          }
 
        for (auto line = triangulation.begin_line();
             line != triangulation.end_line();
             ++line)
          if (line->user_flag_set())
            {
              Assert(line->has_children() == false, ExcInternalError());
              n_lines_in_pairs += 2;
              needed_vertices += 1;
            }
 
        triangulation.faces->lines.allocate_end(n_lines_in_pairs, 0);
 
        needed_vertices += std::count(triangulation.vertices_used.begin(),
                                      triangulation.vertices_used.end(),
                                      true);
 
        if (needed_vertices > triangulation.vertices.size())
          {
            triangulation.vertices.resize(needed_vertices, Point<spacedim>());
            triangulation.vertices_used.resize(needed_vertices, false);
          }
 
        unsigned int next_unused_vertex = 0;
 
        {
          typename Triangulation<dim, spacedim>::active_line_iterator
            line = triangulation.begin_active_line(),
            endl = triangulation.end_line();
          typename Triangulation<dim, spacedim>::raw_line_iterator
            next_unused_line = triangulation.begin_raw_line();
 
          for (; line != endl; ++line)
            if (line->user_flag_set())
              {
                // This line needs to be refined. Find the next unused vertex
                // and set it appropriately
                while (triangulation.vertices_used[next_unused_vertex] == true)
                  ++next_unused_vertex;
                Assert(next_unused_vertex < triangulation.vertices.size(),
                       ExcMessage(
                         "Internal error: During refinement, the triangulation "
                         "wants to access an element of the 'vertices' array "
                         "but it turns out that the array is not large "
                         "enough."));
                triangulation.vertices_used[next_unused_vertex] = true;
 
                triangulation.vertices[next_unused_vertex] = line->center(true);
 
                bool pair_found = false;
                for (; next_unused_line != endl; ++next_unused_line)
                  if (!next_unused_line->used() &&
                      !(++next_unused_line)->used())
                    {
                      --next_unused_line;
                      pair_found = true;
                      break;
                    }
                Assert(pair_found, ExcInternalError());
 
                line->set_children(0, next_unused_line->index());
 
                const typename Triangulation<dim, spacedim>::raw_line_iterator
                  children[2] = {next_unused_line, ++next_unused_line};
 
                AssertIsNotUsed(children[0]);
                AssertIsNotUsed(children[1]);
 
                children[0]->set_bounding_object_indices(
                  {line->vertex_index(0), next_unused_vertex});
                children[1]->set_bounding_object_indices(
                  {next_unused_vertex, line->vertex_index(1)});
 
                for (auto &child : children)
                  {
                    child->set_used_flag();
                    child->clear_children();
                    child->clear_user_data();
                    child->clear_user_flag();
                    child->set_boundary_id_internal(line->boundary_id());
                    child->set_manifold_id(line->manifold_id());
                    // Line orientation is relative to the cell it is on so
                    // those cannot be set at this point.
                  }
 
                line->clear_user_flag();
              }
        }
 
        triangulation.faces->lines.allocate_end(0, n_single_lines);
 
 
        typename Triangulation<dim, spacedim>::DistortedCellList
          cells_with_distorted_children;
 
        typename Triangulation<dim, spacedim>::raw_line_iterator
          next_unused_line = triangulation.begin_raw_line();
 
        const auto create_children = [](auto         &triangulation,
                                        unsigned int &next_unused_vertex,
                                        auto         &next_unused_line,
                                        auto         &next_unused_cell,
                                        const auto   &cell) {
          const auto ref_case = cell->refine_flag_set();
          cell->clear_refine_flag();
 
          unsigned int n_new_vertices = 0;
          if (cell->reference_cell() == ReferenceCells::Triangle)
            n_new_vertices = 6;
          else if (cell->reference_cell() == ReferenceCells::Quadrilateral)
            n_new_vertices = 9;
          else
            AssertThrow(false, ExcNotImplemented());
 
          std_cxx26::inplace_vector<unsigned int, 9> new_vertices(
            n_new_vertices, numbers::invalid_unsigned_int);
          for (unsigned int vertex_no = 0; vertex_no < cell->n_vertices();
               ++vertex_no)
            new_vertices[vertex_no] = cell->vertex_index(vertex_no);
          for (unsigned int line_no = 0; line_no < cell->n_lines(); ++line_no)
            if (cell->line(line_no)->has_children())
              new_vertices[cell->n_vertices() + line_no] =
                cell->line(line_no)->child(0)->vertex_index(1);
 
          if (cell->reference_cell() == ReferenceCells::Quadrilateral)
            {
              while (triangulation.vertices_used[next_unused_vertex] == true)
                ++next_unused_vertex;
              Assert(
                next_unused_vertex < triangulation.vertices.size(),
                ExcMessage(
                  "Internal error: During refinement, the triangulation wants "
                  "to access an element of the 'vertices' array but it turns "
                  "out that the array is not large enough."));
              triangulation.vertices_used[next_unused_vertex] = true;
 
              new_vertices[8] = next_unused_vertex;
 
              triangulation.vertices[next_unused_vertex] =
                cell->center(true, true);
            }
 
          std::array<typename Triangulation<dim, spacedim>::raw_line_iterator,
                     12>
                                                       new_lines;
          std::array<types::geometric_orientation, 12> inherited_orientations;
          inherited_orientations.fill(numbers::default_geometric_orientation);
          unsigned int lmin = 0;
          unsigned int lmax = 0;
 
          if (cell->reference_cell() == ReferenceCells::Triangle)
            {
              lmin = 6;
              lmax = 9;
              // For triangles, the innermost faces are always reversed for the
              // first three children and are in the standard orientation for
              // the last one.
              std::fill(inherited_orientations.begin() + lmin,
                        inherited_orientations.begin() + lmax,
                        numbers::reverse_line_orientation);
            }
          else if (cell->reference_cell() == ReferenceCells::Quadrilateral)
            {
              lmin = 8;
              lmax = 12;
            }
          else
            {
              AssertThrow(false, ExcNotImplemented());
            }
 
          for (unsigned int l = lmin; l < lmax; ++l)
            {
              while (next_unused_line->used() == true)
                ++next_unused_line;
              new_lines[l] = next_unused_line;
              ++next_unused_line;
 
              AssertIsNotUsed(new_lines[l]);
            }
 
          // set up lines which have parents:
          for (const unsigned int face_no : cell->face_indices())
            {
              // Check the face (line) orientation to ensure that the (six or
              // eight) outer lines in new_lines are indexed in the default
              // orientation. This way we can index into this array in the
              // without special casing orientations (e.g., quadrilateral child
              // 3 will always have lines 9, 3, 11, 7) when setting child lines.
              const auto combined_orientation =
                cell->combined_face_orientation(face_no);
              Assert(combined_orientation ==
                         numbers::default_geometric_orientation ||
                       combined_orientation ==
                         numbers::reverse_line_orientation,
                     ExcInternalError());
              for (unsigned int c = 0; c < 2; ++c)
                {
                  new_lines[2 * face_no + c] = cell->line(face_no)->child(c);
                  inherited_orientations[2 * face_no + c] =
                    cell->combined_face_orientation(face_no);
                }
              if (combined_orientation == numbers::reverse_line_orientation)
                std::swap(new_lines[2 * face_no], new_lines[2 * face_no + 1]);
            }
 
          // set up lines which do not have parents:
          if (cell->reference_cell() == ReferenceCells::Triangle)
            {
              new_lines[6]->set_bounding_object_indices(
                {new_vertices[3], new_vertices[4]});
              new_lines[7]->set_bounding_object_indices(
                {new_vertices[4], new_vertices[5]});
              new_lines[8]->set_bounding_object_indices(
                {new_vertices[5], new_vertices[3]});
            }
          else if (cell->reference_cell() == ReferenceCells::Quadrilateral)
            {
              new_lines[8]->set_bounding_object_indices(
                {new_vertices[6], new_vertices[8]});
              new_lines[9]->set_bounding_object_indices(
                {new_vertices[8], new_vertices[7]});
              new_lines[10]->set_bounding_object_indices(
                {new_vertices[4], new_vertices[8]});
              new_lines[11]->set_bounding_object_indices(
                {new_vertices[8], new_vertices[5]});
            }
          else
            {
              AssertThrow(false, ExcNotImplemented());
            }
 
          for (unsigned int l = lmin; l < lmax; ++l)
            {
              new_lines[l]->set_used_flag();
              new_lines[l]->clear_user_flag();
              new_lines[l]->clear_user_data();
              new_lines[l]->clear_children();
              // new lines are always internal.
              new_lines[l]->set_boundary_id_internal(
                numbers::internal_face_boundary_id);
              new_lines[l]->set_manifold_id(cell->manifold_id());
            }
 
          typename Triangulation<dim, spacedim>::raw_cell_iterator
            subcells[ReferenceCells::max_n_children<dim>()];
          while (next_unused_cell->used() == true)
            ++next_unused_cell;
 
          unsigned int n_children = 0;
          if (cell->reference_cell() == ReferenceCells::Triangle)
            n_children = 4;
          else if (cell->reference_cell() == ReferenceCells::Quadrilateral)
            n_children = 4;
          else
            AssertThrow(false, ExcNotImplemented());
 
          for (unsigned int i = 0; i < n_children; ++i)
            {
              AssertIsNotUsed(next_unused_cell);
              subcells[i] = next_unused_cell;
              ++next_unused_cell;
              if (i % 2 == 1 && i < n_children - 1)
                while (next_unused_cell->used() == true)
                  ++next_unused_cell;
            }
 
          // Assign lines to child cells:
          constexpr unsigned int X = numbers::invalid_unsigned_int;
          constexpr ::ndarray<unsigned int, 4, 4> tri_child_lines = {
            {{{0, 8, 5, X}}, //
             {{1, 2, 6, X}}, //
             {{7, 3, 4, X}}, //
             {{6, 7, 8, X}}}};
          constexpr ::ndarray<unsigned int, 4, 4> quad_child_lines = {
            {{{0, 8, 4, 10}},
             {{8, 2, 5, 11}},
             {{1, 9, 10, 6}},
             {{9, 3, 11, 7}}}};
          // Here and below we assume that child cells have the same reference
          // cell type as the parent.
          const auto &child_lines =
            cell->reference_cell() == ReferenceCells::Triangle ?
              tri_child_lines :
              quad_child_lines;
          for (unsigned int i = 0; i < n_children; ++i)
            {
              if (cell->reference_cell() == ReferenceCells::Triangle)
                subcells[i]->set_bounding_object_indices(
                  {new_lines[child_lines[i][0]]->index(),
                   new_lines[child_lines[i][1]]->index(),
                   new_lines[child_lines[i][2]]->index()});
              else
                subcells[i]->set_bounding_object_indices(
                  {new_lines[child_lines[i][0]]->index(),
                   new_lines[child_lines[i][1]]->index(),
                   new_lines[child_lines[i][2]]->index(),
                   new_lines[child_lines[i][3]]->index()});
 
              subcells[i]->set_used_flag();
              subcells[i]->clear_refine_flag();
              subcells[i]->clear_user_flag();
              subcells[i]->clear_user_data();
              subcells[i]->clear_children();
              // inherit material properties
              subcells[i]->set_material_id(cell->material_id());
              subcells[i]->set_manifold_id(cell->manifold_id());
              subcells[i]->set_subdomain_id(cell->subdomain_id());
 
              triangulation.levels[subcells[i]->level()]
                ->reference_cell[subcells[i]->index()] = cell->reference_cell();
 
              // Finally, now that children are marked as used, we can set
              // orientation flags (if needed):
              if (triangulation.levels[subcells[i]->level()]
                    ->face_orientations.n_objects() > 0)
                for (unsigned int face_no : cell->face_indices())
                  subcells[i]->set_combined_face_orientation(
                    face_no, inherited_orientations[child_lines[i][face_no]]);
              else
                // It should not be possible to inherit a nonstandard
                // orientation when orientations are not set up
                for (unsigned int face_no : cell->face_indices())
                  Assert(inherited_orientations[child_lines[i][face_no]] ==
                           numbers::default_geometric_orientation,
                         ExcInternalError());
 
              // We only store the parent for every second cell. That's because
              // cells are created during refinement in multiples of two, and so
              // two successive cells always share the same parent. As a
              // consequence, we can save a bit of work by skipping setting
              // parent indices for the odd children.
              if (i % 2 == 0)
                subcells[i]->set_parent(cell->index());
            }
 
          // Unlike the same lines on other children, the innermost triangle's
          // faces are all in the default orientation:
          if (cell->reference_cell() == ReferenceCells::Triangle)
            for (unsigned int face_no : cell->face_indices())
              subcells[3]->set_combined_face_orientation(
                face_no, numbers::default_geometric_orientation);
 
          for (unsigned int i = 0; i < n_children / 2; ++i)
            cell->set_children(2 * i, subcells[2 * i]->index());
 
          cell->set_refinement_case(ref_case);
 
          if (dim == spacedim - 1)
            for (unsigned int c = 0; c < n_children; ++c)
              cell->child(c)->set_direction_flag(cell->direction_flag());
        };
 
        for (int level = 0;
             level < static_cast<int>(triangulation.levels.size()) - 1;
             ++level)
          {
            typename Triangulation<dim, spacedim>::raw_cell_iterator
              next_unused_cell = triangulation.begin_raw(level + 1);
 
            for (const auto &cell :
                 triangulation.active_cell_iterators_on_level(level))
              if (cell->refine_flag_set())
                {
                  create_children(triangulation,
                                  next_unused_vertex,
                                  next_unused_line,
                                  next_unused_cell,
                                  cell);
 
                  if (cell->reference_cell() == ReferenceCells::Quadrilateral &&
                      check_for_distorted_cells &&
                      has_distorted_children<dim, spacedim>(cell))
                    cells_with_distorted_children.distorted_cells.push_back(
                      cell);
 
                  triangulation.signals.post_refinement_on_cell(cell);
                }
          }
 
        return cells_with_distorted_children;
      }
 
 
 
      template <int spacedim>
      static typename Triangulation<1, spacedim>::DistortedCellList
      execute_refinement(Triangulation<1, spacedim> &triangulation,
                         const bool /*check_for_distorted_cells*/)
      {
        const unsigned int dim = 1;
 
        // Check whether a new level is needed. We have to check for
        // this on the highest level only
        for (const auto &cell : triangulation.active_cell_iterators_on_level(
               triangulation.levels.size() - 1))
          if (cell->refine_flag_set())
            {
              triangulation.levels.push_back(
                std::make_unique<internal::TriangulationImplementation::
                                   TriaLevel<dim, spacedim>>(
                  triangulation.strides.max_children_per_cell,
                  triangulation.strides.max_faces_per_cell,
                  triangulation.strides.max_vertices_per_cell));
              break;
            }
 
        // check how much space is needed on every level. We need not
        // check the highest level since either - on the highest level
        // no cells are flagged for refinement - there are, but
        // prepare_refinement added another empty level
        std::size_t needed_vertices = 0;
        for (int level_no = triangulation.levels.size() - 2; level_no >= 0;
             --level_no)
          {
            std::size_t needed_cells = 0;
 
            for (const auto &cell :
                 triangulation.active_cell_iterators_on_level(level_no))
              if (cell->refine_flag_set())
                needed_cells += ReferenceCells::Line.n_isotropic_children();
 
            // count number of used cells on the next higher level
            auto      &next_level = *triangulation.levels[level_no + 1];
            const auto used_cells = std::count(next_level.cells.used.begin(),
                                               next_level.cells.used.end(),
                                               true);
 
            if (used_cells + needed_cells > next_level.size())
              next_level.allocate_end((used_cells + needed_cells) -
                                        next_level.size(),
                                      // We are in 1d so there are no tetrahedra
                                      // and we do not need the orientation
                                      false,
                                      false);
 
            // TODO: perhaps we can merge this with TriaLevel::allocate_end()
            next_level.cells.allocate_end(needed_cells, 0);
            needed_vertices += needed_cells / 2;
          }
 
        // add to needed vertices how many
        // vertices are already in use
        needed_vertices += std::count(triangulation.vertices_used.begin(),
                                      triangulation.vertices_used.end(),
                                      true);
        // if we need more vertices: create them, if not: leave the
        // array as is, since shrinking is not really possible because
        // some of the vertices at the end may be in use
        if (needed_vertices > triangulation.vertices.size())
          {
            triangulation.vertices.resize(needed_vertices, Point<spacedim>());
            triangulation.vertices_used.resize(needed_vertices, false);
          }
 
 
        // Do REFINEMENT on every level; exclude highest level as
        // above
 
        // index of next unused vertex
        unsigned int next_unused_vertex = 0;
 
        for (int level = triangulation.levels.size() - 2; level >= 0; --level)
          {
            typename Triangulation<dim, spacedim>::raw_cell_iterator
              next_unused_cell = triangulation.begin_raw(level + 1);
 
            for (const auto &cell :
                 triangulation.active_cell_iterators_on_level(level))
              if (cell->refine_flag_set())
                {
                  // clear refinement flag
                  cell->clear_refine_flag();
 
                  // search for next unused
                  // vertex
                  while (triangulation.vertices_used[next_unused_vertex] ==
                         true)
                    ++next_unused_vertex;
                  Assert(
                    next_unused_vertex < triangulation.vertices.size(),
                    ExcMessage(
                      "Internal error: During refinement, the triangulation "
                      "wants to access an element of the 'vertices' array "
                      "but it turns out that the array is not large enough."));
 
                  // Now we always ask the cell itself where to put
                  // the new point. The cell in turn will query the
                  // manifold object internally.
                  triangulation.vertices[next_unused_vertex] =
                    cell->center(true);
 
                  triangulation.vertices_used[next_unused_vertex] = true;
 
                  // search for next two unused cell (++ takes care of
                  // the end of the vector)
                  typename Triangulation<dim, spacedim>::raw_cell_iterator
                    first_child,
                    second_child;
                  while (next_unused_cell->used() == true)
                    ++next_unused_cell;
                  first_child = next_unused_cell;
                  first_child->set_used_flag();
                  first_child->clear_user_data();
                  ++next_unused_cell;
                  AssertIsNotUsed(next_unused_cell);
                  second_child = next_unused_cell;
                  second_child->set_used_flag();
                  second_child->clear_user_data();
 
                  types::subdomain_id subdomainid = cell->subdomain_id();
 
                  // insert first child
                  cell->set_children(0, first_child->index());
                  first_child->clear_children();
                  first_child->set_bounding_object_indices(
                    {cell->vertex_index(0), next_unused_vertex});
                  first_child->set_material_id(cell->material_id());
                  first_child->set_manifold_id(cell->manifold_id());
                  first_child->set_subdomain_id(subdomainid);
                  if (dim == spacedim - 1)
                    first_child->set_direction_flag(cell->direction_flag());
 
                  // We only store the parent for every second cell. That's
                  // because cells are created during refinement in multiples of
                  // two, and so two successive cells always share the same
                  // parent. As a consequence, we can save a bit of work by
                  // skipping setting parent indices for the second child.
                  first_child->set_parent(cell->index());
 
                  // Set manifold id of the right face. Only do this
                  // on the first child.
                  first_child->face(1)->set_manifold_id(cell->manifold_id());
 
                  // reset neighborship info (refer to
                  // internal::TriangulationImplementation::TriaLevel for
                  // details)
                  first_child->set_neighbor(1, second_child);
                  if (cell->neighbor(0).state() != IteratorState::valid)
                    first_child->set_neighbor(0, cell->neighbor(0));
                  else if (cell->neighbor(0)->is_active())
                    {
                      // since the neighbors level is always <=level,
                      // if the cell is active, then there are no
                      // cells to the left which may want to know
                      // about this new child cell.
                      Assert(cell->neighbor(0)->level() <= cell->level(),
                             ExcInternalError());
                      first_child->set_neighbor(0, cell->neighbor(0));
                    }
                  else
                    // left neighbor is refined
                    {
                      // set neighbor to cell on same level
                      const unsigned int nbnb = cell->neighbor_of_neighbor(0);
                      first_child->set_neighbor(0,
                                                cell->neighbor(0)->child(nbnb));
 
                      // reset neighbor info of all right descendant
                      // of the left neighbor of cell
                      typename Triangulation<dim, spacedim>::cell_iterator
                        left_neighbor = cell->neighbor(0);
                      while (left_neighbor->has_children())
                        {
                          left_neighbor = left_neighbor->child(nbnb);
                          left_neighbor->set_neighbor(nbnb, first_child);
                        }
                    }
 
                  // insert second child
                  second_child->clear_children();
                  second_child->set_bounding_object_indices(
                    {next_unused_vertex, cell->vertex_index(1)});
                  second_child->set_neighbor(0, first_child);
                  second_child->set_material_id(cell->material_id());
                  second_child->set_manifold_id(cell->manifold_id());
                  second_child->set_subdomain_id(subdomainid);
                  if (dim == spacedim - 1)
                    second_child->set_direction_flag(cell->direction_flag());
 
                  if (cell->neighbor(1).state() != IteratorState::valid)
                    second_child->set_neighbor(1, cell->neighbor(1));
                  else if (cell->neighbor(1)->is_active())
                    {
                      Assert(cell->neighbor(1)->level() <= cell->level(),
                             ExcInternalError());
                      second_child->set_neighbor(1, cell->neighbor(1));
                    }
                  else
                    // right neighbor is refined same as above
                    {
                      const unsigned int nbnb = cell->neighbor_of_neighbor(1);
                      second_child->set_neighbor(
                        1, cell->neighbor(1)->child(nbnb));
 
                      typename Triangulation<dim, spacedim>::cell_iterator
                        right_neighbor = cell->neighbor(1);
                      while (right_neighbor->has_children())
                        {
                          right_neighbor = right_neighbor->child(nbnb);
                          right_neighbor->set_neighbor(nbnb, second_child);
                        }
                    }
                  // inform all listeners that cell refinement is done
                  triangulation.signals.post_refinement_on_cell(cell);
                }
          }
 
        // in 1d, we can not have distorted children unless the parent
        // was already distorted (that is because we don't use
        // boundary information for 1d triangulations). so return an
        // empty list
        return typename Triangulation<1, spacedim>::DistortedCellList();
      }
 
 
      template <int spacedim>
      static typename Triangulation<2, spacedim>::DistortedCellList
      execute_refinement(Triangulation<2, spacedim> &triangulation,
                         const bool                  check_for_distorted_cells)
      {
        const unsigned int dim = 2;
 
        // First check whether we can get away with isotropic refinement, or
        // whether we need to run through the full anisotropic algorithm
        bool do_isotropic_refinement = true;
        for (const auto &cell : triangulation.active_cell_iterators())
          if (cell->refine_flag_set() == RefinementCase<dim>::cut_x ||
              cell->refine_flag_set() == RefinementCase<dim>::cut_y)
            {
              do_isotropic_refinement = false;
              break;
            }
 
        if (do_isotropic_refinement)
          return execute_refinement_isotropic(triangulation,
                                              check_for_distorted_cells);
        else
          {
            // If we get here, we are doing anisotropic refinement.
 
            // Check whether a new level is needed. We have to check for
            // this on the highest level only
            for (const auto &cell :
                 triangulation.active_cell_iterators_on_level(
                   triangulation.levels.size() - 1))
              if (cell->refine_flag_set())
                {
                  triangulation.levels.push_back(
                    std::make_unique<internal::TriangulationImplementation::
                                       TriaLevel<dim, spacedim>>(
                      triangulation.strides.max_children_per_cell,
                      triangulation.strides.max_faces_per_cell,
                      triangulation.strides.max_vertices_per_cell));
                  break;
                }
 
            // TODO[WB]: we clear user flags and pointers of lines; we're going
            // to use them to flag which lines need refinement
            for (typename Triangulation<dim, spacedim>::line_iterator line =
                   triangulation.begin_line();
                 line != triangulation.end_line();
                 ++line)
              {
                line->clear_user_flag();
                line->clear_user_data();
              }
 
            // running over all cells and lines count the number
            // n_single_lines of lines which can be stored as single
            // lines, e.g. inner lines
            unsigned int n_single_lines = 0;
 
            // New lines to be created: number lines which are stored in
            // pairs (the children of lines must be stored in pairs)
            unsigned int n_lines_in_pairs = 0;
 
            // check how much space is needed on every level. We need not
            // check the highest level since either - on the highest level
            // no cells are flagged for refinement - there are, but
            // prepare_refinement added another empty level
            unsigned int needed_vertices = 0;
 
            const bool orientation_needed =
              !triangulation.all_reference_cells_are_hyper_cube();
 
            for (int level_no = triangulation.levels.size() - 2; level_no >= 0;
                 --level_no)
              {
                // count number of flagged cells on this level and compute
                // how many new vertices and new lines will be needed
                std::size_t needed_cells = 0;
 
                for (const auto &cell :
                     triangulation.active_cell_iterators_on_level(level_no))
                  if (cell->refine_flag_set())
                    {
                      Assert(cell->reference_cell().is_hyper_cube(),
                             ExcNotImplemented());
                      if (cell->refine_flag_set() ==
                          RefinementCase<dim>::cut_xy)
                        {
                          needed_cells += 4;
 
                          // new vertex at center of cell is needed in any
                          // case
                          ++needed_vertices;
 
                          // the four inner lines can be stored as singles
                          n_single_lines += 4;
                        }
                      else // cut_x || cut_y
                        {
                          // set the flag showing that anisotropic
                          // refinement is used for at least one cell
                          triangulation.anisotropic_refinement = true;
 
                          needed_cells += 2;
                          // no vertex at center
 
                          // the inner line can be stored as single
                          n_single_lines += 1;
                        }
 
                      // mark all faces (lines) for refinement; checking
                      // locally whether the neighbor would also like to
                      // refine them is rather difficult for lines so we
                      // only flag them and after visiting all cells, we
                      // decide which lines need refinement;
                      for (const unsigned int line_no :
                           GeometryInfo<dim>::face_indices())
                        {
                          if (GeometryInfo<dim>::face_refinement_case(
                                cell->refine_flag_set(), line_no) ==
                              RefinementCase<1>::cut_x)
                            {
                              typename Triangulation<dim,
                                                     spacedim>::line_iterator
                                line = cell->line(line_no);
                              if (line->has_children() == false)
                                line->set_user_flag();
                            }
                        }
                    }
 
                auto      &next_level = *triangulation.levels[level_no + 1];
                const auto used_cells =
                  std::count(next_level.cells.used.begin(),
                             next_level.cells.used.end(),
                             true);
                if (used_cells + needed_cells > next_level.size())
                  next_level.allocate_end(
                    (used_cells + needed_cells) - next_level.size(),
                    orientation_needed,
                    // We are in 2d so there are no tetrahedra
                    false);
 
                // TODO: perhaps we can merge this with
                // TriaLevel::allocate_end()
                next_level.cells.allocate_end(needed_cells, 0);
              }
 
            // now count the lines which were flagged for refinement
            for (typename Triangulation<dim, spacedim>::line_iterator line =
                   triangulation.begin_line();
                 line != triangulation.end_line();
                 ++line)
              if (line->user_flag_set())
                {
                  Assert(line->has_children() == false, ExcInternalError());
                  n_lines_in_pairs += 2;
                  needed_vertices += 1;
                }
            // reserve space for n_lines_in_pairs new lines.  note, that
            // we can't reserve space for the single lines here as well,
            // as all the space reserved for lines in pairs would be
            // counted as unused and we would end up with too little space
            // to store all lines. memory reservation for n_single_lines
            // can only be done AFTER we refined the lines of the current
            // cells
            triangulation.faces->lines.allocate_end(n_lines_in_pairs, 0);
 
            // add to needed vertices how many vertices are already in use
            needed_vertices += std::count(triangulation.vertices_used.begin(),
                                          triangulation.vertices_used.end(),
                                          true);
            // if we need more vertices: create them, if not: leave the
            // array as is, since shrinking is not really possible because
            // some of the vertices at the end may be in use
            if (needed_vertices > triangulation.vertices.size())
              {
                triangulation.vertices.resize(needed_vertices,
                                              Point<spacedim>());
                triangulation.vertices_used.resize(needed_vertices, false);
              }
 
 
            // Do REFINEMENT on every level; exclude highest level as
            // above
 
            //  index of next unused vertex
            unsigned int next_unused_vertex = 0;
 
            // first the refinement of lines.  children are stored
            // pairwise
            {
              // only active objects can be refined further
              typename Triangulation<dim, spacedim>::active_line_iterator
                line = triangulation.begin_active_line(),
                endl = triangulation.end_line();
              typename Triangulation<dim, spacedim>::raw_line_iterator
                next_unused_line = triangulation.begin_raw_line();
 
              for (; line != endl; ++line)
                if (line->user_flag_set())
                  {
                    // this line needs to be refined
 
                    // find the next unused vertex and set it
                    // appropriately
                    while (triangulation.vertices_used[next_unused_vertex] ==
                           true)
                      ++next_unused_vertex;
                    Assert(
                      next_unused_vertex < triangulation.vertices.size(),
                      ExcMessage(
                        "Internal error: During refinement, the triangulation wants to access an element of the 'vertices' array but it turns out that the array is not large enough."));
                    triangulation.vertices_used[next_unused_vertex] = true;
 
                    triangulation.vertices[next_unused_vertex] =
                      line->center(true);
 
                    // now that we created the right point, make up the
                    // two child lines.  To this end, find a pair of
                    // unused lines
                    bool pair_found = false;
                    for (; next_unused_line != endl; ++next_unused_line)
                      if (!next_unused_line->used() &&
                          !(++next_unused_line)->used())
                        {
                          // go back to the first of the two unused
                          // lines
                          --next_unused_line;
                          pair_found = true;
                          break;
                        }
                    Assert(pair_found, ExcInternalError());
 
                    // there are now two consecutive unused lines, such
                    // that the children of a line will be consecutive.
                    // then set the child pointer of the present line
                    line->set_children(0, next_unused_line->index());
 
                    // set the two new lines
                    const typename Triangulation<dim,
                                                 spacedim>::raw_line_iterator
                      children[2] = {next_unused_line, ++next_unused_line};
                    // some tests; if any of the iterators should be
                    // invalid, then already dereferencing will fail
                    AssertIsNotUsed(children[0]);
                    AssertIsNotUsed(children[1]);
 
                    children[0]->set_bounding_object_indices(
                      {line->vertex_index(0), next_unused_vertex});
                    children[1]->set_bounding_object_indices(
                      {next_unused_vertex, line->vertex_index(1)});
 
                    children[0]->set_used_flag();
                    children[1]->set_used_flag();
                    children[0]->clear_children();
                    children[1]->clear_children();
                    children[0]->clear_user_data();
                    children[1]->clear_user_data();
                    children[0]->clear_user_flag();
                    children[1]->clear_user_flag();
 
 
                    children[0]->set_boundary_id_internal(line->boundary_id());
                    children[1]->set_boundary_id_internal(line->boundary_id());
 
                    children[0]->set_manifold_id(line->manifold_id());
                    children[1]->set_manifold_id(line->manifold_id());
 
                    // finally clear flag indicating the need for
                    // refinement
                    line->clear_user_flag();
                  }
            }
 
 
            // Now set up the new cells
 
            // reserve space for inner lines (can be stored as single
            // lines)
            triangulation.faces->lines.allocate_end(0, n_single_lines);
 
            typename Triangulation<2, spacedim>::DistortedCellList
              cells_with_distorted_children;
 
            // reset next_unused_line, as now also single empty places in
            // the vector can be used
            typename Triangulation<dim, spacedim>::raw_line_iterator
              next_unused_line = triangulation.begin_raw_line();
 
            for (int level = 0;
                 level < static_cast<int>(triangulation.levels.size()) - 1;
                 ++level)
              {
                typename Triangulation<dim, spacedim>::raw_cell_iterator
                  next_unused_cell = triangulation.begin_raw(level + 1);
 
                for (const auto &cell :
                     triangulation.active_cell_iterators_on_level(level))
                  if (cell->refine_flag_set())
                    {
                      // actually set up the children and update neighbor
                      // information
                      create_children(triangulation,
                                      next_unused_vertex,
                                      next_unused_line,
                                      next_unused_cell,
                                      cell);
 
                      if (check_for_distorted_cells &&
                          has_distorted_children<dim, spacedim>(cell))
                        cells_with_distorted_children.distorted_cells.push_back(
                          cell);
                      // inform all listeners that cell refinement is done
                      triangulation.signals.post_refinement_on_cell(cell);
                    }
              }
 
            return cells_with_distorted_children;
          }
      }
 
 
      template <int spacedim>
      static typename Triangulation<3, spacedim>::DistortedCellList
      execute_refinement_isotropic(Triangulation<3, spacedim> &triangulation,
                                   const bool check_for_distorted_cells)
      {
        constexpr int          dim = 3;
        constexpr unsigned int X   = numbers::invalid_unsigned_int;
        using raw_line_iterator =
          typename Triangulation<dim, spacedim>::raw_line_iterator;
        using raw_quad_iterator =
          typename Triangulation<dim, spacedim>::raw_quad_iterator;
 
        Assert(spacedim == 3, ExcNotImplemented());
 
        Assert(triangulation.vertices.size() ==
                 triangulation.vertices_used.size(),
               ExcInternalError());
 
        // Check whether a new level is needed. We have to check for
        // this on the highest level only
        for (const auto &cell : triangulation.active_cell_iterators_on_level(
               triangulation.levels.size() - 1))
          if (cell->refine_flag_set())
            {
              AssertThrow(
                triangulation.levels.size() < numbers::max_n_levels,
                (typename Triangulation<3, spacedim>::ExcTooManyLevels()));
              triangulation.levels.push_back(
                std::make_unique<internal::TriangulationImplementation::
                                   TriaLevel<dim, spacedim>>(
                  triangulation.strides.max_children_per_cell,
                  triangulation.strides.max_faces_per_cell,
                  triangulation.strides.max_vertices_per_cell));
              break;
            }
 
        // first clear user flags for quads and lines; we're going to
        // use them to flag which lines and quads need refinement
        triangulation.faces->quads.clear_user_data();
        triangulation.faces->lines.clear_user_flags();
        triangulation.faces->quads.clear_user_flags();
 
        // check how much space is needed on every level. We need not
        // check the highest level since either
        // - on the highest level no cells are flagged for refinement
        // - there are, but prepare_refinement added another empty
        //   level which then is the highest level
 
        // Variables to hold the number of newly to be created
        // vertices, lines, and faces. As these are stored globally,
        // declare them outside the loop over all levels. We need lines
        // and faces in pairs for refinement of old lines/face. And lines and
        // faces stored individually for the ones created in the interior
        // of an existing cell
        { // STORAGE
          unsigned int needed_vertices     = 0;
          unsigned int needed_lines_single = 0;
          unsigned int needed_faces_single = 0;
          unsigned int needed_lines_pair   = 0;
          unsigned int needed_faces_pair   = 0;
          bool         contains_pyramid    = false;
          for (int level_no = triangulation.levels.size() - 2; level_no >= 0;
               --level_no)
            {
              std::size_t needed_cells = 0;
 
              for (const auto &cell :
                   triangulation.active_cell_iterators_on_level(level_no))
                if (cell->refine_flag_set())
                  {
                    // Only support isotropic refinement
                    Assert(cell->refine_flag_set() ==
                             RefinementCase<dim>::cut_xyz,
                           ExcInternalError());
 
                    // get reference cell type of current cell
                    const auto cell_reference_cell = cell->reference_cell();
 
                    // Now count up how many new cells, faces, edges, and
                    // vertices we will need to allocate to do this refinement.
                    needed_cells += cell_reference_cell.n_isotropic_children();
 
                    switch (cell_reference_cell)
                      {
                        case ReferenceCells::Tetrahedron:
                          needed_lines_single += 1;
                          needed_faces_single += 8;
                          break;
 
                        case ReferenceCells::Pyramid:
                          // TODO: Pyramid
                          // - No vertices (all provided by face refinement)
                          // - 5 faces for center pyramid
                          // - 4 times 2 for the remaining tets
                          needed_lines_single += 4;
                          needed_faces_single += 13;
                          contains_pyramid = true;
                          break;
 
                        case ReferenceCells::Wedge:
                          // - We won't need vertices (all required vertices
                          //   will be provided by the refined faces)
                          // - 3 lines will be required to build up the
                          //   "refined" middle triangle
                          // - 4 faces for the new refined middle triangle + 6
                          //   faces for the quads building the wedges in
                          //   z-direction. Rest will be provided via refinement
                          //   of faces. Store them all as singles. See also
                          //   https://link.springer.com/article/10.1007/BF01221213
                          needed_lines_single += 3;
                          needed_faces_single += 10;
                          break;
 
                        case ReferenceCells::Hexahedron:
                          // One vertex in the center of the parent
                          ++needed_vertices;
                          needed_lines_single += 6;
                          needed_faces_single += 12;
                          break;
 
                        default:
                          DEAL_II_ASSERT_UNREACHABLE();
                      }
 
                    // Also check whether we have to refine any of the faces and
                    // edges that bound this cell. They may of course already be
                    // refined, so we only *mark* them for refinement by setting
                    // the user flags
 
                    // Faces
                    for (const auto face : cell->face_indices())
                      if (cell->face(face)->n_children() == 0)
                        cell->face(face)->set_user_flag();
                      else
                        // Check that the child of the cell has enough children
                        Assert(cell->face(face)->n_children() ==
                                 cell_reference_cell.face_reference_cell(face)
                                   .n_isotropic_children(),
                               ExcInternalError());
 
                    // Edges
                    for (const auto line : cell->line_indices())
                      if (cell->line(line)->has_children() == false)
                        cell->line(line)->set_user_flag();
                      else
                        Assert(cell->line(line)->n_children() == 2,
                               ExcInternalError());
                  }
 
              auto      &next_level = *triangulation.levels[level_no + 1];
              const auto used_cells = std::count(next_level.cells.used.begin(),
                                                 next_level.cells.used.end(),
                                                 true);
              if (used_cells + needed_cells > next_level.size())
                next_level.allocate_end(
                  (used_cells + needed_cells) - next_level.size(),
                  // We unconditionally store the orientation in 3d
                  true,
                  // TODO: we can get rid of this check (i.e., we don't need to
                  // store this value for purely wedge meshes). This will be
                  // simpler to do once we merge IsotropicRefinementChoice and
                  // RefinementCase.
                  !triangulation.all_reference_cells_are_hyper_cube());
 
              // TODO: perhaps we can merge this with TriaLevel::allocate_end()
              next_level.cells.allocate_end(needed_cells, 0);
            }
 
          // In case a pyramid is going to be refined we will get additional
          // tets in the refined triangulation, hence we need to inform the
          // triangulation that this happens. Note: if coarsening gets
          // implemented this step needs to be reverted. Maybe switch to a
          // refinement counter for pyramids and track its value to revert this
          // change.
          if (contains_pyramid)
            // Prevent multiple occurrences of tet inside the vector
            if (std::find(triangulation.reference_cells.begin(),
                          triangulation.reference_cells.end(),
                          ReferenceCells::Tetrahedron) ==
                triangulation.reference_cells.end())
              // Add it to start of vector to keep sorting intact.
              triangulation.reference_cells.insert(
                triangulation.reference_cells.begin(),
                ReferenceCells::Tetrahedron);
 
          // now count the faces and lines which were flagged for
          // refinement
          for (typename Triangulation<dim, spacedim>::quad_iterator quad =
                 triangulation.begin_quad();
               quad != triangulation.end_quad();
               ++quad)
            {
              if (quad->user_flag_set() == false)
                continue;
 
              if (quad->reference_cell() == ReferenceCells::Quadrilateral)
                {
                  needed_faces_pair += 4;
                  needed_lines_pair += 4;
                  needed_vertices += 1;
                }
              else if (quad->reference_cell() == ReferenceCells::Triangle)
                {
                  needed_faces_pair += 4;
                  needed_lines_single += 3;
                }
              else
                {
                  DEAL_II_ASSERT_UNREACHABLE();
                }
            }
 
          for (typename Triangulation<dim, spacedim>::line_iterator line =
                 triangulation.begin_line();
               line != triangulation.end_line();
               ++line)
            {
              if (line->user_flag_set() == false)
                continue;
 
              needed_lines_pair += 2;
              needed_vertices += 1;
            }
 
          triangulation.faces->allocate_end(needed_faces_pair,
                                            needed_faces_single);
          triangulation.faces->lines.allocate_end(needed_lines_pair,
                                                  needed_lines_single);
 
          // add to needed vertices how many vertices are already in use
          needed_vertices += std::count(triangulation.vertices_used.begin(),
                                        triangulation.vertices_used.end(),
                                        true);
 
          if (needed_vertices > triangulation.vertices.size())
            {
              triangulation.vertices.resize(needed_vertices, Point<spacedim>());
              triangulation.vertices_used.resize(needed_vertices, false);
            }
        } // STORAGE
 
        //-----------------------------------------
        // Before we start with the actual refinement, we do some
        // sanity checks if in debug mode. especially, we try to catch
        // the notorious problem with lines being twice refined,
        // i.e. there are cells adjacent at one line ("around the
        // edge", but not at a face), with two cells differing by more
        // than one refinement level
        //
        // this check is very simple to implement here, since we have
        // all lines flagged if they shall be refined
        if constexpr (running_in_debug_mode())
          {
            for (const auto &cell : triangulation.active_cell_iterators())
              if (!cell->refine_flag_set())
                for (unsigned int line_n = 0; line_n < cell->n_lines();
                     ++line_n)
                  if (cell->line(line_n)->has_children())
                    for (unsigned int c = 0; c < 2; ++c)
                      Assert(cell->line(line_n)->child(c)->user_flag_set() ==
                               false,
                             ExcInternalError());
          }
 
        unsigned int current_vertex = 0;
 
        // helper function - find the next available vertex number and mark it
        // as used.
        auto get_next_unused_vertex = [](const unsigned int current_vertex,
                                         std::vector<bool> &vertices_used) {
          unsigned int next_vertex = current_vertex;
          while (next_vertex < vertices_used.size() &&
                 vertices_used[next_vertex] == true)
            ++next_vertex;
          Assert(next_vertex < vertices_used.size(), ExcInternalError());
          vertices_used[next_vertex] = true;
 
          return next_vertex;
        };
 
 
        { // LINES
          typename Triangulation<dim, spacedim>::active_line_iterator
            line = triangulation.begin_active_line(),
            endl = triangulation.end_line();
          raw_line_iterator next_unused_line = triangulation.begin_raw_line();
 
          for (; line != endl; ++line)
            {
              if (line->user_flag_set() == false)
                continue;
 
              next_unused_line =
                triangulation.faces->lines.template next_free_pair_object<1>(
                  triangulation);
              Assert(next_unused_line.state() == IteratorState::valid,
                     ExcInternalError());
 
              // now we found two consecutive unused lines, such
              // that the children of a line will be consecutive.
              // then set the child pointer of the present line
              line->set_children(0, next_unused_line->index());
 
              const std::array<raw_line_iterator, 2> children{
                {next_unused_line, ++next_unused_line}};
 
              AssertIsNotUsed(children[0]);
              AssertIsNotUsed(children[1]);
 
              current_vertex =
                get_next_unused_vertex(current_vertex,
                                       triangulation.vertices_used);
              triangulation.vertices[current_vertex] = line->center(true);
 
              children[0]->set_bounding_object_indices(
                {line->vertex_index(0), current_vertex});
              children[1]->set_bounding_object_indices(
                {current_vertex, line->vertex_index(1)});
 
              const auto manifold_id = line->manifold_id();
              const auto boundary_id = line->boundary_id();
              for (const auto &child : children)
                {
                  child->set_used_flag();
                  child->clear_children();
                  child->clear_user_data();
                  child->clear_user_flag();
                  child->set_boundary_id_internal(boundary_id);
                  child->set_manifold_id(manifold_id);
                }
 
              line->clear_user_flag();
            }
        }
 
        { // FACES
          // (i.e., quads or triangles, or both)
          typename Triangulation<dim, spacedim>::face_iterator
            face = triangulation.begin_face(),
            endf = triangulation.end_face();
 
          for (; face != endf; ++face)
            {
              if (face->user_flag_set() == false)
                continue;
 
              const auto reference_face_type = face->reference_cell();
              const auto n_lines_per_face    = reference_face_type.n_lines();
 
              // 1) Create new lines (property is set later).
              // Maximum of 4 new lines (4 quadrilateral, 3 triangle).
              std::array<raw_line_iterator, 4> new_lines;
              switch (reference_face_type)
                {
                  case ReferenceCells::Triangle:
                    for (unsigned int l = 0; l < 3; ++l)
                      new_lines[l] =
                        triangulation.faces->lines
                          .template next_free_single_object<1>(triangulation);
                    break;
 
                  case ReferenceCells::Quadrilateral:
                    for (unsigned int l = 0; l < 2; ++l)
                      {
                        auto next_unused_line =
                          triangulation.faces->lines
                            .template next_free_pair_object<1>(triangulation);
                        new_lines[2 * l]     = next_unused_line;
                        new_lines[2 * l + 1] = ++next_unused_line;
                      }
                    break;
 
                  default:
                    DEAL_II_NOT_IMPLEMENTED();
                }
 
              if constexpr (running_in_debug_mode())
                {
                  for (const unsigned int line : face->line_indices())
                    AssertIsNotUsed(new_lines[line]);
                }
 
              // 2) Create new face (properties are set below).
              // Both triangles and quads are divided in four. (For historical
              // reasons, we only have `raw_quad_iterator`, not
              // `raw_face_iterator`, but the former also works if a face is
              // actually a triangle.)
              std::array<raw_quad_iterator, 4> new_faces;
              for (unsigned int f = 0; f < 2; ++f)
                {
                  auto next_unused_quad =
                    triangulation.faces->quads
                      .template next_free_pair_object<2>(triangulation);
 
                  new_faces[2 * f]     = next_unused_quad;
                  new_faces[2 * f + 1] = ++next_unused_quad;
 
                  face->set_children(2 * f, new_faces[2 * f]->index());
                }
              // Tell the original face that is has been isotropically refined.
              face->set_refinement_case(RefinementCase<2>::cut_xy);
 
              if constexpr (running_in_debug_mode())
                {
                  for (const auto &quad : new_faces)
                    AssertIsNotUsed(quad);
                }
 
              // 3) Create an ordered list of vertices.
              // The order is determined by the physical position of the
              // vertices within the face reference cell, enabling easy
              // identification/lookup by the vertex's index in this list.
              // Additionally the center vertex required for quadrilaterals is
              // created.
 
              // Maximum of 9 vertices per refined face (9 for Quadrilateral, 6
              // for Triangle)
              std::array<unsigned int, 9> vertex_indices = {};
              unsigned int                k              = 0;
              for (const auto i : face->vertex_indices())
                vertex_indices[k++] = face->vertex_index(i);
 
              for (unsigned int l = 0; l < n_lines_per_face; ++l)
                vertex_indices[k++] = face->line(l)->child(0)->vertex_index(1);
 
              if (reference_face_type == ReferenceCells::Quadrilateral)
                {
                  current_vertex =
                    get_next_unused_vertex(current_vertex,
                                           triangulation.vertices_used);
                  vertex_indices[k++] = current_vertex;
 
                  triangulation.vertices[current_vertex] =
                    face->center(true, true);
                }
 
              // 4) Set new lines on these faces and their properties.
              // 'magic numbers' in `line_vertices_X` are indices in the list
              // created in step 3).
              std::array<raw_line_iterator, 12> lines;
              unsigned int                      n_lines = 0;
              for (unsigned int l = 0; l < n_lines_per_face; ++l)
                for (unsigned int c = 0; c < 2; ++c)
                  lines[n_lines++] = face->line(l)->child(
                    child_line_index(c, face->line_orientation(l)));
 
              for (unsigned int l = 0; l < n_lines_per_face; ++l)
                lines[n_lines++] = new_lines[l];
 
              std::array<int, 12> line_indices;
              for (unsigned int i = 0; i < n_lines; ++i)
                line_indices[i] = lines[i]->index();
 
              // 2---7---3   .-6-.-7-.   .---.---.
              // |   |   |   1   9   3   | 2 | 3 |
              // 4---8---5   .-10.11-.   .---.---.
              // |   |   |   0   8   2   | 0 | 1 |
              // 0---6---1   .-4-.-5-.   .---.---.
              constexpr ::ndarray<unsigned int, 12, 2> line_vertices_quad{
                {{{0, 4}},
                 {{4, 2}},
                 {{1, 5}},
                 {{5, 3}},
                 {{0, 6}},
                 {{6, 1}},
                 {{2, 7}},
                 {{7, 3}},
                 {{6, 8}},
                 {{8, 7}},
                 {{4, 8}},
                 {{8, 5}}}};
 
              constexpr ::ndarray<unsigned int, 4, 4> quad_lines_quad{
                {{{0, 8, 4, 10}},
                 {{8, 2, 5, 11}},
                 {{1, 9, 10, 6}},
                 {{9, 3, 11, 7}}}};
 
              //  2                *                *             .
              //  |\               |\               |\            .
              //  v  ^             4  3             |  \          .
              //  |    \           |    \           | 2  \        .
              //  5--<--4          *--7--*          *-----*       .
              //  |\    |\         |\    |\         |\  3 |\      .
              //  v  ^  ^  ^       5  8  6  2       |  \  |  \    .
              //  |    \|    \     |    \|    \     | 0  \| 1  \  .
              //  0-->--3-->--1    *--0--*--1--*    *-----*-----* .
              constexpr ::ndarray<unsigned int, 12, 2> line_vertices_tri{
                {{{0, 3}},
                 {{3, 1}},
                 {{1, 4}},
                 {{4, 2}},
                 {{2, 5}},
                 {{5, 0}},
                 {{3, 4}},
                 {{4, 5}},
                 {{3, 5}},
                 {{X, X}},
                 {{X, X}},
                 {{X, X}}}};
 
              constexpr ::ndarray<unsigned int, 4, 4> tri_lines_tri{
                {{{0, 8, 5, X}},
                 {{1, 2, 6, X}},
                 {{7, 3, 4, X}},
                 {{6, 7, 8, X}}}};
 
              // The defined lines in `line_vertices_tri` do not satisfy the
              // expected orientations of all the children's reference cells.
              // This table specifies the expected line orientations (i.e.
              // vertex order) necessary for `set_line_orientation(...)`.
              constexpr ::ndarray<unsigned int, 4, 4, 2>
                tri_line_vertices_tri{
                  {{{{{0, 3}}, {{3, 5}}, {{5, 0}}, {{X, X}}}},
                   {{{{3, 1}}, {{1, 4}}, {{4, 3}}, {{X, X}}}},
                   {{{{5, 4}}, {{4, 2}}, {{2, 5}}, {{X, X}}}},
                   {{{{3, 4}}, {{4, 5}}, {{5, 3}}, {{X, X}}}}}};
 
              // Select lookup table according to reference cell of parent.
              const auto &line_vertices =
                (reference_face_type == ReferenceCells::Quadrilateral) ?
                  line_vertices_quad :
                  line_vertices_tri;
              const auto &face_lines =
                (reference_face_type == ReferenceCells::Quadrilateral) ?
                  quad_lines_quad :
                  tri_lines_tri;
 
              // The first 2*face->n_lines() lines are already created by
              // refining the original lines on the face. Therefore, only the
              // subsequent face->n_lines() lines need to be processed now.
              for (unsigned int i = 0, j = 2 * n_lines_per_face;
                   i < n_lines_per_face;
                   ++i, ++j)
                {
                  auto &new_line = new_lines[i];
                  new_line->set_bounding_object_indices(
                    {vertex_indices[line_vertices[j][0]],
                     vertex_indices[line_vertices[j][1]]});
                  new_line->set_used_flag();
                  new_line->clear_user_flag();
                  new_line->clear_user_data();
                  new_line->clear_children();
                  new_line->set_boundary_id_internal(face->boundary_id());
                  new_line->set_manifold_id(face->manifold_id());
                }
 
              // 5) Set properties of faces
              for (unsigned int i = 0; i < new_faces.size(); ++i)
                {
                  // 5.a) Create faces
                  auto &new_face = new_faces[i];
 
                  // We assume here that all children have the same type as the
                  // parent face.
                  triangulation.faces->set_quad_type(new_face->index(),
                                                     reference_face_type);
 
                  switch (reference_face_type)
                    {
                      case ReferenceCells::Triangle:
                        new_face->set_bounding_object_indices(
                          {line_indices[face_lines[i][0]],
                           line_indices[face_lines[i][1]],
                           line_indices[face_lines[i][2]]});
                        break;
 
                      case ReferenceCells::Quadrilateral:
                        new_face->set_bounding_object_indices(
                          {line_indices[face_lines[i][0]],
                           line_indices[face_lines[i][1]],
                           line_indices[face_lines[i][2]],
                           line_indices[face_lines[i][3]]});
                        break;
 
                      default:
                        DEAL_II_NOT_IMPLEMENTED();
                    }
 
                  new_face->set_used_flag();
                  new_face->clear_user_flag();
                  new_face->clear_user_data();
                  new_face->clear_children();
                  new_face->set_boundary_id_internal(face->boundary_id());
                  new_face->set_manifold_id(face->manifold_id());
 
                  [[maybe_unused]] std::set<unsigned int> s;
 
                  // 5.b.I) Fix orientation of lines of face
                  // For triangles an expensive algorithm is used,
                  // quadrilaterals are treated a few lines below by a cheaper
                  // algorithm
                  if (reference_face_type == ReferenceCells::Triangle)
                    {
                      for (const auto f : new_face->line_indices())
                        {
                          const std::array<unsigned int, 2> vertices_0 = {
                            {lines[face_lines[i][f]]->vertex_index(0),
                             lines[face_lines[i][f]]->vertex_index(1)}};
 
                          const std::array<unsigned int, 2> vertices_1 = {
                            {vertex_indices[tri_line_vertices_tri[i][f][0]],
                             vertex_indices[tri_line_vertices_tri[i][f][1]]}};
 
                          const auto orientation =
                            ReferenceCells::Line.get_combined_orientation(
                              make_array_view(vertices_0),
                              make_array_view(vertices_1));
 
                          if constexpr (library_build_mode ==
                                        LibraryBuildMode::debug)
                            {
                              for (const auto i : vertices_0)
                                s.insert(i);
                              for (const auto i : vertices_1)
                                s.insert(i);
                            }
 
                          new_face->set_line_orientation(f, orientation);
                        }
                      if constexpr (library_build_mode ==
                                    LibraryBuildMode::debug)
                        {
                          AssertDimension(s.size(), 3);
                        }
                    }
                }
 
              // 5.b.II) Fix orientation of lines of faces for quadrilaterals
              // with cheap algorithm.
              if (reference_face_type == ReferenceCells::Quadrilateral)
                {
                  constexpr ::ndarray<unsigned int, 4, 2>
                    quad_child_line_vertices{{{{0, 2}}, //
                                              {{1, 3}}, //
                                              {{0, 1}}, //
                                              {{2, 3}}}};
 
                  for (unsigned int i = 0; i < 4; ++i)
                    for (unsigned int j = 0; j < 2; ++j)
                      new_faces[quad_child_line_vertices[i][j]]
                        ->set_line_orientation(i, face->line_orientation(i));
                }
 
              face->clear_user_flag();
            }
        } // FACES
 
        // Cells
        typename Triangulation<3, spacedim>::DistortedCellList
          cells_with_distorted_children;
 
        typename Triangulation<dim, spacedim>::active_cell_iterator cell =
          triangulation.begin_active(0);
        for (unsigned int level = 0; level != triangulation.levels.size() - 1;
             ++level)
          {
            typename Triangulation<dim, spacedim>::raw_cell_iterator
              next_unused_cell = triangulation.begin_raw(level + 1);
            Assert(cell == triangulation.end() ||
                     cell->level() >= static_cast<int>(level),
                   ExcInternalError());
 
            // Iterate over all active cells in the current level
            for (; cell != triangulation.end() &&
                   cell->level() == static_cast<int>(level);
                 ++cell)
              {
                if (cell->refine_flag_set() ==
                    RefinementCase<dim>::no_refinement)
                  continue;
 
                // Copy the requested refinement case to the cell's actual
                // refinement flag, informing the cell how it has been refined.
                const RefinementCase<dim> ref_case = cell->refine_flag_set();
                cell->clear_refine_flag();
                cell->set_refinement_case(ref_case);
 
                const auto cell_reference_cell = cell->reference_cell();
 
                unsigned int       n_new_lines = 0;
                unsigned int       n_new_faces = 0;
                const unsigned int n_new_cells =
                  cell_reference_cell.n_isotropic_children();
 
                // Must match the information used close the beginning of this
                // function.
                switch (cell_reference_cell)
                  {
                    case ReferenceCells::Tetrahedron:
                      n_new_lines = 1;
                      n_new_faces = 8;
                      break;
 
                    case ReferenceCells::Pyramid:
                      n_new_lines = 4;
                      n_new_faces = 13;
                      break;
 
                    case ReferenceCells::Wedge:
                      n_new_lines = 3;
                      n_new_faces = 10;
                      break;
 
                    case ReferenceCells::Hexahedron:
                      n_new_lines = 6;
                      n_new_faces = 12;
                      break;
 
                    default:
                      DEAL_II_ASSERT_UNREACHABLE();
                  }
 
                // the following numbers should be the max numbers of the above.
                // They are used for sizing of arrays.
                constexpr unsigned int max_n_new_lines       = 6;
                constexpr unsigned int max_n_new_faces       = 13;
                constexpr unsigned int max_n_new_children    = 10;
                constexpr unsigned int max_relevant_vertices = 27;
                constexpr unsigned int max_relevant_lines    = 30;
                constexpr unsigned int max_face_indices      = 36;
 
                std::array<raw_line_iterator, max_n_new_lines> new_lines;
                for (unsigned int i = 0; i < n_new_lines; ++i)
                  {
                    new_lines[i] =
                      triangulation.faces->lines
                        .template next_free_single_object<1>(triangulation);
 
                    AssertIsNotUsed(new_lines[i]);
                    new_lines[i]->set_used_flag();
                    new_lines[i]->clear_user_flag();
                    new_lines[i]->clear_user_data();
                    new_lines[i]->clear_children();
                    new_lines[i]->set_boundary_id_internal(
                      numbers::internal_face_boundary_id);
                    new_lines[i]->set_manifold_id(cell->manifold_id());
                  }
 
                std::array<raw_quad_iterator, max_n_new_faces> new_faces;
                for (unsigned int i = 0; i < n_new_faces; ++i)
                  {
                    new_faces[i] =
                      triangulation.faces->quads
                        .template next_free_single_object<2>(triangulation);
 
                    auto &new_face = new_faces[i];
 
                    AssertIsNotUsed(new_face);
                    new_face->set_used_flag();
                    new_face->clear_user_flag();
                    new_face->clear_user_data();
                    new_face->clear_children();
                    new_face->set_boundary_id_internal(
                      numbers::internal_face_boundary_id);
                    new_face->set_manifold_id(cell->manifold_id());
 
                    // At this point the face doesn't have its ReferenceCell
                    // set, so rely on lower-level functionality to reset
                    // per-line data
                    for (unsigned int j = 0;
                         j < triangulation.faces->lines_per_quad;
                         ++j)
                      new_face->set_line_orientation(
                        j, numbers::default_geometric_orientation);
                  }
 
 
                { // CREATE_CELLS
                  // Build a list of all relevant vertices required for
                  // - The creation of additional faces not generated by the
                  //   refinement of the parent faces.
                  // - The definition of the child cells.
                  //
                  // Number of meaningful entries in this list is
                  //   10  Tetrahedron    18 Wedge
                  //   14  Pyramid        27 Hexahedron
                  // In case the maximum changes, edit `max_relevant_vertices` a
                  // few lines above (or in case it was moved to ReferenceCell,
                  // update it there).
                  //
                  // The following algorithm is used to get the vertices.
                  //  1. Get vertices of the parent as provided.
                  //  https://dealii.org/developer/doxygen/deal.II/group__simplex.html#simplex_reference_cells.
                  //
                  //  2. Get the center vertices of the (refined) bounding lines
                  //     of the parent's faces (quad and tet (i.e. all) faces)
                  //
                  //     .---3---.         .
                  //     |       |        /|
                  //     0       1  or   2 1
                  //     |       |      /  |
                  //     .---2---.     .-0-.            n: vertices retrieved
                  //
                  //  3. Get the remaining new vertices that were created in the
                  //     center of the parent's bounding faces during refinement
                  //     (quad faces)
                  //
                  //     .-------.
                  //     |       |
                  //     .---O   |
                  //     |   |   |
                  //     .---.---.                        O: vertex retrieved
                  //
                  //  4. Get (or, better yet, create) the newly required
                  //     vertices. (i.e. the new vertex in the middle of a hex,
                  //     only relevant for hexes)
                  //
                  //       .-------.
                  //      /|       |
                  //     . |   /   |
                  //     | - -O    |
                  //     | .--:----.
                  //     |/   :   /
                  //     .-------.
 
                  std::array<unsigned int, max_relevant_vertices>
                    vertex_indices = {};
 
                  { // GET_VERTICES
                    // continuous counter variable
                    unsigned int k = 0;
 
                    // avoid a compiler warning by fixing max number of
                    // iterations to 8.
                    const unsigned int n_vertices =
                      std::min(cell->n_vertices(), 8u);
 
                    // Step 1 Get vertices of the parent
                    for (unsigned int i = 0; i < n_vertices; ++i)
                      vertex_indices[k++] = cell->vertex_index(i);
 
                    // Step 2 (relevant for all faces)
 
                    // Avoid a compiler warning by fixing the max number of loop
                    // iterations to 12 (max number of new lines)
                    const unsigned int n_lines = std::min(cell->n_lines(), 12u);
 
                    const std::array<unsigned int, 12> line_indices =
                      TriaAccessorImplementation::Implementation::
                        get_line_indices_of_cell(*cell);
 
                    for (unsigned int l = 0; l < n_lines; ++l)
                      {
                        raw_line_iterator line(&triangulation,
                                               0,
                                               line_indices[l]);
                        vertex_indices[k++] = line->child(0)->vertex_index(1);
                      }
 
                    // Step 3 (depends on the reference cell type of face, only
                    // quads affected)
                    for (auto quad_face_index :
                         cell_reference_cell.face_indices_by_type(
                           ReferenceCells::Quadrilateral))
                      {
                        vertex_indices[k++] = cell->face(quad_face_index)
                                                ->child(0)
                                                ->vertex_index(3);
                      }
 
                    // Step 4 Create the center vertex for hex cells
                    if (cell_reference_cell == ReferenceCells::Hexahedron)
                      {
                        // Set single new vertex in the center
                        current_vertex =
                          get_next_unused_vertex(current_vertex,
                                                 triangulation.vertices_used);
                        vertex_indices[k++] = current_vertex;
 
                        triangulation.vertices[current_vertex] =
                          cell->center(true, true);
                      }
                  } // GET_VERTICES
 
 
                  unsigned int chosen_line_tetrahedron = 0;
 
                  // set up new lines
                  switch (cell_reference_cell)
                    {
                      case ReferenceCells::Tetrahedron:
                        {
                          // in the tetrahedron case, we have the three
                          // possibilities (6,8), (5,7), (4,9) -> pick the
                          // shortest line to guarantee the best possible aspect
                          // ratios
                          constexpr ::ndarray<unsigned int, 3, 2>
                            new_line_vertices = {{{{6, 8}}, //
                                                  {{5, 7}}, //
                                                  {{4, 9}}}};
 
                          // choose line to cut either by refinement case or by
                          // shortest distance between edge midpoints
                          std::uint8_t refinement_choice =
                            cell->refine_choice();
                          if (refinement_choice ==
                              static_cast<char>(IsotropicRefinementChoice::
                                                  isotropic_refinement))
                            {
                              const auto &vertices =
                                triangulation.get_vertices();
                              double min_distance =
                                std::numeric_limits<double>::infinity();
                              for (unsigned int i = 0;
                                   i < new_line_vertices.size();
                                   ++i)
                                {
                                  const double current_distance =
                                    vertices[vertex_indices
                                               [new_line_vertices[i][0]]]
                                      .distance(
                                        vertices[vertex_indices
                                                   [new_line_vertices[i][1]]]);
                                  if (current_distance < min_distance)
                                    {
                                      chosen_line_tetrahedron = i;
                                      min_distance = current_distance;
                                    }
                                }
                            }
                          else if (refinement_choice ==
                                   static_cast<char>(
                                     IsotropicRefinementChoice::cut_tet_68))
                            chosen_line_tetrahedron = 0;
                          else if (refinement_choice ==
                                   static_cast<char>(
                                     IsotropicRefinementChoice::cut_tet_57))
                            chosen_line_tetrahedron = 1;
                          else if (refinement_choice ==
                                   static_cast<char>(
                                     IsotropicRefinementChoice::cut_tet_49))
                            chosen_line_tetrahedron = 2;
                          else
                            DEAL_II_NOT_IMPLEMENTED();
 
                          cell->set_refinement_case(
                            RefinementCase<dim>(chosen_line_tetrahedron + 1));
 
                          new_lines[0]->set_bounding_object_indices(
                            {vertex_indices
                               [new_line_vertices[chosen_line_tetrahedron][0]],
                             vertex_indices[new_line_vertices
                                              [chosen_line_tetrahedron][1]]});
                        }
                        break;
 
                      case ReferenceCells::Pyramid:
                        {
                          // Directions so that middle bottom pyramids all have
                          // lines with default orientation. Order as children
                          // of refined bottom quad.
                          static constexpr ::ndarray<unsigned int, 4, 2>
                            new_line_vertices = {{{{13, 9}},  //
                                                  {{13, 10}}, //
                                                  {{13, 11}}, //
                                                  {{13, 12}}}};
 
                          for (unsigned int i = 0; i < n_new_lines; ++i)
                            new_lines[i]->set_bounding_object_indices(
                              {vertex_indices[new_line_vertices[i][0]],
                               vertex_indices[new_line_vertices[i][1]]});
                        }
                        break;
 
                      case ReferenceCells::Wedge:
                        {
                          // Directions so that middle tri looks like a 'normal'
                          // refined tri (refined quads define outer lines)
                          constexpr ::ndarray<unsigned int, 3, 2>
                            new_line_vertices = {{{{15, 16}}, //
                                                  {{16, 17}}, //
                                                  {{15, 17}}}};
 
                          for (unsigned int i = 0; i < n_new_lines; ++i)
                            new_lines[i]->set_bounding_object_indices(
                              {vertex_indices[new_line_vertices[i][0]],
                               vertex_indices[new_line_vertices[i][1]]});
                        }
                        break;
 
                      case ReferenceCells::Hexahedron:
                        {
                          constexpr ::ndarray<unsigned int, 6, 2>
                            new_line_vertices = {{{{22, 26}},
                                                  {{26, 23}},
                                                  {{20, 26}},
                                                  {{26, 21}},
                                                  {{24, 26}},
                                                  {{26, 25}}}};
                          for (unsigned int i = 0; i < n_new_lines; ++i)
                            new_lines[i]->set_bounding_object_indices(
                              {vertex_indices[new_line_vertices[i][0]],
                               vertex_indices[new_line_vertices[i][1]]});
                        }
                        break;
 
                      default:
                        DEAL_II_ASSERT_UNREACHABLE();
                    }
 
                  // Get the corrected orientation of the child indices of
                  // `child` for a refined face, given the `face_ref_cell_type`
                  // and the `face_orientation` as stored in the cell it bounds.
                  auto standard_to_real_quad_child_index =
                    [](const unsigned int                 child,
                       const types::geometric_orientation face_orientation,
                       const ReferenceCell<dim - 1>       face_ref_cell_type) {
                      // The parent vertex indices correlate nicely with the
                      // child indices of a tri and quad, as shown in the
                      // drawing for 2D face refinement (see the curly brackets
                      // tagged {} // FACES). We can thus apply the existing
                      // algorithm for orientation correction of vertex indices
                      // and reinterpret the output. The only problem is child 3
                      // in triangles. However it is always in the center so no
                      // correction is necessary at all.
                      switch (face_ref_cell_type)
                        {
                          case ReferenceCells::Triangle:
                            // A complicated way to query the private attribute
                            // `ReferenceCell::triangle_vertex_permutations[face_orientation][vertex]`
                            if (child != 3)
                              return ReferenceCells::Tetrahedron
                                .standard_to_real_face_vertex(child,
                                                              0,
                                                              face_orientation);
                            else
                              return 3u;
 
                          case ReferenceCells::Quadrilateral:
                            // `ReferenceCell::quadrilateral_vertex_permutations[face_orientation][vertex]`.
                            return ReferenceCells::Hexahedron
                              .standard_to_real_face_vertex(child,
                                                            0,
                                                            face_orientation);
                          default:
                            DEAL_II_NOT_IMPLEMENTED();
                        }
                    };
 
                  // set up new faces
                  {
                    // Similar to `vertex_indices` above the `relevant_lines`
                    // are filled in multiple steps. The order is once again
                    // determined by the reference cell and the refinement
                    // algorithms used by the faces. (For Tetrahedron
                    // `chosen_line_tetrahedron` is also relevant).
                    //  1. Get the lines from the faces that are Quadrilaterals.
                    //
                    //     .- -.- -.
                    //     :   1   :
                    //     .-2-.-3-.
                    //     :   0   :
                    //     .- - - -.                         n: extracted lines
                    //
                    //  2. Get the lines from the faces that are Triangles.
                    //
                    //     |    \      .
                    //     *--1--*     .
                    //     |\    |\    .
                    //     |  2  0  \  .
                    //     |    \|     .
                    //     *-----*---  .
                    //
                    //  3. Newly created lines (see above)
                    //  4. Load indices for the extracted lines
                    //
                    // Number of meaningful entries in this list is
                    //   13  Tetrahedron    21 Wedge
                    //   20  Pyramid        30 Hexahedron
                    // In case the maximum changes, edit `max_relevant_lines` a
                    // few lines above (or in case it was moved to
                    // ReferenceCell, update it there).
 
                    std::array<raw_line_iterator, max_relevant_lines>
                                                        relevant_lines;
                    std::array<int, max_relevant_lines> relevant_line_indices;
                    unsigned int relevant_lines_counter = 0;
                    // 1. Get relevant lines from quads
                    for (auto f : cell_reference_cell.face_indices_by_type(
                           ReferenceCells::Quadrilateral))
                      {
                        // To retrieve the newly created lines within a quad, it
                        // is sufficient to query child 0 and 3 (or 1 and 2), as
                        // they border all relevant lines. Specifically, we are
                        // interested in lines 1 and 3 of child 0, and lines
                        // 0 and 2 of child 3. This is currently implemented in
                        // `quad_relevant_children_and_lines` below (where the
                        // first index is the child and the second is the
                        // child's line index). A more readable implementation
                        // is provided right below. However, it cannot be
                        // (nicely) used to mimic the existing table's behavior.
                        // Modifications to this behavior would result in
                        // extensive rewriting of tables in `ReferenceCell`.
                        //
                        // constexpr std::
                        //   pair<unsigned int, std::array<unsigned int, 2>>
                        //     quad_relevant_children_and_lines = {{0, {1, 3}},
                        //                                         {3, {0, 2}}};
                        constexpr ::ndarray<unsigned int, 4, 2>
                          quad_relevant_children_and_lines = {{{{0, 1}}, //
                                                               {{3, 0}}, //
                                                               {{0, 3}}, //
                                                               {{3, 2}}}};
                        for (unsigned int l = 0; l < 4;
                             ++l, ++relevant_lines_counter)
                          {
                            // Use previously defined helper function to find
                            // children in an orientation-corrected way.
                            const unsigned int relevant_child =
                              standard_to_real_quad_child_index(
                                quad_relevant_children_and_lines[l][0],
                                cell->combined_face_orientation(f),
                                ReferenceCells::Quadrilateral);
 
                            // Get the orientation corrected line index of the
                            // child while using the fact that quad children
                            // share their physical orientation with the parent.
                            const unsigned int relevant_child_line =
                              ReferenceCells::Hexahedron
                                .standard_to_real_face_line(
                                  quad_relevant_children_and_lines[l][1],
                                  // face irrelevant -> function doesn't use it.
                                  0,
                                  cell->combined_face_orientation(f));
 
 
                            // extract and store line
                            relevant_lines[relevant_lines_counter] =
                              cell->face(f)
                                ->isotropic_child(relevant_child)
                                ->line(relevant_child_line);
                          }
                      }
 
                    // 2. Get lines from tris
                    for (auto f : cell_reference_cell.face_indices_by_type(
                           ReferenceCells::Triangle))
                      {
                        for (unsigned int l = 0; l < 3;
                             ++l, ++relevant_lines_counter)
                          {
                            // Permutations of reference cell indices against
                            // real world indices given the real world
                            // orientation of the face.
                            const std::array<std::array<unsigned int, 3>, 6>
                              // Extract newly created lines surrounding the
                              // center child (child 3) of the refined
                              // triangular faces in a predictable manner (i.e.,
                              // the lines always appear at the same location
                              // within the to be refined 3d reference cell),
                              // the orientation of the face must be considered.
                              // This orientation correction is implemented via
                              // the table below relating the lines of the
                              // oriented face to their counterparts on the
                              // reference cell face.
                              tri_line_perm = {{{{0, 1, 2}}, // 0
                                                {{1, 0, 2}},
                                                {{2, 0, 1}}, // 2
                                                {{0, 2, 1}},
                                                {{1, 2, 0}}, // 4
                                                {{2, 1, 0}}}};
 
                            const auto combined_orientation =
                              cell->combined_face_orientation(f);
                            relevant_lines[relevant_lines_counter] =
                              cell->face(f)
                                ->child(3 /*center triangle*/)
                                ->line(tri_line_perm[combined_orientation][l]);
                          }
                      }
 
                    // 3. Fill end of `relevant_lines` with `new_lines`.
                    for (unsigned int i = 0; i < n_new_lines;
                         ++i, ++relevant_lines_counter)
                      relevant_lines[relevant_lines_counter] = new_lines[i];
 
 
                    // 4. Extract and store indices of the relevant lines.
                    for (unsigned int i = 0; i < relevant_lines_counter; ++i)
                      relevant_line_indices[i] = relevant_lines[i]->index();
 
 
                    // New faces (internal to the parent) are required for the
                    // creation of children. The lines bounding these new faces
                    // are stored in `relevant_lines`. The exact assignment of
                    // these lines is then determined using the lookup table
                    // retrieved below, by providing the appropriate indices
                    // into `relevant_lines`.
                    const auto &new_face_lines =
                      cell_reference_cell.new_isotropic_child_face_lines(
                        chosen_line_tetrahedron);
 
                    // The created lines might not be oriented according to the
                    // expectation of the face's reference cell. Consequently,
                    // an additional lookup table is necessary to define the
                    // correct line connectivity desired by the reference cell.
                    // This is very similar to the refinement of triangles in 2d
                    // where the same problem occurred.
                    const auto &new_face_lines_vert =
                      cell_reference_cell
                        .new_isotropic_child_face_line_vertices(
                          chosen_line_tetrahedron);
 
                    // When fixing the orientation of lines for hexes the
                    // calculated orientation of one line can be used for some
                    // other lines as well. This connection is described in this
                    // table.
                    constexpr ::ndarray<unsigned int, 4, 2>
                      representative_lines{{{{0, 2}}, //
                                            {{2, 0}}, //
                                            {{3, 3}}, //
                                            {{1, 1}}}};
 
                    for (unsigned int q = 0; q < n_new_faces; ++q)
                      {
                        auto &new_face = new_faces[q];
 
                        if (new_face_lines[q][3] == X)
                          {
                            // Set the face type implicitly, based on whether
                            // the fourth line is `X`.
                            triangulation.faces->set_quad_type(
                              new_face->index(), ReferenceCells::Triangle);
 
                            new_face->set_bounding_object_indices(
                              {relevant_line_indices[new_face_lines[q][0]],
                               relevant_line_indices[new_face_lines[q][1]],
                               relevant_line_indices[new_face_lines[q][2]]});
                          }
                        else
                          {
                            triangulation.faces->set_quad_type(
                              new_face->index(), ReferenceCells::Quadrilateral);
 
                            new_face->set_bounding_object_indices(
                              {relevant_line_indices[new_face_lines[q][0]],
                               relevant_line_indices[new_face_lines[q][1]],
                               relevant_line_indices[new_face_lines[q][2]],
                               relevant_line_indices[new_face_lines[q][3]]});
                          }
 
                        // On hexes, we must only determine a single line
                        // according to the representative_lines array above
                        // (this saves expensive operations), for tets we do
                        // all lines manually
                        const unsigned int n_compute_lines =
                          cell_reference_cell == ReferenceCells::Hexahedron ?
                            1 :
                            new_face->n_lines();
                        for (unsigned int line = 0; line < n_compute_lines;
                             ++line)
                          {
                            const unsigned int l =
                              (cell_reference_cell ==
                               ReferenceCells::Hexahedron) ?
                                representative_lines[q % 4][0] :
                                line;
 
                            const std::array<unsigned int, 2> vertices_0 = {
                              {relevant_lines[new_face_lines[q][l]]
                                 ->vertex_index(0),
                               relevant_lines[new_face_lines[q][l]]
                                 ->vertex_index(1)}};
 
                            const std::array<unsigned int, 2> vertices_1 = {
                              {vertex_indices[new_face_lines_vert[q][l][0]],
                               vertex_indices[new_face_lines_vert[q][l][1]]}};
 
                            const auto orientation =
                              ReferenceCells::Line.get_combined_orientation(
                                make_array_view(vertices_0),
                                make_array_view(vertices_1));
 
                            new_face->set_line_orientation(l, orientation);
 
                            // on a hex, inject the status of the current line
                            // also to the line on the other quad along the
                            // same direction
                            if (cell_reference_cell ==
                                ReferenceCells::Hexahedron)
                              new_faces[representative_lines[q % 4][1] + q -
                                        (q % 4)]
                                ->set_line_orientation(l, orientation);
                          }
                      }
                  }
 
                  // set up new cell
                  {
                    // Similar to the steps before we now use an algorithm to
                    // extract all the faces.
                    //  1. Add newly created faces to the beginning.
                    //  2. Get children of refined quad faces.
                    //     for numbering see {} // FACE.
                    //  3. Get children of refined tri faces.
                    //     for numbering see {} // FACE.
                    //
                    // Number of meaningful entries in this list is
                    //   24  Tetrahedron    30 Wedge
                    //   33  Pyramid        36 Hexahedron
                    // In case the maximum changes, edit `max_face_indices` a
                    // few lines above (or in case it was moved to
                    // ReferenceCell, update it there).
 
                    std::array<int, max_face_indices> face_indices;
 
                    unsigned int face_indices_counter = 0;
 
                    // 1. Get new face's ids.
                    for (; face_indices_counter < n_new_faces;
                         ++face_indices_counter)
                      face_indices[face_indices_counter] =
                        new_faces[face_indices_counter]->index();
 
                    // 2. Now the faces of the parent's refined quads.
                    for (auto f_q : cell_reference_cell.face_indices_by_type(
                           ReferenceCells::Quadrilateral))
                      {
                        for (unsigned int c = 0; c < 4;
                             ++c, ++face_indices_counter)
                          {
                            const auto combined_orientation =
                              cell->combined_face_orientation(f_q);
 
                            // Correct once again for orientation.
                            face_indices[face_indices_counter] =
                              cell->face(f_q)->isotropic_child_index(
                                standard_to_real_quad_child_index(
                                  c,
                                  combined_orientation,
                                  ReferenceCells::Quadrilateral));
                          }
                      }
 
                    // 3. Now the faces of the parent's refined tris
                    for (auto f_t : cell_reference_cell.face_indices_by_type(
                           ReferenceCells::Triangle))
                      {
                        for (unsigned int c = 0; c < 4;
                             ++c, ++face_indices_counter)
                          {
                            const auto combined_orientation =
                              cell->combined_face_orientation(f_t);
 
                            // Correct once again for orientation.
                            face_indices[face_indices_counter] =
                              cell->face(f_t)->child_index(
                                standard_to_real_quad_child_index(
                                  c,
                                  combined_orientation,
                                  ReferenceCells::Triangle));
                          }
                      }
 
                    // Similar to the faces described above, we need to identify
                    // which faces (specified by their indices in
                    // `face_indices`) bound the new cells. This information is
                    // stored in the ReferenceCell and retrieved here.
                    const auto cell_faces =
                      cell_reference_cell.new_isotropic_child_cell_faces(
                        chosen_line_tetrahedron);
 
                    // We also need to account for potential orientation
                    // mismatches between the face and the cell's ReferenceCell.
                    // (See the discussion above in new face creation for more
                    // details.)
                    const auto new_cells_vertices =
                      cell_reference_cell.new_isotropic_child_cell_vertices(
                        chosen_line_tetrahedron);
 
                    // 1. Preserve data that can only be extracted from active
                    //    cells.
                    //    Parent becomes inactive after setting the first child!
                    const ::types::subdomain_id cell_subdomain_id =
                      cell->subdomain_id();
 
                    // Create the new cells
                    std::array<
                      typename Triangulation<dim, spacedim>::raw_cell_iterator,
                      max_n_new_children>
                      new_cells;
                    for (unsigned int c = 0; c < n_new_cells; ++c)
                      {
                        // Since we search for pairs of hexes (next_free_hex
                        // returns a pair) we need to search only every other
                        // time.
                        if (c % 2 == 0)
                          next_unused_cell =
                            triangulation.levels[level + 1]
                              ->cells.next_free_hex(triangulation, level + 1);
                        else
                          ++next_unused_cell;
 
                        new_cells[c] = next_unused_cell;
 
                        auto &new_cell = new_cells[c];
 
                        AssertIsNotUsed(new_cell);
 
                        // 2. Copy parent data to child.
                        new_cell->set_used_flag();
                        new_cell->clear_user_flag();
                        new_cell->clear_user_data();
                        new_cell->clear_children();
                        new_cell->set_material_id(cell->material_id());
                        new_cell->set_manifold_id(cell->manifold_id());
                        new_cell->set_subdomain_id(cell_subdomain_id);
 
                        // 3. Setup child parent relation (can only happen after
                        //    call to `set_used_flag()`)
                        if (c % 2 == 0)
                          {
                            new_cell->set_parent(cell->index());
                            cell->set_children(c, new_cell->index());
                          }
 
                        // 4.a Determine the cell reference cell using the
                        //     number of vertices it has according to
                        //     `new_cells_vertices`.
                        const ReferenceCell child_reference_cell =
                            (new_cells_vertices[c][4] == X) ? ReferenceCells::Tetrahedron : // Tet
                            (new_cells_vertices[c][5] == X) ? ReferenceCells::Pyramid :     // Pyramid
                            (new_cells_vertices[c][6] == X) ? ReferenceCells::Wedge :       // Wedge
                            ReferenceCells::Hexahedron; // Hex
 
                        // 4.b Set the reference cell
                        triangulation.levels[new_cell->level()]
                          ->reference_cell[new_cell->index()] =
                          child_reference_cell;
 
                        // 5. Set the bounding faces of the new cells
                        //    This seems to be the only option since
                        //    set_bounding_object_indices() takes an
                        //    std::initializer_list as argument
                        switch (child_reference_cell)
                          {
                            case ReferenceCells::Tetrahedron:
                              new_cell->set_bounding_object_indices(
                                {face_indices[cell_faces[c][0]],
                                 face_indices[cell_faces[c][1]],
                                 face_indices[cell_faces[c][2]],
                                 face_indices[cell_faces[c][3]]});
                              break;
 
                            // Pyramids and Wedges share same number of faces
                            case ReferenceCells::Pyramid:
                            case ReferenceCells::Wedge:
                              new_cell->set_bounding_object_indices(
                                {face_indices[cell_faces[c][0]],
                                 face_indices[cell_faces[c][1]],
                                 face_indices[cell_faces[c][2]],
                                 face_indices[cell_faces[c][3]],
                                 face_indices[cell_faces[c][4]]});
                              break;
 
                            case ReferenceCells::Hexahedron:
                              new_cell->set_bounding_object_indices(
                                {face_indices[cell_faces[c][0]],
                                 face_indices[cell_faces[c][1]],
                                 face_indices[cell_faces[c][2]],
                                 face_indices[cell_faces[c][3]],
                                 face_indices[cell_faces[c][4]],
                                 face_indices[cell_faces[c][5]]});
                              break;
 
                            default:
                              DEAL_II_ASSERT_UNREACHABLE();
                          }
 
                        // 6. Fix orientation of faces.
                        // For all reference cells except for Hexes, we need to
                        // go through the faces and figure the orientation out
                        // the hard way
 
                        // Set the orientation flag to its default state for
                        // all faces initially. later on go the other way
                        // round and reset faces that are at the boundary of
                        // the mother cube.
                        // TODO: This might only be necessary for Hexes. For
                        // all other cells the orientation is calculated for
                        // all faces one by one. (Only a guess though).
                        for (const auto f : new_cell->face_indices())
                          new_cell->set_combined_face_orientation(
                            f, numbers::default_geometric_orientation);
 
                        if (child_reference_cell != ReferenceCells::Hexahedron)
                          {
                            for (const auto f : new_cell->face_indices())
                              {
                                const auto &face = new_cell->face(f);
 
                                // load correct vertices of cell
                                auto new_cell_vertices = new_cells_vertices[c];
 
                                // Load correct indices of vertices of face `f`
                                std::array<unsigned int, 4> vertices_0,
                                  vertices_1;
                                vertices_0.fill(numbers::invalid_unsigned_int);
                                vertices_1.fill(numbers::invalid_unsigned_int);
 
                                for (unsigned int face_vertex_no :
                                     face->vertex_indices())
                                  {
                                    // Calculate index for vertex
                                    // `face_vertex_no` of face `f` as an index
                                    // in the cells reference cells indices.
                                    const auto cell_vertex_no =
                                      child_reference_cell
                                        .face_to_cell_vertices(
                                          f,
                                          face_vertex_no,
                                          numbers::
                                            default_geometric_orientation);
 
                                    // Get according index from
                                    // `vertex_indices`. Use `cell_vertex_no` as
                                    // the index in the table describing which
                                    // vertices belong to which cell and then
                                    // get that index. Use `cell_vertex_no` to
                                    // look up the specific vertex (more
                                    // precisely the index in `vertex_indices`)
                                    // within the cell's connectivity table
                                    // `new_cell_vertices` and retrieve the
                                    // corresponding vertex (index) from
                                    // `vertex_indices`.
                                    vertices_0[face_vertex_no] = vertex_indices
                                      [new_cell_vertices[cell_vertex_no]];
                                  }
 
                                // max 4 vertices (if face is quad)
                                for (const auto i : face->vertex_indices())
                                  vertices_1[i] = face->vertex_index(i);
 
                                // Calculate combined orientation as permutation
                                // of desired face vertex indices an actual
                                // vertex indices of the face.
                                new_cell->set_combined_face_orientation(
                                  f,
                                  face->reference_cell()
                                    .get_combined_orientation(
                                      make_array_view(vertices_0.cbegin(),
                                                      vertices_0.cbegin() +
                                                        face->n_vertices()),
                                      make_array_view(vertices_1.cbegin(),
                                                      vertices_1.cbegin() +
                                                        face->n_vertices())));
                              }
                          }
                      }
 
                    // For hexes, we can simply inherit the orientation values
                    // from the parent on the outer faces; the inner faces can
                    // be skipped as their orientation is always the default
                    // one set above.
                    constexpr ::ndarray<unsigned int, 6, 4>
                      face_to_child_indices_hex{{{{0, 2, 4, 6}},
                                                 {{1, 3, 5, 7}},
                                                 {{0, 1, 4, 5}},
                                                 {{2, 3, 6, 7}},
                                                 {{0, 1, 2, 3}},
                                                 {{4, 5, 6, 7}}}};
                    if (cell_reference_cell == ReferenceCells::Hexahedron)
                      for (const auto f : cell->face_indices())
                        {
                          const auto combined_orientation =
                            cell->combined_face_orientation(f);
                          for (unsigned int c = 0; c < 4; ++c)
                            new_cells[face_to_child_indices_hex[f][c]]
                              ->set_combined_face_orientation(
                                f, combined_orientation);
                        }
                  }
                } // CREATE_CELLS
 
                // Check for distorted children
                if (check_for_distorted_cells &&
                    has_distorted_children<dim, spacedim>(cell))
                  cells_with_distorted_children.distorted_cells.push_back(cell);
 
                triangulation.signals.post_refinement_on_cell(cell);
              }
          }
 
        triangulation.faces->quads.clear_user_data();
 
        return cells_with_distorted_children;
      }
 
      template <int spacedim>
      static typename Triangulation<3, spacedim>::DistortedCellList
      execute_refinement(Triangulation<3, spacedim> &triangulation,
                         const bool                  check_for_distorted_cells)
      {
        const unsigned int dim = 3;
 
        {
          bool flag_isotropic_mesh = true;
          typename Triangulation<dim, spacedim>::raw_cell_iterator
            cell = triangulation.begin(),
            endc = triangulation.end();
          for (; cell != endc; ++cell)
            if (cell->used())
              if (triangulation.get_anisotropic_refinement_flag() ||
                  cell->refine_flag_set() == RefinementCase<dim>::cut_x ||
                  cell->refine_flag_set() == RefinementCase<dim>::cut_y ||
                  cell->refine_flag_set() == RefinementCase<dim>::cut_z ||
                  cell->refine_flag_set() == RefinementCase<dim>::cut_xy ||
                  cell->refine_flag_set() == RefinementCase<dim>::cut_xz ||
                  cell->refine_flag_set() == RefinementCase<dim>::cut_yz)
                {
                  flag_isotropic_mesh = false;
                  break;
                }
 
          if (flag_isotropic_mesh)
            return execute_refinement_isotropic(triangulation,
                                                check_for_distorted_cells);
        }
 
        // this function probably also works for spacedim>3 but it
        // isn't tested. it will probably be necessary to pull new
        // vertices onto the manifold just as we do for the other
        // functions above.
        Assert(spacedim == 3, ExcNotImplemented());
 
        // Check whether a new level is needed. We have to check for
        // this on the highest level only
        for (const auto &cell : triangulation.active_cell_iterators_on_level(
               triangulation.levels.size() - 1))
          if (cell->refine_flag_set())
            {
              AssertThrow(
                triangulation.levels.size() < numbers::max_n_levels,
                (typename Triangulation<3, spacedim>::ExcTooManyLevels()));
              triangulation.levels.push_back(
                std::make_unique<internal::TriangulationImplementation::
                                   TriaLevel<dim, spacedim>>(
                  triangulation.strides.max_children_per_cell,
                  triangulation.strides.max_faces_per_cell,
                  triangulation.strides.max_vertices_per_cell));
              break;
            }
 
 
        // first clear user flags for quads and lines; we're going to
        // use them to flag which lines and quads need refinement
        triangulation.faces->quads.clear_user_data();
 
        for (typename Triangulation<dim, spacedim>::line_iterator line =
               triangulation.begin_line();
             line != triangulation.end_line();
             ++line)
          line->clear_user_flag();
        for (typename Triangulation<dim, spacedim>::quad_iterator quad =
               triangulation.begin_quad();
             quad != triangulation.end_quad();
             ++quad)
          quad->clear_user_flag();
 
        // create an array of face refine cases. User indices of faces
        // will be set to values corresponding with indices in this
        // array.
        const RefinementCase<dim - 1> face_refinement_cases[4] = {
          RefinementCase<dim - 1>::no_refinement,
          RefinementCase<dim - 1>::cut_x,
          RefinementCase<dim - 1>::cut_y,
          RefinementCase<dim - 1>::cut_xy};
 
        // check how much space is needed on every level. We need not
        // check the highest level since either
        // - on the highest level no cells are flagged for refinement
        // - there are, but prepare_refinement added another empty
        //   level which then is the highest level
 
        // variables to hold the number of newly to be created
        // vertices, lines and quads. as these are stored globally,
        // declare them outside the loop over al levels. we need lines
        // and quads in pairs for refinement of old ones and lines and
        // quads, that can be stored as single ones, as they are newly
        // created in the inside of an existing cell
        unsigned int needed_vertices     = 0;
        unsigned int needed_lines_single = 0;
        unsigned int needed_quads_single = 0;
        unsigned int needed_lines_pair   = 0;
        unsigned int needed_quads_pair   = 0;
        for (int level_no = triangulation.levels.size() - 2; level_no >= 0;
             --level_no)
          {
            // count number of flagged cells on this level and compute
            // how many new vertices and new lines will be needed
            std::size_t needed_cells = 0;
 
            for (const auto &acell :
                 triangulation.active_cell_iterators_on_level(level_no))
              if (acell->refine_flag_set())
                {
                  RefinementCase<dim> ref_case = acell->refine_flag_set();
 
                  // now for interior vertices, lines and quads, which
                  // are needed in any case
                  if (ref_case == RefinementCase<dim>::cut_x ||
                      ref_case == RefinementCase<dim>::cut_y ||
                      ref_case == RefinementCase<dim>::cut_z)
                    {
                      ++needed_quads_single;
                      needed_cells += 2;
                      triangulation.anisotropic_refinement = true;
                    }
                  else if (ref_case == RefinementCase<dim>::cut_xy ||
                           ref_case == RefinementCase<dim>::cut_xz ||
                           ref_case == RefinementCase<dim>::cut_yz)
                    {
                      ++needed_lines_single;
                      needed_quads_single += 4;
                      needed_cells += 4;
                      triangulation.anisotropic_refinement = true;
                    }
                  else if (ref_case == RefinementCase<dim>::cut_xyz)
                    {
                      ++needed_vertices;
                      needed_lines_single += 6;
                      needed_quads_single += 12;
                      needed_cells += 8;
                    }
                  else
                    {
                      // we should never get here
                      DEAL_II_ASSERT_UNREACHABLE();
                    }
 
                  // mark all faces for refinement; checking locally
                  // if and how the neighbor would like to refine
                  // these is difficult so we only flag them and after
                  // visiting all cells, we decide which faces need
                  // which refinement;
                  for (const unsigned int face :
                       GeometryInfo<dim>::face_indices())
                    {
                      typename Triangulation<dim, spacedim>::face_iterator
                        aface = acell->face(face);
                      // get the RefineCase this faces has for the
                      // given RefineCase of the cell
                      RefinementCase<dim - 1> face_ref_case =
                        GeometryInfo<dim>::face_refinement_case(
                          ref_case,
                          face,
                          acell->face_orientation(face),
                          acell->face_flip(face),
                          acell->face_rotation(face));
                      // only do something, if this face has to be
                      // refined
                      if (face_ref_case)
                        {
                          if (face_ref_case ==
                              RefinementCase<dim - 1>::isotropic_refinement)
                            {
                              if (aface->n_active_descendants() < 4)
                                // we use user_flags to denote needed
                                // isotropic refinement
                                aface->set_user_flag();
                            }
                          else if (aface->refinement_case() != face_ref_case)
                            // we use user_indices to denote needed
                            // anisotropic refinement. note, that we
                            // can have at most one anisotropic
                            // refinement case for this face, as
                            // otherwise prepare_refinement() would
                            // have changed one of the cells to yield
                            // isotropic refinement at this
                            // face. therefore we set the user_index
                            // uniquely
                            {
                              Assert(aface->refinement_case() ==
                                         RefinementCase<
                                           dim - 1>::isotropic_refinement ||
                                       aface->refinement_case() ==
                                         RefinementCase<dim - 1>::no_refinement,
                                     ExcInternalError());
                              aface->set_user_index(face_ref_case);
                            }
                        }
                    } // for all faces
 
                  // flag all lines, that have to be refined
                  for (unsigned int line = 0;
                       line < GeometryInfo<dim>::lines_per_cell;
                       ++line)
                    if (GeometryInfo<dim>::line_refinement_case(ref_case,
                                                                line) &&
                        !acell->line(line)->has_children())
                      acell->line(line)->set_user_flag();
 
                } // if refine_flag set and for all cells on this level
 
            // count number of used cells on the next higher level
            auto      &next_level = *triangulation.levels[level_no + 1];
            const auto used_cells = std::count(next_level.cells.used.begin(),
                                               next_level.cells.used.end(),
                                               true);
            if (used_cells + needed_cells > next_level.size())
              next_level.allocate_end(
                (used_cells + needed_cells) - next_level.size(),
                // We unconditionally store the orientation in 3d
                true,
                // TODO: we can get rid of this check (i.e., we don't need to
                // store this value for purely wedge meshes). This will be
                // simpler to do once we merge IsotropicRefinementChoice and
                // RefinementCase.
                !triangulation.all_reference_cells_are_hyper_cube());
 
            // TODO: perhaps we can merge this with TriaLevel::allocate_end()
            next_level.cells.allocate_end(needed_cells, 0);
          } // for all levels
        // now count the quads and lines which were flagged for
        // refinement
        for (typename Triangulation<dim, spacedim>::quad_iterator quad =
               triangulation.begin_quad();
             quad != triangulation.end_quad();
             ++quad)
          {
            if (quad->user_flag_set())
              {
                // isotropic refinement: 1 interior vertex, 4 quads
                // and 4 interior lines. we store the interior lines
                // in pairs in case the face is already or will be
                // refined anisotropically
                needed_quads_pair += 4;
                needed_lines_pair += 4;
                needed_vertices += 1;
              }
            if (quad->user_index())
              {
                // anisotropic refinement: 1 interior
                // line and two quads
                needed_quads_pair += 2;
                needed_lines_single += 1;
                // there is a kind of complicated situation here which
                // requires our attention. if the quad is refined
                // isotropcally, two of the interior lines will get a
                // new mother line - the interior line of our
                // anisotropically refined quad. if those two lines
                // are not consecutive, we cannot do so and have to
                // replace them by two lines that are consecutive. we
                // try to avoid that situation, but it may happen
                // nevertheless through repeated refinement and
                // coarsening. thus we have to check here, as we will
                // need some additional space to store those new lines
                // in case we need them...
                if (quad->has_children())
                  {
                    Assert(quad->refinement_case() ==
                             RefinementCase<dim - 1>::isotropic_refinement,
                           ExcInternalError());
                    if ((face_refinement_cases[quad->user_index()] ==
                           RefinementCase<dim - 1>::cut_x &&
                         (quad->child(0)->line_index(1) + 1 !=
                          quad->child(2)->line_index(1))) ||
                        (face_refinement_cases[quad->user_index()] ==
                           RefinementCase<dim - 1>::cut_y &&
                         (quad->child(0)->line_index(3) + 1 !=
                          quad->child(1)->line_index(3))))
                      needed_lines_pair += 2;
                  }
              }
          }
 
        for (typename Triangulation<dim, spacedim>::line_iterator line =
               triangulation.begin_line();
             line != triangulation.end_line();
             ++line)
          if (line->user_flag_set())
            {
              needed_lines_pair += 2;
              needed_vertices += 1;
            }
 
        // reserve space for needed_lines new lines stored in pairs
        triangulation.faces->lines.allocate_end(needed_lines_pair,
                                                needed_lines_single);
 
        // reserve space for needed_quads new quads stored in pairs
        triangulation.faces->allocate_end(needed_quads_pair,
                                          needed_quads_single);
 
        // add to needed vertices how many vertices are already in use
        needed_vertices += std::count(triangulation.vertices_used.begin(),
                                      triangulation.vertices_used.end(),
                                      true);
        // if we need more vertices: create them, if not: leave the
        // array as is, since shrinking is not really possible because
        // some of the vertices at the end may be in use
        if (needed_vertices > triangulation.vertices.size())
          {
            triangulation.vertices.resize(needed_vertices, Point<spacedim>());
            triangulation.vertices_used.resize(needed_vertices, false);
          }
 
 
        //-----------------------------------------
        // Before we start with the actual refinement, we do some
        // sanity checks if in debug mode. especially, we try to catch
        // the notorious problem with lines being twice refined,
        // i.e. there are cells adjacent at one line ("around the
        // edge", but not at a face), with two cells differing by more
        // than one refinement level
        //
        // this check is very simple to implement here, since we have
        // all lines flagged if they shall be refined
        if constexpr (running_in_debug_mode())
          {
            for (const auto &cell : triangulation.active_cell_iterators())
              if (!cell->refine_flag_set())
                for (unsigned int line = 0;
                     line < GeometryInfo<dim>::lines_per_cell;
                     ++line)
                  if (cell->line(line)->has_children())
                    for (unsigned int c = 0; c < 2; ++c)
                      Assert(cell->line(line)->child(c)->user_flag_set() ==
                               false,
                             ExcInternalError());
          }
 
        //-----------------------------------------
        // Do refinement on every level
        //
        // To make life a bit easier, we first refine those lines and
        // quads that were flagged for refinement and then compose the
        // newly to be created cells.
        //
        // index of next unused vertex
        unsigned int next_unused_vertex = 0;
 
        // first for lines
        {
          // only active objects can be refined further
          typename Triangulation<dim, spacedim>::active_line_iterator
            line = triangulation.begin_active_line(),
            endl = triangulation.end_line();
          typename Triangulation<dim, spacedim>::raw_line_iterator
            next_unused_line = triangulation.begin_raw_line();
 
          for (; line != endl; ++line)
            if (line->user_flag_set())
              {
                // this line needs to be refined
 
                // find the next unused vertex and set it
                // appropriately
                while (triangulation.vertices_used[next_unused_vertex] == true)
                  ++next_unused_vertex;
                Assert(
                  next_unused_vertex < triangulation.vertices.size(),
                  ExcMessage(
                    "Internal error: During refinement, the triangulation wants to access an element of the 'vertices' array but it turns out that the array is not large enough."));
                triangulation.vertices_used[next_unused_vertex] = true;
 
                triangulation.vertices[next_unused_vertex] = line->center(true);
 
                // now that we created the right point, make up the
                // two child lines (++ takes care of the end of the
                // vector)
                next_unused_line =
                  triangulation.faces->lines.template next_free_pair_object<1>(
                    triangulation);
                Assert(next_unused_line.state() == IteratorState::valid,
                       ExcInternalError());
 
                // now we found two consecutive unused lines, such
                // that the children of a line will be consecutive.
                // then set the child pointer of the present line
                line->set_children(0, next_unused_line->index());
 
                // set the two new lines
                const typename Triangulation<dim, spacedim>::raw_line_iterator
                  children[2] = {next_unused_line, ++next_unused_line};
 
                // some tests; if any of the iterators should be
                // invalid, then already dereferencing will fail
                AssertIsNotUsed(children[0]);
                AssertIsNotUsed(children[1]);
 
                children[0]->set_bounding_object_indices(
                  {line->vertex_index(0), next_unused_vertex});
                children[1]->set_bounding_object_indices(
                  {next_unused_vertex, line->vertex_index(1)});
 
                children[0]->set_used_flag();
                children[1]->set_used_flag();
                children[0]->clear_children();
                children[1]->clear_children();
                children[0]->clear_user_data();
                children[1]->clear_user_data();
                children[0]->clear_user_flag();
                children[1]->clear_user_flag();
 
                children[0]->set_boundary_id_internal(line->boundary_id());
                children[1]->set_boundary_id_internal(line->boundary_id());
 
                children[0]->set_manifold_id(line->manifold_id());
                children[1]->set_manifold_id(line->manifold_id());
 
                // finally clear flag
                // indicating the need
                // for refinement
                line->clear_user_flag();
              }
        }
 
 
        //-------------------------------------
        // now refine marked quads
        //-------------------------------------
 
        // here we encounter several cases:
 
        // a) the quad is unrefined and shall be refined isotropically
 
        // b) the quad is unrefined and shall be refined
        // anisotropically
 
        // c) the quad is unrefined and shall be refined both
        // anisotropically and isotropically (this is reduced to case
        // b) and then case b) for the children again)
 
        // d) the quad is refined anisotropically and shall be refined
        // isotropically (this is reduced to case b) for the
        // anisotropic children)
 
        // e) the quad is refined isotropically and shall be refined
        // anisotropically (this is transformed to case c), however we
        // might have to renumber/rename children...)
 
        // we need a loop in cases c) and d), as the anisotropic
        // children might have a lower index than the mother quad
        for (unsigned int loop = 0; loop < 2; ++loop)
          {
            // usually, only active objects can be refined
            // further. however, in cases d) and e) that is not true,
            // so we have to use 'normal' iterators here
            typename Triangulation<dim, spacedim>::quad_iterator
              quad = triangulation.begin_quad(),
              endq = triangulation.end_quad();
            typename Triangulation<dim, spacedim>::raw_line_iterator
              next_unused_line = triangulation.begin_raw_line();
            typename Triangulation<dim, spacedim>::raw_quad_iterator
              next_unused_quad = triangulation.begin_raw_quad();
 
            for (; quad != endq; ++quad)
              {
                if (quad->user_index())
                  {
                    RefinementCase<dim - 1> aniso_quad_ref_case =
                      face_refinement_cases[quad->user_index()];
                    // there is one unlikely event here, where we
                    // already have refind the face: if the face was
                    // refined anisotropically and we want to refine
                    // it isotropically, both children are flagged for
                    // anisotropic refinement. however, if those
                    // children were already flagged for anisotropic
                    // refinement, they might already be processed and
                    // refined.
                    if (aniso_quad_ref_case == quad->refinement_case())
                      continue;
 
                    Assert(quad->refinement_case() ==
                               RefinementCase<dim - 1>::cut_xy ||
                             quad->refinement_case() ==
                               RefinementCase<dim - 1>::no_refinement,
                           ExcInternalError());
 
                    // this quad needs to be refined anisotropically
                    Assert(quad->user_index() ==
                               RefinementCase<dim - 1>::cut_x ||
                             quad->user_index() ==
                               RefinementCase<dim - 1>::cut_y,
                           ExcInternalError());
 
                    // make the new line interior to the quad
                    typename Triangulation<dim, spacedim>::raw_line_iterator
                      new_line;
 
                    new_line =
                      triangulation.faces->lines
                        .template next_free_single_object<1>(triangulation);
                    AssertIsNotUsed(new_line);
 
                    // first collect the
                    // indices of the vertices:
                    // *--1--*
                    // |  |  |
                    // |  |  |    cut_x
                    // |  |  |
                    // *--0--*
                    //
                    // *-----*
                    // |     |
                    // 0-----1    cut_y
                    // |     |
                    // *-----*
                    unsigned int vertex_indices[2];
                    if (aniso_quad_ref_case == RefinementCase<dim - 1>::cut_x)
                      {
                        vertex_indices[0] =
                          quad->line(2)->child(0)->vertex_index(1);
                        vertex_indices[1] =
                          quad->line(3)->child(0)->vertex_index(1);
                      }
                    else
                      {
                        vertex_indices[0] =
                          quad->line(0)->child(0)->vertex_index(1);
                        vertex_indices[1] =
                          quad->line(1)->child(0)->vertex_index(1);
                      }
 
                    new_line->set_bounding_object_indices(
                      {vertex_indices[0], vertex_indices[1]});
                    new_line->set_used_flag();
                    new_line->clear_user_flag();
                    new_line->clear_user_data();
                    new_line->clear_children();
                    new_line->set_boundary_id_internal(quad->boundary_id());
                    new_line->set_manifold_id(quad->manifold_id());
 
                    // find some space (consecutive) for the two newly
                    // to be created quads.
                    typename Triangulation<dim, spacedim>::raw_quad_iterator
                      new_quads[2];
 
                    next_unused_quad =
                      triangulation.faces->quads
                        .template next_free_pair_object<2>(triangulation);
                    new_quads[0] = next_unused_quad;
                    AssertIsNotUsed(new_quads[0]);
 
                    ++next_unused_quad;
                    new_quads[1] = next_unused_quad;
                    AssertIsNotUsed(new_quads[1]);
 
                    if (aniso_quad_ref_case == RefinementCase<dim - 1>::cut_x)
                      {
                        new_quads[0]->set_bounding_object_indices(
                          {static_cast<int>(quad->line_index(0)),
                           new_line->index(),
                           quad->line(2)
                             ->child(
                               child_line_index(0, quad->line_orientation(2)))
                             ->index(),
                           quad->line(3)
                             ->child(
                               child_line_index(0, quad->line_orientation(3)))
                             ->index()});
                        new_quads[1]->set_bounding_object_indices(
                          {new_line->index(),
                           static_cast<int>(quad->line_index(1)),
                           quad->line(2)
                             ->child(
                               child_line_index(1, quad->line_orientation(2)))
                             ->index(),
                           quad->line(3)
                             ->child(
                               child_line_index(1, quad->line_orientation(3)))
                             ->index()});
                      }
                    else
                      {
                        new_quads[0]->set_bounding_object_indices(
                          {quad->line(0)
                             ->child(
                               child_line_index(0, quad->line_orientation(0)))
                             ->index(),
                           quad->line(1)
                             ->child(
                               child_line_index(0, quad->line_orientation(1)))
                             ->index(),
                           static_cast<int>(quad->line_index(2)),
                           new_line->index()});
                        new_quads[1]->set_bounding_object_indices(
                          {quad->line(0)
                             ->child(
                               child_line_index(1, quad->line_orientation(0)))
                             ->index(),
                           quad->line(1)
                             ->child(
                               child_line_index(1, quad->line_orientation(1)))
                             ->index(),
                           new_line->index(),
                           static_cast<int>(quad->line_index(3))});
                      }
 
                    for (const auto &new_quad : new_quads)
                      {
                        new_quad->set_used_flag();
                        new_quad->clear_user_flag();
                        new_quad->clear_user_data();
                        new_quad->clear_children();
                        new_quad->set_boundary_id_internal(quad->boundary_id());
                        new_quad->set_manifold_id(quad->manifold_id());
                        // set all line orientations to true, change
                        // this after the loop, as we have to consider
                        // different lines for each child
                        for (unsigned int j = 0;
                             j < GeometryInfo<dim>::lines_per_face;
                             ++j)
                          new_quad->set_line_orientation(
                            j, numbers::default_geometric_orientation);
                      }
                    // now set the line orientation of children of
                    // outer lines correctly, the lines in the
                    // interior of the refined quad are automatically
                    // oriented conforming to the standard
                    new_quads[0]->set_line_orientation(
                      0, quad->line_orientation(0));
                    new_quads[0]->set_line_orientation(
                      2, quad->line_orientation(2));
                    new_quads[1]->set_line_orientation(
                      1, quad->line_orientation(1));
                    new_quads[1]->set_line_orientation(
                      3, quad->line_orientation(3));
                    if (aniso_quad_ref_case == RefinementCase<dim - 1>::cut_x)
                      {
                        new_quads[0]->set_line_orientation(
                          3, quad->line_orientation(3));
                        new_quads[1]->set_line_orientation(
                          2, quad->line_orientation(2));
                      }
                    else
                      {
                        new_quads[0]->set_line_orientation(
                          1, quad->line_orientation(1));
                        new_quads[1]->set_line_orientation(
                          0, quad->line_orientation(0));
                      }
 
                    // test, whether this face is refined
                    // isotropically already. if so, set the correct
                    // children pointers.
                    if (quad->refinement_case() ==
                        RefinementCase<dim - 1>::cut_xy)
                      {
                        // we will put a new refinemnt level of
                        // anisotropic refinement between the
                        // unrefined and isotropically refined quad
                        // ending up with the same fine quads but
                        // introducing anisotropically refined ones as
                        // children of the unrefined quad and mother
                        // cells of the original fine ones.
 
                        // this process includes the creation of a new
                        // middle line which we will assign as the
                        // mother line of two of the existing inner
                        // lines. If those inner lines are not
                        // consecutive in memory, we won't find them
                        // later on, so we have to create new ones
                        // instead and replace all occurrences of the
                        // old ones with those new ones. As this is
                        // kind of ugly, we hope we don't have to do
                        // it often...
                        typename Triangulation<dim, spacedim>::line_iterator
                          old_child[2];
                        if (aniso_quad_ref_case ==
                            RefinementCase<dim - 1>::cut_x)
                          {
                            old_child[0] = quad->child(0)->line(1);
                            old_child[1] = quad->child(2)->line(1);
                          }
                        else
                          {
                            Assert(aniso_quad_ref_case ==
                                     RefinementCase<dim - 1>::cut_y,
                                   ExcInternalError());
 
                            old_child[0] = quad->child(0)->line(3);
                            old_child[1] = quad->child(1)->line(3);
                          }
 
                        if (old_child[0]->index() + 1 != old_child[1]->index())
                          {
                            // this is exactly the ugly case we talked
                            // about. so, no complaining, lets get
                            // two new lines and copy all info
                            typename Triangulation<dim,
                                                   spacedim>::raw_line_iterator
                              new_child[2];
 
                            new_child[0] = new_child[1] =
                              triangulation.faces->lines
                                .template next_free_pair_object<1>(
                                  triangulation);
                            ++new_child[1];
 
                            new_child[0]->set_used_flag();
                            new_child[1]->set_used_flag();
 
                            const int old_index_0 = old_child[0]->index(),
                                      old_index_1 = old_child[1]->index(),
                                      new_index_0 = new_child[0]->index(),
                                      new_index_1 = new_child[1]->index();
 
                            // loop over all quads and replace the old
                            // lines
                            for (unsigned int q = 0;
                                 q < triangulation.faces->quads.n_objects();
                                 ++q)
                              for (unsigned int l = 0;
                                   l < GeometryInfo<dim>::lines_per_face;
                                   ++l)
                                {
                                  const int this_index =
                                    triangulation.faces->quads
                                      .get_bounding_object_indices(q)[l];
                                  if (this_index == old_index_0)
                                    triangulation.faces->quads
                                      .get_bounding_object_indices(q)[l] =
                                      new_index_0;
                                  else if (this_index == old_index_1)
                                    triangulation.faces->quads
                                      .get_bounding_object_indices(q)[l] =
                                      new_index_1;
                                }
                            // now we have to copy all information of
                            // the two lines
                            for (unsigned int i = 0; i < 2; ++i)
                              {
                                Assert(!old_child[i]->has_children(),
                                       ExcInternalError());
 
                                new_child[i]->set_bounding_object_indices(
                                  {old_child[i]->vertex_index(0),
                                   old_child[i]->vertex_index(1)});
                                new_child[i]->set_boundary_id_internal(
                                  old_child[i]->boundary_id());
                                new_child[i]->set_manifold_id(
                                  old_child[i]->manifold_id());
                                new_child[i]->set_user_index(
                                  old_child[i]->user_index());
                                if (old_child[i]->user_flag_set())
                                  new_child[i]->set_user_flag();
                                else
                                  new_child[i]->clear_user_flag();
 
                                new_child[i]->clear_children();
 
                                old_child[i]->clear_user_flag();
                                old_child[i]->clear_user_index();
                                old_child[i]->clear_used_flag();
                              }
                          }
                        // now that we cared about the lines, go on
                        // with the quads themselves, where we might
                        // encounter similar situations...
                        if (aniso_quad_ref_case ==
                            RefinementCase<dim - 1>::cut_x)
                          {
                            new_line->set_children(
                              0, quad->child(0)->line_index(1));
                            Assert(new_line->child(1) ==
                                     quad->child(2)->line(1),
                                   ExcInternalError());
                            // now evereything is quite
                            // complicated. we have the children
                            // numbered according to
                            //
                            // *---*---*
                            // |n+2|n+3|
                            // *---*---*
                            // | n |n+1|
                            // *---*---*
                            //
                            // from the original isotropic
                            // refinement. we have to reorder them as
                            //
                            // *---*---*
                            // |n+1|n+3|
                            // *---*---*
                            // | n |n+2|
                            // *---*---*
                            //
                            // such that n and n+1 are consecutive
                            // children of m and n+2 and n+3 are
                            // consecutive children of m+1, where m
                            // and m+1 are given as in
                            //
                            // *---*---*
                            // |   |   |
                            // | m |m+1|
                            // |   |   |
                            // *---*---*
                            //
                            // this is a bit ugly, of course: loop
                            // over all cells on all levels and look
                            // for faces n+1 (switch_1) and n+2
                            // (switch_2).
                            const typename Triangulation<dim, spacedim>::
                              quad_iterator switch_1 = quad->child(1),
                                            switch_2 = quad->child(2);
                            const int switch_1_index = switch_1->index();
                            const int switch_2_index = switch_2->index();
                            for (unsigned int l = 0;
                                 l < triangulation.levels.size();
                                 ++l)
                              for (unsigned int h = 0;
                                   h <
                                   triangulation.levels[l]->cells.n_objects();
                                   ++h)
                                for (const unsigned int q :
                                     GeometryInfo<dim>::face_indices())
                                  {
                                    const int face_index =
                                      triangulation.levels[l]
                                        ->cells.get_bounding_object_indices(
                                          h)[q];
                                    if (face_index == switch_1_index)
                                      triangulation.levels[l]
                                        ->cells.get_bounding_object_indices(
                                          h)[q] = switch_2_index;
                                    else if (face_index == switch_2_index)
                                      triangulation.levels[l]
                                        ->cells.get_bounding_object_indices(
                                          h)[q] = switch_1_index;
                                  }
                            // now we have to copy all information of
                            // the two quads
                            int switch_1_lines[4], switch_2_lines[4];
                            for (int i = 0; i < 4; ++i)
                              {
                                switch_1_lines[i] = switch_1->line(i)->index();
                                switch_2_lines[i] = switch_2->line(i)->index();
                              }
                            const types::geometric_orientation
                              switch_1_line_orientations[4] = {
                                switch_1->line_orientation(0),
                                switch_1->line_orientation(1),
                                switch_1->line_orientation(2),
                                switch_1->line_orientation(3)};
                            const types::boundary_id switch_1_boundary_id =
                              switch_1->boundary_id();
                            const unsigned int switch_1_user_index =
                              switch_1->user_index();
                            const bool switch_1_user_flag =
                              switch_1->user_flag_set();
                            const RefinementCase<dim - 1>
                              switch_1_refinement_case =
                                switch_1->refinement_case();
                            const int switch_1_first_child_pair =
                              (switch_1_refinement_case ?
                                 switch_1->child_index(0) :
                                 -1);
                            const int switch_1_second_child_pair =
                              (switch_1_refinement_case ==
                                   RefinementCase<dim - 1>::cut_xy ?
                                 switch_1->child_index(2) :
                                 -1);
 
                            switch_1->set_bounding_object_indices(
                              {switch_2_lines[0],
                               switch_2_lines[1],
                               switch_2_lines[2],
                               switch_2_lines[3]});
                            switch_1->set_line_orientation(
                              0, switch_2->line_orientation(0));
                            switch_1->set_line_orientation(
                              1, switch_2->line_orientation(1));
                            switch_1->set_line_orientation(
                              2, switch_2->line_orientation(2));
                            switch_1->set_line_orientation(
                              3, switch_2->line_orientation(3));
                            switch_1->set_boundary_id_internal(
                              switch_2->boundary_id());
                            switch_1->set_manifold_id(switch_2->manifold_id());
                            switch_1->set_user_index(switch_2->user_index());
                            if (switch_2->user_flag_set())
                              switch_1->set_user_flag();
                            else
                              switch_1->clear_user_flag();
                            switch_1->clear_refinement_case();
                            switch_1->set_refinement_case(
                              switch_2->refinement_case());
                            switch_1->clear_children();
                            if (switch_2->refinement_case())
                              switch_1->set_children(0,
                                                     switch_2->child_index(0));
                            if (switch_2->refinement_case() ==
                                RefinementCase<dim - 1>::cut_xy)
                              switch_1->set_children(2,
                                                     switch_2->child_index(2));
 
                            switch_2->set_bounding_object_indices(
                              {switch_1_lines[0],
                               switch_1_lines[1],
                               switch_1_lines[2],
                               switch_1_lines[3]});
                            switch_2->set_line_orientation(
                              0, switch_1_line_orientations[0]);
                            switch_2->set_line_orientation(
                              1, switch_1_line_orientations[1]);
                            switch_2->set_line_orientation(
                              2, switch_1_line_orientations[2]);
                            switch_2->set_line_orientation(
                              3, switch_1_line_orientations[3]);
                            switch_2->set_boundary_id_internal(
                              switch_1_boundary_id);
                            switch_2->set_manifold_id(switch_1->manifold_id());
                            switch_2->set_user_index(switch_1_user_index);
                            if (switch_1_user_flag)
                              switch_2->set_user_flag();
                            else
                              switch_2->clear_user_flag();
                            switch_2->clear_refinement_case();
                            switch_2->set_refinement_case(
                              switch_1_refinement_case);
                            switch_2->clear_children();
                            switch_2->set_children(0,
                                                   switch_1_first_child_pair);
                            switch_2->set_children(2,
                                                   switch_1_second_child_pair);
 
                            new_quads[0]->set_refinement_case(
                              RefinementCase<2>::cut_y);
                            new_quads[0]->set_children(0, quad->child_index(0));
                            new_quads[1]->set_refinement_case(
                              RefinementCase<2>::cut_y);
                            new_quads[1]->set_children(0, quad->child_index(2));
                          }
                        else
                          {
                            new_quads[0]->set_refinement_case(
                              RefinementCase<2>::cut_x);
                            new_quads[0]->set_children(0, quad->child_index(0));
                            new_quads[1]->set_refinement_case(
                              RefinementCase<2>::cut_x);
                            new_quads[1]->set_children(0, quad->child_index(2));
                            new_line->set_children(
                              0, quad->child(0)->line_index(3));
                            Assert(new_line->child(1) ==
                                     quad->child(1)->line(3),
                                   ExcInternalError());
                          }
                        quad->clear_children();
                      }
 
                    // note these quads as children to the present one
                    quad->set_children(0, new_quads[0]->index());
 
                    quad->set_refinement_case(aniso_quad_ref_case);
 
                    // finally clear flag indicating the need for
                    // refinement
                    quad->clear_user_data();
                  } // if (anisotropic refinement)
 
                if (quad->user_flag_set())
                  {
                    // this quad needs to be refined isotropically
 
                    // first of all: we only get here in the first run
                    // of the loop
                    Assert(loop == 0, ExcInternalError());
 
                    // find the next unused vertex. we'll need this in
                    // any case
                    while (triangulation.vertices_used[next_unused_vertex] ==
                           true)
                      ++next_unused_vertex;
                    Assert(
                      next_unused_vertex < triangulation.vertices.size(),
                      ExcMessage(
                        "Internal error: During refinement, the triangulation wants to access an element of the 'vertices' array but it turns out that the array is not large enough."));
 
                    // now: if the quad is refined anisotropically
                    // already, set the anisotropic refinement flag
                    // for both children. Additionally, we have to
                    // refine the inner line, as it is an outer line
                    // of the two (anisotropic) children
                    const RefinementCase<dim - 1> quad_ref_case =
                      quad->refinement_case();
 
                    if (quad_ref_case == RefinementCase<dim - 1>::cut_x ||
                        quad_ref_case == RefinementCase<dim - 1>::cut_y)
                      {
                        // set the 'opposite' refine case for children
                        quad->child(0)->set_user_index(
                          RefinementCase<dim - 1>::cut_xy - quad_ref_case);
                        quad->child(1)->set_user_index(
                          RefinementCase<dim - 1>::cut_xy - quad_ref_case);
                        // refine the inner line
                        typename Triangulation<dim, spacedim>::line_iterator
                          middle_line;
                        if (quad_ref_case == RefinementCase<dim - 1>::cut_x)
                          middle_line = quad->child(0)->line(1);
                        else
                          middle_line = quad->child(0)->line(3);
 
                        // if the face has been refined
                        // anisotropically in the last refinement step
                        // it might be, that it is flagged already and
                        // that the middle line is thus refined
                        // already. if not create children.
                        if (!middle_line->has_children())
                          {
                            // set the middle vertex
                            // appropriately. double refinement of
                            // quads can only happen in the interior
                            // of the domain, so we need not care
                            // about boundary quads here
                            triangulation.vertices[next_unused_vertex] =
                              middle_line->center(true);
                            triangulation.vertices_used[next_unused_vertex] =
                              true;
 
                            // now search a slot for the two
                            // child lines
                            next_unused_line =
                              triangulation.faces->lines
                                .template next_free_pair_object<1>(
                                  triangulation);
 
                            // set the child pointer of the present
                            // line
                            middle_line->set_children(
                              0, next_unused_line->index());
 
                            // set the two new lines
                            const typename Triangulation<dim, spacedim>::
                              raw_line_iterator children[2] = {
                                next_unused_line, ++next_unused_line};
 
                            // some tests; if any of the iterators
                            // should be invalid, then already
                            // dereferencing will fail
                            AssertIsNotUsed(children[0]);
                            AssertIsNotUsed(children[1]);
 
                            children[0]->set_bounding_object_indices(
                              {middle_line->vertex_index(0),
                               next_unused_vertex});
                            children[1]->set_bounding_object_indices(
                              {next_unused_vertex,
                               middle_line->vertex_index(1)});
 
                            children[0]->set_used_flag();
                            children[1]->set_used_flag();
                            children[0]->clear_children();
                            children[1]->clear_children();
                            children[0]->clear_user_data();
                            children[1]->clear_user_data();
                            children[0]->clear_user_flag();
                            children[1]->clear_user_flag();
 
                            children[0]->set_boundary_id_internal(
                              middle_line->boundary_id());
                            children[1]->set_boundary_id_internal(
                              middle_line->boundary_id());
 
                            children[0]->set_manifold_id(
                              middle_line->manifold_id());
                            children[1]->set_manifold_id(
                              middle_line->manifold_id());
                          }
                        // now remove the flag from the quad and go to
                        // the next quad, the actual refinement of the
                        // quad takes place later on in this pass of
                        // the loop or in the next one
                        quad->clear_user_flag();
                        continue;
                      } // if (several refinement cases)
 
                    // if we got here, we have an unrefined quad and
                    // have to do the usual work like in an purely
                    // isotropic refinement
                    Assert(quad_ref_case ==
                             RefinementCase<dim - 1>::no_refinement,
                           ExcInternalError());
 
                    // set the middle vertex appropriately: it might be that
                    // the quad itself is not at the boundary, but that one of
                    // its lines actually is. in this case, the newly created
                    // vertices at the centers of the lines are not
                    // necessarily the mean values of the adjacent vertices,
                    // so do not compute the new vertex as the mean value of
                    // the 4 vertices of the face, but rather as a weighted
                    // mean value of the 8 vertices which we already have (the
                    // four old ones, and the four ones inserted as middle
                    // points for the four lines). summing up some more points
                    // is generally cheaper than first asking whether one of
                    // the lines is at the boundary
                    //
                    // note that the exact weights are chosen such as to
                    // minimize the distortion of the four new quads from the
                    // optimal shape. their description uses the formulas
                    // underlying the TransfiniteInterpolationManifold
                    // implementation
                    triangulation.vertices[next_unused_vertex] =
                      quad->center(true, true);
                    triangulation.vertices_used[next_unused_vertex] = true;
 
                    // now that we created the right point, make up
                    // the four lines interior to the quad (++ takes
                    // care of the end of the vector)
                    typename Triangulation<dim, spacedim>::raw_line_iterator
                      new_lines[4];
 
                    for (unsigned int i = 0; i < 4; ++i)
                      {
                        if (i % 2 == 0)
                          // search a free pair of lines for 0. and
                          // 2. line, so that two of them end up
                          // together, which is necessary if later on
                          // we want to refine the quad
                          // anisotropically and the two lines end up
                          // as children of new line
                          next_unused_line =
                            triangulation.faces->lines
                              .template next_free_pair_object<1>(triangulation);
 
                        new_lines[i] = next_unused_line;
                        ++next_unused_line;
 
                        AssertIsNotUsed(new_lines[i]);
                      }
 
                    // set the data of the four lines.  first collect
                    // the indices of the five vertices:
                    //
                    // *--3--*
                    // |  |  |
                    // 0--4--1
                    // |  |  |
                    // *--2--*
                    //
                    // the lines are numbered as follows:
                    //
                    // *--*--*
                    // |  1  |
                    // *2-*-3*
                    // |  0  |
                    // *--*--*
 
                    const unsigned int vertex_indices[5] = {
                      quad->line(0)->child(0)->vertex_index(1),
                      quad->line(1)->child(0)->vertex_index(1),
                      quad->line(2)->child(0)->vertex_index(1),
                      quad->line(3)->child(0)->vertex_index(1),
                      next_unused_vertex};
 
                    new_lines[0]->set_bounding_object_indices(
                      {vertex_indices[2], vertex_indices[4]});
                    new_lines[1]->set_bounding_object_indices(
                      {vertex_indices[4], vertex_indices[3]});
                    new_lines[2]->set_bounding_object_indices(
                      {vertex_indices[0], vertex_indices[4]});
                    new_lines[3]->set_bounding_object_indices(
                      {vertex_indices[4], vertex_indices[1]});
 
                    for (const auto &new_line : new_lines)
                      {
                        new_line->set_used_flag();
                        new_line->clear_user_flag();
                        new_line->clear_user_data();
                        new_line->clear_children();
                        new_line->set_boundary_id_internal(quad->boundary_id());
                        new_line->set_manifold_id(quad->manifold_id());
                      }
 
                    // now for the quads. again, first collect some
                    // data about the indices of the lines, with the
                    // following numbering:
                    //
                    //   .-6-.-7-.
                    //   1   9   3
                    //   .-10.11-.
                    //   0   8   2
                    //   .-4-.-5-.
 
                    const int line_indices[12] = {
                      quad->line(0)
                        ->child(child_line_index(0, quad->line_orientation(0)))
                        ->index(),
                      quad->line(0)
                        ->child(child_line_index(1, quad->line_orientation(0)))
                        ->index(),
                      quad->line(1)
                        ->child(child_line_index(0, quad->line_orientation(1)))
                        ->index(),
                      quad->line(1)
                        ->child(child_line_index(1, quad->line_orientation(1)))
                        ->index(),
                      quad->line(2)
                        ->child(child_line_index(0, quad->line_orientation(2)))
                        ->index(),
                      quad->line(2)
                        ->child(child_line_index(1, quad->line_orientation(2)))
                        ->index(),
                      quad->line(3)
                        ->child(child_line_index(0, quad->line_orientation(3)))
                        ->index(),
                      quad->line(3)
                        ->child(child_line_index(1, quad->line_orientation(3)))
                        ->index(),
                      new_lines[0]->index(),
                      new_lines[1]->index(),
                      new_lines[2]->index(),
                      new_lines[3]->index()};
 
                    // find some space (consecutive)
                    // for the first two newly to be
                    // created quads.
                    typename Triangulation<dim, spacedim>::raw_quad_iterator
                      new_quads[4];
 
                    next_unused_quad =
                      triangulation.faces->quads
                        .template next_free_pair_object<2>(triangulation);
 
                    new_quads[0] = next_unused_quad;
                    AssertIsNotUsed(new_quads[0]);
 
                    ++next_unused_quad;
                    new_quads[1] = next_unused_quad;
                    AssertIsNotUsed(new_quads[1]);
 
                    next_unused_quad =
                      triangulation.faces->quads
                        .template next_free_pair_object<2>(triangulation);
                    new_quads[2] = next_unused_quad;
                    AssertIsNotUsed(new_quads[2]);
 
                    ++next_unused_quad;
                    new_quads[3] = next_unused_quad;
                    AssertIsNotUsed(new_quads[3]);
 
                    // note these quads as children to the present one
                    quad->set_children(0, new_quads[0]->index());
                    quad->set_children(2, new_quads[2]->index());
                    quad->set_refinement_case(RefinementCase<2>::cut_xy);
 
                    new_quads[0]->set_bounding_object_indices(
                      {line_indices[0],
                       line_indices[8],
                       line_indices[4],
                       line_indices[10]});
                    new_quads[1]->set_bounding_object_indices(
                      {line_indices[8],
                       line_indices[2],
                       line_indices[5],
                       line_indices[11]});
                    new_quads[2]->set_bounding_object_indices(
                      {line_indices[1],
                       line_indices[9],
                       line_indices[10],
                       line_indices[6]});
                    new_quads[3]->set_bounding_object_indices(
                      {line_indices[9],
                       line_indices[3],
                       line_indices[11],
                       line_indices[7]});
                    for (const auto &new_quad : new_quads)
                      {
                        new_quad->set_used_flag();
                        new_quad->clear_user_flag();
                        new_quad->clear_user_data();
                        new_quad->clear_children();
                        new_quad->set_boundary_id_internal(quad->boundary_id());
                        new_quad->set_manifold_id(quad->manifold_id());
                        // set all line orientations to true, change
                        // this after the loop, as we have to consider
                        // different lines for each child
                        for (unsigned int j = 0;
                             j < GeometryInfo<dim>::lines_per_face;
                             ++j)
                          new_quad->set_line_orientation(
                            j, numbers::default_geometric_orientation);
                      }
                    // now set the line orientation of children of
                    // outer lines correctly, the lines in the
                    // interior of the refined quad are automatically
                    // oriented conforming to the standard
                    new_quads[0]->set_line_orientation(
                      0, quad->line_orientation(0));
                    new_quads[0]->set_line_orientation(
                      2, quad->line_orientation(2));
                    new_quads[1]->set_line_orientation(
                      1, quad->line_orientation(1));
                    new_quads[1]->set_line_orientation(
                      2, quad->line_orientation(2));
                    new_quads[2]->set_line_orientation(
                      0, quad->line_orientation(0));
                    new_quads[2]->set_line_orientation(
                      3, quad->line_orientation(3));
                    new_quads[3]->set_line_orientation(
                      1, quad->line_orientation(1));
                    new_quads[3]->set_line_orientation(
                      3, quad->line_orientation(3));
 
                    // finally clear flag indicating the need for
                    // refinement
                    quad->clear_user_flag();
                  } // if (isotropic refinement)
              }     // for all quads
          }         // looped two times over all quads, all quads refined now
 
        //---------------------------------
        // Now, finally, set up the new
        // cells
        //---------------------------------
 
        typename Triangulation<3, spacedim>::DistortedCellList
          cells_with_distorted_children;
 
        for (unsigned int level = 0; level != triangulation.levels.size() - 1;
             ++level)
          {
            // only active objects can be refined further; remember
            // that we won't operate on the finest level, so
            // triangulation.begin_*(level+1) is allowed
            typename Triangulation<dim, spacedim>::active_hex_iterator
              hex  = triangulation.begin_active_hex(level),
              endh = triangulation.begin_active_hex(level + 1);
            typename Triangulation<dim, spacedim>::raw_hex_iterator
              next_unused_hex = triangulation.begin_raw_hex(level + 1);
 
            for (; hex != endh; ++hex)
              if (hex->refine_flag_set())
                {
                  // this hex needs to be refined
 
                  // clear flag indicating the need for refinement. do
                  // it here already, since we can't do it anymore
                  // once the cell has children
                  const RefinementCase<dim> ref_case = hex->refine_flag_set();
                  hex->clear_refine_flag();
                  hex->set_refinement_case(ref_case);
 
                  // depending on the refine case we might have to
                  // create additional vertices, lines and quads
                  // interior of the hex before the actual children
                  // can be set up.
 
                  // in a first step: reserve the needed space for
                  // lines, quads and hexes and initialize them
                  // correctly
 
                  unsigned int n_new_lines = 0;
                  unsigned int n_new_quads = 0;
                  unsigned int n_new_hexes = 0;
                  switch (ref_case)
                    {
                      case RefinementCase<dim>::cut_x:
                      case RefinementCase<dim>::cut_y:
                      case RefinementCase<dim>::cut_z:
                        n_new_lines = 0;
                        n_new_quads = 1;
                        n_new_hexes = 2;
                        break;
                      case RefinementCase<dim>::cut_xy:
                      case RefinementCase<dim>::cut_xz:
                      case RefinementCase<dim>::cut_yz:
                        n_new_lines = 1;
                        n_new_quads = 4;
                        n_new_hexes = 4;
                        break;
                      case RefinementCase<dim>::cut_xyz:
                        n_new_lines = 6;
                        n_new_quads = 12;
                        n_new_hexes = 8;
                        break;
                      default:
                        DEAL_II_ASSERT_UNREACHABLE();
                        break;
                    }
 
                  // find some space for the newly to be created
                  // interior lines and initialize them.
                  std::vector<
                    typename Triangulation<dim, spacedim>::raw_line_iterator>
                    new_lines(n_new_lines);
                  for (unsigned int i = 0; i < n_new_lines; ++i)
                    {
                      new_lines[i] =
                        triangulation.faces->lines
                          .template next_free_single_object<1>(triangulation);
 
                      AssertIsNotUsed(new_lines[i]);
                      new_lines[i]->set_used_flag();
                      new_lines[i]->clear_user_flag();
                      new_lines[i]->clear_user_data();
                      new_lines[i]->clear_children();
                      // interior line
                      new_lines[i]->set_boundary_id_internal(
                        numbers::internal_face_boundary_id);
                      // they inherit geometry description of the hex they
                      // belong to
                      new_lines[i]->set_manifold_id(hex->manifold_id());
                    }
 
                  // find some space for the newly to be created
                  // interior quads and initialize them.
                  std::vector<
                    typename Triangulation<dim, spacedim>::raw_quad_iterator>
                    new_quads(n_new_quads);
                  for (unsigned int i = 0; i < n_new_quads; ++i)
                    {
                      new_quads[i] =
                        triangulation.faces->quads
                          .template next_free_single_object<2>(triangulation);
 
                      AssertIsNotUsed(new_quads[i]);
                      new_quads[i]->set_used_flag();
                      new_quads[i]->clear_user_flag();
                      new_quads[i]->clear_user_data();
                      new_quads[i]->clear_children();
                      // interior quad
                      new_quads[i]->set_boundary_id_internal(
                        numbers::internal_face_boundary_id);
                      // they inherit geometry description of the hex they
                      // belong to
                      new_quads[i]->set_manifold_id(hex->manifold_id());
                      // set all line orientation flags to true by
                      // default, change this afterwards, if necessary
                      for (unsigned int j = 0;
                           j < GeometryInfo<dim>::lines_per_face;
                           ++j)
                        new_quads[i]->set_line_orientation(
                          j, numbers::default_geometric_orientation);
                    }
 
                  types::subdomain_id subdomainid = hex->subdomain_id();
 
                  // find some space for the newly to be created hexes
                  // and initialize them.
                  std::vector<
                    typename Triangulation<dim, spacedim>::raw_hex_iterator>
                    new_hexes(n_new_hexes);
                  for (unsigned int i = 0; i < n_new_hexes; ++i)
                    {
                      if (i % 2 == 0)
                        next_unused_hex =
                          triangulation.levels[level + 1]->cells.next_free_hex(
                            triangulation, level + 1);
                      else
                        ++next_unused_hex;
 
                      new_hexes[i] = next_unused_hex;
 
                      AssertIsNotUsed(new_hexes[i]);
                      new_hexes[i]->set_used_flag();
                      new_hexes[i]->clear_user_flag();
                      new_hexes[i]->clear_user_data();
                      new_hexes[i]->clear_children();
                      // inherit material
                      // properties
                      new_hexes[i]->set_material_id(hex->material_id());
                      new_hexes[i]->set_manifold_id(hex->manifold_id());
                      new_hexes[i]->set_subdomain_id(subdomainid);
 
                      // We only store the parent for every second cell. That's
                      // because cells are created during refinement in
                      // multiples of two, and so two successive cells always
                      // share the same parent.
                      if (i % 2 == 0)
                        new_hexes[i]->set_parent(hex->index());
 
                      // set the face_orientation flag to true for all
                      // faces initially, as this is the default value
                      // which is true for all faces interior to the
                      // hex. later on go the other way round and
                      // reset faces that are at the boundary of the
                      // mother cube
                      //
                      // the same is true for the face_flip and
                      // face_rotation flags. however, the latter two
                      // are set to false by default as this is the
                      // standard value
                      for (const unsigned int f :
                           GeometryInfo<dim>::face_indices())
                        new_hexes[i]->set_combined_face_orientation(
                          f, numbers::default_geometric_orientation);
                    }
                  // note these hexes as children to the present cell
                  for (unsigned int i = 0; i < n_new_hexes / 2; ++i)
                    hex->set_children(2 * i, new_hexes[2 * i]->index());
 
                  // we have to take into account whether the
                  // different faces are oriented correctly or in the
                  // opposite direction, so store that up front
 
                  // face_orientation
                  const bool f_or[6] = {hex->face_orientation(0),
                                        hex->face_orientation(1),
                                        hex->face_orientation(2),
                                        hex->face_orientation(3),
                                        hex->face_orientation(4),
                                        hex->face_orientation(5)};
 
                  // face_flip
                  const bool f_fl[6] = {hex->face_flip(0),
                                        hex->face_flip(1),
                                        hex->face_flip(2),
                                        hex->face_flip(3),
                                        hex->face_flip(4),
                                        hex->face_flip(5)};
 
                  // face_rotation
                  const bool f_ro[6] = {hex->face_rotation(0),
                                        hex->face_rotation(1),
                                        hex->face_rotation(2),
                                        hex->face_rotation(3),
                                        hex->face_rotation(4),
                                        hex->face_rotation(5)};
 
                  // combined orientation
                  const types::geometric_orientation f_co[6] = {
                    hex->combined_face_orientation(0),
                    hex->combined_face_orientation(1),
                    hex->combined_face_orientation(2),
                    hex->combined_face_orientation(3),
                    hex->combined_face_orientation(4),
                    hex->combined_face_orientation(5)};
 
                  // little helper table, indicating, whether the
                  // child with index 0 or with index 1 can be found
                  // at the standard origin of an anisotropically
                  // refined quads in real orientation index 1:
                  // (RefineCase - 1) index 2: face_flip
 
                  // index 3: face rotation
                  // note: face orientation has no influence
                  const unsigned int child_at_origin[2][2][2] = {
                    {{0, 0},   // RefinementCase<dim>::cut_x, face_flip=false,
                               // face_rotation=false and true
                     {1, 1}},  // RefinementCase<dim>::cut_x, face_flip=true,
                               // face_rotation=false and true
                    {{0, 1},   // RefinementCase<dim>::cut_y, face_flip=false,
                               // face_rotation=false and true
                     {1, 0}}}; // RefinementCase<dim>::cut_y, face_flip=true,
                               // face_rotation=false and true
 
                  //-------------------------------------
                  //
                  // in the following we will do the same thing for
                  // each refinement case: create a new vertex (if
                  // needed), create new interior lines (if needed),
                  // create new interior quads and afterwards build
                  // the children hexes out of these and the existing
                  // subfaces of the outer quads (which have been
                  // created above). However, even if the steps are
                  // quite similar, the actual work strongly depends
                  // on the actual refinement case. therefore, we use
                  // separate blocks of code for each of these cases,
                  // which hopefully increases the readability to some
                  // extend.
 
                  switch (ref_case)
                    {
                      case RefinementCase<dim>::cut_x:
                        {
                          //----------------------------
                          //
                          //     RefinementCase<dim>::cut_x
                          //
                          // the refined cube will look
                          // like this:
                          //
                          //        *----*----*
                          //       /    /    /|
                          //      /    /    / |
                          //     /    /    /  |
                          //    *----*----*   |
                          //    |    |    |   |
                          //    |    |    |   *
                          //    |    |    |  /
                          //    |    |    | /
                          //    |    |    |/
                          //    *----*----*
                          //
                          // again, first collect some data about the
                          // indices of the lines, with the following
                          // numbering:
 
                          // face 2: front plane
                          //   (note: x,y exchanged)
                          //   *---*---*
                          //   |   |   |
                          //   |   0   |
                          //   |   |   |
                          //   *---*---*
                          //       m0
                          // face 3: back plane
                          //   (note: x,y exchanged)
                          //       m1
                          //   *---*---*
                          //   |   |   |
                          //   |   1   |
                          //   |   |   |
                          //   *---*---*
                          // face 4: bottom plane
                          //       *---*---*
                          //      /   /   /
                          //     /   2   /
                          //    /   /   /
                          //   *---*---*
                          //       m0
                          // face 5: top plane
                          //           m1
                          //       *---*---*
                          //      /   /   /
                          //     /   3   /
                          //    /   /   /
                          //   *---*---*
 
                          // set up a list of line iterators first. from
                          // this, construct lists of line_indices and
                          // line orientations later on
                          const typename Triangulation<dim, spacedim>::
                            raw_line_iterator lines[4] = {
                              hex->face(2)->child(0)->line(
                                (hex->face(2)->refinement_case() ==
                                 RefinementCase<2>::cut_x) ?
                                  1 :
                                  3), // 0
                              hex->face(3)->child(0)->line(
                                (hex->face(3)->refinement_case() ==
                                 RefinementCase<2>::cut_x) ?
                                  1 :
                                  3), // 1
                              hex->face(4)->child(0)->line(
                                (hex->face(4)->refinement_case() ==
                                 RefinementCase<2>::cut_x) ?
                                  1 :
                                  3), // 2
                              hex->face(5)->child(0)->line(
                                (hex->face(5)->refinement_case() ==
                                 RefinementCase<2>::cut_x) ?
                                  1 :
                                  3) // 3
                            };
 
                          int line_indices[4];
                          for (unsigned int i = 0; i < 4; ++i)
                            line_indices[i] = lines[i]->index();
 
                          // the orientation of lines for the inner quads
                          // is quite tricky. as these lines are newly
                          // created ones and thus have no parents, they
                          // cannot inherit this property. set up an array
                          // and fill it with the respective values
                          types::geometric_orientation line_orientation[4]{};
 
                          // the middle vertex marked as m0 above is the
                          // start vertex for lines 0 and 2 in standard
                          // orientation, whereas m1 is the end vertex of
                          // lines 1 and 3 in standard orientation
                          const unsigned int middle_vertices[2] = {
                            hex->line(2)->child(0)->vertex_index(1),
                            hex->line(7)->child(0)->vertex_index(1)};
 
                          for (unsigned int i = 0; i < 4; ++i)
                            if (lines[i]->vertex_index(i % 2) ==
                                middle_vertices[i % 2])
                              line_orientation[i] =
                                numbers::default_geometric_orientation;
                            else
                              {
                                // it must be the other way round then
                                Assert(lines[i]->vertex_index((i + 1) % 2) ==
                                         middle_vertices[i % 2],
                                       ExcInternalError());
                                line_orientation[i] =
                                  numbers::reverse_line_orientation;
                              }
 
                          // set up the new quad, line numbering is as
                          // indicated above
                          new_quads[0]->set_bounding_object_indices(
                            {line_indices[0],
                             line_indices[1],
                             line_indices[2],
                             line_indices[3]});
 
                          new_quads[0]->set_line_orientation(
                            0, line_orientation[0]);
                          new_quads[0]->set_line_orientation(
                            1, line_orientation[1]);
                          new_quads[0]->set_line_orientation(
                            2, line_orientation[2]);
                          new_quads[0]->set_line_orientation(
                            3, line_orientation[3]);
 
                          // the quads are numbered as follows:
                          //
                          // planes in the interior of the old hex:
                          //
                          //      *
                          //     /|
                          //    / | x
                          //   /  | *-------*      *---------*
                          //  *   | |       |     /         /
                          //  | 0 | |       |    /         /
                          //  |   * |       |   /         /
                          //  |  /  *-------*y *---------*x
                          //  | /
                          //  |/
                          //  *
                          //
                          // children of the faces of the old hex
                          //
                          //      *---*---*        *---*---*
                          //     /|   |   |       /   /   /|
                          //    / |   |   |      / 9 / 10/ |
                          //   /  | 5 | 6 |     /   /   /  |
                          //  *   |   |   |    *---*---*   |
                          //  | 1 *---*---*    |   |   | 2 *
                          //  |  /   /   /     |   |   |  /
                          //  | / 7 / 8 /      | 3 | 4 | /
                          //  |/   /   /       |   |   |/
                          //  *---*---*        *---*---*
                          //
                          // note that we have to take care of the
                          // orientation of faces.
                          const int quad_indices[11] = {
                            new_quads[0]->index(), // 0
 
                            hex->face(0)->index(), // 1
 
                            hex->face(1)->index(), // 2
 
                            hex->face(2)->child_index(
                              child_at_origin[hex->face(2)->refinement_case() -
                                              1][f_fl[2]][f_ro[2]]), // 3
                            hex->face(2)->child_index(
                              1 -
                              child_at_origin[hex->face(2)->refinement_case() -
                                              1][f_fl[2]][f_ro[2]]),
 
                            hex->face(3)->child_index(
                              child_at_origin[hex->face(3)->refinement_case() -
                                              1][f_fl[3]][f_ro[3]]), // 5
                            hex->face(3)->child_index(
                              1 -
                              child_at_origin[hex->face(3)->refinement_case() -
                                              1][f_fl[3]][f_ro[3]]),
 
                            hex->face(4)->child_index(
                              child_at_origin[hex->face(4)->refinement_case() -
                                              1][f_fl[4]][f_ro[4]]), // 7
                            hex->face(4)->child_index(
                              1 -
                              child_at_origin[hex->face(4)->refinement_case() -
                                              1][f_fl[4]][f_ro[4]]),
 
                            hex->face(5)->child_index(
                              child_at_origin[hex->face(5)->refinement_case() -
                                              1][f_fl[5]][f_ro[5]]), // 9
                            hex->face(5)->child_index(
                              1 -
                              child_at_origin[hex->face(5)->refinement_case() -
                                              1][f_fl[5]][f_ro[5]])
 
                          };
 
                          new_hexes[0]->set_bounding_object_indices(
                            {quad_indices[1],
                             quad_indices[0],
                             quad_indices[3],
                             quad_indices[5],
                             quad_indices[7],
                             quad_indices[9]});
                          new_hexes[1]->set_bounding_object_indices(
                            {quad_indices[0],
                             quad_indices[2],
                             quad_indices[4],
                             quad_indices[6],
                             quad_indices[8],
                             quad_indices[10]});
                          break;
                        }
 
                      case RefinementCase<dim>::cut_y:
                        {
                          //----------------------------
                          //
                          //     RefinementCase<dim>::cut_y
                          //
                          // the refined cube will look like this:
                          //
                          //        *---------*
                          //       /         /|
                          //      *---------* |
                          //     /         /| |
                          //    *---------* | |
                          //    |         | | |
                          //    |         | | *
                          //    |         | |/
                          //    |         | *
                          //    |         |/
                          //    *---------*
                          //
                          // again, first collect some data about the
                          // indices of the lines, with the following
                          // numbering:
 
                          // face 0: left plane
                          //       *
                          //      /|
                          //     * |
                          //    /| |
                          //   * | |
                          //   | 0 |
                          //   | | *
                          //   | |/
                          //   | *m0
                          //   |/
                          //   *
                          // face 1: right plane
                          //       *
                          //      /|
                          //   m1* |
                          //    /| |
                          //   * | |
                          //   | 1 |
                          //   | | *
                          //   | |/
                          //   | *
                          //   |/
                          //   *
                          // face 4: bottom plane
                          //       *-------*
                          //      /       /
                          //   m0*---2---*
                          //    /       /
                          //   *-------*
                          // face 5: top plane
                          //       *-------*
                          //      /       /
                          //     *---3---*m1
                          //    /       /
                          //   *-------*
 
                          // set up a list of line iterators first. from
                          // this, construct lists of line_indices and
                          // line orientations later on
                          const typename Triangulation<dim, spacedim>::
                            raw_line_iterator lines[4] = {
                              hex->face(0)->child(0)->line(
                                (hex->face(0)->refinement_case() ==
                                 RefinementCase<2>::cut_x) ?
                                  1 :
                                  3), // 0
                              hex->face(1)->child(0)->line(
                                (hex->face(1)->refinement_case() ==
                                 RefinementCase<2>::cut_x) ?
                                  1 :
                                  3), // 1
                              hex->face(4)->child(0)->line(
                                (hex->face(4)->refinement_case() ==
                                 RefinementCase<2>::cut_x) ?
                                  1 :
                                  3), // 2
                              hex->face(5)->child(0)->line(
                                (hex->face(5)->refinement_case() ==
                                 RefinementCase<2>::cut_x) ?
                                  1 :
                                  3) // 3
                            };
 
                          int line_indices[4];
                          for (unsigned int i = 0; i < 4; ++i)
                            line_indices[i] = lines[i]->index();
 
                          // the orientation of lines for the inner quads
                          // is quite tricky. as these lines are newly
                          // created ones and thus have no parents, they
                          // cannot inherit this property. set up an array
                          // and fill it with the respective values
                          types::geometric_orientation line_orientation[4]{};
 
                          // the middle vertex marked as m0 above is the
                          // start vertex for lines 0 and 2 in standard
                          // orientation, whereas m1 is the end vertex of
                          // lines 1 and 3 in standard orientation
                          const unsigned int middle_vertices[2] = {
                            hex->line(0)->child(0)->vertex_index(1),
                            hex->line(5)->child(0)->vertex_index(1)};
 
                          for (unsigned int i = 0; i < 4; ++i)
                            if (lines[i]->vertex_index(i % 2) ==
                                middle_vertices[i % 2])
                              line_orientation[i] =
                                numbers::default_geometric_orientation;
                            else
                              {
                                // it must be the other way round then
                                Assert(lines[i]->vertex_index((i + 1) % 2) ==
                                         middle_vertices[i % 2],
                                       ExcInternalError());
                                line_orientation[i] =
                                  numbers::reverse_line_orientation;
                              }
 
                          // set up the new quad, line numbering is as
                          // indicated above
                          new_quads[0]->set_bounding_object_indices(
                            {line_indices[2],
                             line_indices[3],
                             line_indices[0],
                             line_indices[1]});
 
                          new_quads[0]->set_line_orientation(
                            0, line_orientation[2]);
                          new_quads[0]->set_line_orientation(
                            1, line_orientation[3]);
                          new_quads[0]->set_line_orientation(
                            2, line_orientation[0]);
                          new_quads[0]->set_line_orientation(
                            3, line_orientation[1]);
 
                          // the quads are numbered as follows:
                          //
                          // planes in the interior of the old hex:
                          //
                          //      *
                          //     /|
                          //    / | x
                          //   /  | *-------*      *---------*
                          //  *   | |       |     /         /
                          //  |   | |   0   |    /         /
                          //  |   * |       |   /         /
                          //  |  /  *-------*y *---------*x
                          //  | /
                          //  |/
                          //  *
                          //
                          // children of the faces of the old hex
                          //
                          //      *-------*        *-------*
                          //     /|       |       /   10  /|
                          //    * |       |      *-------* |
                          //   /| |   6   |     /   9   /| |
                          //  * |2|       |    *-------* |4|
                          //  | | *-------*    |       | | *
                          //  |1|/   8   /     |       |3|/
                          //  | *-------*      |   5   | *
                          //  |/   7   /       |       |/
                          //  *-------*        *-------*
                          //
                          // note that we have to take care of the
                          // orientation of faces.
                          const int quad_indices[11] = {
                            new_quads[0]->index(), // 0
 
                            hex->face(0)->child_index(
                              child_at_origin[hex->face(0)->refinement_case() -
                                              1][f_fl[0]][f_ro[0]]), // 1
                            hex->face(0)->child_index(
                              1 -
                              child_at_origin[hex->face(0)->refinement_case() -
                                              1][f_fl[0]][f_ro[0]]),
 
                            hex->face(1)->child_index(
                              child_at_origin[hex->face(1)->refinement_case() -
                                              1][f_fl[1]][f_ro[1]]), // 3
                            hex->face(1)->child_index(
                              1 -
                              child_at_origin[hex->face(1)->refinement_case() -
                                              1][f_fl[1]][f_ro[1]]),
 
                            hex->face(2)->index(), // 5
 
                            hex->face(3)->index(), // 6
 
                            hex->face(4)->child_index(
                              child_at_origin[hex->face(4)->refinement_case() -
                                              1][f_fl[4]][f_ro[4]]), // 7
                            hex->face(4)->child_index(
                              1 -
                              child_at_origin[hex->face(4)->refinement_case() -
                                              1][f_fl[4]][f_ro[4]]),
 
                            hex->face(5)->child_index(
                              child_at_origin[hex->face(5)->refinement_case() -
                                              1][f_fl[5]][f_ro[5]]), // 9
                            hex->face(5)->child_index(
                              1 -
                              child_at_origin[hex->face(5)->refinement_case() -
                                              1][f_fl[5]][f_ro[5]])
 
                          };
 
                          new_hexes[0]->set_bounding_object_indices(
                            {quad_indices[1],
                             quad_indices[3],
                             quad_indices[5],
                             quad_indices[0],
                             quad_indices[7],
                             quad_indices[9]});
                          new_hexes[1]->set_bounding_object_indices(
                            {quad_indices[2],
                             quad_indices[4],
                             quad_indices[0],
                             quad_indices[6],
                             quad_indices[8],
                             quad_indices[10]});
                          break;
                        }
 
                      case RefinementCase<dim>::cut_z:
                        {
                          //----------------------------
                          //
                          //     RefinementCase<dim>::cut_z
                          //
                          // the refined cube will look like this:
                          //
                          //        *---------*
                          //       /         /|
                          //      /         / |
                          //     /         /  *
                          //    *---------*  /|
                          //    |         | / |
                          //    |         |/  *
                          //    *---------*  /
                          //    |         | /
                          //    |         |/
                          //    *---------*
                          //
                          // again, first collect some data about the
                          // indices of the lines, with the following
                          // numbering:
 
                          // face 0: left plane
                          //       *
                          //      /|
                          //     / |
                          //    /  *
                          //   *  /|
                          //   | 0 |
                          //   |/  *
                          // m0*  /
                          //   | /
                          //   |/
                          //   *
                          // face 1: right plane
                          //       *
                          //      /|
                          //     / |
                          //    /  *m1
                          //   *  /|
                          //   | 1 |
                          //   |/  *
                          //   *  /
                          //   | /
                          //   |/
                          //   *
                          // face 2: front plane
                          //   (note: x,y exchanged)
                          //   *-------*
                          //   |       |
                          // m0*---2---*
                          //   |       |
                          //   *-------*
                          // face 3: back plane
                          //   (note: x,y exchanged)
                          //   *-------*
                          //   |       |
                          //   *---3---*m1
                          //   |       |
                          //   *-------*
 
                          // set up a list of line iterators first. from
                          // this, construct lists of line_indices and
                          // line orientations later on
                          const typename Triangulation<dim, spacedim>::
                            raw_line_iterator lines[4] = {
                              hex->face(0)->child(0)->line(
                                (hex->face(0)->refinement_case() ==
                                 RefinementCase<2>::cut_x) ?
                                  1 :
                                  3), // 0
                              hex->face(1)->child(0)->line(
                                (hex->face(1)->refinement_case() ==
                                 RefinementCase<2>::cut_x) ?
                                  1 :
                                  3), // 1
                              hex->face(2)->child(0)->line(
                                (hex->face(2)->refinement_case() ==
                                 RefinementCase<2>::cut_x) ?
                                  1 :
                                  3), // 2
                              hex->face(3)->child(0)->line(
                                (hex->face(3)->refinement_case() ==
                                 RefinementCase<2>::cut_x) ?
                                  1 :
                                  3) // 3
                            };
 
                          int line_indices[4];
                          for (unsigned int i = 0; i < 4; ++i)
                            line_indices[i] = lines[i]->index();
 
                          // the orientation of lines for the inner quads
                          // is quite tricky. as these lines are newly
                          // created ones and thus have no parents, they
                          // cannot inherit this property. set up an array
                          // and fill it with the respective values
                          types::geometric_orientation line_orientation[4]{};
 
                          // the middle vertex marked as m0 above is the
                          // start vertex for lines 0 and 2 in standard
                          // orientation, whereas m1 is the end vertex of
                          // lines 1 and 3 in standard orientation
                          const unsigned int middle_vertices[2] = {
                            middle_vertex_index<dim, spacedim>(hex->line(8)),
                            middle_vertex_index<dim, spacedim>(hex->line(11))};
 
                          for (unsigned int i = 0; i < 4; ++i)
                            if (lines[i]->vertex_index(i % 2) ==
                                middle_vertices[i % 2])
                              line_orientation[i] =
                                numbers::default_geometric_orientation;
                            else
                              {
                                // it must be the other way round then
                                Assert(lines[i]->vertex_index((i + 1) % 2) ==
                                         middle_vertices[i % 2],
                                       ExcInternalError());
                                line_orientation[i] =
                                  numbers::reverse_line_orientation;
                              }
 
                          // set up the new quad, line numbering is as
                          // indicated above
                          new_quads[0]->set_bounding_object_indices(
                            {line_indices[0],
                             line_indices[1],
                             line_indices[2],
                             line_indices[3]});
 
                          new_quads[0]->set_line_orientation(
                            0, line_orientation[0]);
                          new_quads[0]->set_line_orientation(
                            1, line_orientation[1]);
                          new_quads[0]->set_line_orientation(
                            2, line_orientation[2]);
                          new_quads[0]->set_line_orientation(
                            3, line_orientation[3]);
 
                          // the quads are numbered as follows:
                          //
                          // planes in the interior of the old hex:
                          //
                          //      *
                          //     /|
                          //    / | x
                          //   /  | *-------*      *---------*
                          //  *   | |       |     /         /
                          //  |   | |       |    /    0    /
                          //  |   * |       |   /         /
                          //  |  /  *-------*y *---------*x
                          //  | /
                          //  |/
                          //  *
                          //
                          // children of the faces of the old hex
                          //
                          //      *---*---*        *-------*
                          //     /|   8   |       /       /|
                          //    / |       |      /   10  / |
                          //   /  *-------*     /       /  *
                          //  * 2/|       |    *-------* 4/|
                          //  | / |   7   |    |   6   | / |
                          //  |/1 *-------*    |       |/3 *
                          //  *  /       /     *-------*  /
                          //  | /   9   /      |       | /
                          //  |/       /       |   5   |/
                          //  *-------*        *---*---*
                          //
                          // note that we have to take care of the
                          // orientation of faces.
                          const int quad_indices[11] = {
                            new_quads[0]->index(), // 0
 
                            hex->face(0)->child_index(
                              child_at_origin[hex->face(0)->refinement_case() -
                                              1][f_fl[0]][f_ro[0]]), // 1
                            hex->face(0)->child_index(
                              1 -
                              child_at_origin[hex->face(0)->refinement_case() -
                                              1][f_fl[0]][f_ro[0]]),
 
                            hex->face(1)->child_index(
                              child_at_origin[hex->face(1)->refinement_case() -
                                              1][f_fl[1]][f_ro[1]]), // 3
                            hex->face(1)->child_index(
                              1 -
                              child_at_origin[hex->face(1)->refinement_case() -
                                              1][f_fl[1]][f_ro[1]]),
 
                            hex->face(2)->child_index(
                              child_at_origin[hex->face(2)->refinement_case() -
                                              1][f_fl[2]][f_ro[2]]), // 5
                            hex->face(2)->child_index(
                              1 -
                              child_at_origin[hex->face(2)->refinement_case() -
                                              1][f_fl[2]][f_ro[2]]),
 
                            hex->face(3)->child_index(
                              child_at_origin[hex->face(3)->refinement_case() -
                                              1][f_fl[3]][f_ro[3]]), // 7
                            hex->face(3)->child_index(
                              1 -
                              child_at_origin[hex->face(3)->refinement_case() -
                                              1][f_fl[3]][f_ro[3]]),
 
                            hex->face(4)->index(), // 9
 
                            hex->face(5)->index() // 10
                          };
 
                          new_hexes[0]->set_bounding_object_indices(
                            {quad_indices[1],
                             quad_indices[3],
                             quad_indices[5],
                             quad_indices[7],
                             quad_indices[9],
                             quad_indices[0]});
                          new_hexes[1]->set_bounding_object_indices(
                            {quad_indices[2],
                             quad_indices[4],
                             quad_indices[6],
                             quad_indices[8],
                             quad_indices[0],
                             quad_indices[10]});
                          break;
                        }
 
                      case RefinementCase<dim>::cut_xy:
                        {
                          //----------------------------
                          //
                          //     RefinementCase<dim>::cut_xy
                          //
                          // the refined cube will look like this:
                          //
                          //        *----*----*
                          //       /    /    /|
                          //      *----*----* |
                          //     /    /    /| |
                          //    *----*----* | |
                          //    |    |    | | |
                          //    |    |    | | *
                          //    |    |    | |/
                          //    |    |    | *
                          //    |    |    |/
                          //    *----*----*
                          //
 
                          // first, create the new internal line
                          new_lines[0]->set_bounding_object_indices(
                            {middle_vertex_index<dim, spacedim>(hex->face(4)),
                             middle_vertex_index<dim, spacedim>(hex->face(5))});
 
                          // again, first collect some data about the
                          // indices of the lines, with the following
                          // numbering:
 
                          // face 0: left plane
                          //       *
                          //      /|
                          //     * |
                          //    /| |
                          //   * | |
                          //   | 0 |
                          //   | | *
                          //   | |/
                          //   | *
                          //   |/
                          //   *
                          // face 1: right plane
                          //       *
                          //      /|
                          //     * |
                          //    /| |
                          //   * | |
                          //   | 1 |
                          //   | | *
                          //   | |/
                          //   | *
                          //   |/
                          //   *
                          // face 2: front plane
                          //   (note: x,y exchanged)
                          //   *---*---*
                          //   |   |   |
                          //   |   2   |
                          //   |   |   |
                          //   *-------*
                          // face 3: back plane
                          //   (note: x,y exchanged)
                          //   *---*---*
                          //   |   |   |
                          //   |   3   |
                          //   |   |   |
                          //   *---*---*
                          // face 4: bottom plane
                          //       *---*---*
                          //      /   5   /
                          //     *-6-*-7-*
                          //    /   4   /
                          //   *---*---*
                          // face 5: top plane
                          //       *---*---*
                          //      /   9   /
                          //     *10-*-11*
                          //    /   8   /
                          //   *---*---*
                          // middle planes
                          //     *-------*   *---*---*
                          //    /       /    |   |   |
                          //   /       /     |   12  |
                          //  /       /      |   |   |
                          // *-------*       *---*---*
 
                          // set up a list of line iterators first. from
                          // this, construct lists of line_indices and
                          // line orientations later on
                          const typename Triangulation<
                            dim,
                            spacedim>::raw_line_iterator lines[13] = {
                            hex->face(0)->child(0)->line(
                              (hex->face(0)->refinement_case() ==
                               RefinementCase<2>::cut_x) ?
                                1 :
                                3), // 0
                            hex->face(1)->child(0)->line(
                              (hex->face(1)->refinement_case() ==
                               RefinementCase<2>::cut_x) ?
                                1 :
                                3), // 1
                            hex->face(2)->child(0)->line(
                              (hex->face(2)->refinement_case() ==
                               RefinementCase<2>::cut_x) ?
                                1 :
                                3), // 2
                            hex->face(3)->child(0)->line(
                              (hex->face(3)->refinement_case() ==
                               RefinementCase<2>::cut_x) ?
                                1 :
                                3), // 3
 
                            hex->face(4)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[4], f_fl[4], f_ro[4]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  1, f_or[4], f_fl[4], f_ro[4])), // 4
                            hex->face(4)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[4], f_fl[4], f_ro[4]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  0, f_or[4], f_fl[4], f_ro[4])), // 5
                            hex->face(4)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[4], f_fl[4], f_ro[4]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  3, f_or[4], f_fl[4], f_ro[4])), // 6
                            hex->face(4)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[4], f_fl[4], f_ro[4]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  2, f_or[4], f_fl[4], f_ro[4])), // 7
 
                            hex->face(5)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[5], f_fl[5], f_ro[5]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  1, f_or[5], f_fl[5], f_ro[5])), // 8
                            hex->face(5)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[5], f_fl[5], f_ro[5]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  0, f_or[5], f_fl[5], f_ro[5])), // 9
                            hex->face(5)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[5], f_fl[5], f_ro[5]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  3, f_or[5], f_fl[5], f_ro[5])), // 10
                            hex->face(5)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[5], f_fl[5], f_ro[5]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  2, f_or[5], f_fl[5], f_ro[5])), // 11
 
                            new_lines[0] // 12
                          };
 
                          int line_indices[13];
                          for (unsigned int i = 0; i < 13; ++i)
                            line_indices[i] = lines[i]->index();
 
                          // the orientation of lines for the inner quads
                          // is quite tricky. as these lines are newly
                          // created ones and thus have no parents, they
                          // cannot inherit this property. set up an array
                          // and fill it with the respective values
                          types::geometric_orientation line_orientation[13]{};
 
                          // the middle vertices of the lines of our
                          // bottom face
                          const unsigned int middle_vertices[4] = {
                            hex->line(0)->child(0)->vertex_index(1),
                            hex->line(1)->child(0)->vertex_index(1),
                            hex->line(2)->child(0)->vertex_index(1),
                            hex->line(3)->child(0)->vertex_index(1),
                          };
 
                          // note: for lines 0 to 3 the orientation of the
                          // line is 'true', if vertex 0 is on the bottom
                          // face
                          for (unsigned int i = 0; i < 4; ++i)
                            if (lines[i]->vertex_index(0) == middle_vertices[i])
                              line_orientation[i] =
                                numbers::default_geometric_orientation;
                            else
                              {
                                // it must be the other way round then
                                Assert(lines[i]->vertex_index(1) ==
                                         middle_vertices[i],
                                       ExcInternalError());
                                line_orientation[i] =
                                  numbers::reverse_line_orientation;
                              }
 
                          // note: for lines 4 to 11 (inner lines of the
                          // outer quads) the following holds: the second
                          // vertex of the even lines in standard
                          // orientation is the vertex in the middle of
                          // the quad, whereas for odd lines the first
                          // vertex is the same middle vertex.
                          for (unsigned int i = 4; i < 12; ++i)
                            if (lines[i]->vertex_index((i + 1) % 2) ==
                                middle_vertex_index<dim, spacedim>(
                                  hex->face(3 + i / 4)))
                              line_orientation[i] =
                                numbers::default_geometric_orientation;
                            else
                              {
                                // it must be the other way round then
                                Assert(lines[i]->vertex_index(i % 2) ==
                                         (middle_vertex_index<dim, spacedim>(
                                           hex->face(3 + i / 4))),
                                       ExcInternalError());
                                line_orientation[i] =
                                  numbers::reverse_line_orientation;
                              }
                          // for the last line the line orientation is
                          // always true, since it was just constructed
                          // that way
                          line_orientation[12] =
                            numbers::default_geometric_orientation;
 
                          // set up the 4 quads, numbered as follows (left
                          // quad numbering, right line numbering
                          // extracted from above)
                          //
                          //      *          *
                          //     /|         9|
                          //    * |        * |
                          //  y/| |       8| 3
                          //  * |1|      * | |
                          //  | | |x     | 12|
                          //  |0| *      | | *
                          //  | |/       2 |5
                          //  | *        | *
                          //  |/         |4
                          //  *          *
                          //
                          //  x
                          //  *---*---*      *10-*-11*
                          //  |   |   |      |   |   |
                          //  | 2 | 3 |      0   12  1
                          //  |   |   |      |   |   |
                          //  *---*---*y     *-6-*-7-*
 
                          new_quads[0]->set_bounding_object_indices(
                            {line_indices[2],
                             line_indices[12],
                             line_indices[4],
                             line_indices[8]});
                          new_quads[1]->set_bounding_object_indices(
                            {line_indices[12],
                             line_indices[3],
                             line_indices[5],
                             line_indices[9]});
                          new_quads[2]->set_bounding_object_indices(
                            {line_indices[6],
                             line_indices[10],
                             line_indices[0],
                             line_indices[12]});
                          new_quads[3]->set_bounding_object_indices(
                            {line_indices[7],
                             line_indices[11],
                             line_indices[12],
                             line_indices[1]});
 
                          new_quads[0]->set_line_orientation(
                            0, line_orientation[2]);
                          new_quads[0]->set_line_orientation(
                            2, line_orientation[4]);
                          new_quads[0]->set_line_orientation(
                            3, line_orientation[8]);
 
                          new_quads[1]->set_line_orientation(
                            1, line_orientation[3]);
                          new_quads[1]->set_line_orientation(
                            2, line_orientation[5]);
                          new_quads[1]->set_line_orientation(
                            3, line_orientation[9]);
 
                          new_quads[2]->set_line_orientation(
                            0, line_orientation[6]);
                          new_quads[2]->set_line_orientation(
                            1, line_orientation[10]);
                          new_quads[2]->set_line_orientation(
                            2, line_orientation[0]);
 
                          new_quads[3]->set_line_orientation(
                            0, line_orientation[7]);
                          new_quads[3]->set_line_orientation(
                            1, line_orientation[11]);
                          new_quads[3]->set_line_orientation(
                            3, line_orientation[1]);
 
                          // the quads are numbered as follows:
                          //
                          // planes in the interior of the old hex:
                          //
                          //      *
                          //     /|
                          //    * | x
                          //   /| | *---*---*      *---------*
                          //  * |1| |   |   |     /         /
                          //  | | | | 2 | 3 |    /         /
                          //  |0| * |   |   |   /         /
                          //  | |/  *---*---*y *---------*x
                          //  | *
                          //  |/
                          //  *
                          //
                          // children of the faces of the old hex
                          //
                          //      *---*---*        *---*---*
                          //     /|   |   |       /18 / 19/|
                          //    * |10 | 11|      /---/---* |
                          //   /| |   |   |     /16 / 17/| |
                          //  * |5|   |   |    *---*---* |7|
                          //  | | *---*---*    |   |   | | *
                          //  |4|/14 / 15/     |   |   |6|/
                          //  | *---/---/      | 8 | 9 | *
                          //  |/12 / 13/       |   |   |/
                          //  *---*---*        *---*---*
                          //
                          // note that we have to take care of the
                          // orientation of faces.
                          const int quad_indices[20] = {
                            new_quads[0]->index(), // 0
                            new_quads[1]->index(),
                            new_quads[2]->index(),
                            new_quads[3]->index(),
 
                            hex->face(0)->child_index(
                              child_at_origin[hex->face(0)->refinement_case() -
                                              1][f_fl[0]][f_ro[0]]), // 4
                            hex->face(0)->child_index(
                              1 -
                              child_at_origin[hex->face(0)->refinement_case() -
                                              1][f_fl[0]][f_ro[0]]),
 
                            hex->face(1)->child_index(
                              child_at_origin[hex->face(1)->refinement_case() -
                                              1][f_fl[1]][f_ro[1]]), // 6
                            hex->face(1)->child_index(
                              1 -
                              child_at_origin[hex->face(1)->refinement_case() -
                                              1][f_fl[1]][f_ro[1]]),
 
                            hex->face(2)->child_index(
                              child_at_origin[hex->face(2)->refinement_case() -
                                              1][f_fl[2]][f_ro[2]]), // 8
                            hex->face(2)->child_index(
                              1 -
                              child_at_origin[hex->face(2)->refinement_case() -
                                              1][f_fl[2]][f_ro[2]]),
 
                            hex->face(3)->child_index(
                              child_at_origin[hex->face(3)->refinement_case() -
                                              1][f_fl[3]][f_ro[3]]), // 10
                            hex->face(3)->child_index(
                              1 -
                              child_at_origin[hex->face(3)->refinement_case() -
                                              1][f_fl[3]][f_ro[3]]),
 
                            hex->face(4)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                0, f_or[4], f_fl[4], f_ro[4])), // 12
                            hex->face(4)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                1, f_or[4], f_fl[4], f_ro[4])),
                            hex->face(4)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                2, f_or[4], f_fl[4], f_ro[4])),
                            hex->face(4)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                3, f_or[4], f_fl[4], f_ro[4])),
 
                            hex->face(5)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                0, f_or[5], f_fl[5], f_ro[5])), // 16
                            hex->face(5)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                1, f_or[5], f_fl[5], f_ro[5])),
                            hex->face(5)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                2, f_or[5], f_fl[5], f_ro[5])),
                            hex->face(5)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                3, f_or[5], f_fl[5], f_ro[5]))};
 
                          new_hexes[0]->set_bounding_object_indices(
                            {quad_indices[4],
                             quad_indices[0],
                             quad_indices[8],
                             quad_indices[2],
                             quad_indices[12],
                             quad_indices[16]});
                          new_hexes[1]->set_bounding_object_indices(
                            {quad_indices[0],
                             quad_indices[6],
                             quad_indices[9],
                             quad_indices[3],
                             quad_indices[13],
                             quad_indices[17]});
                          new_hexes[2]->set_bounding_object_indices(
                            {quad_indices[5],
                             quad_indices[1],
                             quad_indices[2],
                             quad_indices[10],
                             quad_indices[14],
                             quad_indices[18]});
                          new_hexes[3]->set_bounding_object_indices(
                            {quad_indices[1],
                             quad_indices[7],
                             quad_indices[3],
                             quad_indices[11],
                             quad_indices[15],
                             quad_indices[19]});
                          break;
                        }
 
                      case RefinementCase<dim>::cut_xz:
                        {
                          //----------------------------
                          //
                          //     RefinementCase<dim>::cut_xz
                          //
                          // the refined cube will look like this:
                          //
                          //        *----*----*
                          //       /    /    /|
                          //      /    /    / |
                          //     /    /    /  *
                          //    *----*----*  /|
                          //    |    |    | / |
                          //    |    |    |/  *
                          //    *----*----*  /
                          //    |    |    | /
                          //    |    |    |/
                          //    *----*----*
                          //
 
                          // first, create the new internal line
                          new_lines[0]->set_bounding_object_indices(
                            {middle_vertex_index<dim, spacedim>(hex->face(2)),
                             middle_vertex_index<dim, spacedim>(hex->face(3))});
 
                          // again, first collect some data about the
                          // indices of the lines, with the following
                          // numbering:
 
                          // face 0: left plane
                          //       *
                          //      /|
                          //     / |
                          //    /  *
                          //   *  /|
                          //   | 0 |
                          //   |/  *
                          //   *  /
                          //   | /
                          //   |/
                          //   *
                          // face 1: right plane
                          //       *
                          //      /|
                          //     / |
                          //    /  *
                          //   *  /|
                          //   | 1 |
                          //   |/  *
                          //   *  /
                          //   | /
                          //   |/
                          //   *
                          // face 2: front plane
                          //   (note: x,y exchanged)
                          //   *---*---*
                          //   |   5   |
                          //   *-6-*-7-*
                          //   |   4   |
                          //   *---*---*
                          // face 3: back plane
                          //   (note: x,y exchanged)
                          //   *---*---*
                          //   |   9   |
                          //   *10-*-11*
                          //   |   8   |
                          //   *---*---*
                          // face 4: bottom plane
                          //       *---*---*
                          //      /   /   /
                          //     /   2   /
                          //    /   /   /
                          //   *---*---*
                          // face 5: top plane
                          //       *---*---*
                          //      /   /   /
                          //     /   3   /
                          //    /   /   /
                          //   *---*---*
                          // middle planes
                          //     *---*---*   *-------*
                          //    /   /   /    |       |
                          //   /   12  /     |       |
                          //  /   /   /      |       |
                          // *---*---*       *-------*
 
                          // set up a list of line iterators first. from
                          // this, construct lists of line_indices and
                          // line orientations later on
                          const typename Triangulation<
                            dim,
                            spacedim>::raw_line_iterator lines[13] = {
                            hex->face(0)->child(0)->line(
                              (hex->face(0)->refinement_case() ==
                               RefinementCase<2>::cut_x) ?
                                1 :
                                3), // 0
                            hex->face(1)->child(0)->line(
                              (hex->face(1)->refinement_case() ==
                               RefinementCase<2>::cut_x) ?
                                1 :
                                3), // 1
                            hex->face(4)->child(0)->line(
                              (hex->face(4)->refinement_case() ==
                               RefinementCase<2>::cut_x) ?
                                1 :
                                3), // 2
                            hex->face(5)->child(0)->line(
                              (hex->face(5)->refinement_case() ==
                               RefinementCase<2>::cut_x) ?
                                1 :
                                3), // 3
 
                            hex->face(2)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[2], f_fl[2], f_ro[2]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  3, f_or[2], f_fl[2], f_ro[2])), // 4
                            hex->face(2)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[2], f_fl[2], f_ro[2]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  2, f_or[2], f_fl[2], f_ro[2])), // 5
                            hex->face(2)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[2], f_fl[2], f_ro[2]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  1, f_or[2], f_fl[2], f_ro[2])), // 6
                            hex->face(2)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[2], f_fl[2], f_ro[2]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  0, f_or[2], f_fl[2], f_ro[2])), // 7
 
                            hex->face(3)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[3], f_fl[3], f_ro[3]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  3, f_or[3], f_fl[3], f_ro[3])), // 8
                            hex->face(3)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[3], f_fl[3], f_ro[3]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  2, f_or[3], f_fl[3], f_ro[3])), // 9
                            hex->face(3)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[3], f_fl[3], f_ro[3]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  1, f_or[3], f_fl[3], f_ro[3])), // 10
                            hex->face(3)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[3], f_fl[3], f_ro[3]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  0, f_or[3], f_fl[3], f_ro[3])), // 11
 
                            new_lines[0] // 12
                          };
 
                          int line_indices[13];
                          for (unsigned int i = 0; i < 13; ++i)
                            line_indices[i] = lines[i]->index();
 
                          // the orientation of lines for the inner quads
                          // is quite tricky. as these lines are newly
                          // created ones and thus have no parents, they
                          // cannot inherit this property. set up an array
                          // and fill it with the respective values
                          types::geometric_orientation line_orientation[13]{};
 
                          // the middle vertices of the
                          // lines of our front face
                          const unsigned int middle_vertices[4] = {
                            hex->line(8)->child(0)->vertex_index(1),
                            hex->line(9)->child(0)->vertex_index(1),
                            hex->line(2)->child(0)->vertex_index(1),
                            hex->line(6)->child(0)->vertex_index(1),
                          };
 
                          // note: for lines 0 to 3 the orientation of the
                          // line is 'true', if vertex 0 is on the front
                          for (unsigned int i = 0; i < 4; ++i)
                            if (lines[i]->vertex_index(0) == middle_vertices[i])
                              line_orientation[i] =
                                numbers::default_geometric_orientation;
                            else
                              {
                                // it must be the other way round then
                                Assert(lines[i]->vertex_index(1) ==
                                         middle_vertices[i],
                                       ExcInternalError());
                                line_orientation[i] =
                                  numbers::reverse_line_orientation;
                              }
 
                          // note: for lines 4 to 11 (inner lines of the
                          // outer quads) the following holds: the second
                          // vertex of the even lines in standard
                          // orientation is the vertex in the middle of
                          // the quad, whereas for odd lines the first
                          // vertex is the same middle vertex.
                          for (unsigned int i = 4; i < 12; ++i)
                            if (lines[i]->vertex_index((i + 1) % 2) ==
                                middle_vertex_index<dim, spacedim>(
                                  hex->face(1 + i / 4)))
                              line_orientation[i] =
                                numbers::default_geometric_orientation;
                            else
                              {
                                // it must be the other way
                                // round then
                                Assert(lines[i]->vertex_index(i % 2) ==
                                         (middle_vertex_index<dim, spacedim>(
                                           hex->face(1 + i / 4))),
                                       ExcInternalError());
                                line_orientation[i] =
                                  numbers::reverse_line_orientation;
                              }
                          // for the last line the line orientation is
                          // always true, since it was just constructed
                          // that way
                          line_orientation[12] =
                            numbers::default_geometric_orientation;
 
                          // set up the 4 quads, numbered as follows (left
                          // quad numbering, right line numbering
                          // extracted from above), the drawings denote
                          // middle planes
                          //
                          //      *          *
                          //     /|         /|
                          //    / |        3 9
                          //  y/  *       /  *
                          //  * 3/|      *  /|
                          //  | / |x     5 12|8
                          //  |/  *      |/  *
                          //  * 2/       *  /
                          //  | /        4 2
                          //  |/         |/
                          //  *          *
                          //
                          //       y
                          //      *----*----*      *-10-*-11-*
                          //     /    /    /      /    /    /
                          //    / 0  /  1 /      0    12   1
                          //   /    /    /      /    /    /
                          //  *----*----*x     *--6-*--7-*
 
                          new_quads[0]->set_bounding_object_indices(
                            {line_indices[0],
                             line_indices[12],
                             line_indices[6],
                             line_indices[10]});
                          new_quads[1]->set_bounding_object_indices(
                            {line_indices[12],
                             line_indices[1],
                             line_indices[7],
                             line_indices[11]});
                          new_quads[2]->set_bounding_object_indices(
                            {line_indices[4],
                             line_indices[8],
                             line_indices[2],
                             line_indices[12]});
                          new_quads[3]->set_bounding_object_indices(
                            {line_indices[5],
                             line_indices[9],
                             line_indices[12],
                             line_indices[3]});
 
                          new_quads[0]->set_line_orientation(
                            0, line_orientation[0]);
                          new_quads[0]->set_line_orientation(
                            2, line_orientation[6]);
                          new_quads[0]->set_line_orientation(
                            3, line_orientation[10]);
 
                          new_quads[1]->set_line_orientation(
                            1, line_orientation[1]);
                          new_quads[1]->set_line_orientation(
                            2, line_orientation[7]);
                          new_quads[1]->set_line_orientation(
                            3, line_orientation[11]);
 
                          new_quads[2]->set_line_orientation(
                            0, line_orientation[4]);
                          new_quads[2]->set_line_orientation(
                            1, line_orientation[8]);
                          new_quads[2]->set_line_orientation(
                            2, line_orientation[2]);
 
                          new_quads[3]->set_line_orientation(
                            0, line_orientation[5]);
                          new_quads[3]->set_line_orientation(
                            1, line_orientation[9]);
                          new_quads[3]->set_line_orientation(
                            3, line_orientation[3]);
 
                          // the quads are numbered as follows:
                          //
                          // planes in the interior of the old hex:
                          //
                          //      *
                          //     /|
                          //    / | x
                          //   /3 * *-------*      *----*----*
                          //  *  /| |       |     /    /    /
                          //  | / | |       |    /  0 /  1 /
                          //  |/  * |       |   /    /    /
                          //  * 2/  *-------*y *----*----*x
                          //  | /
                          //  |/
                          //  *
                          //
                          // children of the faces
                          // of the old hex
                          //      *---*---*        *---*---*
                          //     /|13 | 15|       /   /   /|
                          //    / |   |   |      /18 / 19/ |
                          //   /  *---*---*     /   /   /  *
                          //  * 5/|   |   |    *---*---* 7/|
                          //  | / |12 | 14|    | 9 | 11| / |
                          //  |/4 *---*---*    |   |   |/6 *
                          //  *  /   /   /     *---*---*  /
                          //  | /16 / 17/      |   |   | /
                          //  |/   /   /       | 8 | 10|/
                          //  *---*---*        *---*---*
                          //
                          // note that we have to take care of the
                          // orientation of faces.
                          const int quad_indices[20] = {
                            new_quads[0]->index(), // 0
                            new_quads[1]->index(),
                            new_quads[2]->index(),
                            new_quads[3]->index(),
 
                            hex->face(0)->child_index(
                              child_at_origin[hex->face(0)->refinement_case() -
                                              1][f_fl[0]][f_ro[0]]), // 4
                            hex->face(0)->child_index(
                              1 -
                              child_at_origin[hex->face(0)->refinement_case() -
                                              1][f_fl[0]][f_ro[0]]),
 
                            hex->face(1)->child_index(
                              child_at_origin[hex->face(1)->refinement_case() -
                                              1][f_fl[1]][f_ro[1]]), // 6
                            hex->face(1)->child_index(
                              1 -
                              child_at_origin[hex->face(1)->refinement_case() -
                                              1][f_fl[1]][f_ro[1]]),
 
                            hex->face(2)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                0, f_or[2], f_fl[2], f_ro[2])), // 8
                            hex->face(2)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                1, f_or[2], f_fl[2], f_ro[2])),
                            hex->face(2)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                2, f_or[2], f_fl[2], f_ro[2])),
                            hex->face(2)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                3, f_or[2], f_fl[2], f_ro[2])),
 
                            hex->face(3)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                0, f_or[3], f_fl[3], f_ro[3])), // 12
                            hex->face(3)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                1, f_or[3], f_fl[3], f_ro[3])),
                            hex->face(3)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                2, f_or[3], f_fl[3], f_ro[3])),
                            hex->face(3)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                3, f_or[3], f_fl[3], f_ro[3])),
 
                            hex->face(4)->child_index(
                              child_at_origin[hex->face(4)->refinement_case() -
                                              1][f_fl[4]][f_ro[4]]), // 16
                            hex->face(4)->child_index(
                              1 -
                              child_at_origin[hex->face(4)->refinement_case() -
                                              1][f_fl[4]][f_ro[4]]),
 
                            hex->face(5)->child_index(
                              child_at_origin[hex->face(5)->refinement_case() -
                                              1][f_fl[5]][f_ro[5]]), // 18
                            hex->face(5)->child_index(
                              1 -
                              child_at_origin[hex->face(5)->refinement_case() -
                                              1][f_fl[5]][f_ro[5]])};
 
                          // due to the exchange of x and y for the front
                          // and back face, we order the children
                          // according to
                          //
                          // *---*---*
                          // | 1 | 3 |
                          // *---*---*
                          // | 0 | 2 |
                          // *---*---*
                          new_hexes[0]->set_bounding_object_indices(
                            {quad_indices[4],
                             quad_indices[2],
                             quad_indices[8],
                             quad_indices[12],
                             quad_indices[16],
                             quad_indices[0]});
                          new_hexes[1]->set_bounding_object_indices(
                            {quad_indices[5],
                             quad_indices[3],
                             quad_indices[9],
                             quad_indices[13],
                             quad_indices[0],
                             quad_indices[18]});
                          new_hexes[2]->set_bounding_object_indices(
                            {quad_indices[2],
                             quad_indices[6],
                             quad_indices[10],
                             quad_indices[14],
                             quad_indices[17],
                             quad_indices[1]});
                          new_hexes[3]->set_bounding_object_indices(
                            {quad_indices[3],
                             quad_indices[7],
                             quad_indices[11],
                             quad_indices[15],
                             quad_indices[1],
                             quad_indices[19]});
                          break;
                        }
 
                      case RefinementCase<dim>::cut_yz:
                        {
                          //----------------------------
                          //
                          //     RefinementCase<dim>::cut_yz
                          //
                          // the refined cube will look like this:
                          //
                          //        *---------*
                          //       /         /|
                          //      *---------* |
                          //     /         /| |
                          //    *---------* |/|
                          //    |         | * |
                          //    |         |/| *
                          //    *---------* |/
                          //    |         | *
                          //    |         |/
                          //    *---------*
                          //
 
                          // first, create the new
                          // internal line
                          new_lines[0]->set_bounding_object_indices(
 
                            {middle_vertex_index<dim, spacedim>(hex->face(0)),
                             middle_vertex_index<dim, spacedim>(hex->face(1))});
 
                          // again, first collect some data about the
                          // indices of the lines, with the following
                          // numbering: (note that face 0 and 1 each are
                          // shown twice for better readability)
 
                          // face 0: left plane
                          //       *            *
                          //      /|           /|
                          //     * |          * |
                          //    /| *         /| *
                          //   * 5/|        * |7|
                          //   | * |        | * |
                          //   |/| *        |6| *
                          //   * 4/         * |/
                          //   | *          | *
                          //   |/           |/
                          //   *            *
                          // face 1: right plane
                          //       *            *
                          //      /|           /|
                          //     * |          * |
                          //    /| *         /| *
                          //   * 9/|        * |11
                          //   | * |        | * |
                          //   |/| *        |10 *
                          //   * 8/         * |/
                          //   | *          | *
                          //   |/           |/
                          //   *            *
                          // face 2: front plane
                          //   (note: x,y exchanged)
                          //   *-------*
                          //   |       |
                          //   *---0---*
                          //   |       |
                          //   *-------*
                          // face 3: back plane
                          //   (note: x,y exchanged)
                          //   *-------*
                          //   |       |
                          //   *---1---*
                          //   |       |
                          //   *-------*
                          // face 4: bottom plane
                          //       *-------*
                          //      /       /
                          //     *---2---*
                          //    /       /
                          //   *-------*
                          // face 5: top plane
                          //       *-------*
                          //      /       /
                          //     *---3---*
                          //    /       /
                          //   *-------*
                          // middle planes
                          //     *-------*   *-------*
                          //    /       /    |       |
                          //   *---12--*     |       |
                          //  /       /      |       |
                          // *-------*       *-------*
 
                          // set up a list of line iterators first. from
                          // this, construct lists of line_indices and
                          // line orientations later on
                          const typename Triangulation<
                            dim,
                            spacedim>::raw_line_iterator lines[13] = {
                            hex->face(2)->child(0)->line(
                              (hex->face(2)->refinement_case() ==
                               RefinementCase<2>::cut_x) ?
                                1 :
                                3), // 0
                            hex->face(3)->child(0)->line(
                              (hex->face(3)->refinement_case() ==
                               RefinementCase<2>::cut_x) ?
                                1 :
                                3), // 1
                            hex->face(4)->child(0)->line(
                              (hex->face(4)->refinement_case() ==
                               RefinementCase<2>::cut_x) ?
                                1 :
                                3), // 2
                            hex->face(5)->child(0)->line(
                              (hex->face(5)->refinement_case() ==
                               RefinementCase<2>::cut_x) ?
                                1 :
                                3), // 3
 
                            hex->face(0)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[0], f_fl[0], f_ro[0]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  1, f_or[0], f_fl[0], f_ro[0])), // 4
                            hex->face(0)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[0], f_fl[0], f_ro[0]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  0, f_or[0], f_fl[0], f_ro[0])), // 5
                            hex->face(0)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[0], f_fl[0], f_ro[0]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  3, f_or[0], f_fl[0], f_ro[0])), // 6
                            hex->face(0)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[0], f_fl[0], f_ro[0]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  2, f_or[0], f_fl[0], f_ro[0])), // 7
 
                            hex->face(1)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[1], f_fl[1], f_ro[1]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  1, f_or[1], f_fl[1], f_ro[1])), // 8
                            hex->face(1)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[1], f_fl[1], f_ro[1]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  0, f_or[1], f_fl[1], f_ro[1])), // 9
                            hex->face(1)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[1], f_fl[1], f_ro[1]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  3, f_or[1], f_fl[1], f_ro[1])), // 10
                            hex->face(1)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[1], f_fl[1], f_ro[1]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  2, f_or[1], f_fl[1], f_ro[1])), // 11
 
                            new_lines[0] // 12
                          };
 
                          int line_indices[13];
                          for (unsigned int i = 0; i < 13; ++i)
                            line_indices[i] = lines[i]->index();
 
                          // the orientation of lines for the inner quads
                          // is quite tricky. as these lines are newly
                          // created ones and thus have no parents, they
                          // cannot inherit this property. set up an array
                          // and fill it with the respective values
                          types::geometric_orientation line_orientation[13]{};
 
                          // the middle vertices of the lines of our front
                          // face
                          const unsigned int middle_vertices[4] = {
                            hex->line(8)->child(0)->vertex_index(1),
                            hex->line(10)->child(0)->vertex_index(1),
                            hex->line(0)->child(0)->vertex_index(1),
                            hex->line(4)->child(0)->vertex_index(1),
                          };
 
                          // note: for lines 0 to 3 the orientation of the
                          // line is 'true', if vertex 0 is on the front
                          for (unsigned int i = 0; i < 4; ++i)
                            if (lines[i]->vertex_index(0) == middle_vertices[i])
                              line_orientation[i] =
                                numbers::default_geometric_orientation;
                            else
                              {
                                // it must be the other way round then
                                Assert(lines[i]->vertex_index(1) ==
                                         middle_vertices[i],
                                       ExcInternalError());
                                line_orientation[i] =
                                  numbers::reverse_line_orientation;
                              }
 
                          // note: for lines 4 to 11 (inner lines of the
                          // outer quads) the following holds: the second
                          // vertex of the even lines in standard
                          // orientation is the vertex in the middle of
                          // the quad, whereas for odd lines the first
                          // vertex is the same middle vertex.
                          for (unsigned int i = 4; i < 12; ++i)
                            if (lines[i]->vertex_index((i + 1) % 2) ==
                                middle_vertex_index<dim, spacedim>(
                                  hex->face(i / 4 - 1)))
                              line_orientation[i] =
                                numbers::default_geometric_orientation;
                            else
                              {
                                // it must be the other way round then
                                Assert(lines[i]->vertex_index(i % 2) ==
                                         (middle_vertex_index<dim, spacedim>(
                                           hex->face(i / 4 - 1))),
                                       ExcInternalError());
                                line_orientation[i] =
                                  numbers::reverse_line_orientation;
                              }
                          // for the last line the line orientation is always
                          // the default, since it was just constructed that way
                          line_orientation[12] =
                            numbers::default_geometric_orientation;
 
                          // set up the 4 quads, numbered as follows (left
                          // quad numbering, right line numbering
                          // extracted from above)
                          //
                          //  x
                          //  *-------*      *---3---*
                          //  |   3   |      5       9
                          //  *-------*      *---12--*
                          //  |   2   |      4       8
                          //  *-------*y     *---2---*
                          //
                          //       y
                          //      *---------*      *----1----*
                          //     /    1    /      7         11
                          //    *---------*      *----12---*
                          //   /    0    /      6         10
                          //  *---------*x     *----0----*
 
                          new_quads[0]->set_bounding_object_indices(
                            {line_indices[6],
                             line_indices[10],
                             line_indices[0],
                             line_indices[12]});
                          new_quads[1]->set_bounding_object_indices(
                            {line_indices[7],
                             line_indices[11],
                             line_indices[12],
                             line_indices[1]});
                          new_quads[2]->set_bounding_object_indices(
                            {line_indices[2],
                             line_indices[12],
                             line_indices[4],
                             line_indices[8]});
                          new_quads[3]->set_bounding_object_indices(
                            {line_indices[12],
                             line_indices[3],
                             line_indices[5],
                             line_indices[9]});
 
                          new_quads[0]->set_line_orientation(
                            0, line_orientation[6]);
                          new_quads[0]->set_line_orientation(
                            1, line_orientation[10]);
                          new_quads[0]->set_line_orientation(
                            2, line_orientation[0]);
 
                          new_quads[1]->set_line_orientation(
                            0, line_orientation[7]);
                          new_quads[1]->set_line_orientation(
                            1, line_orientation[11]);
                          new_quads[1]->set_line_orientation(
                            3, line_orientation[1]);
 
                          new_quads[2]->set_line_orientation(
                            0, line_orientation[2]);
                          new_quads[2]->set_line_orientation(
                            2, line_orientation[4]);
                          new_quads[2]->set_line_orientation(
                            3, line_orientation[8]);
 
                          new_quads[3]->set_line_orientation(
                            1, line_orientation[3]);
                          new_quads[3]->set_line_orientation(
                            2, line_orientation[5]);
                          new_quads[3]->set_line_orientation(
                            3, line_orientation[9]);
 
                          // the quads are numbered as follows:
                          //
                          // planes in the interior of the old hex:
                          //
                          //      *
                          //     /|
                          //    / | x
                          //   /  | *-------*      *---------*
                          //  *   | |   3   |     /    1    /
                          //  |   | *-------*    *---------*
                          //  |   * |   2   |   /    0    /
                          //  |  /  *-------*y *---------*x
                          //  | /
                          //  |/
                          //  *
                          //
                          // children of the faces
                          // of the old hex
                          //      *-------*        *-------*
                          //     /|       |       /  19   /|
                          //    * |  15   |      *-------* |
                          //   /|7*-------*     /  18   /|11
                          //  * |/|       |    *-------* |/|
                          //  |6* |  14   |    |       10* |
                          //  |/|5*-------*    |  13   |/|9*
                          //  * |/  17   /     *-------* |/
                          //  |4*-------*      |       |8*
                          //  |/  16   /       |  12   |/
                          //  *-------*        *-------*
                          //
                          // note that we have to take care of the
                          // orientation of faces.
                          const int quad_indices[20] = {
                            new_quads[0]->index(), // 0
                            new_quads[1]->index(),
                            new_quads[2]->index(),
                            new_quads[3]->index(),
 
                            hex->face(0)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                0, f_or[0], f_fl[0], f_ro[0])), // 4
                            hex->face(0)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                1, f_or[0], f_fl[0], f_ro[0])),
                            hex->face(0)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                2, f_or[0], f_fl[0], f_ro[0])),
                            hex->face(0)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                3, f_or[0], f_fl[0], f_ro[0])),
 
                            hex->face(1)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                0, f_or[1], f_fl[1], f_ro[1])), // 8
                            hex->face(1)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                1, f_or[1], f_fl[1], f_ro[1])),
                            hex->face(1)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                2, f_or[1], f_fl[1], f_ro[1])),
                            hex->face(1)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                3, f_or[1], f_fl[1], f_ro[1])),
 
                            hex->face(2)->child_index(
                              child_at_origin[hex->face(2)->refinement_case() -
                                              1][f_fl[2]][f_ro[2]]), // 12
                            hex->face(2)->child_index(
                              1 -
                              child_at_origin[hex->face(2)->refinement_case() -
                                              1][f_fl[2]][f_ro[2]]),
 
                            hex->face(3)->child_index(
                              child_at_origin[hex->face(3)->refinement_case() -
                                              1][f_fl[3]][f_ro[3]]), // 14
                            hex->face(3)->child_index(
                              1 -
                              child_at_origin[hex->face(3)->refinement_case() -
                                              1][f_fl[3]][f_ro[3]]),
 
                            hex->face(4)->child_index(
                              child_at_origin[hex->face(4)->refinement_case() -
                                              1][f_fl[4]][f_ro[4]]), // 16
                            hex->face(4)->child_index(
                              1 -
                              child_at_origin[hex->face(4)->refinement_case() -
                                              1][f_fl[4]][f_ro[4]]),
 
                            hex->face(5)->child_index(
                              child_at_origin[hex->face(5)->refinement_case() -
                                              1][f_fl[5]][f_ro[5]]), // 18
                            hex->face(5)->child_index(
                              1 -
                              child_at_origin[hex->face(5)->refinement_case() -
                                              1][f_fl[5]][f_ro[5]])};
 
                          new_hexes[0]->set_bounding_object_indices(
                            {quad_indices[4],
                             quad_indices[8],
                             quad_indices[12],
                             quad_indices[2],
                             quad_indices[16],
                             quad_indices[0]});
                          new_hexes[1]->set_bounding_object_indices(
                            {quad_indices[5],
                             quad_indices[9],
                             quad_indices[2],
                             quad_indices[14],
                             quad_indices[17],
                             quad_indices[1]});
                          new_hexes[2]->set_bounding_object_indices(
                            {quad_indices[6],
                             quad_indices[10],
                             quad_indices[13],
                             quad_indices[3],
                             quad_indices[0],
                             quad_indices[18]});
                          new_hexes[3]->set_bounding_object_indices(
                            {quad_indices[7],
                             quad_indices[11],
                             quad_indices[3],
                             quad_indices[15],
                             quad_indices[1],
                             quad_indices[19]});
                          break;
                        }
 
                      case RefinementCase<dim>::cut_xyz:
                        {
                          //----------------------------
                          //
                          //     RefinementCase<dim>::cut_xyz
                          //     isotropic refinement
                          //
                          // the refined cube will look
                          // like this:
                          //
                          //        *----*----*
                          //       /    /    /|
                          //      *----*----* |
                          //     /    /    /| *
                          //    *----*----* |/|
                          //    |    |    | * |
                          //    |    |    |/| *
                          //    *----*----* |/
                          //    |    |    | *
                          //    |    |    |/
                          //    *----*----*
                          //
 
                          // find the next unused vertex and set it
                          // appropriately
                          while (
                            triangulation.vertices_used[next_unused_vertex] ==
                            true)
                            ++next_unused_vertex;
                          Assert(
                            next_unused_vertex < triangulation.vertices.size(),
                            ExcMessage(
                              "Internal error: During refinement, the triangulation wants to access an element of the 'vertices' array but it turns out that the array is not large enough."));
                          triangulation.vertices_used[next_unused_vertex] =
                            true;
 
                          // the new vertex is definitely in the interior,
                          // so we need not worry about the
                          // boundary. However we need to worry about
                          // Manifolds. Let the cell compute its own
                          // center, by querying the underlying manifold
                          // object.
                          triangulation.vertices[next_unused_vertex] =
                            hex->center(true, true);
 
                          // set the data of the six lines.  first collect
                          // the indices of the seven vertices (consider
                          // the two planes to be crossed to form the
                          // planes cutting the hex in two vertically and
                          // horizontally)
                          //
                          //     *--3--*   *--5--*
                          //    /  /  /    |  |  |
                          //   0--6--1     0--6--1
                          //  /  /  /      |  |  |
                          // *--2--*       *--4--*
                          // the lines are numbered
                          // as follows:
                          //     *--*--*   *--*--*
                          //    /  1  /    |  5  |
                          //   *2-*-3*     *2-*-3*
                          //  /  0  /      |  4  |
                          // *--*--*       *--*--*
                          //
                          const unsigned int vertex_indices[7] = {
                            middle_vertex_index<dim, spacedim>(hex->face(0)),
                            middle_vertex_index<dim, spacedim>(hex->face(1)),
                            middle_vertex_index<dim, spacedim>(hex->face(2)),
                            middle_vertex_index<dim, spacedim>(hex->face(3)),
                            middle_vertex_index<dim, spacedim>(hex->face(4)),
                            middle_vertex_index<dim, spacedim>(hex->face(5)),
                            next_unused_vertex};
 
                          new_lines[0]->set_bounding_object_indices(
                            {vertex_indices[2], vertex_indices[6]});
                          new_lines[1]->set_bounding_object_indices(
                            {vertex_indices[6], vertex_indices[3]});
                          new_lines[2]->set_bounding_object_indices(
                            {vertex_indices[0], vertex_indices[6]});
                          new_lines[3]->set_bounding_object_indices(
                            {vertex_indices[6], vertex_indices[1]});
                          new_lines[4]->set_bounding_object_indices(
                            {vertex_indices[4], vertex_indices[6]});
                          new_lines[5]->set_bounding_object_indices(
                            {vertex_indices[6], vertex_indices[5]});
 
                          // again, first collect some data about the
                          // indices of the lines, with the following
                          // numbering: (note that face 0 and 1 each are
                          // shown twice for better readability)
 
                          // face 0: left plane
                          //       *            *
                          //      /|           /|
                          //     * |          * |
                          //    /| *         /| *
                          //   * 1/|        * |3|
                          //   | * |        | * |
                          //   |/| *        |2| *
                          //   * 0/         * |/
                          //   | *          | *
                          //   |/           |/
                          //   *            *
                          // face 1: right plane
                          //       *            *
                          //      /|           /|
                          //     * |          * |
                          //    /| *         /| *
                          //   * 5/|        * |7|
                          //   | * |        | * |
                          //   |/| *        |6| *
                          //   * 4/         * |/
                          //   | *          | *
                          //   |/           |/
                          //   *            *
                          // face 2: front plane
                          //   (note: x,y exchanged)
                          //   *---*---*
                          //   |   11  |
                          //   *-8-*-9-*
                          //   |   10  |
                          //   *---*---*
                          // face 3: back plane
                          //   (note: x,y exchanged)
                          //   *---*---*
                          //   |   15  |
                          //   *12-*-13*
                          //   |   14  |
                          //   *---*---*
                          // face 4: bottom plane
                          //       *---*---*
                          //      /  17   /
                          //     *18-*-19*
                          //    /   16  /
                          //   *---*---*
                          // face 5: top plane
                          //       *---*---*
                          //      /  21   /
                          //     *22-*-23*
                          //    /   20  /
                          //   *---*---*
                          // middle planes
                          //     *---*---*   *---*---*
                          //    /  25   /    |   29  |
                          //   *26-*-27*     *26-*-27*
                          //  /   24  /      |   28  |
                          // *---*---*       *---*---*
 
                          // set up a list of line iterators first. from
                          // this, construct lists of line_indices and
                          // line orientations later on
                          const typename Triangulation<
                            dim,
                            spacedim>::raw_line_iterator lines[30] = {
                            hex->face(0)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[0], f_fl[0], f_ro[0]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  1, f_or[0], f_fl[0], f_ro[0])), // 0
                            hex->face(0)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[0], f_fl[0], f_ro[0]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  0, f_or[0], f_fl[0], f_ro[0])), // 1
                            hex->face(0)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[0], f_fl[0], f_ro[0]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  3, f_or[0], f_fl[0], f_ro[0])), // 2
                            hex->face(0)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[0], f_fl[0], f_ro[0]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  2, f_or[0], f_fl[0], f_ro[0])), // 3
 
                            hex->face(1)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[1], f_fl[1], f_ro[1]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  1, f_or[1], f_fl[1], f_ro[1])), // 4
                            hex->face(1)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[1], f_fl[1], f_ro[1]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  0, f_or[1], f_fl[1], f_ro[1])), // 5
                            hex->face(1)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[1], f_fl[1], f_ro[1]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  3, f_or[1], f_fl[1], f_ro[1])), // 6
                            hex->face(1)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[1], f_fl[1], f_ro[1]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  2, f_or[1], f_fl[1], f_ro[1])), // 7
 
                            hex->face(2)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[2], f_fl[2], f_ro[2]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  1, f_or[2], f_fl[2], f_ro[2])), // 8
                            hex->face(2)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[2], f_fl[2], f_ro[2]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  0, f_or[2], f_fl[2], f_ro[2])), // 9
                            hex->face(2)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[2], f_fl[2], f_ro[2]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  3, f_or[2], f_fl[2], f_ro[2])), // 10
                            hex->face(2)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[2], f_fl[2], f_ro[2]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  2, f_or[2], f_fl[2], f_ro[2])), // 11
 
                            hex->face(3)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[3], f_fl[3], f_ro[3]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  1, f_or[3], f_fl[3], f_ro[3])), // 12
                            hex->face(3)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[3], f_fl[3], f_ro[3]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  0, f_or[3], f_fl[3], f_ro[3])), // 13
                            hex->face(3)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[3], f_fl[3], f_ro[3]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  3, f_or[3], f_fl[3], f_ro[3])), // 14
                            hex->face(3)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[3], f_fl[3], f_ro[3]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  2, f_or[3], f_fl[3], f_ro[3])), // 15
 
                            hex->face(4)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[4], f_fl[4], f_ro[4]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  1, f_or[4], f_fl[4], f_ro[4])), // 16
                            hex->face(4)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[4], f_fl[4], f_ro[4]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  0, f_or[4], f_fl[4], f_ro[4])), // 17
                            hex->face(4)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[4], f_fl[4], f_ro[4]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  3, f_or[4], f_fl[4], f_ro[4])), // 18
                            hex->face(4)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[4], f_fl[4], f_ro[4]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  2, f_or[4], f_fl[4], f_ro[4])), // 19
 
                            hex->face(5)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[5], f_fl[5], f_ro[5]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  1, f_or[5], f_fl[5], f_ro[5])), // 20
                            hex->face(5)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[5], f_fl[5], f_ro[5]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  0, f_or[5], f_fl[5], f_ro[5])), // 21
                            hex->face(5)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  0, f_or[5], f_fl[5], f_ro[5]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  3, f_or[5], f_fl[5], f_ro[5])), // 22
                            hex->face(5)
                              ->isotropic_child(
                                GeometryInfo<dim>::standard_to_real_face_vertex(
                                  3, f_or[5], f_fl[5], f_ro[5]))
                              ->line(
                                GeometryInfo<dim>::standard_to_real_face_line(
                                  2, f_or[5], f_fl[5], f_ro[5])), // 23
 
                            new_lines[0], // 24
                            new_lines[1], // 25
                            new_lines[2], // 26
                            new_lines[3], // 27
                            new_lines[4], // 28
                            new_lines[5]  // 29
                          };
 
                          int line_indices[30];
                          for (unsigned int i = 0; i < 30; ++i)
                            line_indices[i] = lines[i]->index();
 
                          // the orientation of lines for the inner quads
                          // is quite tricky. as these lines are newly
                          // created ones and thus have no parents, they
                          // cannot inherit this property. set up an array
                          // and fill it with the respective values
                          types::geometric_orientation line_orientation[30]{};
 
                          // note: for the first 24 lines (inner lines of
                          // the outer quads) the following holds: the
                          // second vertex of the even lines in standard
                          // orientation is the vertex in the middle of
                          // the quad, whereas for odd lines the first
                          // vertex is the same middle vertex.
                          for (unsigned int i = 0; i < 24; ++i)
                            if (lines[i]->vertex_index((i + 1) % 2) ==
                                vertex_indices[i / 4])
                              line_orientation[i] =
                                numbers::default_geometric_orientation;
                            else
                              {
                                // it must be the other way
                                // round then
                                Assert(lines[i]->vertex_index(i % 2) ==
                                         vertex_indices[i / 4],
                                       ExcInternalError());
                                line_orientation[i] =
                                  numbers::reverse_line_orientation;
                              }
                          // for the last 6 lines the line orientation is
                          // always true, since they were just constructed
                          // that way
                          for (unsigned int i = 24; i < 30; ++i)
                            line_orientation[i] =
                              numbers::default_geometric_orientation;
 
                          // set up the 12 quads, numbered as follows
                          // (left quad numbering, right line numbering
                          // extracted from above)
                          //
                          //      *          *
                          //     /|        21|
                          //    * |        * 15
                          //  y/|3*      20| *
                          //  * |/|      * |/|
                          //  |2* |x    11 * 14
                          //  |/|1*      |/| *
                          //  * |/       * |17
                          //  |0*       10 *
                          //  |/         |16
                          //  *          *
                          //
                          //  x
                          //  *---*---*      *22-*-23*
                          //  | 5 | 7 |      1  29   5
                          //  *---*---*      *26-*-27*
                          //  | 4 | 6 |      0  28   4
                          //  *---*---*y     *18-*-19*
                          //
                          //       y
                          //      *----*----*      *-12-*-13-*
                          //     / 10 / 11 /      3   25    7
                          //    *----*----*      *-26-*-27-*
                          //   / 8  / 9  /      2   24    6
                          //  *----*----*x     *--8-*--9-*
 
                          new_quads[0]->set_bounding_object_indices(
                            {line_indices[10],
                             line_indices[28],
                             line_indices[16],
                             line_indices[24]});
                          new_quads[1]->set_bounding_object_indices(
                            {line_indices[28],
                             line_indices[14],
                             line_indices[17],
                             line_indices[25]});
                          new_quads[2]->set_bounding_object_indices(
                            {line_indices[11],
                             line_indices[29],
                             line_indices[24],
                             line_indices[20]});
                          new_quads[3]->set_bounding_object_indices(
                            {line_indices[29],
                             line_indices[15],
                             line_indices[25],
                             line_indices[21]});
                          new_quads[4]->set_bounding_object_indices(
                            {line_indices[18],
                             line_indices[26],
                             line_indices[0],
                             line_indices[28]});
                          new_quads[5]->set_bounding_object_indices(
                            {line_indices[26],
                             line_indices[22],
                             line_indices[1],
                             line_indices[29]});
                          new_quads[6]->set_bounding_object_indices(
                            {line_indices[19],
                             line_indices[27],
                             line_indices[28],
                             line_indices[4]});
                          new_quads[7]->set_bounding_object_indices(
                            {line_indices[27],
                             line_indices[23],
                             line_indices[29],
                             line_indices[5]});
                          new_quads[8]->set_bounding_object_indices(
                            {line_indices[2],
                             line_indices[24],
                             line_indices[8],
                             line_indices[26]});
                          new_quads[9]->set_bounding_object_indices(
                            {line_indices[24],
                             line_indices[6],
                             line_indices[9],
                             line_indices[27]});
                          new_quads[10]->set_bounding_object_indices(
                            {line_indices[3],
                             line_indices[25],
                             line_indices[26],
                             line_indices[12]});
                          new_quads[11]->set_bounding_object_indices(
                            {line_indices[25],
                             line_indices[7],
                             line_indices[27],
                             line_indices[13]});
 
                          // now reset the line_orientation flags of outer
                          // lines as they cannot be set in a loop (at
                          // least not easily)
                          new_quads[0]->set_line_orientation(
                            0, line_orientation[10]);
                          new_quads[0]->set_line_orientation(
                            2, line_orientation[16]);
 
                          new_quads[1]->set_line_orientation(
                            1, line_orientation[14]);
                          new_quads[1]->set_line_orientation(
                            2, line_orientation[17]);
 
                          new_quads[2]->set_line_orientation(
                            0, line_orientation[11]);
                          new_quads[2]->set_line_orientation(
                            3, line_orientation[20]);
 
                          new_quads[3]->set_line_orientation(
                            1, line_orientation[15]);
                          new_quads[3]->set_line_orientation(
                            3, line_orientation[21]);
 
                          new_quads[4]->set_line_orientation(
                            0, line_orientation[18]);
                          new_quads[4]->set_line_orientation(
                            2, line_orientation[0]);
 
                          new_quads[5]->set_line_orientation(
                            1, line_orientation[22]);
                          new_quads[5]->set_line_orientation(
                            2, line_orientation[1]);
 
                          new_quads[6]->set_line_orientation(
                            0, line_orientation[19]);
                          new_quads[6]->set_line_orientation(
                            3, line_orientation[4]);
 
                          new_quads[7]->set_line_orientation(
                            1, line_orientation[23]);
                          new_quads[7]->set_line_orientation(
                            3, line_orientation[5]);
 
                          new_quads[8]->set_line_orientation(
                            0, line_orientation[2]);
                          new_quads[8]->set_line_orientation(
                            2, line_orientation[8]);
 
                          new_quads[9]->set_line_orientation(
                            1, line_orientation[6]);
                          new_quads[9]->set_line_orientation(
                            2, line_orientation[9]);
 
                          new_quads[10]->set_line_orientation(
                            0, line_orientation[3]);
                          new_quads[10]->set_line_orientation(
                            3, line_orientation[12]);
 
                          new_quads[11]->set_line_orientation(
                            1, line_orientation[7]);
                          new_quads[11]->set_line_orientation(
                            3, line_orientation[13]);
 
                          //-------------------------------
                          // create the eight new hexes
                          //
                          // again first collect some data.  here, we need
                          // the indices of a whole lotta quads.
 
                          // the quads are numbered as follows:
                          //
                          // planes in the interior of the old hex:
                          //
                          //      *
                          //     /|
                          //    * |
                          //   /|3*  *---*---*      *----*----*
                          //  * |/|  | 5 | 7 |     / 10 / 11 /
                          //  |2* |  *---*---*    *----*----*
                          //  |/|1*  | 4 | 6 |   / 8  / 9  /
                          //  * |/   *---*---*y *----*----*x
                          //  |0*
                          //  |/
                          //  *
                          //
                          // children of the faces
                          // of the old hex
                          //      *-------*        *-------*
                          //     /|25   27|       /34   35/|
                          //    15|       |      /       /19
                          //   /  |       |     /32   33/  |
                          //  *   |24   26|    *-------*18 |
                          //  1413*-------*    |21   23| 17*
                          //  |  /30   31/     |       |  /
                          //  12/       /      |       |16
                          //  |/28   29/       |20   22|/
                          //  *-------*        *-------*
                          //
                          // note that we have to
                          // take care of the
                          // orientation of
                          // faces.
                          const int quad_indices[36] = {
                            new_quads[0]->index(), // 0
                            new_quads[1]->index(),
                            new_quads[2]->index(),
                            new_quads[3]->index(),
                            new_quads[4]->index(),
                            new_quads[5]->index(),
                            new_quads[6]->index(),
                            new_quads[7]->index(),
                            new_quads[8]->index(),
                            new_quads[9]->index(),
                            new_quads[10]->index(),
                            new_quads[11]->index(), // 11
 
                            hex->face(0)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                0, f_or[0], f_fl[0], f_ro[0])), // 12
                            hex->face(0)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                1, f_or[0], f_fl[0], f_ro[0])),
                            hex->face(0)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                2, f_or[0], f_fl[0], f_ro[0])),
                            hex->face(0)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                3, f_or[0], f_fl[0], f_ro[0])),
 
                            hex->face(1)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                0, f_or[1], f_fl[1], f_ro[1])), // 16
                            hex->face(1)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                1, f_or[1], f_fl[1], f_ro[1])),
                            hex->face(1)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                2, f_or[1], f_fl[1], f_ro[1])),
                            hex->face(1)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                3, f_or[1], f_fl[1], f_ro[1])),
 
                            hex->face(2)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                0, f_or[2], f_fl[2], f_ro[2])), // 20
                            hex->face(2)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                1, f_or[2], f_fl[2], f_ro[2])),
                            hex->face(2)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                2, f_or[2], f_fl[2], f_ro[2])),
                            hex->face(2)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                3, f_or[2], f_fl[2], f_ro[2])),
 
                            hex->face(3)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                0, f_or[3], f_fl[3], f_ro[3])), // 24
                            hex->face(3)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                1, f_or[3], f_fl[3], f_ro[3])),
                            hex->face(3)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                2, f_or[3], f_fl[3], f_ro[3])),
                            hex->face(3)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                3, f_or[3], f_fl[3], f_ro[3])),
 
                            hex->face(4)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                0, f_or[4], f_fl[4], f_ro[4])), // 28
                            hex->face(4)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                1, f_or[4], f_fl[4], f_ro[4])),
                            hex->face(4)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                2, f_or[4], f_fl[4], f_ro[4])),
                            hex->face(4)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                3, f_or[4], f_fl[4], f_ro[4])),
 
                            hex->face(5)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                0, f_or[5], f_fl[5], f_ro[5])), // 32
                            hex->face(5)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                1, f_or[5], f_fl[5], f_ro[5])),
                            hex->face(5)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                2, f_or[5], f_fl[5], f_ro[5])),
                            hex->face(5)->isotropic_child_index(
                              GeometryInfo<dim>::standard_to_real_face_vertex(
                                3, f_or[5], f_fl[5], f_ro[5]))};
 
                          // bottom children
                          new_hexes[0]->set_bounding_object_indices(
                            {quad_indices[12],
                             quad_indices[0],
                             quad_indices[20],
                             quad_indices[4],
                             quad_indices[28],
                             quad_indices[8]});
                          new_hexes[1]->set_bounding_object_indices(
                            {quad_indices[0],
                             quad_indices[16],
                             quad_indices[22],
                             quad_indices[6],
                             quad_indices[29],
                             quad_indices[9]});
                          new_hexes[2]->set_bounding_object_indices(
                            {quad_indices[13],
                             quad_indices[1],
                             quad_indices[4],
                             quad_indices[24],
                             quad_indices[30],
                             quad_indices[10]});
                          new_hexes[3]->set_bounding_object_indices(
                            {quad_indices[1],
                             quad_indices[17],
                             quad_indices[6],
                             quad_indices[26],
                             quad_indices[31],
                             quad_indices[11]});
 
                          // top children
                          new_hexes[4]->set_bounding_object_indices(
                            {quad_indices[14],
                             quad_indices[2],
                             quad_indices[21],
                             quad_indices[5],
                             quad_indices[8],
                             quad_indices[32]});
                          new_hexes[5]->set_bounding_object_indices(
                            {quad_indices[2],
                             quad_indices[18],
                             quad_indices[23],
                             quad_indices[7],
                             quad_indices[9],
                             quad_indices[33]});
                          new_hexes[6]->set_bounding_object_indices(
                            {quad_indices[15],
                             quad_indices[3],
                             quad_indices[5],
                             quad_indices[25],
                             quad_indices[10],
                             quad_indices[34]});
                          new_hexes[7]->set_bounding_object_indices(
                            {quad_indices[3],
                             quad_indices[19],
                             quad_indices[7],
                             quad_indices[27],
                             quad_indices[11],
                             quad_indices[35]});
                          break;
                        }
                      default:
                        // all refinement cases have been treated, there
                        // only remains
                        // RefinementCase<dim>::no_refinement as
                        // untreated enumeration value. However, in that
                        // case we should have aborted much
                        // earlier. thus we should never get here
                        DEAL_II_ASSERT_UNREACHABLE();
                        break;
                    } // switch (ref_case)
 
                  // and set face orientation flags. note that new
                  // faces in the interior of the mother cell always
                  // have a correctly oriented face, but the ones on
                  // the outer faces will inherit this flag
                  //
                  // the flag have been set to true for all faces
                  // initially, now go the other way round and reset
                  // faces that are at the boundary of the mother cube
                  //
                  // the same is true for the face_flip and
                  // face_rotation flags. however, the latter two are
                  // set to false by default as this is the standard
                  // value
 
                  // loop over all faces and all (relevant) subfaces
                  // of that in order to set the correct values for
                  // face_orientation, face_flip and face_rotation,
                  // which are inherited from the corresponding face
                  // of the mother cube
                  for (const unsigned int f : GeometryInfo<dim>::face_indices())
                    for (unsigned int s = 0;
                         s < std::max(GeometryInfo<dim - 1>::n_children(
                                        GeometryInfo<dim>::face_refinement_case(
                                          ref_case, f)),
                                      1U);
                         ++s)
                      {
                        const unsigned int current_child =
                          GeometryInfo<dim>::child_cell_on_face(
                            ref_case,
                            f,
                            s,
                            f_or[f],
                            f_fl[f],
                            f_ro[f],
                            GeometryInfo<dim>::face_refinement_case(
                              ref_case, f, f_or[f], f_fl[f], f_ro[f]));
                        new_hexes[current_child]->set_combined_face_orientation(
                          f, f_co[f]);
                      }
 
                  // now see if we have created cells that are
                  // distorted and if so add them to our list
                  if (check_for_distorted_cells &&
                      has_distorted_children<dim, spacedim>(hex))
                    cells_with_distorted_children.distorted_cells.push_back(
                      hex);
 
                  // note that the refinement flag was already cleared
                  // at the beginning of this loop
 
                  // inform all listeners that cell refinement is done
                  triangulation.signals.post_refinement_on_cell(hex);
                }
          }
 
        // clear user data on quads. we used some of this data to
        // indicate anisotropic refinemnt cases on faces. all data
        // should be cleared by now, but the information whether we
        // used indices or pointers is still present. reset it now to
        // enable the user to use whichever they like later on.
        triangulation.faces->quads.clear_user_data();
 
        // return the list with distorted children
        return cells_with_distorted_children;
      }
 
 
      template <int spacedim>
      static void
      prevent_distorted_boundary_cells(Triangulation<1, spacedim> &)
      {}
 
 
 
      template <int dim, int spacedim>
      static void
      prevent_distorted_boundary_cells(
        Triangulation<dim, spacedim> &triangulation)
      {
        // If the codimension is one, we cannot perform this check
        // yet.
        if (spacedim > dim)
          return;
 
        for (const auto &cell : triangulation.cell_iterators())
          if (cell->at_boundary() && cell->refine_flag_set() &&
              cell->refine_flag_set() !=
                RefinementCase<dim>::isotropic_refinement)
            {
              // The cell is at the boundary and it is flagged for
              // anisotropic refinement. Therefore, we have a closer
              // look
              const RefinementCase<dim> ref_case = cell->refine_flag_set();
              for (const unsigned int face_no :
                   GeometryInfo<dim>::face_indices())
                if (cell->face(face_no)->at_boundary())
                  {
                    // this is the critical face at the boundary.
                    if (GeometryInfo<dim>::face_refinement_case(ref_case,
                                                                face_no) !=
                        RefinementCase<dim - 1>::isotropic_refinement)
                      {
                        // up to now, we do not want to refine this
                        // cell along the face under consideration
                        // here.
                        const typename Triangulation<dim,
                                                     spacedim>::face_iterator
                          face = cell->face(face_no);
                        // the new point on the boundary would be this
                        // one.
                        const Point<spacedim> new_bound = face->center(true);
                        // to check it, transform to the unit cell
                        // with a linear mapping
                        const Point<dim> new_unit =
                          cell->reference_cell()
                            .template get_default_linear_mapping<spacedim>()
                            .transform_real_to_unit_cell(cell, new_bound);
 
                        // Now, we have to calculate the distance from
                        // the face in the unit cell.
 
                        // take the correct coordinate direction (0
                        // for faces 0 and 1, 1 for faces 2 and 3, 2
                        // for faces 4 and 5) and subtract the correct
                        // boundary value of the face (0 for faces 0,
                        // 2, and 4; 1 for faces 1, 3 and 5)
                        const double dist =
                          std::fabs(new_unit[face_no / 2] - face_no % 2);
 
                        // compare this with the empirical value
                        // allowed. if it is too big, flag the face
                        // for isotropic refinement
                        const double allowed = 0.25;
 
                        if (dist > allowed)
                          cell->flag_for_face_refinement(face_no);
                      } // if flagged for anistropic refinement
                  }     // if (cell->face(face)->at_boundary())
            }           // for all cells
      }
 
 
      template <int dim, int spacedim>
      static void
      prepare_refinement_dim_dependent(const Triangulation<dim, spacedim> &)
      {
        Assert(dim < 3,
               ExcMessage("Wrong function called -- there should "
                          "be a specialization."));
      }
 
 
      template <int spacedim>
      static void
      prepare_refinement_dim_dependent(
        Triangulation<3, spacedim> &triangulation)
      {
        const unsigned int dim = 3;
        using raw_line_iterator =
          typename Triangulation<dim, spacedim>::raw_line_iterator;
 
        // variable to store whether the mesh was changed in the
        // present loop and in the whole process
        bool mesh_changed = false;
 
        do
          {
            mesh_changed = false;
 
            // for this following, we need to know which cells are
            // going to be coarsened, if we had to make a
            // decision. the following function sets these flags:
            triangulation.fix_coarsen_flags();
 
            // first clear flags on lines, since we need them to determine
            // which lines will be refined
            triangulation.clear_user_flags_line();
 
            // flag those lines that are refined and will not be
            // coarsened and those that will be refined
            for (const auto &cell : triangulation.cell_iterators())
              if (cell->refine_flag_set())
                {
                  const std::array<unsigned int, 12> line_indices =
                    TriaAccessorImplementation::Implementation::
                      get_line_indices_of_cell(*cell);
                  for (unsigned int l = 0; l < cell->n_lines(); ++l)
                    if (GeometryInfo<dim>::line_refinement_case(
                          cell->refine_flag_set(), l) ==
                        RefinementCase<1>::cut_x)
                      {
                        raw_line_iterator line(&triangulation,
                                               0,
                                               line_indices[l]);
                        // flag a line, that will be refined
                        line->set_user_flag();
                      }
                }
              else if (cell->has_children() &&
                       !cell->child(0)->coarsen_flag_set())
                {
                  const std::array<unsigned int, 12> line_indices =
                    TriaAccessorImplementation::Implementation::
                      get_line_indices_of_cell(*cell);
                  for (unsigned int l = 0; l < cell->n_lines(); ++l)
                    if (GeometryInfo<dim>::line_refinement_case(
                          cell->refinement_case(), l) ==
                        RefinementCase<1>::cut_x)
                      {
                        raw_line_iterator line(&triangulation,
                                               0,
                                               line_indices[l]);
                        // flag a line, that is refined and will stay so
                        line->set_user_flag();
                      }
                }
              else if (cell->has_children() &&
                       cell->child(0)->coarsen_flag_set())
                cell->set_user_flag();
 
 
            // now check whether there are cells with lines that are
            // more than once refined or that will be more than once
            // refined. The first thing should never be the case, in
            // the second case we flag the cell for refinement
            for (typename Triangulation<dim, spacedim>::active_cell_iterator
                   cell = triangulation.last_active();
                 cell != triangulation.end();
                 --cell)
              {
                const std::array<unsigned int, 12> line_indices =
                  TriaAccessorImplementation::Implementation::
                    get_line_indices_of_cell(*cell);
                for (unsigned int l = 0; l < cell->n_lines(); ++l)
                  {
                    raw_line_iterator line(&triangulation, 0, line_indices[l]);
                    if (line->has_children())
                      {
                        // if this line is refined, its children should
                        // not have further children
                        //
                        // however, if any of the children is flagged
                        // for further refinement, we need to refine
                        // this cell also (at least, if the cell is not
                        // already flagged)
                        bool offending_line_found = false;
 
                        for (unsigned int c = 0; c < 2; ++c)
                          {
                            Assert(line->child(c)->has_children() == false,
                                   ExcInternalError());
 
                            if (line->child(c)->user_flag_set() &&
                                (GeometryInfo<dim>::line_refinement_case(
                                   cell->refine_flag_set(), l) ==
                                 RefinementCase<1>::no_refinement))
                              {
                                // tag this cell for refinement
                                cell->clear_coarsen_flag();
                                // if anisotropic coarsening is allowed:
                                // extend the refine_flag in the needed
                                // direction, else set refine_flag
                                // (isotropic)
                                if (triangulation.smooth_grid &
                                    Triangulation<dim, spacedim>::
                                      allow_anisotropic_smoothing)
                                  cell->flag_for_line_refinement(l);
                                else
                                  cell->set_refine_flag();
 
                                for (unsigned int k = 0; k < cell->n_lines();
                                     ++k)
                                  if (GeometryInfo<dim>::line_refinement_case(
                                        cell->refine_flag_set(), l) ==
                                      RefinementCase<1>::cut_x)
                                    // flag a line, that will be refined
                                    raw_line_iterator(&triangulation,
                                                      0,
                                                      line_indices[k])
                                      ->set_user_flag();
 
                                // note that we have changed the grid
                                offending_line_found = true;
 
                                // it may save us several loop
                                // iterations if we flag all lines of
                                // this cell now (and not at the outset
                                // of the next iteration) for refinement
                                for (unsigned int k = 0; k < cell->n_lines();
                                     ++k)
                                  {
                                    const auto line =
                                      raw_line_iterator(&triangulation,
                                                        0,
                                                        line_indices[k]);
                                    if (!line->has_children() &&
                                        (GeometryInfo<dim>::
                                           line_refinement_case(
                                             cell->refine_flag_set(), k) !=
                                         RefinementCase<1>::no_refinement))
                                      line->set_user_flag();
                                  }
 
                                break;
                              }
                          }
 
                        if (offending_line_found)
                          {
                            mesh_changed = true;
                            break;
                          }
                      }
                  }
              }
 
 
            // there is another thing here: if any of the lines will
            // be refined, then we may not coarsen the present cell
            // similarly, if any of the lines *is* already refined, we
            // may not coarsen the current cell. however, there's a
            // catch: if the line is refined, but the cell behind it
            // is going to be coarsened, then the situation
            // changes. if we forget this second condition, the
            // refine_and_coarsen_3d test will start to fail. note
            // that to know which cells are going to be coarsened, the
            // call for fix_coarsen_flags above is necessary
            for (typename Triangulation<dim, spacedim>::cell_iterator cell =
                   triangulation.last();
                 cell != triangulation.end();
                 --cell)
              if (cell->user_flag_set())
                {
                  const std::array<unsigned int, 12> line_indices =
                    TriaAccessorImplementation::Implementation::
                      get_line_indices_of_cell(*cell);
                  for (unsigned int l = 0; l < cell->n_lines(); ++l)
                    {
                      raw_line_iterator line(&triangulation,
                                             0,
                                             line_indices[l]);
                      if (line->has_children() &&
                          (line->child(0)->user_flag_set() ||
                           line->child(1)->user_flag_set()))
                        {
                          for (unsigned int c = 0; c < cell->n_children(); ++c)
                            cell->child(c)->clear_coarsen_flag();
                          cell->clear_user_flag();
                          for (unsigned int k = 0; k < cell->n_lines(); ++k)
                            if (GeometryInfo<dim>::line_refinement_case(
                                  cell->refinement_case(), k) ==
                                RefinementCase<1>::cut_x)
                              // flag a line, that is refined and will
                              // stay so
                              raw_line_iterator(&triangulation,
                                                0,
                                                line_indices[k])
                                ->set_user_flag();
                          mesh_changed = true;
                          break;
                        }
                    }
                }
          }
        while (mesh_changed == true);
      }
 
 
 
      template <int dim, int spacedim>
      static bool
      coarsening_allowed(
        const typename Triangulation<dim, spacedim>::cell_iterator &cell)
      {
        // in 1d, coarsening is always allowed since we don't enforce
        // the 2:1 constraint there
        if (dim == 1)
          return true;
 
        const RefinementCase<dim> ref_case = cell->refinement_case();
        for (const unsigned int n : GeometryInfo<dim>::face_indices())
          {
            // if the cell is not refined along that face, coarsening
            // will not change anything, so do nothing. the same
            // applies, if the face is at the boundary
            const RefinementCase<dim - 1> face_ref_case =
              GeometryInfo<dim>::face_refinement_case(cell->refinement_case(),
                                                      n);
 
            const unsigned int n_subfaces =
              GeometryInfo<dim - 1>::n_children(face_ref_case);
 
            if (n_subfaces == 0 || cell->at_boundary(n))
              continue;
            for (unsigned int c = 0; c < n_subfaces; ++c)
              {
                const typename Triangulation<dim, spacedim>::cell_iterator
                  child = cell->child(
                    GeometryInfo<dim>::child_cell_on_face(ref_case, n, c));
 
                const typename Triangulation<dim, spacedim>::cell_iterator
                  child_neighbor = child->neighbor(n);
                if (!child->neighbor_is_coarser(n))
                  {
                    // in 2d, if the child's neighbor is coarser, then it has
                    // no children. however, in 3d it might be
                    // otherwise. consider for example, that our face might be
                    // refined with cut_x, but the neighbor is refined with
                    // cut_xy at that face. then the neighbor pointers of the
                    // children of our cell will point to the common neighbor
                    // cell, not to its children. what we really want to know
                    // in the following is, whether the neighbor cell is
                    // refined twice with reference to our cell. that only
                    // has to be asked, if the child's neighbor is not a
                    // coarser one. we check whether some of the children on
                    // the neighbor are not flagged for coarsening, in that
                    // case we may not coarsen. it is enough to check the
                    // first child because we have already fixed the coarsen
                    // flags on finer levels
                    if (child_neighbor->has_children() &&
                        !(child_neighbor->child(0)->is_active() &&
                          child_neighbor->child(0)->coarsen_flag_set()))
                      return false;
 
                    // the same applies, if the neighbors children are not
                    // refined but will be after refinement
                    if (child_neighbor->refine_flag_set())
                      return false;
                  }
              }
          }
        return true;
      }
    };
 
 
    struct ImplementationMixedMesh
    {
      template <int spacedim>
      static void
      update_neighbors(Triangulation<1, spacedim> &)
      {}
 
      template <int dim, int spacedim>
      static void
      update_neighbors(Triangulation<dim, spacedim> &triangulation)
      {
        std::vector<std::pair<int, int>> adjacent_cells(
          2 * triangulation.n_raw_faces(), {-1, -1});
 
        const auto set_entry = [&](const auto &face_index, const auto &cell) {
          const std::pair<int, int> cell_pair = {cell->level(), cell->index()};
 
          int index = numbers::invalid_unsigned_int;
          if (adjacent_cells[2 * face_index].first == -1 &&
              adjacent_cells[2 * face_index].second == -1)
            {
              index = 2 * face_index + 0;
            }
          else
            {
              Assert(((adjacent_cells[2 * face_index + 1].first == -1) &&
                      (adjacent_cells[2 * face_index + 1].second == -1)),
                     ExcNotImplemented());
              index = 2 * face_index + 1;
            }
 
          adjacent_cells[index] = cell_pair;
        };
 
        const auto get_entry =
          [&](const auto &face_index,
              const auto &cell) -> TriaIterator<CellAccessor<dim, spacedim>> {
          auto test = adjacent_cells[2 * face_index];
 
          if (test == std::make_pair(cell->level(), cell->index()))
            test = adjacent_cells[2 * face_index + 1];
 
          return TriaIterator<CellAccessor<dim, spacedim>>(&triangulation,
                                                           test.first,
                                                           test.second);
        };
 
        for (const auto &cell : triangulation.cell_iterators())
          for (const auto &face : cell->face_iterators())
            {
              set_entry(face->index(), cell);
 
              if (cell->is_active() && face->has_children())
                for (unsigned int c = 0; c < face->n_children(); ++c)
                  set_entry(face->child(c)->index(), cell);
            }
 
        for (const auto &cell : triangulation.cell_iterators())
          for (auto f : cell->face_indices())
            cell->set_neighbor(f, get_entry(cell->face(f)->index(), cell));
      }
 
      template <int dim, int spacedim>
      static void
      delete_children(
        Triangulation<dim, spacedim> & /*triangulation*/,
        typename Triangulation<dim, spacedim>::cell_iterator & /*cell*/,
        std::vector<unsigned int> & /*line_cell_count*/,
        std::vector<unsigned int> & /*quad_cell_count*/)
      {
        AssertThrow(false, ExcNotImplemented());
      }
 
      template <int dim, int spacedim>
      static typename Triangulation<dim, spacedim>::DistortedCellList
      execute_refinement(Triangulation<dim, spacedim> &triangulation,
                         const bool check_for_distorted_cells)
      {
        return Implementation::execute_refinement_isotropic(
          triangulation, check_for_distorted_cells);
      }
 
      template <int dim, int spacedim>
      static void
      prevent_distorted_boundary_cells(
        Triangulation<dim, spacedim> & /*triangulation*/)
      {
        // nothing to do since anisotropy is not supported
      }
 
      template <int dim, int spacedim>
      static void
      prepare_refinement_dim_dependent(
        Triangulation<dim, spacedim> &triangulation)
      {
        Implementation::prepare_refinement_dim_dependent(triangulation);
      }
 
      template <int dim, int spacedim>
      static bool
      coarsening_allowed(
        const typename Triangulation<dim, spacedim>::cell_iterator &)
      {
        AssertThrow(false, ExcNotImplemented());
 
        return false;
      }
    };
  } // namespace TriangulationImplementation
} // namespace internal
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
const unsigned int Triangulation<dim, spacedim>::dimension;
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
Triangulation<dim, spacedim>::Triangulation(
  const MeshSmoothing smooth_grid,
  const bool          check_for_distorted_cells)
  : cell_attached_data({0, 0, {}, {}})
  , smooth_grid(smooth_grid)
  , default_flat_manifold(std::make_unique<const FlatManifold<dim, spacedim>>())
  , anisotropic_refinement(false)
  , check_for_distorted_cells(check_for_distorted_cells)
{
  if (dim == 1)
    {
      vertex_to_boundary_id_map_1d =
        std::make_unique<std::map<unsigned int, types::boundary_id>>();
      vertex_to_manifold_id_map_1d =
        std::make_unique<std::map<unsigned int, types::manifold_id>>();
    }
 
  // connect the any_change signal to the other top level signals
  signals.create.connect(signals.any_change);
  signals.post_refinement.connect(signals.any_change);
  signals.clear.connect(signals.any_change);
  signals.mesh_movement.connect(signals.any_change);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
Triangulation<dim, spacedim>::Triangulation(
  Triangulation<dim, spacedim> &&tria) noexcept
  : EnableObserverPointer(std::move(tria))
  , cell_attached_data(std::move(tria.cell_attached_data))
  , smooth_grid(tria.smooth_grid)
  , reference_cells(std::move(tria.reference_cells))
  , strides(tria.strides)
  , periodic_face_pairs_level_0(std::move(tria.periodic_face_pairs_level_0))
  , periodic_face_map(std::move(tria.periodic_face_map))
  , levels(std::move(tria.levels))
  , faces(std::move(tria.faces))
  , vertices(std::move(tria.vertices))
  , vertices_used(std::move(tria.vertices_used))
  , manifolds(std::move(tria.manifolds))
  , default_flat_manifold(std::make_unique<const FlatManifold<dim, spacedim>>())
  , anisotropic_refinement(tria.anisotropic_refinement)
  , check_for_distorted_cells(tria.check_for_distorted_cells)
  , number_cache(std::move(tria.number_cache))
  , vertex_to_boundary_id_map_1d(std::move(tria.vertex_to_boundary_id_map_1d))
  , vertex_to_manifold_id_map_1d(std::move(tria.vertex_to_manifold_id_map_1d))
{
  tria.number_cache = internal::TriangulationImplementation::NumberCache<dim>();
  tria.strides      = {};
 
  if (tria.policy)
    this->policy = tria.policy->clone();
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
Triangulation<dim, spacedim> &Triangulation<dim, spacedim>::operator=(
  Triangulation<dim, spacedim> &&tria) noexcept
{
  EnableObserverPointer::operator=(std::move(tria));
 
  cell_attached_data           = std::move(tria.cell_attached_data);
  smooth_grid                  = tria.smooth_grid;
  reference_cells              = std::move(tria.reference_cells);
  strides                      = tria.strides;
  periodic_face_pairs_level_0  = std::move(tria.periodic_face_pairs_level_0);
  periodic_face_map            = std::move(tria.periodic_face_map);
  levels                       = std::move(tria.levels);
  faces                        = std::move(tria.faces);
  vertices                     = std::move(tria.vertices);
  vertices_used                = std::move(tria.vertices_used);
  manifolds                    = std::move(tria.manifolds);
  anisotropic_refinement       = tria.anisotropic_refinement;
  number_cache                 = tria.number_cache;
  vertex_to_boundary_id_map_1d = std::move(tria.vertex_to_boundary_id_map_1d);
  vertex_to_manifold_id_map_1d = std::move(tria.vertex_to_manifold_id_map_1d);
 
  tria.number_cache = internal::TriangulationImplementation::NumberCache<dim>();
  tria.strides      = {};
 
  if (tria.policy)
    this->policy = tria.policy->clone();
 
  return *this;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
Triangulation<dim, spacedim>::~Triangulation()
{
  // notify listeners that the triangulation is going down...
  try
    {
      signals.clear();
    }
  catch (...)
    {}
 
  levels.clear();
 
  // the vertex_to_boundary_id_map_1d field should be unused except in
  // 1d. double check this here, as destruction is a good place to
  // ensure that what we've done over the course of the lifetime of
  // this object makes sense
  AssertNothrow((dim == 1) || (vertex_to_boundary_id_map_1d == nullptr),
                ExcInternalError());
 
  // the vertex_to_manifold_id_map_1d field should be also unused
  // except in 1d. check this as well
  AssertNothrow((dim == 1) || (vertex_to_manifold_id_map_1d == nullptr),
                ExcInternalError());
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::clear()
{
  // notify listeners that the triangulation is going down...
  signals.clear();
 
  // ...and then actually clear all content of it
  clear_despite_subscriptions();
  periodic_face_pairs_level_0.clear();
  periodic_face_map.clear();
  reference_cells.clear();
 
  cell_attached_data = {0, 0, {}, {}};
  data_serializer.clear();
}
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
MPI_Comm Triangulation<dim, spacedim>::get_mpi_communicator() const
{
  return MPI_COMM_SELF;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
std::weak_ptr<const Utilities::MPI::Partitioner> Triangulation<dim, spacedim>::
  global_active_cell_index_partitioner() const
{
  return number_cache.active_cell_index_partitioner;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
std::weak_ptr<const Utilities::MPI::Partitioner> Triangulation<dim, spacedim>::
  global_level_cell_index_partitioner(const unsigned int level) const
{
  AssertIndexRange(level, this->n_levels());
 
  return number_cache.level_cell_index_partitioners[level];
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::set_mesh_smoothing(
  const MeshSmoothing mesh_smoothing)
{
  smooth_grid = mesh_smoothing;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
const typename Triangulation<dim, spacedim>::MeshSmoothing
  &Triangulation<dim, spacedim>::get_mesh_smoothing() const
{
  return smooth_grid;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::set_manifold(
  const types::manifold_id       m_number,
  const Manifold<dim, spacedim> &manifold_object)
{
  AssertIndexRange(m_number, numbers::flat_manifold_id);
 
  manifolds[m_number] = manifold_object.clone();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::reset_manifold(
  const types::manifold_id m_number)
{
  AssertIndexRange(m_number, numbers::flat_manifold_id);
 
  // delete the entry located at number.
  manifolds[m_number] = default_flat_manifold->clone();
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::reset_all_manifolds()
{
  for (auto &m : manifolds)
    m.second = default_flat_manifold->clone();
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::set_all_manifold_ids(
  const types::manifold_id m_number)
{
  Assert(
    n_cells() > 0,
    ExcMessage(
      "Error: set_all_manifold_ids() can not be called on an empty Triangulation."));
 
  for (const auto &cell : this->active_cell_iterators())
    cell->set_all_manifold_ids(m_number);
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::set_all_manifold_ids_on_boundary(
  const types::manifold_id m_number)
{
  Assert(
    n_cells() > 0,
    ExcMessage(
      "Error: set_all_manifold_ids_on_boundary() can not be called on an empty Triangulation."));
 
  for (const auto &cell : this->active_cell_iterators())
    for (auto f : GeometryInfo<dim>::face_indices())
      if (cell->face(f)->at_boundary())
        cell->face(f)->set_all_manifold_ids(m_number);
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::set_all_manifold_ids_on_boundary(
  const types::boundary_id b_id,
  const types::manifold_id m_number)
{
  Assert(
    n_cells() > 0,
    ExcMessage(
      "Error: set_all_manifold_ids_on_boundary() can not be called on an empty Triangulation."));
 
  bool boundary_found = false;
  for (const auto &cell : this->active_cell_iterators())
    {
      // loop on faces
      for (auto f : GeometryInfo<dim>::face_indices())
        if (cell->face(f)->at_boundary() &&
            cell->face(f)->boundary_id() == b_id)
          {
            boundary_found = true;
            cell->face(f)->set_manifold_id(m_number);
          }
 
      // loop on edges if dim >= 3
      if (dim >= 3)
        for (unsigned int e = 0; e < GeometryInfo<dim>::lines_per_cell; ++e)
          if (cell->line(e)->at_boundary() &&
              cell->line(e)->boundary_id() == b_id)
            {
              boundary_found = true;
              cell->line(e)->set_manifold_id(m_number);
            }
    }
 
  Assert(boundary_found, ExcBoundaryIdNotFound(b_id));
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
const Manifold<dim, spacedim> &Triangulation<dim, spacedim>::get_manifold(
  const types::manifold_id m_number) const
{
  // check if flat manifold has been queried
  if (m_number == numbers::flat_manifold_id)
    return *default_flat_manifold;
 
  // look, if there is a manifold stored at
  // manifold_id number.
  const auto it = manifolds.find(m_number);
 
  if (it != manifolds.end())
    {
      // if we have found an entry, return it
      return *(it->second);
    }
 
  Assert(
    false,
    ExcMessage(
      "No manifold of the manifold id " + std::to_string(m_number) +
      " has been attached to the triangulation. "
      "Please attach the right manifold with Triangulation::set_manifold()."));
 
  return *default_flat_manifold; // never reached
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
std::vector<types::boundary_id> Triangulation<dim, spacedim>::get_boundary_ids()
  const
{
  std::set<types::boundary_id> boundary_ids;
  for (const auto &cell : active_cell_iterators())
    if (cell->is_locally_owned())
      for (const auto &face : cell->face_indices())
        if (cell->at_boundary(face))
          boundary_ids.insert(cell->face(face)->boundary_id());
 
  return {boundary_ids.begin(), boundary_ids.end()};
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
std::vector<types::manifold_id> Triangulation<dim, spacedim>::get_manifold_ids()
  const
{
  std::set<types::manifold_id> m_ids;
  for (const auto &cell : active_cell_iterators())
    if (cell->is_locally_owned())
      {
        m_ids.insert(cell->manifold_id());
        for (const auto &face : cell->face_iterators())
          m_ids.insert(face->manifold_id());
        if (dim == 3)
          {
            const auto line_indices = internal::TriaAccessorImplementation::
              Implementation::get_line_indices_of_cell(*cell);
            for (unsigned int l = 0; l < cell->n_lines(); ++l)
              {
                raw_line_iterator line(this, 0, line_indices[l]);
                m_ids.insert(line->manifold_id());
              }
          }
      }
  return {m_ids.begin(), m_ids.end()};
}
 
#endif
/*-----------------------------------------------------------------*/
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::copy_triangulation(
  const Triangulation<dim, spacedim> &other_tria)
{
  Assert((vertices.empty()) && (levels.empty()) && (faces == nullptr),
         ExcTriangulationNotEmpty(vertices.size(), levels.size()));
  Assert((other_tria.levels.size() != 0) && (other_tria.vertices.size() != 0) &&
           (dim == 1 || other_tria.faces != nullptr),
         ExcMessage(
           "When calling Triangulation::copy_triangulation(), "
           "the target triangulation must be empty but the source "
           "triangulation (the argument to this function) must contain "
           "something. Here, it seems like the source does not "
           "contain anything at all."));
 
 
  // copy normal elements
  vertices               = other_tria.vertices;
  vertices_used          = other_tria.vertices_used;
  anisotropic_refinement = other_tria.anisotropic_refinement;
  smooth_grid            = other_tria.smooth_grid;
  reference_cells        = other_tria.reference_cells;
  strides                = other_tria.strides;
 
  if (dim > 1)
    faces =
      std::make_unique<internal::TriangulationImplementation::TriaFaces<dim>>(
        *other_tria.faces);
 
  for (const auto &p : other_tria.manifolds)
    set_manifold(p.first, *p.second);
 
 
  Assert(other_tria.levels.size() <= numbers::max_n_levels, ExcInternalError());
  levels.reserve(other_tria.levels.size());
  for (const auto &level : other_tria.levels)
    levels.push_back(
      std::make_unique<
        internal::TriangulationImplementation::TriaLevel<dim, spacedim>>(
        *level));
 
  number_cache = other_tria.number_cache;
 
  if (dim == 1)
    {
      vertex_to_boundary_id_map_1d =
        std::make_unique<std::map<unsigned int, types::boundary_id>>(
          *other_tria.vertex_to_boundary_id_map_1d);
 
      vertex_to_manifold_id_map_1d =
        std::make_unique<std::map<unsigned int, types::manifold_id>>(
          *other_tria.vertex_to_manifold_id_map_1d);
    }
 
  if (other_tria.policy)
    this->policy = other_tria.policy->clone();
 
  // periodic faces
  this->periodic_face_pairs_level_0.reserve(
    other_tria.periodic_face_pairs_level_0.size());
 
  for (const auto &other_entry : other_tria.periodic_face_pairs_level_0)
    {
      auto entry = other_entry;
      entry.cell[0] =
        cell_iterator(this, entry.cell[0]->level(), entry.cell[0]->index());
      entry.cell[1] =
        cell_iterator(this, entry.cell[1]->level(), entry.cell[1]->index());
      periodic_face_pairs_level_0.emplace_back(entry);
    }
 
  for (auto [first_cell_, second_cell_and_orientation] :
       other_tria.periodic_face_map)
    {
      auto first_cell  = first_cell_; // make copy since key is const
      first_cell.first = cell_iterator(this,
                                       first_cell.first->level(),
                                       first_cell.first->index());
      second_cell_and_orientation.first.first =
        cell_iterator(this,
                      second_cell_and_orientation.first.first->level(),
                      second_cell_and_orientation.first.first->index());
 
      this->periodic_face_map[first_cell] = second_cell_and_orientation;
    }
 
  // inform those who are listening on other_tria of the copy operation
  other_tria.signals.copy(*this);
  // also inform all listeners of the current triangulation that the
  // triangulation has been created
  signals.create();
 
  // note that we need not copy the
  // subscriptor!
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::reset_policy()
{
  if (this->all_reference_cells_are_hyper_cube())
    {
      this->policy =
        std::make_unique<internal::TriangulationImplementation::PolicyWrapper<
          dim,
          spacedim,
          internal::TriangulationImplementation::Implementation>>();
    }
  else
    {
      this->policy =
        std::make_unique<internal::TriangulationImplementation::PolicyWrapper<
          dim,
          spacedim,
          internal::TriangulationImplementation::ImplementationMixedMesh>>();
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::create_triangulation(
  const std::vector<Point<spacedim>> &v,
  const std::vector<CellData<dim>>   &cells,
  const SubCellData                  &subcelldata)
{
  Assert((vertices.empty()) && (levels.empty()) && (faces == nullptr),
         ExcTriangulationNotEmpty(vertices.size(), levels.size()));
  // check that no forbidden arrays
  // are used
  Assert(subcelldata.check_consistency(dim), ExcInternalError());
 
  // try to create a triangulation; if this fails, we still want to
  // throw an exception but if we just do so we'll get into trouble
  // because sometimes other objects are already attached to it:
  try
    {
      internal::TriangulationImplementation::Implementation::
        create_triangulation(v, cells, subcelldata, *this);
    }
  catch (...)
    {
      clear_despite_subscriptions();
      throw;
    }
 
  reset_policy();
 
  // update our counts of the various elements of a triangulation, and set
  // active_cell_indices of all cells
  reset_cell_vertex_indices_cache();
  internal::TriangulationImplementation::Implementation::compute_number_cache(
    *this, levels.size(), number_cache);
  reset_active_cell_indices();
  reset_global_cell_indices();
 
  // now verify that there are indeed no distorted cells. as per the
  // documentation of this class, we first collect all distorted cells
  // and then throw an exception if there are any
  if (check_for_distorted_cells)
    {
      DistortedCellList distorted_cells = collect_distorted_coarse_cells(*this);
      // throw the array (and fill the various location fields) if
      // there are distorted cells. otherwise, just fall off the end
      // of the function
      AssertThrow(distorted_cells.distorted_cells.empty(), distorted_cells);
    }
 
 
  /*
      When the triangulation is a manifold (dim < spacedim) and made of
      quadrilaterals, the normal field provided from the map class depends on
      the order of the vertices. It may happen that this normal field is
      discontinuous. The following code takes care that this is not the case by
      setting the cell direction flag on those cell that produce the wrong
      orientation.
 
      To determine if 2 neighbors have the same or opposite orientation we use
      a truth table. Its entries are indexed by the local indices of the
      common face. For example if two elements share a face, and this face is
      face 0 for element 0 and face 1 for element 1, then table(0,1) will tell
      whether the orientation are the same (true) or opposite (false).
 
      Even though there may be a combinatorial/graph theory argument to get this
      table in any dimension, I tested by hand all the different possible cases
      in 1D and 2D to generate the table.
 
      Assuming that a surface respects the standard orientation for 2d meshes,
      the truth tables are symmetric and their true values are the following
 
      - 1D curves:  (0,1)
      - 2D surface: (0,1),(0,2),(1,3),(2,3)
 
      We store this data using an n_faces x n_faces full matrix, which is
      actually much bigger than the minimal data required, but it makes the code
      more readable.
 
    */
  if ((dim == spacedim - 1) && all_reference_cells_are_hyper_cube())
    {
      Table<2, bool> correct(GeometryInfo<dim>::faces_per_cell,
                             GeometryInfo<dim>::faces_per_cell);
      switch (dim)
        {
          case 1:
            {
              const bool values[][2] = {{false, true}, {true, false}};
              for (const unsigned int i : GeometryInfo<dim>::face_indices())
                for (const unsigned int j : GeometryInfo<dim>::face_indices())
                  correct(i, j) = values[i][j];
              break;
            }
          case 2:
            {
              const bool values[][4] = {{false, true, true, false},
                                        {true, false, false, true},
                                        {true, false, false, true},
                                        {false, true, true, false}};
              for (const unsigned int i : GeometryInfo<dim>::face_indices())
                for (const unsigned int j : GeometryInfo<dim>::face_indices())
                  correct(i, j) = (values[i][j]);
              break;
            }
          default:
            DEAL_II_NOT_IMPLEMENTED();
        }
 
 
      std::list<active_cell_iterator> this_round, next_round;
      active_cell_iterator            neighbor;
 
      // Start with the first cell and (arbitrarily) decide that its
      // direction flag should be 'true':
      this_round.push_back(begin_active());
      begin_active()->set_direction_flag(true);
      begin_active()->set_user_flag();
 
      while (this_round.size() > 0)
        {
          for (const auto &cell : this_round)
            {
              for (const unsigned int i : cell->face_indices())
                {
                  if (cell->face(i)->at_boundary() == false)
                    {
                      // Consider the i'th neighbor of a cell for
                      // which we have already set the direction:
                      neighbor = cell->neighbor(i);
 
                      const unsigned int nb_of_nb =
                        cell->neighbor_of_neighbor(i);
 
                      // If we already saw this neighboring cell,
                      // check that everything is fine:
                      if (neighbor->user_flag_set())
                        {
                          Assert(
                            !(correct(i, nb_of_nb) ^
                              (neighbor->direction_flag() ==
                               cell->direction_flag())),
                            ExcMessage(
                              "The triangulation you are trying to create is not orientable."));
                        }
                      else
                        {
                          // We had not seen this cell yet. Set its
                          // orientation flag (if necessary), mark it
                          // as treated via the user flag, and push it
                          // onto the list of cells to start work from
                          // the next time around:
                          if (correct(i, nb_of_nb) ^
                              (neighbor->direction_flag() ==
                               cell->direction_flag()))
                            neighbor->set_direction_flag(
                              !neighbor->direction_flag());
                          neighbor->set_user_flag();
                          next_round.push_back(neighbor);
                        }
                    }
                }
            }
 
          // Before we quit let's check that if the triangulation is
          // disconnected that we still get all cells by starting
          // again from the first cell we haven't treated yet -- that
          // is, the first cell of the next disconnected component we
          // had not yet touched.
          if (next_round.empty())
            for (const auto &cell : this->active_cell_iterators())
              if (cell->user_flag_set() == false)
                {
                  next_round.push_back(cell);
                  cell->set_direction_flag(true);
                  cell->set_user_flag();
                  break;
                }
 
          // Go on to the next round:
          next_round.swap(this_round);
          next_round.clear();
        }
      clear_user_flags();
    }
 
  this->update_cell_relations();
 
  // inform all listeners that the triangulation has been created
  signals.create();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::create_triangulation(
  const TriangulationDescription::Description<dim, spacedim> &construction_data)
{
  // 1) create coarse grid
  create_triangulation(construction_data.coarse_cell_vertices,
                       construction_data.coarse_cells,
                       SubCellData());
 
  // create a copy of cell_infos such that we can sort them
  auto cell_infos = construction_data.cell_infos;
 
  // sort cell_infos on each level separately
  for (auto &cell_info : cell_infos)
    std::sort(
      cell_info.begin(),
      cell_info.end(),
      [&](const TriangulationDescription::CellData<dim> &a,
          const TriangulationDescription::CellData<dim> &b) {
        const CellId a_id(a.id);
        const CellId b_id(b.id);
 
        const auto a_coarse_cell_index =
          this->coarse_cell_id_to_coarse_cell_index(a_id.get_coarse_cell_id());
        const auto b_coarse_cell_index =
          this->coarse_cell_id_to_coarse_cell_index(b_id.get_coarse_cell_id());
 
        // according to their coarse-cell index and if that is
        // same according to their cell id (the result is that
        // cells on each level are sorted according to their
        // index on that level - what we need in the following
        // operations)
        if (a_coarse_cell_index != b_coarse_cell_index)
          return a_coarse_cell_index < b_coarse_cell_index;
        else
          return a_id < b_id;
      });
 
  // 2) create all levels via a sequence of refinements. note that
  //    we must make sure that we actually have cells on this level,
  //    which is not clear in a parallel context for some processes
  for (unsigned int level = 0;
       level < cell_infos.size() && !cell_infos[level].empty();
       ++level)
    {
      // a) set material and manifold ids here (because new vertices
      //    have to be positioned correctly during each refinement step)
      {
        auto cell      = this->begin(level);
        auto cell_info = cell_infos[level].begin();
        for (; cell_info != cell_infos[level].end(); ++cell_info)
          {
            while (cell_info->id != cell->id().template to_binary<dim>())
              ++cell;
 
            if (level == 0)
              {
                Assert(cell->material_id() == cell_info->material_id,
                       ExcMessage(
                         "The coarsest level material_ids must match."));
                Assert(cell->manifold_id() == cell_info->manifold_id,
                       ExcMessage(
                         "The coarsest level manifold_ids must match."));
              }
 
            cell->set_material_id(cell_info->material_id);
            if (dim == 2)
              for (const auto face : cell->face_indices())
                cell->face(face)->set_manifold_id(
                  cell_info->manifold_line_ids[face]);
            else if (dim == 3)
              {
                for (const auto face : cell->face_indices())
                  cell->face(face)->set_manifold_id(
                    cell_info->manifold_quad_ids[face]);
 
                const auto line_indices = internal::TriaAccessorImplementation::
                  Implementation::get_line_indices_of_cell(*cell);
                for (unsigned int l = 0; l < cell->n_lines(); ++l)
                  {
                    raw_line_iterator line(this, 0, line_indices[l]);
                    line->set_manifold_id(cell_info->manifold_line_ids[l]);
                  }
              }
 
            cell->set_manifold_id(cell_info->manifold_id);
          }
      }
 
      // b) perform refinement on all levels but on the finest
      if (level + 1 != cell_infos.size())
        {
          // find cells that should have children and mark them for
          // refinement
          auto coarse_cell    = this->begin(level);
          auto fine_cell_info = cell_infos[level + 1].begin();
 
          // loop over all cells on the next level
          for (; fine_cell_info != cell_infos[level + 1].end();
               ++fine_cell_info)
            {
              // find the parent of that cell
              while (
                !coarse_cell->id().is_parent_of(CellId(fine_cell_info->id)))
                ++coarse_cell;
 
              // set parent for refinement
              coarse_cell->set_refine_flag();
            }
 
          // execute refinement
          ::Triangulation<dim,
                                spacedim>::execute_coarsening_and_refinement();
        }
    }
 
  // 3) set boundary ids
  for (unsigned int level = 0;
       level < cell_infos.size() && !cell_infos[level].empty();
       ++level)
    {
      auto cell      = this->begin(level);
      auto cell_info = cell_infos[level].begin();
      for (; cell_info != cell_infos[level].end(); ++cell_info)
        {
          // find cell that has the correct cell
          while (cell_info->id != cell->id().template to_binary<dim>())
            ++cell;
 
          // boundary ids
          for (auto pair : cell_info->boundary_ids)
            if (cell->face(pair.first)->at_boundary())
              cell->face(pair.first)->set_boundary_id(pair.second);
        }
    }
 
  // inform all listeners that the triangulation has been created
  signals.create();
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::flip_all_direction_flags()
{
  AssertThrow(dim + 1 == spacedim,
              ExcMessage(
                "This function can only be called if dim == spacedim-1."));
  for (const auto &cell : this->active_cell_iterators())
    cell->set_direction_flag(!cell->direction_flag());
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::compute_line_to_adjacent_cells_map()
{
  if constexpr (dim == 3)
    {
      // Compute the vertex to cell map
      const std::vector<std::set<active_cell_iterator>> vertex_to_cell =
        GridTools::vertex_to_cell_map(*this);
 
      // Compute the line-neighbors
      if (this->line_to_adjacent_cells_map)
        // If the std::optional ready has a value attached to it, just change
        // the size of the table stored inside it.
        this->line_to_adjacent_cells_map->reinit(
          this->n_active_cells(),
          (is_mixed_mesh() ?
             GeometryInfo<dim>::lines_per_cell : // err on the safe side: choose
                                                 // the largest number of lines
                                                 // per cell
                                                 get_reference_cells()[0]
               .n_lines()) // choose the right number of  lines per cell for the
                           // mesh type used
        );
      else
        // Else if the std::optional has no value attached to it, create a table
        // of the corresponding size and store it inside the std::optional.
        this->line_to_adjacent_cells_map =
          std::make_optional<Table<2, std::set<active_cell_iterator>>>(
            Table<2, std::set<active_cell_iterator>>(
              this->n_active_cells(),
              (is_mixed_mesh() ? GeometryInfo<dim>::lines_per_cell :
                                 get_reference_cells()[0].n_lines())));
 
 
      // Loop over all cells -> lines -> vertices
      for (const auto &cell : this->active_cell_iterators())
        for (unsigned int line : cell->line_indices())
          {
            const unsigned int vertex_0 = cell->vertex_index(
              GeometryInfo<dim>::line_to_cell_vertices(line, 0));
            const unsigned int vertex_1 = cell->vertex_index(
              GeometryInfo<dim>::line_to_cell_vertices(line, 1));
            const std::set<
              typename Triangulation<dim, spacedim>::active_cell_iterator>
              &adjacent_cells_to_vertex_0 = vertex_to_cell[vertex_0];
            const std::set<
              typename Triangulation<dim, spacedim>::active_cell_iterator>
              &adjacent_cells_to_vertex_1 = vertex_to_cell[vertex_1];
 
            // add all cells that are adjacent to vertex_0 and vertex_1
            std::set_intersection(
              adjacent_cells_to_vertex_0.begin(),
              adjacent_cells_to_vertex_0.end(),
              adjacent_cells_to_vertex_1.begin(),
              adjacent_cells_to_vertex_1.end(),
              std::inserter(line_to_adjacent_cells_map
                              .value()[cell->active_cell_index()][line],
                            line_to_adjacent_cells_map
                              .value()[cell->active_cell_index()][line]
                              .begin()));
          }
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::set_all_refine_flags()
{
  Assert(n_cells() > 0,
         ExcMessage("Error: An empty Triangulation can not be refined."));
 
  for (const auto &cell : this->active_cell_iterators())
    {
      cell->clear_coarsen_flag();
      cell->set_refine_flag();
      cell->set_refine_choice();
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::refine_global(const unsigned int times)
{
  Assert(n_cells() > 0,
         ExcMessage("Error: An empty Triangulation can not be refined."));
 
  for (unsigned int i = 0; i < times; ++i)
    {
      set_all_refine_flags();
      execute_coarsening_and_refinement();
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::coarsen_global(const unsigned int times)
{
  for (unsigned int i = 0; i < times; ++i)
    {
      for (const auto &cell : this->active_cell_iterators())
        {
          cell->clear_refine_flag();
          cell->set_coarsen_flag();
        }
      execute_coarsening_and_refinement();
    }
}
 
 
#endif
/*-------------------- refine/coarsen flags -------------------------*/
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_refine_flags(std::vector<bool> &v) const
{
  v.resize(dim * n_active_cells(), false);
  std::vector<bool>::iterator i = v.begin();
 
  for (const auto &cell : this->active_cell_iterators())
    for (unsigned int j = 0; j < dim; ++j, ++i)
      if (cell->refine_flag_set() & (1 << j))
        *i = true;
 
  Assert(i == v.end(), ExcInternalError());
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_refine_flags(std::ostream &out) const
{
  std::vector<bool> v;
  save_refine_flags(v);
  write_bool_vector(mn_tria_refine_flags_begin,
                    v,
                    mn_tria_refine_flags_end,
                    out);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_refine_flags(std::istream &in)
{
  std::vector<bool> v;
  read_bool_vector(mn_tria_refine_flags_begin, v, mn_tria_refine_flags_end, in);
  load_refine_flags(v);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_refine_flags(const std::vector<bool> &v)
{
  AssertThrow(v.size() == dim * n_active_cells(), ExcGridReadError());
 
  std::vector<bool>::const_iterator i = v.begin();
  for (const auto &cell : this->active_cell_iterators())
    {
      unsigned int ref_case = 0;
 
      for (unsigned int j = 0; j < dim; ++j, ++i)
        if (*i == true)
          ref_case += 1 << j;
      Assert(ref_case < RefinementCase<dim>::isotropic_refinement + 1,
             ExcGridReadError());
      if (ref_case > 0)
        cell->set_refine_flag(RefinementCase<dim>(ref_case));
      else
        cell->clear_refine_flag();
    }
 
  Assert(i == v.end(), ExcInternalError());
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_coarsen_flags(
  std::vector<bool> &v) const
{
  v.resize(n_active_cells(), false);
  std::vector<bool>::iterator i = v.begin();
  for (const auto &cell : this->active_cell_iterators())
    {
      *i = cell->coarsen_flag_set();
      ++i;
    }
 
  Assert(i == v.end(), ExcInternalError());
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_coarsen_flags(std::ostream &out) const
{
  std::vector<bool> v;
  save_coarsen_flags(v);
  write_bool_vector(mn_tria_coarsen_flags_begin,
                    v,
                    mn_tria_coarsen_flags_end,
                    out);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_coarsen_flags(std::istream &in)
{
  std::vector<bool> v;
  read_bool_vector(mn_tria_coarsen_flags_begin,
                   v,
                   mn_tria_coarsen_flags_end,
                   in);
  load_coarsen_flags(v);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_coarsen_flags(
  const std::vector<bool> &v)
{
  Assert(v.size() == n_active_cells(), ExcGridReadError());
 
  std::vector<bool>::const_iterator i = v.begin();
  for (const auto &cell : this->active_cell_iterators())
    {
      if (*i == true)
        cell->set_coarsen_flag();
      else
        cell->clear_coarsen_flag();
      ++i;
    }
 
  Assert(i == v.end(), ExcInternalError());
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
bool Triangulation<dim, spacedim>::get_anisotropic_refinement_flag() const
{
  return anisotropic_refinement;
}
 
 
#endif
 
namespace internal
{
  namespace
  {
    template <int dim, int spacedim>
    std::vector<std::vector<bool>>
    extract_raw_coarsen_flags(
      const std::vector<
        std::unique_ptr<::internal::TriangulationImplementation::
                          TriaLevel<dim, spacedim>>> &levels)
    {
      std::vector<std::vector<bool>> coarsen_flags(levels.size());
      for (unsigned int level = 0; level < levels.size(); ++level)
        coarsen_flags[level] = levels[level]->coarsen_flags;
      return coarsen_flags;
    }
 
    template <int dim, int spacedim>
    std::vector<std::vector<std::uint8_t>>
    extract_raw_refine_flags(
      const std::vector<
        std::unique_ptr<::internal::TriangulationImplementation::
                          TriaLevel<dim, spacedim>>> &levels)
    {
      std::vector<std::vector<std::uint8_t>> refine_flags(levels.size());
      for (unsigned int level = 0; level < levels.size(); ++level)
        refine_flags[level] = levels[level]->refine_flags;
      return refine_flags;
    }
  } // namespace
} // namespace internal
 
 
/*-------------------- user data/flags -------------------------*/
 
 
namespace
{
  // clear user data of cells
  template <int dim, int spacedim>
  void
  clear_user_data(
    std::vector<std::unique_ptr<
      internal::TriangulationImplementation::TriaLevel<dim, spacedim>>> &levels)
  {
    for (auto &level : levels)
      level->cells.clear_user_data();
  }
 
 
  // clear user data of faces
  template <int dim>
  void
  clear_user_data(internal::TriangulationImplementation::TriaFaces<dim> *faces)
  {
    if (dim == 2)
      {
        faces->lines.clear_user_data();
      }
 
 
    if (dim == 3)
      {
        faces->lines.clear_user_data();
        faces->quads.clear_user_data();
      }
  }
} // namespace
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::clear_user_data()
{
  // let functions in anonymous namespace do their work
  ::clear_user_data(levels);
  if (dim > 1)
    ::clear_user_data(faces.get());
}
 
 
 
namespace
{
  template <int dim, int spacedim>
  void
  clear_user_flags_line(
    std::vector<std::unique_ptr<
      internal::TriangulationImplementation::TriaLevel<dim, spacedim>>> &levels,
    internal::TriangulationImplementation::TriaFaces<dim>               *faces)
  {
    if (dim == 1)
      {
        for (const auto &level : levels)
          level->cells.clear_user_flags();
      }
    else if (dim == 2 || dim == 3)
      {
        faces->lines.clear_user_flags();
      }
    else
      {
        DEAL_II_NOT_IMPLEMENTED();
      }
  }
} // namespace
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::clear_user_flags_line()
{
  ::clear_user_flags_line(levels, faces.get());
}
 
 
 
namespace
{
  template <int dim, int spacedim>
  void
  clear_user_flags_quad(
    std::vector<std::unique_ptr<
      internal::TriangulationImplementation::TriaLevel<dim, spacedim>>> &levels,
    internal::TriangulationImplementation::TriaFaces<dim>               *faces)
  {
    if (dim == 1)
      {
        // nothing to do in 1d
      }
    else if (dim == 2)
      {
        for (const auto &level : levels)
          level->cells.clear_user_flags();
      }
    else if (dim == 3)
      {
        faces->quads.clear_user_flags();
      }
    else
      {
        DEAL_II_NOT_IMPLEMENTED();
      }
  }
} // namespace
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::clear_user_flags_quad()
{
  ::clear_user_flags_quad(levels, faces.get());
}
 
 
 
namespace
{
  template <int dim, int spacedim>
  void
  clear_user_flags_hex(
    std::vector<std::unique_ptr<
      internal::TriangulationImplementation::TriaLevel<dim, spacedim>>> &levels,
    internal::TriangulationImplementation::TriaFaces<dim> *)
  {
    if (dim == 1)
      {
        // nothing to do in 1d
      }
    else if (dim == 2)
      {
        // nothing to do in 2d
      }
    else if (dim == 3)
      {
        for (const auto &level : levels)
          level->cells.clear_user_flags();
      }
    else
      {
        DEAL_II_NOT_IMPLEMENTED();
      }
  }
} // namespace
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::clear_user_flags_hex()
{
  ::clear_user_flags_hex(levels, faces.get());
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::clear_user_flags()
{
  clear_user_flags_line();
  clear_user_flags_quad();
  clear_user_flags_hex();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_flags(std::ostream &out) const
{
  save_user_flags_line(out);
 
  if (dim >= 2)
    save_user_flags_quad(out);
 
  if (dim >= 3)
    save_user_flags_hex(out);
 
  if (dim >= 4)
    DEAL_II_NOT_IMPLEMENTED();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_flags(std::vector<bool> &v) const
{
  // clear vector and append
  // all the stuff later on
  v.clear();
 
  std::vector<bool> tmp;
 
  save_user_flags_line(tmp);
  v.insert(v.end(), tmp.begin(), tmp.end());
 
  if (dim >= 2)
    {
      save_user_flags_quad(tmp);
      v.insert(v.end(), tmp.begin(), tmp.end());
    }
 
  if (dim >= 3)
    {
      save_user_flags_hex(tmp);
      v.insert(v.end(), tmp.begin(), tmp.end());
    }
 
  if (dim >= 4)
    DEAL_II_NOT_IMPLEMENTED();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_flags(std::istream &in)
{
  load_user_flags_line(in);
 
  if (dim >= 2)
    load_user_flags_quad(in);
 
  if (dim >= 3)
    load_user_flags_hex(in);
 
  if (dim >= 4)
    DEAL_II_NOT_IMPLEMENTED();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_flags(const std::vector<bool> &v)
{
  Assert(v.size() == n_lines() + n_quads() + n_hexs(), ExcInternalError());
  std::vector<bool> tmp;
 
  // first extract the flags
  // belonging to lines
  tmp.insert(tmp.end(), v.begin(), v.begin() + n_lines());
  // and set the lines
  load_user_flags_line(tmp);
 
  if (dim >= 2)
    {
      tmp.clear();
      tmp.insert(tmp.end(),
                 v.begin() + n_lines(),
                 v.begin() + n_lines() + n_quads());
      load_user_flags_quad(tmp);
    }
 
  if (dim >= 3)
    {
      tmp.clear();
      tmp.insert(tmp.end(),
                 v.begin() + n_lines() + n_quads(),
                 v.begin() + n_lines() + n_quads() + n_hexs());
      load_user_flags_hex(tmp);
    }
 
  if (dim >= 4)
    DEAL_II_NOT_IMPLEMENTED();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_flags_line(
  std::vector<bool> &v) const
{
  v.resize(n_lines(), false);
  std::vector<bool>::iterator i    = v.begin();
  line_iterator               line = begin_line(), endl = end_line();
  for (; line != endl; ++line, ++i)
    *i = line->user_flag_set();
 
  Assert(i == v.end(), ExcInternalError());
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_flags_line(std::ostream &out) const
{
  std::vector<bool> v;
  save_user_flags_line(v);
  write_bool_vector(mn_tria_line_user_flags_begin,
                    v,
                    mn_tria_line_user_flags_end,
                    out);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_flags_line(std::istream &in)
{
  std::vector<bool> v;
  read_bool_vector(mn_tria_line_user_flags_begin,
                   v,
                   mn_tria_line_user_flags_end,
                   in);
  load_user_flags_line(v);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_flags_line(
  const std::vector<bool> &v)
{
  Assert(v.size() == n_lines(), ExcGridReadError());
 
  line_iterator                     line = begin_line(), endl = end_line();
  std::vector<bool>::const_iterator i = v.begin();
  for (; line != endl; ++line, ++i)
    if (*i == true)
      line->set_user_flag();
    else
      line->clear_user_flag();
 
  Assert(i == v.end(), ExcInternalError());
}
 
#endif
 
namespace
{
  template <typename Iterator>
  bool
  get_user_flag(const Iterator &i)
  {
    return i->user_flag_set();
  }
 
 
 
  template <int structdim, int dim, int spacedim>
  bool
  get_user_flag(const TriaIterator<InvalidAccessor<structdim, dim, spacedim>> &)
  {
    DEAL_II_ASSERT_UNREACHABLE();
    return false;
  }
 
 
 
  template <typename Iterator>
  void
  set_user_flag(const Iterator &i)
  {
    i->set_user_flag();
  }
 
 
 
  template <int structdim, int dim, int spacedim>
  void
  set_user_flag(const TriaIterator<InvalidAccessor<structdim, dim, spacedim>> &)
  {
    DEAL_II_ASSERT_UNREACHABLE();
  }
 
 
 
  template <typename Iterator>
  void
  clear_user_flag(const Iterator &i)
  {
    i->clear_user_flag();
  }
 
 
 
  template <int structdim, int dim, int spacedim>
  void
  clear_user_flag(
    const TriaIterator<InvalidAccessor<structdim, dim, spacedim>> &)
  {
    DEAL_II_ASSERT_UNREACHABLE();
  }
} // namespace
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_flags_quad(
  std::vector<bool> &v) const
{
  v.resize(n_quads(), false);
 
  if (dim >= 2)
    {
      std::vector<bool>::iterator i    = v.begin();
      quad_iterator               quad = begin_quad(), endq = end_quad();
      for (; quad != endq; ++quad, ++i)
        *i = get_user_flag(quad);
 
      Assert(i == v.end(), ExcInternalError());
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_flags_quad(std::ostream &out) const
{
  std::vector<bool> v;
  save_user_flags_quad(v);
  write_bool_vector(mn_tria_quad_user_flags_begin,
                    v,
                    mn_tria_quad_user_flags_end,
                    out);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_flags_quad(std::istream &in)
{
  std::vector<bool> v;
  read_bool_vector(mn_tria_quad_user_flags_begin,
                   v,
                   mn_tria_quad_user_flags_end,
                   in);
  load_user_flags_quad(v);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_flags_quad(
  const std::vector<bool> &v)
{
  Assert(v.size() == n_quads(), ExcGridReadError());
 
  if (dim >= 2)
    {
      quad_iterator                     quad = begin_quad(), endq = end_quad();
      std::vector<bool>::const_iterator i = v.begin();
      for (; quad != endq; ++quad, ++i)
        if (*i == true)
          set_user_flag(quad);
        else
          clear_user_flag(quad);
 
      Assert(i == v.end(), ExcInternalError());
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_flags_hex(
  std::vector<bool> &v) const
{
  v.resize(n_hexs(), false);
 
  if (dim >= 3)
    {
      std::vector<bool>::iterator i   = v.begin();
      hex_iterator                hex = begin_hex(), endh = end_hex();
      for (; hex != endh; ++hex, ++i)
        *i = get_user_flag(hex);
 
      Assert(i == v.end(), ExcInternalError());
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_flags_hex(std::ostream &out) const
{
  std::vector<bool> v;
  save_user_flags_hex(v);
  write_bool_vector(mn_tria_hex_user_flags_begin,
                    v,
                    mn_tria_hex_user_flags_end,
                    out);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_flags_hex(std::istream &in)
{
  std::vector<bool> v;
  read_bool_vector(mn_tria_hex_user_flags_begin,
                   v,
                   mn_tria_hex_user_flags_end,
                   in);
  load_user_flags_hex(v);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_flags_hex(
  const std::vector<bool> &v)
{
  Assert(v.size() == n_hexs(), ExcGridReadError());
 
  if (dim >= 3)
    {
      hex_iterator                      hex = begin_hex(), endh = end_hex();
      std::vector<bool>::const_iterator i = v.begin();
      for (; hex != endh; ++hex, ++i)
        if (*i == true)
          set_user_flag(hex);
        else
          clear_user_flag(hex);
 
      Assert(i == v.end(), ExcInternalError());
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_indices(
  std::vector<unsigned int> &v) const
{
  // clear vector and append all the
  // stuff later on
  v.clear();
 
  std::vector<unsigned int> tmp;
 
  save_user_indices_line(tmp);
  v.insert(v.end(), tmp.begin(), tmp.end());
 
  if (dim >= 2)
    {
      save_user_indices_quad(tmp);
      v.insert(v.end(), tmp.begin(), tmp.end());
    }
 
  if (dim >= 3)
    {
      save_user_indices_hex(tmp);
      v.insert(v.end(), tmp.begin(), tmp.end());
    }
 
  if (dim >= 4)
    DEAL_II_NOT_IMPLEMENTED();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_indices(
  const std::vector<unsigned int> &v)
{
  Assert(v.size() == n_lines() + n_quads() + n_hexs(), ExcInternalError());
  std::vector<unsigned int> tmp;
 
  // first extract the indices
  // belonging to lines
  tmp.insert(tmp.end(), v.begin(), v.begin() + n_lines());
  // and set the lines
  load_user_indices_line(tmp);
 
  if (dim >= 2)
    {
      tmp.clear();
      tmp.insert(tmp.end(),
                 v.begin() + n_lines(),
                 v.begin() + n_lines() + n_quads());
      load_user_indices_quad(tmp);
    }
 
  if (dim >= 3)
    {
      tmp.clear();
      tmp.insert(tmp.end(),
                 v.begin() + n_lines() + n_quads(),
                 v.begin() + n_lines() + n_quads() + n_hexs());
      load_user_indices_hex(tmp);
    }
 
  if (dim >= 4)
    DEAL_II_NOT_IMPLEMENTED();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save(const std::string &file_basename) const
{
  // Save triangulation information.
  {
    std::ofstream ofs_tria(file_basename + "_triangulation.data");
    AssertThrow(ofs_tria.fail() == false, ExcIO());
 
    boost::archive::text_oarchive oa(ofs_tria, boost::archive::no_header);
    save(oa,
         internal::CellAttachedDataSerializer<dim, spacedim>::version_number);
  }
 
  // Save attached data.
  {
    std::ofstream ofs_info(file_basename + ".info");
    ofs_info
      << "version nproc n_attached_fixed_size_objs n_attached_variable_size_objs n_active_cells"
      << std::endl
      << internal::CellAttachedDataSerializer<dim, spacedim>::version_number
      << " " << 1 << " " << this->cell_attached_data.pack_callbacks_fixed.size()
      << " " << this->cell_attached_data.pack_callbacks_variable.size() << " "
      << this->n_global_active_cells() << std::endl;
  }
 
  this->save_attached_data(0, this->n_global_active_cells(), file_basename);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load(const std::string &file_basename)
{
  // It's probably prudent to first get rid of any all content of the
  // triangulation, rather than hope that the deserialization below
  // overwrites everything:
  clear();
 
  // Load triangulation information.
  {
    std::ifstream ifs_tria(file_basename + "_triangulation.data");
    AssertThrow(ifs_tria.fail() == false, ExcIO());
 
    boost::archive::text_iarchive ia(ifs_tria, boost::archive::no_header);
    load(ia,
         internal::CellAttachedDataSerializer<dim, spacedim>::version_number);
  }
 
  // Load attached data.
  unsigned int version, numcpus, attached_count_fixed, attached_count_variable,
    n_global_active_cells;
  {
    std::ifstream ifs_info(std::string(file_basename) + ".info");
    AssertThrow(ifs_info.fail() == false, ExcIO());
    std::string firstline;
    std::getline(ifs_info, firstline);
    ifs_info >> version >> numcpus >> attached_count_fixed >>
      attached_count_variable >> n_global_active_cells;
  }
 
  AssertThrow(numcpus == 1,
              ExcMessage("Incompatible number of CPUs found in .info file."));
 
  const auto expected_version =
    ::internal::CellAttachedDataSerializer<dim,
                                                   spacedim>::version_number;
  AssertThrow(version == expected_version,
              ExcMessage(
                "The information saved in the file you are trying "
                "to read the triangulation from was written with an "
                "incompatible file format version and cannot be read."));
  Assert(this->n_global_active_cells() == n_global_active_cells,
         ExcMessage("The number of cells of the triangulation differs "
                    "from the number of cells written into the .info file."));
 
  // Clear all of the callback data, as explained in the documentation of
  // register_data_attach().
  this->cell_attached_data.n_attached_data_sets = 0;
  this->cell_attached_data.n_attached_deserialize =
    attached_count_fixed + attached_count_variable;
 
  this->load_attached_data(0,
                           this->n_global_active_cells(),
                           this->n_active_cells(),
                           file_basename,
                           attached_count_fixed,
                           attached_count_variable);
 
  this->update_cell_relations();
}
 
#endif
namespace
{
  template <typename Iterator>
  unsigned int
  get_user_index(const Iterator &i)
  {
    return i->user_index();
  }
 
 
 
  template <int structdim, int dim, int spacedim>
  unsigned int
  get_user_index(
    const TriaIterator<InvalidAccessor<structdim, dim, spacedim>> &)
  {
    DEAL_II_ASSERT_UNREACHABLE();
    return numbers::invalid_unsigned_int;
  }
 
 
 
  template <typename Iterator>
  void
  set_user_index(const Iterator &i, const unsigned int x)
  {
    i->set_user_index(x);
  }
 
 
 
  template <int structdim, int dim, int spacedim>
  void
  set_user_index(
    const TriaIterator<InvalidAccessor<structdim, dim, spacedim>> &,
    const unsigned int)
  {
    DEAL_II_ASSERT_UNREACHABLE();
  }
} // namespace
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_indices_line(
  std::vector<unsigned int> &v) const
{
  v.resize(n_lines(), 0);
  std::vector<unsigned int>::iterator i    = v.begin();
  line_iterator                       line = begin_line(), endl = end_line();
  for (; line != endl; ++line, ++i)
    *i = line->user_index();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_indices_line(
  const std::vector<unsigned int> &v)
{
  Assert(v.size() == n_lines(), ExcGridReadError());
 
  line_iterator line = begin_line(), endl = end_line();
  std::vector<unsigned int>::const_iterator i = v.begin();
  for (; line != endl; ++line, ++i)
    line->set_user_index(*i);
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_indices_quad(
  std::vector<unsigned int> &v) const
{
  v.resize(n_quads(), 0);
 
  if (dim >= 2)
    {
      std::vector<unsigned int>::iterator i = v.begin();
      quad_iterator quad = begin_quad(), endq = end_quad();
      for (; quad != endq; ++quad, ++i)
        *i = get_user_index(quad);
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_indices_quad(
  const std::vector<unsigned int> &v)
{
  Assert(v.size() == n_quads(), ExcGridReadError());
 
  if (dim >= 2)
    {
      quad_iterator quad = begin_quad(), endq = end_quad();
      std::vector<unsigned int>::const_iterator i = v.begin();
      for (; quad != endq; ++quad, ++i)
        set_user_index(quad, *i);
    }
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_indices_hex(
  std::vector<unsigned int> &v) const
{
  v.resize(n_hexs(), 0);
 
  if (dim >= 3)
    {
      std::vector<unsigned int>::iterator i   = v.begin();
      hex_iterator                        hex = begin_hex(), endh = end_hex();
      for (; hex != endh; ++hex, ++i)
        *i = get_user_index(hex);
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_indices_hex(
  const std::vector<unsigned int> &v)
{
  Assert(v.size() == n_hexs(), ExcGridReadError());
 
  if (dim >= 3)
    {
      hex_iterator hex = begin_hex(), endh = end_hex();
      std::vector<unsigned int>::const_iterator i = v.begin();
      for (; hex != endh; ++hex, ++i)
        set_user_index(hex, *i);
    }
}
 
#endif
 
 
//---------------- user pointers ----------------------------------------//
 
 
namespace
{
  template <typename Iterator>
  void *
  get_user_pointer(const Iterator &i)
  {
    return i->user_pointer();
  }
 
 
 
  template <int structdim, int dim, int spacedim>
  void *
  get_user_pointer(
    const TriaIterator<InvalidAccessor<structdim, dim, spacedim>> &)
  {
    DEAL_II_ASSERT_UNREACHABLE();
    return nullptr;
  }
 
 
 
  template <typename Iterator>
  void
  set_user_pointer(const Iterator &i, void *x)
  {
    i->set_user_pointer(x);
  }
 
 
 
  template <int structdim, int dim, int spacedim>
  void
  set_user_pointer(
    const TriaIterator<InvalidAccessor<structdim, dim, spacedim>> &,
    void *)
  {
    DEAL_II_ASSERT_UNREACHABLE();
  }
} // namespace
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_pointers(
  std::vector<void *> &v) const
{
  // clear vector and append all the
  // stuff later on
  v.clear();
 
  std::vector<void *> tmp;
 
  save_user_pointers_line(tmp);
  v.insert(v.end(), tmp.begin(), tmp.end());
 
  if (dim >= 2)
    {
      save_user_pointers_quad(tmp);
      v.insert(v.end(), tmp.begin(), tmp.end());
    }
 
  if (dim >= 3)
    {
      save_user_pointers_hex(tmp);
      v.insert(v.end(), tmp.begin(), tmp.end());
    }
 
  if (dim >= 4)
    DEAL_II_NOT_IMPLEMENTED();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_pointers(
  const std::vector<void *> &v)
{
  Assert(v.size() == n_lines() + n_quads() + n_hexs(), ExcInternalError());
  std::vector<void *> tmp;
 
  // first extract the pointers
  // belonging to lines
  tmp.insert(tmp.end(), v.begin(), v.begin() + n_lines());
  // and set the lines
  load_user_pointers_line(tmp);
 
  if (dim >= 2)
    {
      tmp.clear();
      tmp.insert(tmp.end(),
                 v.begin() + n_lines(),
                 v.begin() + n_lines() + n_quads());
      load_user_pointers_quad(tmp);
    }
 
  if (dim >= 3)
    {
      tmp.clear();
      tmp.insert(tmp.end(),
                 v.begin() + n_lines() + n_quads(),
                 v.begin() + n_lines() + n_quads() + n_hexs());
      load_user_pointers_hex(tmp);
    }
 
  if (dim >= 4)
    DEAL_II_NOT_IMPLEMENTED();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_pointers_line(
  std::vector<void *> &v) const
{
  v.resize(n_lines(), nullptr);
  std::vector<void *>::iterator i    = v.begin();
  line_iterator                 line = begin_line(), endl = end_line();
  for (; line != endl; ++line, ++i)
    *i = line->user_pointer();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_pointers_line(
  const std::vector<void *> &v)
{
  Assert(v.size() == n_lines(), ExcGridReadError());
 
  line_iterator                       line = begin_line(), endl = end_line();
  std::vector<void *>::const_iterator i = v.begin();
  for (; line != endl; ++line, ++i)
    line->set_user_pointer(*i);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_pointers_quad(
  std::vector<void *> &v) const
{
  v.resize(n_quads(), nullptr);
 
  if (dim >= 2)
    {
      std::vector<void *>::iterator i    = v.begin();
      quad_iterator                 quad = begin_quad(), endq = end_quad();
      for (; quad != endq; ++quad, ++i)
        *i = get_user_pointer(quad);
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_pointers_quad(
  const std::vector<void *> &v)
{
  Assert(v.size() == n_quads(), ExcGridReadError());
 
  if (dim >= 2)
    {
      quad_iterator quad = begin_quad(), endq = end_quad();
      std::vector<void *>::const_iterator i = v.begin();
      for (; quad != endq; ++quad, ++i)
        set_user_pointer(quad, *i);
    }
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_user_pointers_hex(
  std::vector<void *> &v) const
{
  v.resize(n_hexs(), nullptr);
 
  if (dim >= 3)
    {
      std::vector<void *>::iterator i   = v.begin();
      hex_iterator                  hex = begin_hex(), endh = end_hex();
      for (; hex != endh; ++hex, ++i)
        *i = get_user_pointer(hex);
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_user_pointers_hex(
  const std::vector<void *> &v)
{
  Assert(v.size() == n_hexs(), ExcGridReadError());
 
  if (dim >= 3)
    {
      hex_iterator                        hex = begin_hex(), endh = end_hex();
      std::vector<void *>::const_iterator i = v.begin();
      for (; hex != endh; ++hex, ++i)
        set_user_pointer(hex, *i);
    }
}
 
#endif
 
/*------------------------ Cell iterator functions ------------------------*/
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::raw_cell_iterator
  Triangulation<dim, spacedim>::begin_raw(const unsigned int level) const
{
  switch (dim)
    {
      case 1:
        return begin_raw_line(level);
      case 2:
        return begin_raw_quad(level);
      case 3:
        return begin_raw_hex(level);
      default:
        DEAL_II_NOT_IMPLEMENTED();
        return raw_cell_iterator();
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::cell_iterator
  Triangulation<dim, spacedim>::begin(const unsigned int level) const
{
  switch (dim)
    {
      case 1:
        return begin_line(level);
      case 2:
        return begin_quad(level);
      case 3:
        return begin_hex(level);
      default:
        Assert(false, ExcImpossibleInDim(dim));
        return cell_iterator();
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::active_cell_iterator
  Triangulation<dim, spacedim>::begin_active(const unsigned int level) const
{
  switch (dim)
    {
      case 1:
        return begin_active_line(level);
      case 2:
        return begin_active_quad(level);
      case 3:
        return begin_active_hex(level);
      default:
        DEAL_II_NOT_IMPLEMENTED();
        return active_cell_iterator();
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::cell_iterator
  Triangulation<dim, spacedim>::last() const
{
  const unsigned int level = levels.size() - 1;
  if (levels[level]->cells.n_objects() == 0)
    return end(level);
 
  // find the last raw iterator on
  // this level
  raw_cell_iterator ri(const_cast<Triangulation<dim, spacedim> *>(this),
                       level,
                       levels[level]->cells.n_objects() - 1);
 
  // then move to the last used one
  if (ri->used() == true)
    return ri;
  while ((--ri).state() == IteratorState::valid)
    if (ri->used() == true)
      return ri;
  return ri;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::active_cell_iterator
  Triangulation<dim, spacedim>::last_active() const
{
  // get the last used cell
  cell_iterator cell = last();
 
  if (cell != end())
    {
      // then move to the last active one
      if (cell->is_active() == true)
        return cell;
      while ((--cell).state() == IteratorState::valid)
        if (cell->is_active() == true)
          return cell;
    }
  return cell;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::cell_iterator
  Triangulation<dim, spacedim>::create_cell_iterator(
    const CellId &cell_id) const
{
  Assert(
    this->contains_cell(cell_id),
    ExcMessage(
      "CellId is invalid for this triangulation.\n"
      "Either the provided CellId does not correspond to a cell in this "
      "triangulation object, or, in case you are using a parallel "
      "triangulation, may correspond to an artificial cell that is less "
      "refined on this processor. In the case of "
      "parallel::fullydistributed::Triangulation, the corresponding coarse "
      "cell might not be accessible by the current process."));
 
  cell_iterator cell(
    this, 0, coarse_cell_id_to_coarse_cell_index(cell_id.get_coarse_cell_id()));
 
  for (const auto &child_index : cell_id.get_child_indices())
    cell = cell->child(static_cast<unsigned int>(child_index));
 
  return cell;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
bool Triangulation<dim, spacedim>::contains_cell(const CellId &cell_id) const
{
  const auto coarse_cell_index =
    coarse_cell_id_to_coarse_cell_index(cell_id.get_coarse_cell_id());
 
  if (coarse_cell_index == numbers::invalid_unsigned_int)
    return false;
 
  cell_iterator cell(this, 0, coarse_cell_index);
 
  for (const auto &child_index : cell_id.get_child_indices())
    {
      if (cell->has_children() == false)
        return false;
      cell = cell->child(static_cast<unsigned int>(child_index));
    }
 
  return true;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::cell_iterator
  Triangulation<dim, spacedim>::end() const
{
  return cell_iterator(const_cast<Triangulation<dim, spacedim> *>(this),
                       -1,
                       -1);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::raw_cell_iterator
  Triangulation<dim, spacedim>::end_raw(const unsigned int level) const
{
  // This function may be called on parallel triangulations on levels
  // that exist globally, but not on the local portion of the
  // triangulation. In that case, just return the end iterator.
  //
  // We need to use levels.size() instead of n_levels() because the
  // latter function uses the cache, but we need to be able to call
  // this function at a time when the cache is not currently up to
  // date.
  if (level >= levels.size())
    {
      Assert(level < n_global_levels(),
             ExcInvalidLevel(level, n_global_levels()));
      return end();
    }
 
  // Query whether the given level is valid for the local portion of the
  // triangulation.
  Assert(level < levels.size(), ExcInvalidLevel(level, levels.size()));
  if (level < levels.size() - 1)
    return begin_raw(level + 1);
  else
    return end();
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::cell_iterator
  Triangulation<dim, spacedim>::end(const unsigned int level) const
{
  // This function may be called on parallel triangulations on levels
  // that exist globally, but not on the local portion of the
  // triangulation. In that case, just return the end iterator.
  //
  // We need to use levels.size() instead of n_levels() because the
  // latter function uses the cache, but we need to be able to call
  // this function at a time when the cache is not currently up to
  // date.
  if (level >= levels.size())
    {
      Assert(level < n_global_levels(),
             ExcInvalidLevel(level, n_global_levels()));
      return end();
    }
 
  // Query whether the given level is valid for the local portion of the
  // triangulation.
  Assert(level < levels.size(), ExcInvalidLevel(level, levels.size()));
  if (level < levels.size() - 1)
    return begin(level + 1);
  else
    return end();
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::active_cell_iterator
  Triangulation<dim, spacedim>::end_active(const unsigned int level) const
{
  // This function may be called on parallel triangulations on levels
  // that exist globally, but not on the local portion of the
  // triangulation. In that case, just return the end iterator.
  //
  // We need to use levels.size() instead of n_levels() because the
  // latter function uses the cache, but we need to be able to call
  // this function at a time when the cache is not currently up to
  // date.
  if (level >= levels.size())
    {
      Assert(level < n_global_levels(),
             ExcInvalidLevel(level, n_global_levels()));
      return end();
    }
 
  // Query whether the given level is valid for the local portion of the
  // triangulation.
  Assert(level < levels.size(), ExcInvalidLevel(level, levels.size()));
  return (level >= levels.size() - 1 ? active_cell_iterator(end()) :
                                       begin_active(level + 1));
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
IteratorRange<typename Triangulation<dim, spacedim>::
                cell_iterator> Triangulation<dim, spacedim>::cell_iterators()
  const
{
  return IteratorRange<typename Triangulation<dim, spacedim>::cell_iterator>(
    begin(), end());
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
IteratorRange<typename Triangulation<dim, spacedim>::
                active_cell_iterator> Triangulation<dim, spacedim>::
  active_cell_iterators() const
{
  return IteratorRange<
    typename Triangulation<dim, spacedim>::active_cell_iterator>(begin_active(),
                                                                 end());
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
IteratorRange<typename Triangulation<dim, spacedim>::
                cell_iterator> Triangulation<dim, spacedim>::
  cell_iterators_on_level(const unsigned int level) const
{
  return IteratorRange<typename Triangulation<dim, spacedim>::cell_iterator>(
    begin(level), end(level));
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
IteratorRange<typename Triangulation<dim, spacedim>::
                active_cell_iterator> Triangulation<dim, spacedim>::
  active_cell_iterators_on_level(const unsigned int level) const
{
  return IteratorRange<
    typename Triangulation<dim, spacedim>::active_cell_iterator>(
    begin_active(level), end_active(level));
}
#endif
 
/*------------------------ Face iterator functions ------------------------*/
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::face_iterator
  Triangulation<dim, spacedim>::begin_face() const
{
  switch (dim)
    {
      case 1:
        Assert(false, ExcImpossibleInDim(1));
        return raw_face_iterator();
      case 2:
        return begin_line();
      case 3:
        return begin_quad();
      default:
        DEAL_II_NOT_IMPLEMENTED();
        return face_iterator();
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::active_face_iterator
  Triangulation<dim, spacedim>::begin_active_face() const
{
  switch (dim)
    {
      case 1:
        Assert(false, ExcImpossibleInDim(1));
        return raw_face_iterator();
      case 2:
        return begin_active_line();
      case 3:
        return begin_active_quad();
      default:
        DEAL_II_NOT_IMPLEMENTED();
        return active_face_iterator();
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::face_iterator
  Triangulation<dim, spacedim>::end_face() const
{
  switch (dim)
    {
      case 1:
        Assert(false, ExcImpossibleInDim(1));
        return raw_face_iterator();
      case 2:
        return end_line();
      case 3:
        return end_quad();
      default:
        DEAL_II_NOT_IMPLEMENTED();
        return raw_face_iterator();
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
IteratorRange<typename Triangulation<dim, spacedim>::
                active_face_iterator> Triangulation<dim, spacedim>::
  active_face_iterators() const
{
  return IteratorRange<
    typename Triangulation<dim, spacedim>::active_face_iterator>(
    begin_active_face(), end_face());
}
 
/*------------------------ Vertex iterator functions ------------------------*/
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::vertex_iterator
  Triangulation<dim, spacedim>::begin_vertex() const
{
  vertex_iterator i =
    raw_vertex_iterator(const_cast<Triangulation<dim, spacedim> *>(this), 0, 0);
  if (i.state() != IteratorState::valid)
    return i;
  // This loop will end because every triangulation has used vertices.
  while (i->used() == false)
    if ((++i).state() != IteratorState::valid)
      return i;
  return i;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::active_vertex_iterator
  Triangulation<dim, spacedim>::begin_active_vertex() const
{
  // every vertex is active
  return begin_vertex();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::vertex_iterator
  Triangulation<dim, spacedim>::end_vertex() const
{
  return raw_vertex_iterator(const_cast<Triangulation<dim, spacedim> *>(this),
                             -1,
                             numbers::invalid_unsigned_int);
}
 
#endif
 
 
/*------------------------ Line iterator functions ------------------------*/
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::raw_line_iterator
  Triangulation<dim, spacedim>::begin_raw_line(const unsigned int level) const
{
  // This function may be called on parallel triangulations on levels
  // that exist globally, but not on the local portion of the
  // triangulation. In that case, just return the end iterator.
  //
  // We need to use levels.size() instead of n_levels() because the
  // latter function uses the cache, but we need to be able to call
  // this function at a time when the cache is not currently up to
  // date.
  if (level >= levels.size())
    {
      Assert(level < n_global_levels(),
             ExcInvalidLevel(level, n_global_levels()));
      return end_line();
    }
 
  switch (dim)
    {
      case 1:
        // Query whether the given level is valid for the local portion of the
        // triangulation.
        Assert(level < levels.size(), ExcInvalidLevel(level, levels.size()));
 
        if (level >= levels.size() || levels[level]->cells.n_objects() == 0)
          return end_line();
 
        return raw_line_iterator(
          const_cast<Triangulation<dim, spacedim> *>(this), level, 0);
 
      default:
        Assert(level == 0, ExcFacesHaveNoLevel());
        return raw_line_iterator(
          const_cast<Triangulation<dim, spacedim> *>(this), 0, 0);
    }
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::line_iterator
  Triangulation<dim, spacedim>::begin_line(const unsigned int level) const
{
  // level is checked in begin_raw
  raw_line_iterator ri = begin_raw_line(level);
  if (ri.state() != IteratorState::valid)
    return ri;
  while (ri->used() == false)
    if ((++ri).state() != IteratorState::valid)
      return ri;
  return ri;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::active_line_iterator
  Triangulation<dim, spacedim>::begin_active_line(
    const unsigned int level) const
{
  // level is checked in begin_raw
  line_iterator i = begin_line(level);
  if (i.state() != IteratorState::valid)
    return i;
  while (i->has_children())
    if ((++i).state() != IteratorState::valid)
      return i;
  return i;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::line_iterator
  Triangulation<dim, spacedim>::end_line() const
{
  return raw_line_iterator(const_cast<Triangulation<dim, spacedim> *>(this),
                           -1,
                           -1);
}
 
#endif
 
/*------------------------ Quad iterator functions ------------------------*/
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::raw_quad_iterator
  Triangulation<dim, spacedim>::begin_raw_quad(const unsigned int level) const
{
  // This function may be called on parallel triangulations on levels
  // that exist globally, but not on the local portion of the
  // triangulation. In that case, just return the end iterator.
  //
  // We need to use levels.size() instead of n_levels() because the
  // latter function uses the cache, but we need to be able to call
  // this function at a time when the cache is not currently up to
  // date.
  if (level >= levels.size())
    {
      Assert(level < n_global_levels(),
             ExcInvalidLevel(level, n_global_levels()));
      return end_quad();
    }
 
  switch (dim)
    {
      case 1:
        Assert(false, ExcImpossibleInDim(1));
        return raw_hex_iterator();
      case 2:
        {
          // Query whether the given level is valid for the local portion of the
          // triangulation.
          Assert(level < levels.size(), ExcInvalidLevel(level, levels.size()));
 
          if (level >= levels.size() || levels[level]->cells.n_objects() == 0)
            return end_quad();
 
          return raw_quad_iterator(
            const_cast<Triangulation<dim, spacedim> *>(this), level, 0);
        }
 
      case 3:
        {
          Assert(level == 0, ExcFacesHaveNoLevel());
 
          return raw_quad_iterator(
            const_cast<Triangulation<dim, spacedim> *>(this), 0, 0);
        }
 
 
      default:
        DEAL_II_NOT_IMPLEMENTED();
        return raw_hex_iterator();
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::quad_iterator
  Triangulation<dim, spacedim>::begin_quad(const unsigned int level) const
{
  // level is checked in begin_raw
  raw_quad_iterator ri = begin_raw_quad(level);
  if (ri.state() != IteratorState::valid)
    return ri;
  while (ri->used() == false)
    if ((++ri).state() != IteratorState::valid)
      return ri;
  return ri;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::active_quad_iterator
  Triangulation<dim, spacedim>::begin_active_quad(
    const unsigned int level) const
{
  // level is checked in begin_raw
  quad_iterator i = begin_quad(level);
  if (i.state() != IteratorState::valid)
    return i;
  while (i->has_children())
    if ((++i).state() != IteratorState::valid)
      return i;
  return i;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::quad_iterator
  Triangulation<dim, spacedim>::end_quad() const
{
  return raw_quad_iterator(const_cast<Triangulation<dim, spacedim> *>(this),
                           -1,
                           -1);
}
 
#endif
 
/*------------------------ Hex iterator functions ------------------------*/
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::raw_hex_iterator
  Triangulation<dim, spacedim>::begin_raw_hex(const unsigned int level) const
{
  // This function may be called on parallel triangulations on levels
  // that exist globally, but not on the local portion of the
  // triangulation. In that case, just return the end iterator.
  //
  // We need to use levels.size() instead of n_levels() because the
  // latter function uses the cache, but we need to be able to call
  // this function at a time when the cache is not currently up to
  // date.
  if (level >= levels.size())
    {
      Assert(level < n_global_levels(),
             ExcInvalidLevel(level, n_global_levels()));
      return end_hex();
    }
 
  switch (dim)
    {
      case 1:
      case 2:
        Assert(false, ExcImpossibleInDim(1));
        return raw_hex_iterator();
      case 3:
        {
          // Query whether the given level is valid for the local portion of the
          // triangulation.
          Assert(level < levels.size(), ExcInvalidLevel(level, levels.size()));
 
          if (level >= levels.size() || levels[level]->cells.n_objects() == 0)
            return end_hex();
 
          return raw_hex_iterator(
            const_cast<Triangulation<dim, spacedim> *>(this), level, 0);
        }
 
      default:
        DEAL_II_NOT_IMPLEMENTED();
        return raw_hex_iterator();
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::hex_iterator
  Triangulation<dim, spacedim>::begin_hex(const unsigned int level) const
{
  // level is checked in begin_raw
  raw_hex_iterator ri = begin_raw_hex(level);
  if (ri.state() != IteratorState::valid)
    return ri;
  while (ri->used() == false)
    if ((++ri).state() != IteratorState::valid)
      return ri;
  return ri;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::active_hex_iterator
  Triangulation<dim, spacedim>::begin_active_hex(const unsigned int level) const
{
  // level is checked in begin_raw
  hex_iterator i = begin_hex(level);
  if (i.state() != IteratorState::valid)
    return i;
  while (i->has_children())
    if ((++i).state() != IteratorState::valid)
      return i;
  return i;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::hex_iterator
  Triangulation<dim, spacedim>::end_hex() const
{
  return raw_hex_iterator(const_cast<Triangulation<dim, spacedim> *>(this),
                          -1,
                          -1);
}
 
#endif
 
// -------------------------------- number of cells etc ---------------
 
 
namespace internal
{
  namespace TriangulationImplementation
  {
    unsigned int
    n_cells(const internal::TriangulationImplementation::NumberCache<1> &c)
    {
      return c.n_lines;
    }
 
 
    unsigned int
    n_active_cells(
      const internal::TriangulationImplementation::NumberCache<1> &c)
    {
      return c.n_active_lines;
    }
 
 
    unsigned int
    n_cells(const internal::TriangulationImplementation::NumberCache<2> &c)
    {
      return c.n_quads;
    }
 
 
    unsigned int
    n_active_cells(
      const internal::TriangulationImplementation::NumberCache<2> &c)
    {
      return c.n_active_quads;
    }
 
 
    unsigned int
    n_cells(const internal::TriangulationImplementation::NumberCache<3> &c)
    {
      return c.n_hexes;
    }
 
 
    unsigned int
    n_active_cells(
      const internal::TriangulationImplementation::NumberCache<3> &c)
    {
      return c.n_active_hexes;
    }
  } // namespace TriangulationImplementation
} // namespace internal
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_cells() const
{
  return internal::TriangulationImplementation::n_cells(number_cache);
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_active_cells() const
{
  return internal::TriangulationImplementation::n_active_cells(number_cache);
}
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
types::global_cell_index
  Triangulation<dim, spacedim>::n_global_active_cells() const
{
  return n_active_cells();
}
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
types::coarse_cell_id
  Triangulation<dim, spacedim>::n_global_coarse_cells() const
{
  return n_cells(0);
}
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_faces() const
{
  switch (dim)
    {
      case 1:
        return n_used_vertices();
      case 2:
        return n_lines();
      case 3:
        return n_quads();
      default:
        DEAL_II_NOT_IMPLEMENTED();
    }
  return 0;
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_raw_faces() const
{
  switch (dim)
    {
      case 1:
        return n_vertices();
      case 2:
        return n_raw_lines();
      case 3:
        return n_raw_quads();
      default:
        DEAL_II_NOT_IMPLEMENTED();
    }
  return 0;
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_active_faces() const
{
  switch (dim)
    {
      case 1:
        return n_used_vertices();
      case 2:
        return n_active_lines();
      case 3:
        return n_active_quads();
      default:
        DEAL_II_NOT_IMPLEMENTED();
    }
  return 0;
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_raw_cells(
  const unsigned int level) const
{
  switch (dim)
    {
      case 1:
        return n_raw_lines(level);
      case 2:
        return n_raw_quads(level);
      case 3:
        return n_raw_hexs(level);
      default:
        DEAL_II_NOT_IMPLEMENTED();
    }
  return 0;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_cells(
  const unsigned int level) const
{
  switch (dim)
    {
      case 1:
        return n_lines(level);
      case 2:
        return n_quads(level);
      case 3:
        return n_hexs(level);
      default:
        DEAL_II_NOT_IMPLEMENTED();
    }
  return 0;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_active_cells(
  const unsigned int level) const
{
  switch (dim)
    {
      case 1:
        return n_active_lines(level);
      case 2:
        return n_active_quads(level);
      case 3:
        return n_active_hexs(level);
      default:
        DEAL_II_NOT_IMPLEMENTED();
    }
  return 0;
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
bool Triangulation<dim, spacedim>::has_hanging_nodes() const
{
  if (anisotropic_refinement == false)
    {
      for (unsigned int lvl = 0; lvl < n_global_levels() - 1; ++lvl)
        if (n_active_cells(lvl) != 0)
          return true;
    }
  else
    {
      for (const auto &cell : active_cell_iterators())
        for (const auto &i : cell->face_indices())
          if (cell->face(i)->has_children())
            return true;
    }
  return false;
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_lines() const
{
  return number_cache.n_lines;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_raw_lines(
  const unsigned int level) const
{
  if (dim == 1)
    {
      AssertIndexRange(level, n_levels());
      return levels[level]->cells.n_objects();
    }
 
  Assert(false, ExcFacesHaveNoLevel());
  return 0;
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_raw_lines() const
{
  if (dim == 1)
    {
      DEAL_II_NOT_IMPLEMENTED();
      return 0;
    }
 
  return faces->lines.n_objects();
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_lines(
  const unsigned int level) const
{
  AssertIndexRange(level, number_cache.n_lines_level.size());
  Assert(dim == 1, ExcFacesHaveNoLevel());
  return number_cache.n_lines_level[level];
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_active_lines() const
{
  return number_cache.n_active_lines;
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_active_lines(
  const unsigned int level) const
{
  AssertIndexRange(level, number_cache.n_lines_level.size());
  Assert(dim == 1, ExcFacesHaveNoLevel());
 
  return number_cache.n_active_lines_level[level];
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_quads() const
{
  if constexpr (dim == 1)
    return 0;
  else
    return number_cache.n_quads;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_quads(
  const unsigned int level) const
{
  (void)level;
  if constexpr (dim == 1)
    return 0;
  else if constexpr (dim == 2)
    {
      AssertIndexRange(level, number_cache.n_quads_level.size());
      return number_cache.n_quads_level[level];
    }
  else if constexpr (dim == 3)
    Assert(dim < 3, ExcFacesHaveNoLevel());
  else
    DEAL_II_NOT_IMPLEMENTED();
 
  return numbers::invalid_unsigned_int;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_raw_quads() const
{
  if constexpr (dim == 1)
    return 0;
  else if constexpr (dim == 2)
    DEAL_II_NOT_IMPLEMENTED();
  else if constexpr (dim == 3)
    return faces->quads.n_objects();
  else
    DEAL_II_NOT_IMPLEMENTED();
 
  return numbers::invalid_unsigned_int;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_raw_quads(
  const unsigned int level) const
{
  (void)level;
  if constexpr (dim == 1)
    return 0;
  else if constexpr (dim == 2)
    {
      AssertIndexRange(level, n_levels());
      return levels[level]->cells.n_objects();
    }
  else if constexpr (dim == 3)
    Assert(dim < 3, ExcFacesHaveNoLevel());
  else
    DEAL_II_NOT_IMPLEMENTED();
 
  return numbers::invalid_unsigned_int;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_active_quads() const
{
  if constexpr (dim == 1)
    return 0;
  else
    return number_cache.n_active_quads;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_active_quads(
  const unsigned int level) const
{
  (void)level;
  if constexpr (dim == 1)
    return 0;
  else if constexpr (dim == 2)
    {
      AssertIndexRange(level, number_cache.n_quads_level.size());
      return number_cache.n_active_quads_level[level];
    }
  else if constexpr (dim == 3)
    Assert(dim < 3, ExcFacesHaveNoLevel());
  else
    DEAL_II_NOT_IMPLEMENTED();
 
  return numbers::invalid_unsigned_int;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_hexs() const
{
  if constexpr (dim == 3)
    return number_cache.n_hexes;
  else
    return 0;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_hexs(
  const unsigned int level) const
{
  (void)level;
  if constexpr (dim == 3)
    {
      AssertIndexRange(level, number_cache.n_hexes_level.size());
      return number_cache.n_hexes_level[level];
    }
  else
    return 0;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_raw_hexs(
  const unsigned int level) const
{
  (void)level;
  if constexpr (dim == 3)
    {
      AssertIndexRange(level, n_levels());
      return levels[level]->cells.n_objects();
    }
  else
    return 0;
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_active_hexs() const
{
  if constexpr (dim == 3)
    return number_cache.n_active_hexes;
  else
    return 0;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_active_hexs(
  const unsigned int level) const
{
  (void)level;
  if constexpr (dim == 3)
    {
      AssertIndexRange(level, number_cache.n_hexes_level.size());
      return number_cache.n_active_hexes_level[level];
    }
  else
    return 0;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::n_used_vertices() const
{
  return std::count(vertices_used.begin(), vertices_used.end(), true);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
const std::vector<bool> &Triangulation<dim, spacedim>::get_used_vertices() const
{
  return vertices_used;
}
 
#endif
 
template <>
unsigned int
Triangulation<1, 1>::max_adjacent_cells() const
{
  return 2;
}
 
 
 
template <>
unsigned int
Triangulation<1, 2>::max_adjacent_cells() const
{
  return 2;
}
 
 
template <>
unsigned int
Triangulation<1, 3>::max_adjacent_cells() const
{
  return 2;
}
 
#ifndef DOXYGEN
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::max_adjacent_cells() const
{
  cell_iterator cell = begin(0),
                endc = (n_levels() > 1 ? begin(1) : cell_iterator(end()));
  // store the largest index of the
  // vertices used on level 0
  unsigned int max_vertex_index = 0;
  for (; cell != endc; ++cell)
    for (const unsigned int vertex : GeometryInfo<dim>::vertex_indices())
      if (cell->vertex_index(vertex) > max_vertex_index)
        max_vertex_index = cell->vertex_index(vertex);
 
  // store the number of times a cell
  // touches a vertex. An unsigned
  // int should suffice, even for
  // larger dimensions
  std::vector<unsigned short int> usage_count(max_vertex_index + 1, 0);
  // touch a vertex's usage count
  // every time we find an adjacent
  // element
  for (cell = begin(); cell != endc; ++cell)
    for (const unsigned int vertex : GeometryInfo<dim>::vertex_indices())
      ++usage_count[cell->vertex_index(vertex)];
 
  return std::max(GeometryInfo<dim>::vertices_per_cell,
                  static_cast<unsigned int>(
                    *std::max_element(usage_count.begin(), usage_count.end())));
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
types::subdomain_id
  Triangulation<dim, spacedim>::locally_owned_subdomain() const
{
  return numbers::invalid_subdomain_id;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
Triangulation<dim, spacedim> &Triangulation<dim, spacedim>::get_triangulation()
{
  return *this;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
const Triangulation<dim, spacedim>
  &Triangulation<dim, spacedim>::get_triangulation() const
{
  return *this;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::add_periodicity(
  const std::vector<GridTools::PeriodicFacePair<cell_iterator>>
    &periodicity_vector)
{
  periodic_face_pairs_level_0.insert(periodic_face_pairs_level_0.end(),
                                     periodicity_vector.begin(),
                                     periodicity_vector.end());
 
  // Now initialize periodic_face_map
  update_periodic_face_map();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
const typename std::map<
  std::pair<typename Triangulation<dim, spacedim>::cell_iterator, unsigned int>,
  std::pair<std::pair<typename Triangulation<dim, spacedim>::cell_iterator,
                      unsigned int>,
            types::geometric_orientation>>
  &Triangulation<dim, spacedim>::get_periodic_face_map() const
{
  return periodic_face_map;
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::update_cell_relations()
{
  // We only update the cell relations here for serial triangulations.
  // For other triangulations, this is done at other stages of
  // mesh creation and mesh refinement.
  if (dynamic_cast<parallel::DistributedTriangulationBase<dim, spacedim> *>(
        this))
    return;
 
  this->local_cell_relations.clear();
  this->local_cell_relations.reserve(this->n_active_cells());
 
  for (const auto &cell : this->active_cell_iterators())
    this->local_cell_relations.emplace_back(
      cell, ::CellStatus::cell_will_persist);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::pack_data_serial()
{
  if (dynamic_cast<parallel::DistributedTriangulationBase<dim, spacedim> *>(
        this))
    return;
 
  // pack data before triangulation gets updated
  if (this->cell_attached_data.n_attached_data_sets > 0)
    {
      this->local_cell_relations.clear();
 
      for (const auto &cell : this->active_cell_iterators())
        {
          const bool children_will_be_coarsened =
            (cell->level() > 0) && (cell->coarsen_flag_set());
 
          if (children_will_be_coarsened == false)
            {
              ::CellStatus status =
                cell->refine_flag_set() ?
                  ::CellStatus::cell_will_be_refined :
                  ::CellStatus::cell_will_persist;
 
              local_cell_relations.emplace_back(cell, status);
            }
          else if (cell->parent()->child_iterator_to_index(cell) == 0)
            {
              local_cell_relations.emplace_back(
                cell->parent(),
                ::CellStatus::children_will_be_coarsened);
 
              for (unsigned int i = 1; i < cell->parent()->n_children(); ++i)
                local_cell_relations.emplace_back(
                  cell->parent(), ::CellStatus::cell_invalid);
            }
        }
 
      // pack data
      this->data_serializer.pack_data(
        this->local_cell_relations,
        this->cell_attached_data.pack_callbacks_fixed,
        this->cell_attached_data.pack_callbacks_variable,
        this->get_mpi_communicator());
 
      // dummy copy of data
      this->data_serializer.dest_data_fixed =
        this->data_serializer.src_data_fixed;
      this->data_serializer.dest_data_variable =
        this->data_serializer.src_data_variable;
      this->data_serializer.dest_sizes_variable =
        this->data_serializer.src_sizes_variable;
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::unpack_data_serial()
{
  if (dynamic_cast<parallel::DistributedTriangulationBase<dim, spacedim> *>(
        this))
    return;
 
  // transfer data after triangulation got updated
  if (this->cell_attached_data.n_attached_data_sets > 0)
    {
      std::vector<typename internal::CellAttachedDataSerializer<dim, spacedim>::
                    cell_relation_t>
        temp;
 
      for (const auto &cell : local_cell_relations)
        {
          if (cell.first->has_children())
            {
              Assert(cell.second == ::CellStatus::cell_will_be_refined,
                     ExcInternalError());
 
              temp.emplace_back(cell.first->child(0),
                                ::CellStatus::cell_will_be_refined);
            }
          else
            temp.push_back(cell);
        }
 
      this->local_cell_relations = std::move(temp);
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::execute_coarsening_and_refinement()
{
  // Call our version of prepare_coarsening_and_refinement() even if a derived
  // class like parallel::distributed::Triangulation overrides it. Their
  // function will be called in their execute_coarsening_and_refinement()
  // function. Even in a distributed computation our job here is to reconstruct
  // the local part of the mesh and as such checking our flags is enough.
  Triangulation<dim, spacedim>::prepare_coarsening_and_refinement();
 
  // verify a case with which we have had
  // some difficulty in the past (see the
  // deal.II/coarsening_* tests)
  if (smooth_grid & limit_level_difference_at_vertices)
    Assert(satisfies_level1_at_vertex_rule(*this), ExcInternalError());
 
  // Inform all listeners about beginning of refinement.
  signals.pre_refinement();
 
  this->pack_data_serial();
 
  execute_coarsening();
 
  const DistortedCellList cells_with_distorted_children = execute_refinement();
 
  // We need to update the cell relations in order to be able to
  // deserialize data. Later on, update_cell_relations is called to mark all
  // active cells with the cell_will_persist status.
  this->unpack_data_serial();
 
  reset_cell_vertex_indices_cache();
 
  // If the line_to_adjacent_cell_map is populated, clear it, as it must
  // be recomputed every time the mesh changes.
  line_to_adjacent_cells_map.reset();
 
  // verify a case with which we have had
  // some difficulty in the past (see the
  // deal.II/coarsening_* tests)
  if (smooth_grid & limit_level_difference_at_vertices)
    Assert(satisfies_level1_at_vertex_rule(*this) == true, ExcInternalError());
 
  // finally build up neighbor connectivity information, and set
  // active cell indices
  this->policy->update_neighbors(*this);
  reset_active_cell_indices();
 
  reset_global_cell_indices(); // TODO: better place?
 
  // Inform all listeners about end of refinement.
  signals.post_refinement();
 
  AssertThrow(cells_with_distorted_children.distorted_cells.empty(),
              cells_with_distorted_children);
 
  update_periodic_face_map();
 
  if (this->cell_attached_data.n_attached_data_sets == 0)
    this->update_cell_relations();
 
  if constexpr (running_in_debug_mode())
    {
      // In debug mode, we want to check for some consistency of the
      // result of this function. Because there are multiple exit
      // paths, put this check into a ScopeExit object that is
      // executed on each of the exit paths.
      //
      // Specifically, check on exit of this function that if a quad
      // cell has been refined, all of its children have neighbors
      // in all directions in which the parent cell has neighbors as
      // well. The children's neighbors are either the parent
      // neighbor or the parent neighbor's children, or simply one of
      // the other children of the current cell. This check is
      // useful because if one creates a triangulation with an
      // inconsistently ordered set of cells (e.g., because one has
      // forgotten to call GridTools::consistently_order_cells()),
      // then this relatively simple invariant is violated -- so the
      // check here can be used to catch that case, at least
      // sometimes.
      //
      // In 1d, this situation cannot happen. In 3d, we have explicit
      // orientation flags to ensure that it is not necessary to re-orient
      // cells at the beginning. But in both cases, the invariant should
      // still hold as long as the cell is a hypercube.
      for (const auto &cell : cell_iterators())
        {
          if (cell->has_children() && cell->reference_cell().is_hyper_cube())
            for (const unsigned int f : cell->face_indices())
              if (cell->at_boundary(f) == false)
                {
                  for (const auto &child : cell->child_iterators())
                    {
                      Assert(
                        child->at_boundary(f) == false,
                        ExcMessage(
                          "We ended up with a triangulation whose child cells "
                          "are not connected to their neighbors as expected. "
                          "When you created the triangulation, did you forget "
                          "to call GridTools::consistently_order_cells() "
                          "before calling Triangulation::create_triangulation()?"));
                    }
                }
        }
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::reset_active_cell_indices()
{
  unsigned int active_cell_index = 0;
  for (raw_cell_iterator cell = begin_raw(); cell != end(); ++cell)
    if ((cell->used() == false) || cell->has_children())
      cell->set_active_cell_index(numbers::invalid_unsigned_int);
    else
      {
        cell->set_active_cell_index(active_cell_index);
        ++active_cell_index;
      }
 
  Assert(active_cell_index == n_active_cells(), ExcInternalError());
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::reset_global_cell_indices()
{
  types::global_cell_index active_cell_index = 0;
  for (unsigned int l = 0; l < levels.size(); ++l)
    {
      types::global_cell_index level_cell_index = 0;
      for (const auto &cell : cell_iterators_on_level(l))
        {
          cell->set_global_level_cell_index(level_cell_index++);
          if (cell->is_active())
            cell->set_global_active_cell_index(active_cell_index++);
        }
    }
  AssertDimension(active_cell_index, this->n_active_cells());
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::reset_cell_vertex_indices_cache()
{
  for (unsigned int l = 0; l < levels.size(); ++l)
    for (const auto &cell : cell_iterators_on_level(l))
      {
        const unsigned int n_vertices = cell->n_vertices();
 
        std_cxx26::inplace_vector<unsigned int,
                                  ReferenceCells::max_n_vertices<dim>()>
          cell_vertices(n_vertices);
        if constexpr (running_in_debug_mode())
          std::fill(cell_vertices.begin(),
                    cell_vertices.end(),
                    numbers::invalid_unsigned_int);
 
        GridTools::internal::extract_vertices_without_cache<dim, spacedim>(
          cell, cell_vertices);
        for (unsigned int vertex_no = 0; vertex_no < n_vertices; ++vertex_no)
          levels[l]->set_cached_vertex_index(cell->index(),
                                             vertex_no,
                                             cell_vertices[vertex_no]);
      }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::update_periodic_face_map()
{
  // first empty the currently stored objects
  periodic_face_map.clear();
 
  typename std::vector<
    GridTools::PeriodicFacePair<cell_iterator>>::const_iterator it;
  for (it = periodic_face_pairs_level_0.begin();
       it != periodic_face_pairs_level_0.end();
       ++it)
    {
      update_periodic_face_map_recursively<dim, spacedim>(it->cell[0],
                                                          it->cell[1],
                                                          it->face_idx[0],
                                                          it->face_idx[1],
                                                          it->orientation,
                                                          periodic_face_map);
 
      const auto face_reference_cell =
        it->cell[0]->reference_cell().face_reference_cell(it->face_idx[0]);
      // for the other way, we need to invert the orientation
      update_periodic_face_map_recursively<dim, spacedim>(
        it->cell[1],
        it->cell[0],
        it->face_idx[1],
        it->face_idx[0],
        face_reference_cell.get_inverse_combined_orientation(it->orientation),
        periodic_face_map);
    }
 
  // check consistency
  typename std::map<std::pair<cell_iterator, unsigned int>,
                    std::pair<std::pair<cell_iterator, unsigned int>,
                              types::geometric_orientation>>::const_iterator
    it_test;
  for (it_test = periodic_face_map.begin(); it_test != periodic_face_map.end();
       ++it_test)
    {
      const Triangulation<dim, spacedim>::cell_iterator cell_1 =
        it_test->first.first;
      const Triangulation<dim, spacedim>::cell_iterator cell_2 =
        it_test->second.first.first;
      if (cell_1->level() == cell_2->level())
        {
          // if both cells have the same neighbor, then the same pair
          // order swapped has to be in the map
          Assert(periodic_face_map[it_test->second.first].first ==
                   it_test->first,
                 ExcInternalError());
        }
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
const std::vector<ReferenceCell<dim>>
  &Triangulation<dim, spacedim>::get_reference_cells() const
{
  return this->reference_cells;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
bool Triangulation<dim, spacedim>::all_reference_cells_are_hyper_cube() const
{
  Assert(this->reference_cells.size() > 0,
         ExcMessage("You can't ask about the kinds of reference "
                    "cells used by this triangulation if the "
                    "triangulation doesn't yet have any cells in it."));
  return (this->reference_cells.size() == 1 &&
          this->reference_cells[0].is_hyper_cube());
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
bool Triangulation<dim, spacedim>::all_reference_cells_are_simplex() const
{
  Assert(this->reference_cells.size() > 0,
         ExcMessage("You can't ask about the kinds of reference "
                    "cells used by this triangulation if the "
                    "triangulation doesn't yet have any cells in it."));
  return (this->reference_cells.size() == 1 &&
          this->reference_cells[0].is_simplex());
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
bool Triangulation<dim, spacedim>::is_mixed_mesh() const
{
  Assert(this->reference_cells.size() > 0,
         ExcMessage("You can't ask about the kinds of reference "
                    "cells used by this triangulation if the "
                    "triangulation doesn't yet have any cells in it."));
  return reference_cells.size() > 1 ||
         ((reference_cells[0].is_hyper_cube() == false) &&
          (reference_cells[0].is_simplex() == false));
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
unsigned int Triangulation<dim, spacedim>::register_data_attach(
  const std::function<std::vector<char>(const cell_iterator &,
                                        const ::CellStatus)>
            &pack_callback,
  const bool returns_variable_size_data)
{
  unsigned int handle = numbers::invalid_unsigned_int;
 
  // Add new callback function to the corresponding register.
  // Encode handles according to returns_variable_size_data.
  if (returns_variable_size_data)
    {
      handle = 2 * this->cell_attached_data.pack_callbacks_variable.size();
      this->cell_attached_data.pack_callbacks_variable.push_back(pack_callback);
    }
  else
    {
      handle = 2 * this->cell_attached_data.pack_callbacks_fixed.size() + 1;
      this->cell_attached_data.pack_callbacks_fixed.push_back(pack_callback);
    }
 
  // Increase overall counter.
  ++this->cell_attached_data.n_attached_data_sets;
 
  return handle;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::notify_ready_to_unpack(
  const unsigned int handle,
  const std::function<
    void(const cell_iterator &,
         const ::CellStatus,
         const boost::iterator_range<std::vector<char>::const_iterator> &)>
    &unpack_callback)
{
  // perform unpacking
  this->data_serializer.unpack_data(this->local_cell_relations,
                                    handle,
                                    unpack_callback);
 
  // decrease counters
  --this->cell_attached_data.n_attached_data_sets;
  if (this->cell_attached_data.n_attached_deserialize > 0)
    --this->cell_attached_data.n_attached_deserialize;
 
  // important: only remove data if we are not in the deserialization
  // process. There, each SolutionTransfer registers and unpacks before
  // the next one does this, so n_attached_data_sets is only 1 here.  This
  // would destroy the saved data before the second SolutionTransfer can
  // get it. This created a bug that is documented in
  // tests/mpi/p4est_save_03 with more than one SolutionTransfer.
 
  if (this->cell_attached_data.n_attached_data_sets == 0 &&
      this->cell_attached_data.n_attached_deserialize == 0)
    {
      // everybody got their data, time for cleanup!
      this->cell_attached_data.pack_callbacks_fixed.clear();
      this->cell_attached_data.pack_callbacks_variable.clear();
      this->data_serializer.clear();
 
      // reset all cell_status entries after coarsening/refinement
      for (auto &cell_rel : this->local_cell_relations)
        cell_rel.second = ::CellStatus::cell_will_persist;
    }
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::save_attached_data(
  const unsigned int global_first_cell,
  const unsigned int global_num_cells,
  const std::string &file_basename) const
{
  // cast away constness
  auto tria = const_cast<Triangulation<dim, spacedim> *>(this);
 
  // each cell should have been flagged `CellStatus::cell_will_persist`
  for (const auto &cell_rel : this->local_cell_relations)
    {
      (void)cell_rel;
      Assert((cell_rel.second == // cell_status
              ::CellStatus::cell_will_persist),
             ExcInternalError());
    }
 
  if (this->cell_attached_data.n_attached_data_sets > 0)
    {
      // pack attached data first
      tria->data_serializer.pack_data(
        tria->local_cell_relations,
        tria->cell_attached_data.pack_callbacks_fixed,
        tria->cell_attached_data.pack_callbacks_variable,
        this->get_mpi_communicator());
 
      // then store buffers in file
      tria->data_serializer.save(global_first_cell,
                                 global_num_cells,
                                 file_basename,
                                 this->get_mpi_communicator());
 
      // and release the memory afterwards
      tria->data_serializer.clear();
    }
 
  // clear all of the callback data, as explained in the documentation of
  // register_data_attach()
  {
    tria->cell_attached_data.n_attached_data_sets = 0;
    tria->cell_attached_data.pack_callbacks_fixed.clear();
    tria->cell_attached_data.pack_callbacks_variable.clear();
  }
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::load_attached_data(
  const unsigned int global_first_cell,
  const unsigned int global_num_cells,
  const unsigned int local_num_cells,
  const std::string &file_basename,
  const unsigned int n_attached_deserialize_fixed,
  const unsigned int n_attached_deserialize_variable)
{
  // load saved data, if any was stored
  if (this->cell_attached_data.n_attached_deserialize > 0)
    {
      this->data_serializer.load(global_first_cell,
                                 global_num_cells,
                                 local_num_cells,
                                 file_basename,
                                 n_attached_deserialize_fixed,
                                 n_attached_deserialize_variable,
                                 this->get_mpi_communicator());
 
      this->data_serializer.unpack_cell_status(this->local_cell_relations);
 
      if constexpr (running_in_debug_mode())
        {
          // the CellStatus of all stored cells should always be
          // CellStatus::cell_will_persist.
          for (const auto &cell_rel : this->local_cell_relations)
            {
              Assert((cell_rel.second == // cell_status
                      ::CellStatus::cell_will_persist),
                     ExcInternalError());
            }
        }
    }
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::clear_despite_subscriptions()
{
  levels.clear();
  faces.reset();
 
  vertices.clear();
  vertices_used.clear();
 
  manifolds.clear();
 
  // In 1d, also reset vertex-to-(boundary|manifold) maps to empty maps
  if (dim == 1)
    {
      Assert(vertex_to_boundary_id_map_1d != nullptr, ExcInternalError());
      vertex_to_boundary_id_map_1d->clear();
 
      Assert(vertex_to_manifold_id_map_1d != nullptr, ExcInternalError());
      vertex_to_manifold_id_map_1d->clear();
    }
  else
    {
      // For dim>1, these maps should simply not exist.
      Assert(vertex_to_boundary_id_map_1d == nullptr, ExcInternalError());
      Assert(vertex_to_manifold_id_map_1d == nullptr, ExcInternalError());
    }
 
 
  number_cache = internal::TriangulationImplementation::NumberCache<dim>();
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
typename Triangulation<dim, spacedim>::DistortedCellList
  Triangulation<dim, spacedim>::execute_refinement()
{
  const DistortedCellList cells_with_distorted_children =
    this->policy->execute_refinement(*this, check_for_distorted_cells);
 
 
 
  // re-compute number of lines
  internal::TriangulationImplementation::Implementation::compute_number_cache(
    *this, levels.size(), number_cache);
 
  if constexpr (running_in_debug_mode())
    {
      for (const auto &level : levels)
        monitor_memory(level->cells, dim);
 
      // check whether really all refinement flags are reset (also of
      // previously non-active cells which we may not have touched. If the
      // refinement flag of a non-active cell is set, something went wrong
      // since the cell-accessors should have caught this)
      for (const auto &cell : this->cell_iterators())
        Assert(!cell->refine_flag_set(), ExcInternalError());
    }
 
  return cells_with_distorted_children;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::execute_coarsening()
{
  // first find out if there are any cells at all to be coarsened in the
  // loop below
  const cell_iterator endc       = end();
  bool                do_coarsen = false;
  if (levels.size() >= 2)
    for (cell_iterator cell = begin(n_levels() - 1); cell != endc; --cell)
      if (!cell->is_active() && cell->child(0)->coarsen_flag_set())
        {
          do_coarsen = true;
          break;
        }
 
  if (!do_coarsen)
    return;
 
  // create a vector counting for each line and quads how many cells contain
  // the respective object. this is used later to decide which lines can be
  // deleted after coarsening a cell.
  std::vector<unsigned int> line_cell_count(dim > 1 ? this->n_raw_lines() : 0);
  std::vector<unsigned int> quad_cell_count(dim > 2 ? this->n_raw_quads() : 0);
  if (dim > 1)
    for (const auto &cell : this->cell_iterators())
      {
        if (dim > 2)
          {
            const auto line_indices = internal::TriaAccessorImplementation::
              Implementation::get_line_indices_of_cell(*cell);
            // avoid a compiler warning by fixing the max number of
            // loop iterations to 12
            const unsigned int n_lines = std::min(cell->n_lines(), 12u);
            for (unsigned int l = 0; l < n_lines; ++l)
              ++line_cell_count[line_indices[l]];
            for (const unsigned int q : cell->face_indices())
              ++quad_cell_count[cell->face_index(q)];
          }
        else
          for (unsigned int l = 0; l < cell->n_lines(); ++l)
            ++line_cell_count[cell->line(l)->index()];
      }
 
  // Since the loop goes over used cells we only need not worry about
  // deleting some cells since the ++operator will then just hop over them
  // if we should hit one. Do the loop in the reverse way since we may
  // only delete some cells if their neighbors have already been deleted
  // (if the latter are on a higher level for example). In effect, only
  // those cells are deleted of which originally all children were flagged
  // and for which all children are on the same refinement level. Note
  // that because of the effects of
  // @p{fix_coarsen_flags}, of a cell either all or no children must be
  // flagged for coarsening, so it is ok to only check the first child
  //
  // since we delete the *children* of cells, we can ignore cells on the
  // highest level, i.e., level must be less than or equal to
  // n_levels()-2.
  if (levels.size() >= 2)
    for (cell_iterator cell = begin(n_levels() - 1); cell != endc; --cell)
      if (!cell->is_active() && cell->child(0)->coarsen_flag_set())
        {
          for (unsigned int child = 0; child < cell->n_children(); ++child)
            {
              Assert(cell->child(child)->coarsen_flag_set(),
                     ExcInternalError());
              cell->child(child)->clear_coarsen_flag();
            }
          // inform all listeners that cell coarsening is going to happen
          signals.pre_coarsening_on_cell(cell);
          // use a separate function, since this is dimension specific
          this->policy->delete_children(*this,
                                        cell,
                                        line_cell_count,
                                        quad_cell_count);
        }
 
  // re-compute number of lines and quads
  internal::TriangulationImplementation::Implementation::compute_number_cache(
    *this, levels.size(), number_cache);
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::fix_coarsen_flags()
{
  // copy a piece of code from prepare_coarsening_and_refinement that
  // ensures that the level difference at vertices is limited if so
  // desired. we need this code here since at least in 1d we don't
  // call the dimension-independent version of
  // prepare_coarsening_and_refinement function. in 2d and 3d, having
  // this hunk here makes our lives a bit easier as well as it takes
  // care of these cases earlier than it would otherwise happen.
  //
  // the main difference to the code in p_c_and_r is that here we
  // absolutely have to make sure that we get things right, i.e. that
  // in particular we set flags right if
  // limit_level_difference_at_vertices is set. to do so we iterate
  // until the flags don't change any more
  auto previous_coarsen_flags = internal::extract_raw_coarsen_flags(levels);
 
  bool continue_iterating = true;
 
  do
    {
      if (smooth_grid & limit_level_difference_at_vertices)
        {
          Assert(!anisotropic_refinement,
                 ExcMessage("In case of anisotropic refinement the "
                            "limit_level_difference_at_vertices flag for "
                            "mesh smoothing must not be set!"));
 
          // store highest level one of the cells adjacent to a vertex
          // belongs to
          std::vector<int> vertex_level(vertices.size(), 0);
          for (const auto &cell : this->active_cell_iterators())
            {
              if (cell->refine_flag_set())
                for (const unsigned int vertex : cell->vertex_indices())
                  vertex_level[cell->vertex_index(vertex)] =
                    std::max(vertex_level[cell->vertex_index(vertex)],
                             cell->level() + 1);
              else if (!cell->coarsen_flag_set())
                for (const unsigned int vertex : cell->vertex_indices())
                  vertex_level[cell->vertex_index(vertex)] =
                    std::max(vertex_level[cell->vertex_index(vertex)],
                             cell->level());
              else
                {
                  // if coarsen flag is set then tentatively assume
                  // that the cell will be coarsened. this isn't
                  // always true (the coarsen flag could be removed
                  // again) and so we may make an error here. we try
                  // to correct this by iterating over the entire
                  // process until we are converged
                  Assert(cell->coarsen_flag_set(), ExcInternalError());
                  for (const unsigned int vertex : cell->vertex_indices())
                    vertex_level[cell->vertex_index(vertex)] =
                      std::max(vertex_level[cell->vertex_index(vertex)],
                               cell->level() - 1);
                }
            }
 
 
          // loop over all cells in reverse order. do so because we
          // can then update the vertex levels on the adjacent
          // vertices and maybe already flag additional cells in this
          // loop
          //
          // note that not only may we have to add additional
          // refinement flags, but we will also have to remove
          // coarsening flags on cells adjacent to vertices that will
          // see refinement
          active_cell_iterator endc = end();
          for (active_cell_iterator cell = last_active(); cell != endc; --cell)
            if (cell->refine_flag_set() == false)
              {
                for (const unsigned int vertex : cell->vertex_indices())
                  if (vertex_level[cell->vertex_index(vertex)] >=
                      cell->level() + 1)
                    {
                      // remove coarsen flag...
                      cell->clear_coarsen_flag();
 
                      // ...and if necessary also refine the current
                      // cell, at the same time updating the level
                      // information about vertices
                      if (vertex_level[cell->vertex_index(vertex)] >
                          cell->level() + 1)
                        {
                          cell->set_refine_flag();
 
                          for (const unsigned int v : cell->vertex_indices())
                            vertex_level[cell->vertex_index(v)] =
                              std::max(vertex_level[cell->vertex_index(v)],
                                       cell->level() + 1);
                        }
 
                      // continue and see whether we may, for example,
                      // go into the inner 'if' above based on a
                      // different vertex
                    }
              }
        }
 
      // loop over all cells and remove the coarsen flags for those cells that
      // have sister cells not marked for coarsening, or where some neighbors
      // are more refined.
 
      // Coarsen flags of cells with no mother cell, i.e. on the
      // coarsest level, are deleted explicitly.
      for (const auto &acell : this->active_cell_iterators_on_level(0))
        acell->clear_coarsen_flag();
 
      const cell_iterator endc = end();
      for (cell_iterator cell = begin(n_levels() - 1); cell != endc; --cell)
        {
          // nothing to do if we are already on the finest level
          if (cell->is_active())
            continue;
 
          const unsigned int n_children       = cell->n_children();
          unsigned int       flagged_children = 0;
          for (unsigned int child = 0; child < n_children; ++child)
            {
              const auto child_cell = cell->child(child);
              if (child_cell->is_active() && child_cell->coarsen_flag_set())
                {
                  ++flagged_children;
                  // clear flag since we don't need it anymore
                  child_cell->clear_coarsen_flag();
                }
            }
 
          // flag the children for coarsening again if all children were
          // flagged and if the policy allows it
          if (flagged_children == n_children &&
              this->policy->coarsening_allowed(cell))
            for (unsigned int c = 0; c < n_children; ++c)
              {
                Assert(cell->child(c)->refine_flag_set() == false,
                       ExcInternalError());
 
                cell->child(c)->set_coarsen_flag();
              }
        }
 
      // now see if anything has changed in the last iteration of this
      // function
      auto current_coarsen_flags = internal::extract_raw_coarsen_flags(levels);
 
      continue_iterating = (current_coarsen_flags != previous_coarsen_flags);
      previous_coarsen_flags.swap(current_coarsen_flags);
    }
  while (continue_iterating == true);
}
 
#endif
 
// TODO: merge the following 3 functions since they are the same
template <>
bool
Triangulation<1, 1>::prepare_coarsening_and_refinement()
{
  // save the flags to determine whether something was changed in the
  // course of this function
  const auto flags_before = internal::extract_raw_coarsen_flags(levels);
 
  // do nothing in 1d, except setting the coarsening flags correctly
  fix_coarsen_flags();
 
  const auto flags_after = internal::extract_raw_coarsen_flags(levels);
 
  return (flags_before != flags_after);
}
 
 
 
template <>
bool
Triangulation<1, 2>::prepare_coarsening_and_refinement()
{
  // save the flags to determine whether something was changed in the
  // course of this function
  const auto flags_before = internal::extract_raw_coarsen_flags(levels);
 
  // do nothing in 1d, except setting the coarsening flags correctly
  fix_coarsen_flags();
 
  const auto flags_after = internal::extract_raw_coarsen_flags(levels);
 
  return (flags_before != flags_after);
}
 
 
 
template <>
bool
Triangulation<1, 3>::prepare_coarsening_and_refinement()
{
  // save the flags to determine whether something was changed in the
  // course of this function
  const auto flags_before = internal::extract_raw_coarsen_flags(levels);
 
  // do nothing in 1d, except setting the coarsening flags correctly
  fix_coarsen_flags();
 
  const auto flags_after = internal::extract_raw_coarsen_flags(levels);
 
  return (flags_before != flags_after);
}
 
 
 
namespace
{
  // check if the given @param cell marked for coarsening would
  // produce an unrefined island. To break up long chains of these
  // cells we recursively check our neighbors in case we change this
  // cell. This reduces the number of outer iterations dramatically.
  template <int dim, int spacedim>
  void
  possibly_do_not_produce_unrefined_islands(
    const typename Triangulation<dim, spacedim>::cell_iterator &cell)
  {
    Assert(cell->has_children(), ExcInternalError());
 
    unsigned int n_neighbors = 0;
    // count all neighbors that will be refined along the face of our
    // cell after the next step
    unsigned int count = 0;
    for (const unsigned int n : GeometryInfo<dim>::face_indices())
      {
        const typename Triangulation<dim, spacedim>::cell_iterator neighbor =
          cell->neighbor(n);
        if (neighbor.state() == IteratorState::valid)
          {
            ++n_neighbors;
            if (face_will_be_refined_by_neighbor(cell, n))
              ++count;
          }
      }
    // clear coarsen flags if either all existing neighbors will be
    // refined or all but one will be and the cell is in the interior
    // of the domain
    if (count == n_neighbors ||
        (count >= n_neighbors - 1 &&
         n_neighbors == GeometryInfo<dim>::faces_per_cell))
      {
        for (unsigned int c = 0; c < cell->n_children(); ++c)
          cell->child(c)->clear_coarsen_flag();
 
        for (const unsigned int face : GeometryInfo<dim>::face_indices())
          if (!cell->at_boundary(face) &&
              (!cell->neighbor(face)->is_active()) &&
              (cell_will_be_coarsened(cell->neighbor(face))))
            possibly_do_not_produce_unrefined_islands<dim, spacedim>(
              cell->neighbor(face));
      }
  }
 
 
  // see if the current cell needs to be refined to avoid unrefined
  // islands.
  //
  // there are sometimes chains of cells that induce refinement of
  // each other. to avoid running the loop in
  // prepare_coarsening_and_refinement over and over again for each
  // one of them, at least for the isotropic refinement case we seek
  // to flag neighboring elements as well as necessary. this takes
  // care of (slightly pathological) cases like
  // deal.II/mesh_smoothing_03
  template <int dim, int spacedim>
  void
  possibly_refine_unrefined_island(
    const typename Triangulation<dim, spacedim>::cell_iterator &cell,
    const bool allow_anisotropic_smoothing)
  {
    Assert(cell->is_active(), ExcInternalError());
 
    if constexpr (running_in_debug_mode())
      {
        // If this is not a parallel::distributed::Triangulation, then we really
        // should only get here if the cell is marked for refinement:
        if (dynamic_cast<
              const parallel::distributed::Triangulation<dim, spacedim> *>(
              &cell->get_triangulation()) == nullptr)
          Assert(cell->refine_flag_set() == false, ExcInternalError());
        else
          // But if this is a p::d::Triangulation, then we don't have that
          // much control and we can get here because mesh smoothing is
          // requested but can not be honored because p4est controls
          // what gets refined. In that case, we can at least provide
          // a better error message.
          Assert(
            cell->refine_flag_set() == false,
            ExcMessage(
              "The triangulation is trying to avoid unrefined islands "
              "during mesh refinement/coarsening, as you had requested "
              " by passing the appropriate 'smoothing flags' to the "
              "constructor of the triangulation. However, for objects "
              "of type parallel::distributed::Triangulation, control "
              "over which cells get refined rests with p4est, not the "
              "deal.II triangulation, and consequently it is not "
              "always possible to avoid unrefined islands in the mesh. "
              "Please remove the constructor argument to the triangulation "
              "object that requests mesh smoothing."));
      }
 
    // now we provide two algorithms. the first one is the standard
    // one, coming from the time, where only isotropic refinement was
    // possible. it simply counts the neighbors that are or will be
    // refined and compares to the number of other ones. the second
    // one does this check independently for each direction: if all
    // neighbors in one direction (normally two, at the boundary only
    // one) are refined, the current cell is flagged to be refined in
    // an according direction.
 
    if (allow_anisotropic_smoothing == false)
      {
        // use first algorithm
        unsigned int refined_neighbors = 0, unrefined_neighbors = 0;
        for (const unsigned int face : GeometryInfo<dim>::face_indices())
          if (!cell->at_boundary(face))
            {
              if (face_will_be_refined_by_neighbor(cell, face))
                ++refined_neighbors;
              else
                ++unrefined_neighbors;
            }
 
        if (unrefined_neighbors < refined_neighbors)
          {
            cell->clear_coarsen_flag();
            cell->set_refine_flag();
 
            // ok, so now we have flagged this cell. if we know that
            // there were any unrefined neighbors at all, see if any
            // of those will have to be refined as well
            if (unrefined_neighbors > 0)
              for (const unsigned int face : GeometryInfo<dim>::face_indices())
                if (!cell->at_boundary(face) &&
                    (face_will_be_refined_by_neighbor(cell, face) == false) &&
                    (cell->neighbor(face)->has_children() == false) &&
                    (cell->neighbor(face)->refine_flag_set() == false))
                  possibly_refine_unrefined_island<dim, spacedim>(
                    cell->neighbor(face), allow_anisotropic_smoothing);
          }
      }
    else
      {
        // variable to store the cell refine case needed to fulfill
        // all smoothing requirements
        RefinementCase<dim> smoothing_cell_refinement_case =
          RefinementCase<dim>::no_refinement;
 
        // use second algorithm, do the check individually for each
        // direction
        for (unsigned int face_pair = 0;
             face_pair < GeometryInfo<dim>::faces_per_cell / 2;
             ++face_pair)
          {
            // variable to store the cell refine case needed to refine
            // at the current face pair in the same way as the
            // neighbors do...
            RefinementCase<dim> directional_cell_refinement_case =
              RefinementCase<dim>::isotropic_refinement;
 
            for (unsigned int face_index = 0; face_index < 2; ++face_index)
              {
                unsigned int face = 2 * face_pair + face_index;
                // variable to store the refine case (to come) of the
                // face under consideration
                RefinementCase<dim - 1> expected_face_ref_case =
                  RefinementCase<dim - 1>::no_refinement;
 
                if (cell->neighbor(face).state() == IteratorState::valid)
                  face_will_be_refined_by_neighbor<dim, spacedim>(
                    cell, face, expected_face_ref_case);
                // now extract which refine case would be necessary to
                // achieve the same face refinement. set the
                // intersection with other requirements for the same
                // direction.
 
                // note: using the intersection is not an obvious
                // decision, we could also argue that it is more
                // natural to use the union. however, intersection is
                // the less aggressive tactic and favours a smaller
                // number of refined cells over an intensive
                // smoothing. this way we try not to lose too much of
                // the effort we put in anisotropic refinement
                // indicators due to overly aggressive smoothing...
                directional_cell_refinement_case =
                  (directional_cell_refinement_case &
                   GeometryInfo<dim>::
                     min_cell_refinement_case_for_face_refinement(
                       expected_face_ref_case,
                       face,
                       cell->face_orientation(face),
                       cell->face_flip(face),
                       cell->face_rotation(face)));
              } // for both face indices
            // if both requirements sum up to something useful, add
            // this to the refine case for smoothing. note: if
            // directional_cell_refinement_case is isotropic still,
            // then something went wrong...
            Assert(directional_cell_refinement_case <
                     RefinementCase<dim>::isotropic_refinement,
                   ExcInternalError());
            smoothing_cell_refinement_case =
              smoothing_cell_refinement_case | directional_cell_refinement_case;
          } // for all face_pairs
        // no we collected contributions from all directions. combine
        // the new flags with the existing refine case, but only if
        // smoothing is required
        if (smoothing_cell_refinement_case)
          {
            cell->clear_coarsen_flag();
            cell->set_refine_flag(cell->refine_flag_set() |
                                  smoothing_cell_refinement_case);
          }
      }
  }
} // namespace
 
#ifndef DOXYGEN
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
bool Triangulation<dim, spacedim>::prepare_coarsening_and_refinement()
{
  // save the flags to determine whether something was changed in the
  // course of this function
  const auto coarsen_flags_before = internal::extract_raw_coarsen_flags(levels);
  const auto refine_flags_before  = internal::extract_raw_refine_flags(levels);
 
  // save the flags at the outset of each loop. we do so in order to
  // find out whether something was changed in the present loop, in
  // which case we would have to re-run the loop. the other
  // possibility to find this out would be to set a flag
  // @p{something_changed} to true each time we change something.
  // however, sometimes one change in one of the parts of the loop is
  // undone by another one, so we might end up in an endless loop. we
  // could be tempted to break this loop at an arbitrary number of
  // runs, but that would not be a clean solution, since we would
  // either have to 1/ break the loop too early, in which case the
  // promise that a second call to this function immediately after the
  // first one does not change anything, would be broken, or 2/ we do
  // as many loops as there are levels. we know that information is
  // transported over one level in each run of the loop, so this is
  // enough. Unfortunately, each loop is rather expensive, so we chose
  // the way presented here
  auto coarsen_flags_before_loop = coarsen_flags_before;
  auto refine_flags_before_loop  = refine_flags_before;
 
  // now for what is done in each loop: we have to fulfill several
  // tasks at the same time, namely several mesh smoothing algorithms
  // and mesh regularization, by which we mean that the next mesh
  // fulfills several requirements such as no double refinement at
  // each face or line, etc.
  //
  // since doing these things at once seems almost impossible (in the
  // first year of this library, they were done in two functions, one
  // for refinement and one for coarsening, and most things within
  // these were done at once, so the code was rather impossible to
  // join into this, only, function), we do them one after each
  // other. the order in which we do them is such that the important
  // tasks, namely regularization, are done last and the least
  // important things are done the first. the following order is
  // chosen:
  //
  // 0/ Only if coarsest_level_1 or patch_level_1 is set: clear all
  //    coarsen flags on level 1 to avoid level 0 cells being created
  //    by coarsening.  As coarsen flags will never be added, this can
  //    be done once and for all before the actual loop starts.
  //
  // 1/ do not coarsen a cell if 'most of the neighbors' will be
  //    refined after the step. This is to prevent occurrence of
  //    unrefined islands.
  //
  // 2/ eliminate refined islands in the interior and at the
  //    boundary. since they don't do much harm besides increasing the
  //    number of degrees of freedom, doing this has a rather low
  //    priority.
  //
  // 3/ limit the level difference of neighboring cells at each
  //    vertex.
  //
  // 4/ eliminate unrefined islands. this has higher priority since
  //    this diminishes the approximation properties not only of the
  //    unrefined island, but also of the surrounding patch.
  //
  // 5/ ensure patch level 1. Then the triangulation consists of
  //    patches, i.e. of cells that are refined once. It follows that
  //    if at least one of the children of a cell is or will be
  //    refined than all children need to be refined. This step only
  //    sets refinement flags and does not set coarsening flags.  If
  //    the patch_level_1 flag is set, then
  //    eliminate_unrefined_islands, eliminate_refined_inner_islands
  //    and eliminate_refined_boundary_islands will be fulfilled
  //    automatically and do not need to be enforced separately.
  //
  // 6/ take care of the requirement that no double refinement is done
  //    at each face
  //
  // 7/ take care that no double refinement is done at each line in 3d
  //    or higher dimensions.
  //
  // 8/ make sure that all children of each cell are either flagged
  //    for coarsening or none of the children is
  //
  // For some of these steps, it is known that they interact. Namely,
  // it is not possible to guarantee that after step 6 another step 5
  // would have no effect; the same holds for the opposite order and
  // also when taking into account step 7. however, it is important to
  // guarantee that step five or six do not undo something that step 5
  // did, and step 7 not something of step 6, otherwise the
  // requirements will not be satisfied even if the loop
  // terminates. this is accomplished by the fact that steps 5 and 6
  // only *add* refinement flags and delete coarsening flags
  // (therefore, step 6 can't undo something that step 4 already did),
  // and step 7 only deletes coarsening flags, never adds some. step 7
  // needs also take care that it won't tag cells for refinement for
  // which some neighbors are more refined or will be refined.
 
  //------------------------------------
  // STEP 0:
  //    Only if coarsest_level_1 or patch_level_1 is set: clear all
  //    coarsen flags on level 1 to avoid level 0 cells being created
  //    by coarsening.
  if (((smooth_grid & coarsest_level_1) || (smooth_grid & patch_level_1)) &&
      n_levels() >= 2)
    {
      for (const auto &cell : active_cell_iterators_on_level(1))
        cell->clear_coarsen_flag();
    }
 
  bool mesh_changed_in_this_loop = false;
  do
    {
      //------------------------------------
      // STEP 1:
      //    do not coarsen a cell if 'most of the neighbors' will be
      //    refined after the step. This is to prevent the occurrence
      //    of unrefined islands.  If patch_level_1 is set, this will
      //    be automatically fulfilled.
      if (smooth_grid & do_not_produce_unrefined_islands &&
          !(smooth_grid & patch_level_1))
        {
          for (const auto &cell : cell_iterators())
            {
              // only do something if this
              // cell will be coarsened
              if (!cell->is_active() && cell_will_be_coarsened(cell))
                possibly_do_not_produce_unrefined_islands<dim, spacedim>(cell);
            }
        }
 
 
      //------------------------------------
      // STEP 2:
      //    eliminate refined islands in the interior and at the
      //    boundary. since they don't do much harm besides increasing
      //    the number of degrees of freedom, doing this has a rather
      //    low priority.  If patch_level_1 is set, this will be
      //    automatically fulfilled.
      //
      //    there is one corner case to consider: if this is a
      //    distributed triangulation, there may be refined islands on
      //    the boundary of which we own only part (e.g. a single cell
      //    in the corner of a domain). the rest of the island is
      //    ghost cells and it *looks* like the area around it
      //    (artificial cells) are coarser but this is only because
      //    they may actually be equally fine on other
      //    processors. it's hard to detect this case but we can do
      //    the following: only set coarsen flags to remove this
      //    refined island if all cells we want to set flags on are
      //    locally owned
      if (smooth_grid & (eliminate_refined_inner_islands |
                         eliminate_refined_boundary_islands) &&
          !(smooth_grid & patch_level_1))
        {
          for (const auto &cell : cell_iterators())
            if (!cell->is_active() ||
                (cell->is_active() && cell->refine_flag_set() &&
                 cell->is_locally_owned()))
              {
                // check whether all children are active, i.e. not
                // refined themselves. This is a precondition that the
                // children may be coarsened away. If the cell is only
                // flagged for refinement, then all future children
                // will be active
                bool all_children_active = true;
                if (!cell->is_active())
                  for (unsigned int c = 0; c < cell->n_children(); ++c)
                    if (!cell->child(c)->is_active() ||
                        cell->child(c)->is_ghost() ||
                        cell->child(c)->is_artificial())
                      {
                        all_children_active = false;
                        break;
                      }
 
                if (all_children_active)
                  {
                    // count number of refined and unrefined neighbors
                    // of cell.  neighbors on lower levels are counted
                    // as unrefined since they can only get to the
                    // same level as this cell by the next refinement
                    // cycle
                    unsigned int unrefined_neighbors = 0, total_neighbors = 0;
 
                    // Keep track if this cell is at a periodic
                    // boundary or not.  TODO: We do not currently run
                    // the algorithm for inner islands at a periodic
                    // boundary (remains to be implemented), but we
                    // also don't want to consider them
                    // boundary_island cells as this can interfere
                    // with 2:1 refinement across periodic faces.
                    // Instead: just ignore those cells for this
                    // smoothing operation below.
                    bool at_periodic_boundary = false;
 
                    for (const unsigned int n : cell->face_indices())
                      {
                        const cell_iterator neighbor = cell->neighbor(n);
                        if (neighbor.state() == IteratorState::valid)
                          {
                            ++total_neighbors;
 
                            if (!face_will_be_refined_by_neighbor(cell, n))
                              ++unrefined_neighbors;
                          }
                        else if (cell->has_periodic_neighbor(n))
                          {
                            ++total_neighbors;
                            at_periodic_boundary = true;
                          }
                      }
 
                    // if all neighbors unrefined: mark this cell for
                    // coarsening or don't refine if marked for that
                    //
                    // also do the distinction between the two
                    // versions of the eliminate_refined_*_islands
                    // flag
                    //
                    // the last check is whether there are any
                    // neighbors at all. if not so, then we are (e.g.)
                    // on the coarsest grid with one cell, for which,
                    // of course, we do not remove the refine flag.
                    if ((unrefined_neighbors == total_neighbors) &&
                        ((!cell->at_boundary() &&
                          (smooth_grid & eliminate_refined_inner_islands)) ||
                         (cell->at_boundary() && !at_periodic_boundary &&
                          (smooth_grid &
                           eliminate_refined_boundary_islands))) &&
                        (total_neighbors != 0))
                      {
                        if (!cell->is_active())
                          for (unsigned int c = 0; c < cell->n_children(); ++c)
                            {
                              cell->child(c)->clear_refine_flag();
                              cell->child(c)->set_coarsen_flag();
                            }
                        else
                          cell->clear_refine_flag();
                      }
                  }
              }
        }
 
      //------------------------------------
      // STEP 3:
      //    limit the level difference of neighboring cells at each
      //    vertex.
      //
      //    in case of anisotropic refinement this does not make
      //    sense. as soon as one cell is anisotropically refined, an
      //    Assertion is thrown. therefore we can ignore this problem
      //    later on
      if (smooth_grid & limit_level_difference_at_vertices)
        {
          Assert(!anisotropic_refinement,
                 ExcMessage("In case of anisotropic refinement the "
                            "limit_level_difference_at_vertices flag for "
                            "mesh smoothing must not be set!"));
 
          // store highest level one of the cells adjacent to a vertex
          // belongs to
          std::vector<int> vertex_level(vertices.size(), 0);
          for (const auto &cell : active_cell_iterators())
            {
              if (cell->refine_flag_set())
                for (const unsigned int vertex : cell->vertex_indices())
                  vertex_level[cell->vertex_index(vertex)] =
                    std::max(vertex_level[cell->vertex_index(vertex)],
                             cell->level() + 1);
              else if (!cell->coarsen_flag_set())
                for (const unsigned int vertex : cell->vertex_indices())
                  vertex_level[cell->vertex_index(vertex)] =
                    std::max(vertex_level[cell->vertex_index(vertex)],
                             cell->level());
              else
                {
                  // if coarsen flag is set then tentatively assume
                  // that the cell will be coarsened. this isn't
                  // always true (the coarsen flag could be removed
                  // again) and so we may make an error here
                  Assert(cell->coarsen_flag_set(), ExcInternalError());
                  for (const unsigned int vertex : cell->vertex_indices())
                    vertex_level[cell->vertex_index(vertex)] =
                      std::max(vertex_level[cell->vertex_index(vertex)],
                               cell->level() - 1);
                }
            }
 
 
          // loop over all cells in reverse order. do so because we
          // can then update the vertex levels on the adjacent
          // vertices and maybe already flag additional cells in this
          // loop
          //
          // note that not only may we have to add additional
          // refinement flags, but we will also have to remove
          // coarsening flags on cells adjacent to vertices that will
          // see refinement
          for (active_cell_iterator cell = last_active(); cell != end(); --cell)
            if (cell->refine_flag_set() == false)
              {
                for (const unsigned int vertex : cell->vertex_indices())
                  if (vertex_level[cell->vertex_index(vertex)] >=
                      cell->level() + 1)
                    {
                      // remove coarsen flag...
                      cell->clear_coarsen_flag();
 
                      // ...and if necessary also refine the current
                      // cell, at the same time updating the level
                      // information about vertices
                      if (vertex_level[cell->vertex_index(vertex)] >
                          cell->level() + 1)
                        {
                          cell->set_refine_flag();
 
                          for (const unsigned int v : cell->vertex_indices())
                            vertex_level[cell->vertex_index(v)] =
                              std::max(vertex_level[cell->vertex_index(v)],
                                       cell->level() + 1);
                        }
 
                      // continue and see whether we may, for example,
                      // go into the inner'if'
                      // above based on a
                      // different vertex
                    }
              }
        }
 
      //-----------------------------------
      // STEP 4:
      //    eliminate unrefined islands. this has higher priority
      //    since this diminishes the approximation properties not
      //    only of the unrefined island, but also of the surrounding
      //    patch.
      //
      //    do the loop from finest to coarsest cells since we may
      //    trigger a cascade by marking cells for refinement which
      //    may trigger more cells further down below
      if (smooth_grid & eliminate_unrefined_islands)
        {
          for (active_cell_iterator cell = last_active(); cell != end(); --cell)
            // only do something if cell is not already flagged for
            // (isotropic) refinement
            if (cell->refine_flag_set() !=
                RefinementCase<dim>::isotropic_refinement)
              possibly_refine_unrefined_island<dim, spacedim>(
                cell, (smooth_grid & allow_anisotropic_smoothing) != 0);
        }
 
      //-------------------------------
      // STEP 5:
      //    ensure patch level 1.
      //
      //    Introduce some terminology:
      //    - a cell that is refined
      //      once is a patch of
      //      level 1 simply called patch.
      //    - a cell that is globally
      //      refined twice is called
      //      a patch of level 2.
      //    - patch level n says that
      //      the triangulation consists
      //      of patches of level n.
      //      This makes sense only
      //      if the grid is already at
      //      least n times globally
      //      refined.
      //
      //    E.g. from patch level 1 follows: if at least one of the
      //    children of a cell is or will be refined than enforce all
      //    children to be refined.
 
      //    This step 4 only sets refinement flags and does not set
      //    coarsening flags.
      if (smooth_grid & patch_level_1)
        {
          // An important assumption (A) is that before calling this
          // function the grid was already of patch level 1.
 
          // loop over all cells whose children are all active.  (By
          // assumption (A) either all or none of the children are
          // active).  If the refine flag of at least one of the
          // children is set then set_refine_flag and
          // clear_coarsen_flag of all children.
          for (const auto &cell : cell_iterators())
            if (!cell->is_active())
              {
                // ensure the invariant. we can then check whether all
                // of its children are further refined or not by
                // simply looking at the first child
                Assert(cell_is_patch_level_1(cell), ExcInternalError());
                if (cell->child(0)->has_children() == true)
                  continue;
 
                // cell is found to be a patch.  combine the refine
                // cases of all children
                RefinementCase<dim> combined_ref_case =
                  RefinementCase<dim>::no_refinement;
                for (unsigned int i = 0; i < cell->n_children(); ++i)
                  combined_ref_case =
                    combined_ref_case | cell->child(i)->refine_flag_set();
                if (combined_ref_case != RefinementCase<dim>::no_refinement)
                  for (unsigned int i = 0; i < cell->n_children(); ++i)
                    {
                      cell_iterator child = cell->child(i);
 
                      child->clear_coarsen_flag();
                      child->set_refine_flag(combined_ref_case);
                    }
              }
 
          // The code above dealt with the case where we may get a
          // non-patch_level_1 mesh from refinement. Now also deal
          // with the case where we could get such a mesh by
          // coarsening.  Coarsen the children (and remove the
          // grandchildren) only if all cell->grandchild(i)
          // ->coarsen_flag_set() are set.
          //
          // for a case where this is a bit tricky, take a look at the
          // mesh_smoothing_0[12] testcases
          for (const auto &cell : cell_iterators())
            {
              // check if this cell has active grandchildren. note
              // that we know that it is patch_level_1, i.e. if one of
              // its children is active then so are all, and it isn't
              // going to have any grandchildren at all:
              if (cell->is_active() || cell->child(0)->is_active())
                continue;
 
              // cell is not active, and so are none of its
              // children. check the grandchildren. note that the
              // children are also patch_level_1, and so we only ever
              // need to check their first child
              const unsigned int n_children               = cell->n_children();
              bool               has_active_grandchildren = false;
 
              for (unsigned int i = 0; i < n_children; ++i)
                if (cell->child(i)->child(0)->is_active())
                  {
                    has_active_grandchildren = true;
                    break;
                  }
 
              if (has_active_grandchildren == false)
                continue;
 
 
              // ok, there are active grandchildren. see if either all
              // or none of them are flagged for coarsening
              unsigned int n_grandchildren = 0;
 
              // count all coarsen flags of the grandchildren.
              unsigned int n_coarsen_flags = 0;
 
              // cell is not a patch (of level 1) as it has a
              // grandchild.  Is cell a patch of level 2??  Therefore:
              // find out whether all cell->child(i) are patches
              for (unsigned int c = 0; c < n_children; ++c)
                {
                  // get at the child. by assumption (A), and the
                  // check by which we got here, the child is not
                  // active
                  cell_iterator child = cell->child(c);
 
                  const unsigned int nn_children = child->n_children();
                  n_grandchildren += nn_children;
 
                  // if child is found to be a patch of active cells
                  // itself, then add up how many of its children are
                  // supposed to be coarsened
                  if (child->child(0)->is_active())
                    for (unsigned int cc = 0; cc < nn_children; ++cc)
                      if (child->child(cc)->coarsen_flag_set())
                        ++n_coarsen_flags;
                }
 
              // if not all grandchildren are supposed to be coarsened
              // (e.g. because some simply don't have the flag set, or
              // because they are not active and therefore cannot
              // carry the flag), then remove the coarsen flag from
              // all of the active grandchildren. note that there may
              // be coarsen flags on the grandgrandchildren -- we
              // don't clear them here, but we'll get to them in later
              // iterations if necessary
              //
              // there is nothing we have to do if no coarsen flags
              // have been set at all
              if ((n_coarsen_flags != n_grandchildren) && (n_coarsen_flags > 0))
                for (unsigned int c = 0; c < n_children; ++c)
                  {
                    const cell_iterator child = cell->child(c);
                    if (child->child(0)->is_active())
                      for (unsigned int cc = 0; cc < child->n_children(); ++cc)
                        child->child(cc)->clear_coarsen_flag();
                  }
            }
        }
 
      //--------------------------------
      //
      //  at the boundary we could end up with cells with negative
      //  volume or at least with a part, that is negative, if the
      //  cell is refined anisotropically. we have to check, whether
      //  that can happen
      this->policy->prevent_distorted_boundary_cells(*this);
 
      //-------------------------------
      // STEP 6:
      //    take care of the requirement that no
      //    double refinement is done at each face
      //
      //    in case of anisotropic refinement it is only likely, but
      //    not sure, that the cells, which are more refined along a
      //    certain face common to two cells are on a higher
      //    level. therefore we cannot be sure, that the requirement
      //    of no double refinement is fulfilled after a single pass
      //    of the following actions. We could just wait for the next
      //    global loop. when this function terminates, the
      //    requirement will be fulfilled. However, it might be faster
      //    to insert an inner loop here.
      bool changed = true;
      while (changed)
        {
          changed                   = false;
          active_cell_iterator cell = last_active(), endc = end();
 
          for (; cell != endc; --cell)
            if (cell->refine_flag_set())
              {
                // loop over neighbors of cell
                for (const auto i : cell->face_indices())
                  {
                    // only do something if the face is not at the
                    // boundary and if the face will be refined with
                    // the RefineCase currently flagged for
                    const bool has_periodic_neighbor =
                      cell->has_periodic_neighbor(i);
                    const bool has_neighbor_or_periodic_neighbor =
                      !cell->at_boundary(i) || has_periodic_neighbor;
                    if (has_neighbor_or_periodic_neighbor &&
                        GeometryInfo<dim>::face_refinement_case(
                          cell->refine_flag_set(), i) !=
                          RefinementCase<dim - 1>::no_refinement)
                      {
                        // 1) if the neighbor has children: nothing to
                        // worry about.  2) if the neighbor is active
                        // and a coarser one, ensure, that its
                        // refine_flag is set 3) if the neighbor is
                        // active and as refined along the face as our
                        // current cell, make sure, that no
                        // coarsen_flag is set. if we remove the
                        // coarsen flag of our neighbor,
                        // fix_coarsen_flags() makes sure, that the
                        // mother cell will not be coarsened
                        if (cell->neighbor_or_periodic_neighbor(i)->is_active())
                          {
                            if ((!has_periodic_neighbor &&
                                 cell->neighbor_is_coarser(i)) ||
                                (has_periodic_neighbor &&
                                 cell->periodic_neighbor_is_coarser(i)))
                              {
                                if (cell->neighbor_or_periodic_neighbor(i)
                                      ->coarsen_flag_set())
                                  cell->neighbor_or_periodic_neighbor(i)
                                    ->clear_coarsen_flag();
                                // we'll set the refine flag for this
                                // neighbor below. we note, that we
                                // have changed something by setting
                                // the changed flag to true. We do not
                                // need to do so, if we just removed
                                // the coarsen flag, as the changed
                                // flag only indicates the need to
                                // re-run the inner loop. however, we
                                // only loop over cells flagged for
                                // refinement here, so nothing to
                                // worry about if we remove coarsen
                                // flags
 
                                if (dim == 2)
                                  {
                                    if (smooth_grid &
                                        allow_anisotropic_smoothing)
                                      changed =
                                        has_periodic_neighbor ?
                                          cell->periodic_neighbor(i)
                                            ->flag_for_face_refinement(
                                              cell
                                                ->periodic_neighbor_of_coarser_periodic_neighbor(
                                                  i)
                                                .first,
                                              RefinementCase<dim - 1>::cut_x) :
                                          cell->neighbor(i)
                                            ->flag_for_face_refinement(
                                              cell
                                                ->neighbor_of_coarser_neighbor(
                                                  i)
                                                .first,
                                              RefinementCase<dim - 1>::cut_x);
                                    else
                                      {
                                        if (!cell
                                               ->neighbor_or_periodic_neighbor(
                                                 i)
                                               ->refine_flag_set())
                                          changed = true;
                                        cell->neighbor_or_periodic_neighbor(i)
                                          ->set_refine_flag();
                                      }
                                  }
                                else // i.e. if (dim==3)
                                  {
                                    // ugly situations might arise here,
                                    // consider the following situation, which
                                    // shows neighboring cells at the common
                                    // face, where the upper right element is
                                    // coarser at the given face. Now the upper
                                    // child element of the lower left wants to
                                    // refine according to cut_z, such that
                                    // there is a 'horizontal' refinement of the
                                    // face marked with #####
                                    //
                                    //                         /               /
                                    //                        /               /
                                    //                       *---------------*
                                    //                       |               |
                                    //                       |               |
                                    //                       |               |
                                    //                       |               |
                                    //                       |               |
                                    //                       |               | /
                                    //                       |               |/
                                    //                       *---------------*
                                    //
                                    //
                                    //     *---------------*
                                    //    /|              /|
                                    //   / |     #####   / |
                                    //     |               |
                                    //     *---------------*
                                    //    /|              /|
                                    //   / |             / |
                                    //     |               |
                                    //     *---------------*
                                    //    /               /
                                    //   /               /
                                    //
                                    // this introduces too many hanging nodes
                                    // and the neighboring (coarser) cell (upper
                                    // right) has to be refined. If it is only
                                    // refined according to cut_z, then
                                    // everything is ok:
                                    //
                                    //                         /               /
                                    //                        /               /
                                    //                       *---------------*
                                    //                       |               |
                                    //                       |               | /
                                    //                       |               |/
                                    //                       *---------------*
                                    //                       |               |
                                    //                       |               | /
                                    //                       |               |/
                                    //                       *---------------*
                                    //
                                    //
                                    //     *---------------*
                                    //    /|              /|
                                    //   / *---------------*
                                    //    /|              /|
                                    //     *---------------*
                                    //    /|              /|
                                    //   / |             / |
                                    //     |               |
                                    //     *---------------*
                                    //    /               /
                                    //   /               /
                                    //
                                    // if however the cell wants to refine
                                    // itself in an other way, or if we disallow
                                    // anisotropic smoothing, then simply
                                    // refining the neighbor isotropically is
                                    // not going to work, since this introduces
                                    // a refinement of face ##### with both
                                    // cut_x and cut_y, which is not possible:
                                    //
                                    //                         /       /       /
                                    //                        /       /       /
                                    //                       *-------*-------*
                                    //                       |       |       |
                                    //                       |       |       | /
                                    //                       |       |       |/
                                    //                       *-------*-------*
                                    //                       |       |       |
                                    //                       |       |       | /
                                    //                       |       |       |/
                                    //                       *-------*-------*
                                    //
                                    //
                                    //     *---------------*
                                    //    /|              /|
                                    //   / *---------------*
                                    //    /|              /|
                                    //     *---------------*
                                    //    /|              /|
                                    //   / |             / |
                                    //     |               |
                                    //     *---------------*
                                    //    /               /
                                    //   /               /
                                    //
                                    // thus, in this case we also need to refine
                                    // our current cell in the new direction:
                                    //
                                    //                         /       /       /
                                    //                        /       /       /
                                    //                       *-------*-------*
                                    //                       |       |       |
                                    //                       |       |       | /
                                    //                       |       |       |/
                                    //                       *-------*-------*
                                    //                       |       |       |
                                    //                       |       |       | /
                                    //                       |       |       |/
                                    //                       *-------*-------*
                                    //
                                    //
                                    //     *-------*-------*
                                    //    /|      /|      /|
                                    //   / *-------*-------*
                                    //    /|      /|      /|
                                    //     *-------*-------*
                                    //    /|      /       /|
                                    //   / |             / |
                                    //     |               |
                                    //     *---------------*
                                    //    /               /
                                    //   /               /
 
                                    std::pair<unsigned int, unsigned int>
                                      nb_indices =
                                        has_periodic_neighbor ?
                                          cell
                                            ->periodic_neighbor_of_coarser_periodic_neighbor(
                                              i) :
                                          cell->neighbor_of_coarser_neighbor(i);
                                    unsigned int refined_along_x       = 0,
                                                 refined_along_y       = 0,
                                                 to_be_refined_along_x = 0,
                                                 to_be_refined_along_y = 0;
 
                                    const int this_face_index =
                                      cell->face_index(i);
 
                                    // step 1: detect, along which axis the face
                                    // is currently refined
 
                                    // first, we need an iterator pointing to
                                    // the parent face. This requires a slight
                                    // detour in case the neighbor is behind a
                                    // periodic face.
                                    const auto parent_face = [&]() {
                                      if (has_periodic_neighbor)
                                        {
                                          const auto neighbor =
                                            cell->periodic_neighbor(i);
                                          const auto parent_face_no =
                                            neighbor
                                              ->periodic_neighbor_of_periodic_neighbor(
                                                nb_indices.first);
                                          auto parent =
                                            neighbor->periodic_neighbor(
                                              nb_indices.first);
                                          return parent->face(parent_face_no);
                                        }
                                      else
                                        return cell->neighbor(i)->face(
                                          nb_indices.first);
                                    }();
 
                                    if ((this_face_index ==
                                         parent_face->child_index(0)) ||
                                        (this_face_index ==
                                         parent_face->child_index(1)))
                                      {
                                        // this might be an
                                        // anisotropic child. get the
                                        // face refine case of the
                                        // neighbors face and count
                                        // refinements in x and y
                                        // direction.
                                        RefinementCase<dim - 1> frc =
                                          parent_face->refinement_case();
                                        if (frc & RefinementCase<dim>::cut_x)
                                          ++refined_along_x;
                                        if (frc & RefinementCase<dim>::cut_y)
                                          ++refined_along_y;
                                      }
                                    else
                                      // this has to be an isotropic
                                      // child
                                      {
                                        ++refined_along_x;
                                        ++refined_along_y;
                                      }
                                    // step 2: detect, along which axis the face
                                    // has to be refined given the current
                                    // refine flag
                                    RefinementCase<dim - 1> flagged_frc =
                                      GeometryInfo<dim>::face_refinement_case(
                                        cell->refine_flag_set(),
                                        i,
                                        cell->face_orientation(i),
                                        cell->face_flip(i),
                                        cell->face_rotation(i));
                                    if (flagged_frc &
                                        RefinementCase<dim>::cut_x)
                                      ++to_be_refined_along_x;
                                    if (flagged_frc &
                                        RefinementCase<dim>::cut_y)
                                      ++to_be_refined_along_y;
 
                                    // step 3: set the refine flag of the
                                    // (coarser and active) neighbor.
                                    if ((smooth_grid &
                                         allow_anisotropic_smoothing) ||
                                        cell->neighbor_or_periodic_neighbor(i)
                                          ->refine_flag_set())
                                      {
                                        if (refined_along_x +
                                              to_be_refined_along_x >
                                            1)
                                          changed |=
                                            cell
                                              ->neighbor_or_periodic_neighbor(i)
                                              ->flag_for_face_refinement(
                                                nb_indices.first,
                                                RefinementCase<dim -
                                                               1>::cut_axis(0));
                                        if (refined_along_y +
                                              to_be_refined_along_y >
                                            1)
                                          changed |=
                                            cell
                                              ->neighbor_or_periodic_neighbor(i)
                                              ->flag_for_face_refinement(
                                                nb_indices.first,
                                                RefinementCase<dim -
                                                               1>::cut_axis(1));
                                      }
                                    else
                                      {
                                        if (cell
                                              ->neighbor_or_periodic_neighbor(i)
                                              ->refine_flag_set() !=
                                            RefinementCase<
                                              dim>::isotropic_refinement)
                                          changed = true;
                                        cell->neighbor_or_periodic_neighbor(i)
                                          ->set_refine_flag();
                                      }
 
                                    // step 4: if necessary (see above) add to
                                    // the refine flag of the current cell
                                    cell_iterator nb =
                                      cell->neighbor_or_periodic_neighbor(i);
                                    RefinementCase<dim - 1> nb_frc =
                                      GeometryInfo<dim>::face_refinement_case(
                                        nb->refine_flag_set(),
                                        nb_indices.first,
                                        nb->face_orientation(nb_indices.first),
                                        nb->face_flip(nb_indices.first),
                                        nb->face_rotation(nb_indices.first));
                                    if ((nb_frc & RefinementCase<dim>::cut_x) &&
                                        !((refined_along_x != 0u) ||
                                          (to_be_refined_along_x != 0u)))
                                      changed |= cell->flag_for_face_refinement(
                                        i,
                                        RefinementCase<dim - 1>::cut_axis(0));
                                    if ((nb_frc & RefinementCase<dim>::cut_y) &&
                                        !((refined_along_y != 0u) ||
                                          (to_be_refined_along_y != 0u)))
                                      changed |= cell->flag_for_face_refinement(
                                        i,
                                        RefinementCase<dim - 1>::cut_axis(1));
                                  }
                              }  // if neighbor is coarser
                            else // -> now the neighbor is not coarser
                              {
                                cell->neighbor_or_periodic_neighbor(i)
                                  ->clear_coarsen_flag();
                                const unsigned int nb_nb =
                                  has_periodic_neighbor ?
                                    cell
                                      ->periodic_neighbor_of_periodic_neighbor(
                                        i) :
                                    cell->neighbor_of_neighbor(i);
                                const cell_iterator neighbor =
                                  cell->neighbor_or_periodic_neighbor(i);
                                RefinementCase<dim - 1> face_ref_case =
                                  GeometryInfo<dim>::face_refinement_case(
                                    neighbor->refine_flag_set(),
                                    nb_nb,
                                    neighbor->face_orientation(nb_nb),
                                    neighbor->face_flip(nb_nb),
                                    neighbor->face_rotation(nb_nb));
                                RefinementCase<dim - 1> needed_face_ref_case =
                                  GeometryInfo<dim>::face_refinement_case(
                                    cell->refine_flag_set(),
                                    i,
                                    cell->face_orientation(i),
                                    cell->face_flip(i),
                                    cell->face_rotation(i));
                                // if the neighbor wants to refine the
                                // face with cut_x and we want cut_y
                                // or vice versa, we have to refine
                                // isotropically at the given face
                                if ((face_ref_case ==
                                       RefinementCase<dim>::cut_x &&
                                     needed_face_ref_case ==
                                       RefinementCase<dim>::cut_y) ||
                                    (face_ref_case ==
                                       RefinementCase<dim>::cut_y &&
                                     needed_face_ref_case ==
                                       RefinementCase<dim>::cut_x))
                                  {
                                    changed = cell->flag_for_face_refinement(
                                      i, face_ref_case);
                                    neighbor->flag_for_face_refinement(
                                      nb_nb, needed_face_ref_case);
                                  }
                              }
                          }
                        else //-> the neighbor is not active
                          {
                            RefinementCase<dim - 1>
                              face_ref_case = cell->face(i)->refinement_case(),
                              needed_face_ref_case =
                                GeometryInfo<dim>::face_refinement_case(
                                  cell->refine_flag_set(),
                                  i,
                                  cell->face_orientation(i),
                                  cell->face_flip(i),
                                  cell->face_rotation(i));
                            // if the face is refined with cut_x and
                            // we want cut_y or vice versa, we have to
                            // refine isotropically at the given face
                            if ((face_ref_case == RefinementCase<dim>::cut_x &&
                                 needed_face_ref_case ==
                                   RefinementCase<dim>::cut_y) ||
                                (face_ref_case == RefinementCase<dim>::cut_y &&
                                 needed_face_ref_case ==
                                   RefinementCase<dim>::cut_x))
                              changed =
                                cell->flag_for_face_refinement(i,
                                                               face_ref_case);
                          }
                      }
                  }
              }
        }
 
      //------------------------------------
      // STEP 7:
      //    take care that no double refinement is done at each line in 3d or
      //    higher dimensions.
      this->policy->prepare_refinement_dim_dependent(*this);
 
      //------------------------------------
      // STEP 8:
      //    make sure that all children of each cell are either flagged for
      //    coarsening or none of the children is
      fix_coarsen_flags();
 
      //------------------------------------
      // Match refinement and coarsening decisions across periodic
      // faces if requested. In a distributed triangulation, only
      // modify locally owned cells; changes to ghost cells are made
      // by their owners after flags are exchanged.
      if (smooth_grid & match_refinement_across_periodic_boundaries)
        for (const auto &cell : active_cell_iterators())
          if (cell->is_locally_owned())
            for (const unsigned int face : cell->face_indices())
              if (cell->has_periodic_neighbor(face))
                {
                  const auto periodic_neighbor = cell->periodic_neighbor(face);
 
                  const int periodic_neighbor_future_level =
                    periodic_neighbor->level() +
                    (periodic_neighbor->refine_flag_set() ? 1 : 0) -
                    (periodic_neighbor->coarsen_flag_set() ? 1 : 0);
 
                  if (cell->level() < periodic_neighbor_future_level)
                    {
                      if (cell->coarsen_flag_set())
                        cell->clear_coarsen_flag();
                      else if (!cell->refine_flag_set())
                        cell->set_refine_flag();
                    }
                  else if (cell->level() == periodic_neighbor_future_level &&
                           cell->coarsen_flag_set())
                    cell->clear_coarsen_flag();
                }
 
      // get the refinement and coarsening flags
      auto coarsen_flags_after_loop =
        internal::extract_raw_coarsen_flags(levels);
      auto refine_flags_after_loop = internal::extract_raw_refine_flags(levels);
 
      // find out whether something was changed in this loop
      mesh_changed_in_this_loop =
        ((coarsen_flags_before_loop != coarsen_flags_after_loop) ||
         (refine_flags_before_loop != refine_flags_after_loop));
 
      // set the flags for the next loop already
      coarsen_flags_before_loop.swap(coarsen_flags_after_loop);
      refine_flags_before_loop.swap(refine_flags_after_loop);
    }
  while (mesh_changed_in_this_loop);
 
 
  // find out whether something was really changed in this
  // function. Note that @p{..._flags_before_loop} represents the state
  // after the last loop, i.e., the present state
  return ((coarsen_flags_before != coarsen_flags_before_loop) ||
          (refine_flags_before != refine_flags_before_loop));
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::write_bool_vector(
  const unsigned int       magic_number1,
  const std::vector<bool> &v,
  const unsigned int       magic_number2,
  std::ostream            &out)
{
  const unsigned int N     = v.size();
  unsigned char     *flags = new unsigned char[N / 8 + 1];
  for (unsigned int i = 0; i < N / 8 + 1; ++i)
    flags[i] = 0;
 
  for (unsigned int position = 0; position < N; ++position)
    flags[position / 8] |= (v[position] ? (1 << (position % 8)) : 0);
 
  AssertThrow(out.fail() == false, ExcIO());
 
  // format:
  // 0. magic number
  // 1. number of flags
  // 2. the flags
  // 3. magic number
  out << magic_number1 << ' ' << N << std::endl;
  for (unsigned int i = 0; i < N / 8 + 1; ++i)
    out << static_cast<unsigned int>(flags[i]) << ' ';
 
  out << std::endl << magic_number2 << std::endl;
 
  delete[] flags;
 
  AssertThrow(out.fail() == false, ExcIO());
}
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
void Triangulation<dim, spacedim>::read_bool_vector(
  const unsigned int magic_number1,
  std::vector<bool> &v,
  const unsigned int magic_number2,
  std::istream      &in)
{
  AssertThrow(in.fail() == false, ExcIO());
 
  unsigned int magic_number;
  in >> magic_number;
  AssertThrow(magic_number == magic_number1, ExcGridReadError());
 
  unsigned int N;
  in >> N;
  v.resize(N);
 
  unsigned char     *flags = new unsigned char[N / 8 + 1];
  unsigned short int tmp;
  for (unsigned int i = 0; i < N / 8 + 1; ++i)
    {
      in >> tmp;
      flags[i] = tmp;
    }
 
  for (unsigned int position = 0; position != N; ++position)
    v[position] = ((flags[position / 8] & (1 << (position % 8))) != 0);
 
  in >> magic_number;
  AssertThrow(magic_number == magic_number2, ExcGridReadError());
 
  delete[] flags;
 
  AssertThrow(in.fail() == false, ExcIO());
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
std::size_t Triangulation<dim, spacedim>::memory_consumption() const
{
  std::size_t mem = 0;
  mem += sizeof(MeshSmoothing);
  mem += MemoryConsumption::memory_consumption(reference_cells);
  mem += MemoryConsumption::memory_consumption(periodic_face_pairs_level_0);
  mem += MemoryConsumption::memory_consumption(levels);
  for (const auto &level : levels)
    mem += MemoryConsumption::memory_consumption(*level);
  mem += MemoryConsumption::memory_consumption(vertices);
  mem += MemoryConsumption::memory_consumption(vertices_used);
  mem += sizeof(default_flat_manifold) + sizeof(*default_flat_manifold);
  mem += sizeof(manifolds);
  mem += sizeof(smooth_grid);
  mem += MemoryConsumption::memory_consumption(number_cache);
  mem += sizeof(faces);
  if (faces)
    mem += MemoryConsumption::memory_consumption(*faces);
 
  return mem;
}
 
 
 
template <int dim, int spacedim>
DEAL_II_CXX20_REQUIRES((concepts::is_valid_dim_spacedim<dim, spacedim>))
Triangulation<dim, spacedim>::DistortedCellList::~DistortedCellList() noexcept =
  default;
 
#endif
 
// explicit instantiations
#include "grid/tria.inst"
 
DEAL_II_NAMESPACE_CLOSE