dof_tools.cc

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// ------------------------------------------------------------------------
//
// SPDX-License-Identifier: LGPL-2.1-or-later
// Copyright (C) 1999 - 2024 by the deal.II authors
//
// This file is part of the deal.II library.
//
// Part of the source code is dual licensed under Apache-2.0 WITH
// LLVM-exception OR LGPL-2.1-or-later. Detailed license information
// governing the source code and code contributions can be found in
// LICENSE.md and CONTRIBUTING.md at the top level directory of deal.II.
//
// ------------------------------------------------------------------------
 
#include <deal.II/base/mpi.h>
#include <deal.II/base/quadrature.h>
#include <deal.II/base/table.h>
#include <deal.II/base/template_constraints.h>
 
#include <deal.II/distributed/shared_tria.h>
#include <deal.II/distributed/tria.h>
 
#include <deal.II/dofs/dof_accessor.h>
#include <deal.II/dofs/dof_handler.h>
#include <deal.II/dofs/dof_tools.h>
 
#include <deal.II/fe/fe.h>
#include <deal.II/fe/fe_tools.h>
#include <deal.II/fe/fe_values.h>
 
#include <deal.II/grid/filtered_iterator.h>
#include <deal.II/grid/grid_tools.h>
#include <deal.II/grid/tria.h>
#include <deal.II/grid/tria_iterator.h>
 
#include <deal.II/hp/fe_collection.h>
#include <deal.II/hp/fe_values.h>
#include <deal.II/hp/mapping_collection.h>
#include <deal.II/hp/q_collection.h>
 
#include <deal.II/lac/affine_constraints.h>
#include <deal.II/lac/sparsity_pattern.h>
#include <deal.II/lac/sparsity_pattern_base.h>
#include <deal.II/lac/vector.h>
 
#include <algorithm>
#include <numeric>
 
DEAL_II_NAMESPACE_OPEN
 
 
 
namespace DoFTools
{
  namespace internal
  {
    template <int dim, typename Number = double>
    struct ComparisonHelper
    {
      bool
      operator()(const Point<dim, Number> &lhs,
                 const Point<dim, Number> &rhs) const
      {
        double downstream_size = 0;
        double weight          = 1.;
        for (unsigned int d = 0; d < dim; ++d)
          {
            downstream_size += (rhs[d] - lhs[d]) * weight;
            weight *= 1e-5;
          }
        if (downstream_size < 0)
          return false;
        else if (downstream_size > 0)
          return true;
        else
          {
            for (unsigned int d = 0; d < dim; ++d)
              {
                if (lhs[d] == rhs[d])
                  continue;
                return lhs[d] < rhs[d];
              }
            return false;
          }
      }
    };
 
 
 
    // return an array that for each dof on the reference cell
    // lists the corresponding vector component.
    //
    // if an element is non-primitive then we assign to each degree of freedom
    // the following component:
    // - if the nonzero components that belong to a shape function are not
    //   selected in the component_mask, then the shape function is assigned
    //   to the first nonzero vector component that corresponds to this
    //   shape function
    // - otherwise, the shape function is assigned the first component selected
    //   in the component_mask that corresponds to this shape function
    template <int dim, int spacedim>
    std::vector<unsigned char>
    get_local_component_association(const FiniteElement<dim, spacedim> &fe,
                                    const ComponentMask &component_mask)
    {
      std::vector<unsigned char> local_component_association(
        fe.n_dofs_per_cell(), static_cast<unsigned char>(-1));
 
      // compute the component each local dof belongs to.
      // if the shape function is primitive, then this
      // is simple and we can just associate it with
      // what system_to_component_index gives us
      for (unsigned int i = 0; i < fe.n_dofs_per_cell(); ++i)
        if (fe.is_primitive(i))
          local_component_association[i] =
            fe.system_to_component_index(i).first;
        else
          // if the shape function is not primitive, then either use the
          // component of the first nonzero component corresponding
          // to this shape function (if the component is not specified
          // in the component_mask), or the first component of this block
          // that is listed in the component_mask (if the block this
          // component corresponds to is indeed specified in the component
          // mask)
          {
            const unsigned int first_comp =
              fe.get_nonzero_components(i).first_selected_component();
 
            if ((fe.get_nonzero_components(i) & component_mask)
                  .n_selected_components(fe.n_components()) == 0)
              local_component_association[i] = first_comp;
            else
              // pick the component selected. we know from the previous 'if'
              // that within the components that are nonzero for this
              // shape function there must be at least one for which the
              // mask is true, so we will for sure run into the break()
              // at one point
              for (unsigned int c = first_comp; c < fe.n_components(); ++c)
                if (component_mask[c] == true)
                  {
                    local_component_association[i] = c;
                    break;
                  }
          }
 
      Assert(std::find(local_component_association.begin(),
                       local_component_association.end(),
                       static_cast<unsigned char>(-1)) ==
               local_component_association.end(),
             ExcInternalError());
 
      return local_component_association;
    }
 
 
    // this internal function assigns to each dof the respective component
    // of the vector system.
    //
    // the output array dofs_by_component lists for each dof the
    // corresponding vector component. if the DoFHandler is based on a
    // parallel distributed triangulation then the output array is index by
    // dof_handler.locally_owned_dofs().index_within_set(indices[i])
    //
    // if an element is non-primitive then we assign to each degree of
    // freedom the following component:
    // - if the nonzero components that belong to a shape function are not
    //   selected in the component_mask, then the shape function is assigned
    //   to the first nonzero vector component that corresponds to this
    //   shape function
    // - otherwise, the shape function is assigned the first component selected
    //   in the component_mask that corresponds to this shape function
    template <int dim, int spacedim>
    void
    get_component_association(
      const DoFHandler<dim, spacedim> &dof_handler,
      const ComponentMask             &component_mask,
      std::vector<unsigned char>      &dofs_by_component,
      const unsigned int               mg_level = numbers::invalid_unsigned_int)
    {
      const ::hp::FECollection<dim, spacedim> &fe_collection =
        dof_handler.get_fe_collection();
      Assert(fe_collection.n_components() < 256, ExcNotImplemented());
 
      const auto &locally_owned_dofs =
        (mg_level == numbers::invalid_unsigned_int) ?
          dof_handler.locally_owned_dofs() :
          dof_handler.locally_owned_mg_dofs(mg_level);
 
      AssertDimension(dofs_by_component.size(),
                      locally_owned_dofs.n_elements());
 
      // next set up a table for the degrees of freedom on each of the
      // cells (regardless of the fact whether it is listed in the
      // component_select argument or not)
      //
      // for each element 'f' of the FECollection,
      // local_component_association[f][d] then returns the vector
      // component that degree of freedom 'd' belongs to
      std::vector<std::vector<unsigned char>> local_component_association(
        fe_collection.size());
      for (unsigned int f = 0; f < fe_collection.size(); ++f)
        {
          const FiniteElement<dim, spacedim> &fe = fe_collection[f];
          local_component_association[f] =
            get_local_component_association(fe, component_mask);
        }
 
      // then loop over all cells and do the work
      std::vector<types::global_dof_index> indices;
 
      const auto runner = [&](const auto &task) {
        if (mg_level == numbers::invalid_unsigned_int)
          {
            for (const auto &cell : dof_handler.active_cell_iterators() |
                                      IteratorFilters::LocallyOwnedCell())
              {
                indices.resize(cell->get_fe().n_dofs_per_cell());
                cell->get_dof_indices(indices);
 
                task(cell);
              }
          }
        else
          {
            for (const auto &cell :
                 dof_handler.cell_iterators_on_level(mg_level))
              if (cell->is_locally_owned_on_level())
                {
                  indices.resize(cell->get_fe().n_dofs_per_cell());
                  cell->get_mg_dof_indices(indices);
 
                  task(cell);
                }
          }
      };
 
      runner([&](const auto &cell) {
        const types::fe_index fe_index = cell->active_fe_index();
        for (unsigned int i = 0; i < cell->get_fe().n_dofs_per_cell(); ++i)
          if (locally_owned_dofs.is_element(indices[i]))
            dofs_by_component[locally_owned_dofs.index_within_set(indices[i])] =
              local_component_association[fe_index][i];
      });
    }
 
 
    // this is the function corresponding to the one above but working on
    // blocks instead of components.
    //
    // the output array dofs_by_block lists for each dof the corresponding
    // vector block. if the DoFHandler is based on a parallel distributed
    // triangulation then the output array is index by
    // dof.locally_owned_dofs().index_within_set(indices[i])
    template <int dim, int spacedim>
    inline void
    get_block_association(const DoFHandler<dim, spacedim> &dof,
                          std::vector<unsigned char>      &dofs_by_block)
    {
      const ::hp::FECollection<dim, spacedim> &fe_collection =
        dof.get_fe_collection();
      Assert(fe_collection.n_components() < 256, ExcNotImplemented());
      Assert(dofs_by_block.size() == dof.n_locally_owned_dofs(),
             ExcDimensionMismatch(dofs_by_block.size(),
                                  dof.n_locally_owned_dofs()));
 
      // next set up a table for the degrees of freedom on each of the
      // cells (regardless of the fact whether it is listed in the
      // component_select argument or not)
      //
      // for each element 'f' of the FECollection,
      // local_block_association[f][d] then returns the vector block that
      // degree of freedom 'd' belongs to
      std::vector<std::vector<unsigned char>> local_block_association(
        fe_collection.size());
      for (unsigned int f = 0; f < fe_collection.size(); ++f)
        {
          const FiniteElement<dim, spacedim> &fe = fe_collection[f];
          local_block_association[f].resize(fe.n_dofs_per_cell(),
                                            static_cast<unsigned char>(-1));
          for (unsigned int i = 0; i < fe.n_dofs_per_cell(); ++i)
            local_block_association[f][i] = fe.system_to_block_index(i).first;
 
          Assert(std::find(local_block_association[f].begin(),
                           local_block_association[f].end(),
                           static_cast<unsigned char>(-1)) ==
                   local_block_association[f].end(),
                 ExcInternalError());
        }
 
      // then loop over all cells and do the work
      std::vector<types::global_dof_index> indices;
      for (const auto &cell : dof.active_cell_iterators())
        if (cell->is_locally_owned())
          {
            const types::fe_index fe_index   = cell->active_fe_index();
            const unsigned int dofs_per_cell = cell->get_fe().n_dofs_per_cell();
            indices.resize(dofs_per_cell);
            cell->get_dof_indices(indices);
            for (unsigned int i = 0; i < dofs_per_cell; ++i)
              if (dof.locally_owned_dofs().is_element(indices[i]))
                dofs_by_block[dof.locally_owned_dofs().index_within_set(
                  indices[i])] = local_block_association[fe_index][i];
          }
    }
  } // namespace internal
 
 
 
  template <int dim, int spacedim, typename Number>
  void
  distribute_cell_to_dof_vector(const DoFHandler<dim, spacedim> &dof_handler,
                                const Vector<Number>            &cell_data,
                                Vector<double>                  &dof_data,
                                const unsigned int               component)
  {
    const Triangulation<dim, spacedim> &tria = dof_handler.get_triangulation();
    (void)tria;
 
    AssertDimension(cell_data.size(), tria.n_active_cells());
    AssertDimension(dof_data.size(), dof_handler.n_dofs());
    const auto &fe_collection = dof_handler.get_fe_collection();
    AssertIndexRange(component, fe_collection.n_components());
    for (unsigned int i = 0; i < fe_collection.size(); ++i)
      {
        Assert(fe_collection[i].is_primitive() == true,
               typename FiniteElement<dim>::ExcFENotPrimitive());
      }
 
    // store a flag whether we should care about different components. this
    // is just a simplification, we could ask for this at every single
    // place equally well
    const bool consider_components =
      (dof_handler.get_fe_collection().n_components() != 1);
 
    // zero out the components that we will touch
    if (consider_components == false)
      dof_data = 0;
    else
      {
        std::vector<unsigned char> component_dofs(
          dof_handler.n_locally_owned_dofs());
        internal::get_component_association(
          dof_handler,
          fe_collection.component_mask(FEValuesExtractors::Scalar(component)),
          component_dofs);
 
        for (unsigned int i = 0; i < dof_data.size(); ++i)
          if (component_dofs[i] == static_cast<unsigned char>(component))
            dof_data(i) = 0;
      }
 
    // count how often we have added a value in the sum for each dof
    std::vector<unsigned char> touch_count(dof_handler.n_dofs(), 0);
 
    std::vector<types::global_dof_index> dof_indices;
    dof_indices.reserve(fe_collection.max_dofs_per_cell());
 
    for (const auto &cell : dof_handler.active_cell_iterators())
      {
        const unsigned int dofs_per_cell = cell->get_fe().n_dofs_per_cell();
        dof_indices.resize(dofs_per_cell);
        cell->get_dof_indices(dof_indices);
 
        for (unsigned int i = 0; i < dofs_per_cell; ++i)
          // consider this dof only if it is the right component. if there
          // is only one component, short cut the test
          if (!consider_components ||
              (cell->get_fe().system_to_component_index(i).first == component))
            {
              // sum up contribution of the present_cell to this dof
              dof_data(dof_indices[i]) += cell_data(cell->active_cell_index());
 
              // note that we added another summand
              ++touch_count[dof_indices[i]];
            }
      }
 
    // compute the mean value on all the dofs by dividing with the number
    // of summands.
    for (types::global_dof_index i = 0; i < dof_handler.n_dofs(); ++i)
      {
        // assert that each dof was used at least once. this needs not be
        // the case if the vector has more than one component
        Assert(consider_components || (touch_count[i] != 0),
               ExcInternalError());
        if (touch_count[i] != 0)
          dof_data(i) /= touch_count[i];
      }
  }
 
 
 
  template <int dim, int spacedim>
  IndexSet
  extract_dofs(const DoFHandler<dim, spacedim> &dof,
               const ComponentMask             &component_mask)
  {
    Assert(component_mask.represents_n_components(
             dof.get_fe_collection().n_components()),
           ExcMessage(
             "The given component mask is not sized correctly to represent the "
             "components of the given finite element."));
 
    // Two special cases: no component is selected, and all components are
    // selected; both rather stupid, but easy to catch
    if (component_mask.n_selected_components(
          dof.get_fe_collection().n_components()) == 0)
      return IndexSet(dof.n_dofs());
    else if (component_mask.n_selected_components(
               dof.get_fe_collection().n_components()) ==
             dof.get_fe_collection().n_components())
      return dof.locally_owned_dofs();
 
    // get the component association of each DoF and then select the ones
    // that match the given set of components
    std::vector<unsigned char> dofs_by_component(dof.n_locally_owned_dofs());
    internal::get_component_association(dof, component_mask, dofs_by_component);
 
    // fill the selected components in a vector
    std::vector<types::global_dof_index> selected_dofs;
    selected_dofs.reserve(dof.n_locally_owned_dofs());
    for (types::global_dof_index i = 0; i < dofs_by_component.size(); ++i)
      if (component_mask[dofs_by_component[i]] == true)
        selected_dofs.push_back(dof.locally_owned_dofs().nth_index_in_set(i));
 
    // fill vector of indices to return argument
    IndexSet result(dof.n_dofs());
    result.add_indices(selected_dofs.begin(), selected_dofs.end());
    return result;
  }
 
 
 
  template <int dim, int spacedim>
  IndexSet
  extract_dofs(const DoFHandler<dim, spacedim> &dof,
               const BlockMask                 &block_mask)
  {
    // simply forward to the function that works based on a component mask
    return extract_dofs<dim, spacedim>(
      dof, dof.get_fe_collection().component_mask(block_mask));
  }
 
 
 
  template <int dim, int spacedim>
  std::vector<IndexSet>
  locally_owned_dofs_per_component(const DoFHandler<dim, spacedim> &dof,
                                   const ComponentMask &component_mask)
  {
    const auto n_comps = dof.get_fe_collection().n_components();
    Assert(component_mask.represents_n_components(n_comps),
           ExcMessage(
             "The given component mask is not sized correctly to represent the "
             "components of the given finite element."));
 
    const auto &locally_owned_dofs = dof.locally_owned_dofs();
 
    // get the component association of each DoF and then select the ones
    // that match the given set of components
    std::vector<unsigned char> dofs_by_component(dof.n_locally_owned_dofs());
    internal::get_component_association(dof, component_mask, dofs_by_component);
 
    std::vector<IndexSet> index_per_comp(n_comps, IndexSet(dof.n_dofs()));
 
    for (types::global_dof_index i = 0; i < dof.n_locally_owned_dofs(); ++i)
      {
        const auto &comp_i = dofs_by_component[i];
        if (component_mask[comp_i])
          index_per_comp[comp_i].add_index(
            locally_owned_dofs.nth_index_in_set(i));
      }
    for (const auto &c : index_per_comp)
      c.compress();
    return index_per_comp;
  }
 
 
 
  template <int dim, int spacedim>
  void
  extract_level_dofs(const unsigned int               level,
                     const DoFHandler<dim, spacedim> &dof,
                     const ComponentMask             &component_mask,
                     std::vector<bool>               &selected_dofs)
  {
    const FiniteElement<dim, spacedim> &fe = dof.get_fe();
 
    Assert(component_mask.represents_n_components(
             dof.get_fe_collection().n_components()),
           ExcMessage(
             "The given component mask is not sized correctly to represent the "
             "components of the given finite element."));
    Assert(selected_dofs.size() == dof.n_dofs(level),
           ExcDimensionMismatch(selected_dofs.size(), dof.n_dofs(level)));
 
    // two special cases: no component is selected, and all components are
    // selected, both rather stupid, but easy to catch
    if (component_mask.n_selected_components(
          dof.get_fe_collection().n_components()) == 0)
      {
        std::fill_n(selected_dofs.begin(), dof.n_dofs(level), false);
        return;
      }
    else if (component_mask.n_selected_components(
               dof.get_fe_collection().n_components()) ==
             dof.get_fe_collection().n_components())
      {
        std::fill_n(selected_dofs.begin(), dof.n_dofs(level), true);
        return;
      }
 
    // preset all values by false
    std::fill_n(selected_dofs.begin(), dof.n_dofs(level), false);
 
    // next set up a table for the degrees of freedom on each of the cells
    // whether it is something interesting or not
    std::vector<unsigned char> local_component_association =
      internal::get_local_component_association(fe, component_mask);
    std::vector<bool> local_selected_dofs(fe.n_dofs_per_cell());
    for (unsigned int i = 0; i < fe.n_dofs_per_cell(); ++i)
      local_selected_dofs[i] = component_mask[local_component_association[i]];
 
    // then loop over all cells and do work
    std::vector<types::global_dof_index> indices(fe.n_dofs_per_cell());
    for (const auto &cell : dof.cell_iterators_on_level(level))
      {
        cell->get_mg_dof_indices(indices);
        for (unsigned int i = 0; i < fe.n_dofs_per_cell(); ++i)
          selected_dofs[indices[i]] = local_selected_dofs[i];
      }
  }
 
 
 
  template <int dim, int spacedim>
  void
  extract_level_dofs(const unsigned int               level,
                     const DoFHandler<dim, spacedim> &dof,
                     const BlockMask                 &block_mask,
                     std::vector<bool>               &selected_dofs)
  {
    // simply defer to the other extract_level_dofs() function
    extract_level_dofs(level,
                       dof,
                       dof.get_fe().component_mask(block_mask),
                       selected_dofs);
  }
 
 
 
  template <int dim, int spacedim>
  void
  extract_boundary_dofs(const DoFHandler<dim, spacedim>    &dof_handler,
                        const ComponentMask                &component_mask,
                        std::vector<bool>                  &selected_dofs,
                        const std::set<types::boundary_id> &boundary_ids)
  {
    Assert((dynamic_cast<
              const parallel::distributed::Triangulation<dim, spacedim> *>(
              &dof_handler.get_triangulation()) == nullptr),
           ExcMessage(
             "This function can not be used with distributed triangulations. "
             "See the documentation for more information."));
 
    IndexSet indices =
      extract_boundary_dofs(dof_handler, component_mask, boundary_ids);
 
    // clear and reset array by default values
    selected_dofs.clear();
    selected_dofs.resize(dof_handler.n_dofs(), false);
 
    // then convert the values computed above to the binary vector
    indices.fill_binary_vector(selected_dofs);
  }
 
 
 
  template <int dim, int spacedim>
  void
  extract_boundary_dofs(const DoFHandler<dim, spacedim>    &dof_handler,
                        const ComponentMask                &component_mask,
                        IndexSet                           &selected_dofs,
                        const std::set<types::boundary_id> &boundary_ids)
  {
    // Simply forward to the other function
    selected_dofs =
      extract_boundary_dofs(dof_handler, component_mask, boundary_ids);
  }
 
 
 
  template <int dim, int spacedim>
  IndexSet
  extract_boundary_dofs(const DoFHandler<dim, spacedim>    &dof_handler,
                        const ComponentMask                &component_mask,
                        const std::set<types::boundary_id> &boundary_ids)
  {
    Assert(component_mask.represents_n_components(
             dof_handler.get_fe_collection().n_components()),
           ExcMessage("Component mask has invalid size."));
    Assert(boundary_ids.find(numbers::internal_face_boundary_id) ==
             boundary_ids.end(),
           ExcInvalidBoundaryIndicator());
 
    IndexSet selected_dofs(dof_handler.n_dofs());
 
    // let's see whether we have to check for certain boundary indicators
    // or whether we can accept all
    const bool check_boundary_id = (boundary_ids.size() != 0);
 
    // also see whether we have to check whether a certain vector component
    // is selected, or all
    const bool check_vector_component =
      ((component_mask.represents_the_all_selected_mask() == false) ||
       (component_mask.n_selected_components(
          dof_handler.get_fe_collection().n_components()) !=
        dof_handler.get_fe_collection().n_components()));
 
    std::vector<types::global_dof_index> face_dof_indices;
    face_dof_indices.reserve(
      dof_handler.get_fe_collection().max_dofs_per_face());
 
    // now loop over all cells and check whether their faces are at the
    // boundary. note that we need not take special care of single lines
    // being at the boundary (using @p{cell->has_boundary_lines}), since we
    // do not support boundaries of dimension dim-2, and so every isolated
    // boundary line is also part of a boundary face which we will be
    // visiting sooner or later
    for (const auto &cell : dof_handler.active_cell_iterators())
      // only work on cells that are either locally owned or at least ghost
      // cells
      if (cell->is_artificial() == false)
        for (const unsigned int face : cell->face_indices())
          if (cell->at_boundary(face))
            if (!check_boundary_id ||
                (boundary_ids.find(cell->face(face)->boundary_id()) !=
                 boundary_ids.end()))
              {
                const FiniteElement<dim, spacedim> &fe = cell->get_fe();
 
                const auto reference_cell = cell->reference_cell();
 
                const unsigned int n_vertices_per_cell =
                  reference_cell.n_vertices();
                const unsigned int n_lines_per_cell = reference_cell.n_lines();
                const unsigned int n_vertices_per_face =
                  reference_cell.face_reference_cell(face).n_vertices();
                const unsigned int n_lines_per_face =
                  reference_cell.face_reference_cell(face).n_lines();
 
                const unsigned int dofs_per_face = fe.n_dofs_per_face(face);
                face_dof_indices.resize(dofs_per_face);
                cell->face(face)->get_dof_indices(face_dof_indices,
                                                  cell->active_fe_index());
 
                for (unsigned int i = 0; i < fe.n_dofs_per_face(face); ++i)
                  if (!check_vector_component)
                    selected_dofs.add_index(face_dof_indices[i]);
                  else
                    // check for component is required. somewhat tricky as
                    // usual for the case that the shape function is
                    // non-primitive, but use usual convention (see docs)
                    {
                      // first get at the cell-global number of a face dof,
                      // to ask the FE certain questions
                      const unsigned int cell_index =
                        (dim == 1 ?
                           i :
                           (dim == 2 ?
                              (i < 2 * fe.n_dofs_per_vertex() ?
                                 i :
                                 i + 2 * fe.n_dofs_per_vertex()) :
                              (dim == 3 ? (i < n_vertices_per_face *
                                                 fe.n_dofs_per_vertex() ?
                                             i :
                                             (i < n_vertices_per_face *
                                                      fe.n_dofs_per_vertex() +
                                                    n_lines_per_face *
                                                      fe.n_dofs_per_line() ?
                                                (i - n_vertices_per_face *
                                                       fe.n_dofs_per_vertex()) +
                                                  n_vertices_per_cell *
                                                    fe.n_dofs_per_vertex() :
                                                (i -
                                                 n_vertices_per_face *
                                                   fe.n_dofs_per_vertex() -
                                                 n_lines_per_face *
                                                   fe.n_dofs_per_line()) +
                                                  n_vertices_per_cell *
                                                    fe.n_dofs_per_vertex() +
                                                  n_lines_per_cell *
                                                    fe.n_dofs_per_line())) :
                                          numbers::invalid_unsigned_int)));
                      if (fe.is_primitive(cell_index))
                        {
                          if (component_mask[fe.face_system_to_component_index(
                                                 i, face)
                                               .first] == true)
                            selected_dofs.add_index(face_dof_indices[i]);
                        }
                      else // not primitive
                        {
                          const unsigned int first_nonzero_comp =
                            fe.get_nonzero_components(cell_index)
                              .first_selected_component();
                          Assert(first_nonzero_comp < fe.n_components(),
                                 ExcInternalError());
 
                          if (component_mask[first_nonzero_comp] == true)
                            selected_dofs.add_index(face_dof_indices[i]);
                        }
                    }
              }
 
    return selected_dofs;
  }
 
 
 
  template <int dim, int spacedim>
  void
  extract_dofs_with_support_on_boundary(
    const DoFHandler<dim, spacedim>    &dof_handler,
    const ComponentMask                &component_mask,
    std::vector<bool>                  &selected_dofs,
    const std::set<types::boundary_id> &boundary_ids)
  {
    Assert(component_mask.represents_n_components(
             dof_handler.get_fe_collection().n_components()),
           ExcMessage("This component mask has the wrong size."));
    Assert(boundary_ids.find(numbers::internal_face_boundary_id) ==
             boundary_ids.end(),
           ExcInvalidBoundaryIndicator());
 
    // let's see whether we have to check for certain boundary indicators
    // or whether we can accept all
    const bool check_boundary_id = (boundary_ids.size() != 0);
 
    // also see whether we have to check whether a certain vector component
    // is selected, or all
    const bool check_vector_component =
      (component_mask.represents_the_all_selected_mask() == false);
 
    // clear and reset array by default values
    selected_dofs.clear();
    selected_dofs.resize(dof_handler.n_dofs(), false);
    std::vector<types::global_dof_index> cell_dof_indices;
    cell_dof_indices.reserve(
      dof_handler.get_fe_collection().max_dofs_per_cell());
 
    // now loop over all cells and check whether their faces are at the
    // boundary. note that we need not take special care of single lines
    // being at the boundary (using @p{cell->has_boundary_lines}), since we
    // do not support boundaries of dimension dim-2, and so every isolated
    // boundary line is also part of a boundary face which we will be
    // visiting sooner or later
    for (const auto &cell : dof_handler.active_cell_iterators())
      for (const unsigned int face : cell->face_indices())
        if (cell->at_boundary(face))
          if (!check_boundary_id ||
              (boundary_ids.find(cell->face(face)->boundary_id()) !=
               boundary_ids.end()))
            {
              const FiniteElement<dim, spacedim> &fe = cell->get_fe();
 
              const unsigned int dofs_per_cell = fe.n_dofs_per_cell();
              cell_dof_indices.resize(dofs_per_cell);
              cell->get_dof_indices(cell_dof_indices);
 
              for (unsigned int i = 0; i < fe.n_dofs_per_cell(); ++i)
                if (fe.has_support_on_face(i, face))
                  {
                    if (!check_vector_component)
                      selected_dofs[cell_dof_indices[i]] = true;
                    else
                      // check for component is required. somewhat tricky
                      // as usual for the case that the shape function is
                      // non-primitive, but use usual convention (see docs)
                      {
                        if (fe.is_primitive(i))
                          selected_dofs[cell_dof_indices[i]] =
                            (component_mask[fe.system_to_component_index(i)
                                              .first] == true);
                        else // not primitive
                          {
                            const unsigned int first_nonzero_comp =
                              fe.get_nonzero_components(i)
                                .first_selected_component();
                            Assert(first_nonzero_comp < fe.n_components(),
                                   ExcInternalError());
 
                            selected_dofs[cell_dof_indices[i]] =
                              (component_mask[first_nonzero_comp] == true);
                          }
                      }
                  }
            }
  }
 
 
 
  template <int dim, int spacedim, typename number>
  IndexSet
  extract_dofs_with_support_contained_within(
    const DoFHandler<dim, spacedim> &dof_handler,
    const std::function<
      bool(const typename DoFHandler<dim, spacedim>::active_cell_iterator &)>
                                    &predicate,
    const AffineConstraints<number> &cm)
  {
    const std::function<bool(
      const typename DoFHandler<dim, spacedim>::active_cell_iterator &)>
      predicate_local =
        [=](
          const typename DoFHandler<dim, spacedim>::active_cell_iterator &cell)
      -> bool { return cell->is_locally_owned() && predicate(cell); };
 
    std::vector<types::global_dof_index> local_dof_indices;
    local_dof_indices.reserve(
      dof_handler.get_fe_collection().max_dofs_per_cell());
 
    // Get all the dofs that live on the subdomain:
    std::set<types::global_dof_index> predicate_dofs;
 
    for (const auto &cell : dof_handler.active_cell_iterators())
      if (!cell->is_artificial() && predicate(cell))
        {
          local_dof_indices.resize(cell->get_fe().n_dofs_per_cell());
          cell->get_dof_indices(local_dof_indices);
          predicate_dofs.insert(local_dof_indices.begin(),
                                local_dof_indices.end());
        }
 
    // Get halo layer and accumulate its DoFs
    std::set<types::global_dof_index> dofs_with_support_on_halo_cells;
 
    const std::vector<typename DoFHandler<dim, spacedim>::active_cell_iterator>
      halo_cells =
        GridTools::compute_active_cell_halo_layer(dof_handler, predicate_local);
    for (typename std::vector<typename DoFHandler<dim, spacedim>::
                                active_cell_iterator>::const_iterator it =
           halo_cells.begin();
         it != halo_cells.end();
         ++it)
      {
        // skip halo cells that satisfy the given predicate and thereby
        // shall be a part of the index set on another MPI core.
        // Those could only be ghost cells with p::d::Tria.
        if (predicate(*it))
          {
            Assert((*it)->is_ghost(), ExcInternalError());
            continue;
          }
 
        const unsigned int dofs_per_cell = (*it)->get_fe().n_dofs_per_cell();
        local_dof_indices.resize(dofs_per_cell);
        (*it)->get_dof_indices(local_dof_indices);
        dofs_with_support_on_halo_cells.insert(local_dof_indices.begin(),
                                               local_dof_indices.end());
      }
 
    // A complication coming from the fact that DoFs living on locally
    // owned cells which satisfy predicate may participate in constraints for
    // DoFs outside of this region.
    if (cm.n_constraints() > 0)
      {
        // Remove DoFs that are part of constraints for DoFs outside
        // of predicate. Since we will subtract halo_dofs from predicate_dofs,
        // achieve this by extending halo_dofs with DoFs to which
        // halo_dofs are constrained.
        std::set<types::global_dof_index> extra_halo;
        for (const auto dof : dofs_with_support_on_halo_cells)
          // if halo DoF is constrained, add all DoFs to which it's constrained
          // because after resolving constraints, the support of the DoFs that
          // constrain the current DoF will extend to the halo cells.
          if (const auto *line_ptr = cm.get_constraint_entries(dof))
            {
              const unsigned int line_size = line_ptr->size();
              for (unsigned int j = 0; j < line_size; ++j)
                extra_halo.insert((*line_ptr)[j].first);
            }
 
        dofs_with_support_on_halo_cells.insert(extra_halo.begin(),
                                               extra_halo.end());
      }
 
    // Rework our std::set's into IndexSet and subtract halo DoFs from the
    // predicate_dofs:
    IndexSet support_set(dof_handler.n_dofs());
    support_set.add_indices(predicate_dofs.begin(), predicate_dofs.end());
    support_set.compress();
 
    IndexSet halo_set(dof_handler.n_dofs());
    halo_set.add_indices(dofs_with_support_on_halo_cells.begin(),
                         dofs_with_support_on_halo_cells.end());
    halo_set.compress();
 
    support_set.subtract_set(halo_set);
 
    // we intentionally do not want to limit the output to locally owned DoFs.
    return support_set;
  }
 
 
 
  namespace internal
  {
    namespace
    {
      template <int spacedim>
      IndexSet
      extract_hanging_node_dofs(const DoFHandler<1, spacedim> &dof_handler)
      {
        // there are no hanging nodes in 1d
        return IndexSet(dof_handler.n_dofs());
      }
 
 
      template <int spacedim>
      IndexSet
      extract_hanging_node_dofs(const DoFHandler<2, spacedim> &dof_handler)
      {
        const unsigned int dim = 2;
 
        IndexSet selected_dofs(dof_handler.n_dofs());
 
        const FiniteElement<dim, spacedim> &fe = dof_handler.get_fe();
 
        // this function is similar to the make_sparsity_pattern function,
        // see there for more information
        for (const auto &cell : dof_handler.active_cell_iterators())
          if (!cell->is_artificial())
            {
              for (const unsigned int face : cell->face_indices())
                if (cell->face(face)->has_children())
                  {
                    const typename DoFHandler<dim, spacedim>::line_iterator
                      line = cell->face(face);
 
                    for (unsigned int dof = 0; dof != fe.n_dofs_per_vertex();
                         ++dof)
                      selected_dofs.add_index(
                        line->child(0)->vertex_dof_index(1, dof));
 
                    for (unsigned int child = 0; child < 2; ++child)
                      {
                        if (cell->neighbor_child_on_subface(face, child)
                              ->is_artificial())
                          continue;
                        for (unsigned int dof = 0; dof != fe.n_dofs_per_line();
                             ++dof)
                          selected_dofs.add_index(
                            line->child(child)->dof_index(dof));
                      }
                  }
            }
 
        selected_dofs.compress();
        return selected_dofs;
      }
 
 
      template <int spacedim>
      IndexSet
      extract_hanging_node_dofs(const DoFHandler<3, spacedim> &dof_handler)
      {
        const unsigned int dim = 3;
 
        IndexSet selected_dofs(dof_handler.n_dofs());
        IndexSet unconstrained_dofs(dof_handler.n_dofs());
 
        const FiniteElement<dim, spacedim> &fe = dof_handler.get_fe();
 
        for (const auto &cell : dof_handler.active_cell_iterators())
          if (!cell->is_artificial())
            for (auto f : cell->face_indices())
              {
                const typename DoFHandler<dim, spacedim>::face_iterator face =
                  cell->face(f);
                if (cell->face(f)->has_children())
                  {
                    for (unsigned int child = 0; child < 4; ++child)
                      if (!cell->neighbor_child_on_subface(f, child)
                             ->is_artificial())
                        {
                          // simply take all DoFs that live on this subface
                          std::vector<types::global_dof_index> ldi(
                            fe.n_dofs_per_face(f, child));
                          face->child(child)->get_dof_indices(ldi);
                          selected_dofs.add_indices(ldi.begin(), ldi.end());
                        }
 
                    // and subtract (in the end) all the indices which a shared
                    // between this face and its subfaces
                    for (unsigned int vertex = 0; vertex < 4; ++vertex)
                      for (unsigned int dof = 0; dof != fe.n_dofs_per_vertex();
                           ++dof)
                        unconstrained_dofs.add_index(
                          face->vertex_dof_index(vertex, dof));
                  }
              }
        selected_dofs.subtract_set(unconstrained_dofs);
        return selected_dofs;
      }
    } // namespace
  }   // namespace internal
 
 
 
  template <int dim, int spacedim>
  IndexSet
  extract_hanging_node_dofs(const DoFHandler<dim, spacedim> &dof_handler)
  {
    return internal::extract_hanging_node_dofs(dof_handler);
  }
 
 
 
  template <int dim, int spacedim>
  void
  extract_subdomain_dofs(const DoFHandler<dim, spacedim> &dof_handler,
                         const types::subdomain_id        subdomain_id,
                         std::vector<bool>               &selected_dofs)
  {
    Assert(selected_dofs.size() == dof_handler.n_dofs(),
           ExcDimensionMismatch(selected_dofs.size(), dof_handler.n_dofs()));
 
    // preset all values by false
    std::fill_n(selected_dofs.begin(), dof_handler.n_dofs(), false);
 
    std::vector<types::global_dof_index> local_dof_indices;
    local_dof_indices.reserve(
      dof_handler.get_fe_collection().max_dofs_per_cell());
 
    // this function is similar to the make_sparsity_pattern function, see
    // there for more information
    for (const auto &cell : dof_handler.active_cell_iterators())
      if (cell->subdomain_id() == subdomain_id)
        {
          const unsigned int dofs_per_cell = cell->get_fe().n_dofs_per_cell();
          local_dof_indices.resize(dofs_per_cell);
          cell->get_dof_indices(local_dof_indices);
          for (unsigned int i = 0; i < dofs_per_cell; ++i)
            selected_dofs[local_dof_indices[i]] = true;
        }
  }
 
 
 
  template <int dim, int spacedim>
  IndexSet
  extract_locally_active_dofs(const DoFHandler<dim, spacedim> &dof_handler)
  {
    // collect all the locally owned dofs
    IndexSet dof_set = dof_handler.locally_owned_dofs();
 
    // add the DoF on the adjacent ghost cells to the IndexSet, cache them
    // in a set. need to check each dof manually because we can't be sure
    // that the dof range of locally_owned_dofs is really contiguous.
    std::vector<types::global_dof_index> dof_indices;
    std::set<types::global_dof_index>    global_dof_indices;
 
    for (const auto &cell : dof_handler.active_cell_iterators() |
                              IteratorFilters::LocallyOwnedCell())
      {
        dof_indices.resize(cell->get_fe().n_dofs_per_cell());
        cell->get_dof_indices(dof_indices);
 
        for (const types::global_dof_index dof_index : dof_indices)
          if (!dof_set.is_element(dof_index))
            global_dof_indices.insert(dof_index);
      }
 
    dof_set.add_indices(global_dof_indices.begin(), global_dof_indices.end());
 
    dof_set.compress();
 
    return dof_set;
  }
 
 
 
  template <int dim, int spacedim>
  void
  extract_locally_active_dofs(const DoFHandler<dim, spacedim> &dof_handler,
                              IndexSet                        &dof_set)
  {
    dof_set = extract_locally_active_dofs(dof_handler);
  }
 
 
 
  template <int dim, int spacedim>
  IndexSet
  extract_locally_active_level_dofs(
    const DoFHandler<dim, spacedim> &dof_handler,
    const unsigned int               level)
  {
    // collect all the locally owned dofs
    IndexSet dof_set = dof_handler.locally_owned_mg_dofs(level);
 
    // add the DoF on the adjacent ghost cells to the IndexSet, cache them
    // in a set. need to check each dof manually because we can't be sure
    // that the dof range of locally_owned_dofs is really contiguous.
    std::vector<types::global_dof_index> dof_indices;
    std::set<types::global_dof_index>    global_dof_indices;
 
    const auto filtered_iterators_range =
      filter_iterators(dof_handler.cell_iterators_on_level(level),
                       ::IteratorFilters::LocallyOwnedLevelCell());
    for (const auto &cell : filtered_iterators_range)
      {
        dof_indices.resize(cell->get_fe().n_dofs_per_cell());
        cell->get_mg_dof_indices(dof_indices);
 
        for (const types::global_dof_index dof_index : dof_indices)
          if (!dof_set.is_element(dof_index))
            global_dof_indices.insert(dof_index);
      }
 
    dof_set.add_indices(global_dof_indices.begin(), global_dof_indices.end());
 
    dof_set.compress();
 
    return dof_set;
  }
 
 
 
  template <int dim, int spacedim>
  void
  extract_locally_active_level_dofs(
    const DoFHandler<dim, spacedim> &dof_handler,
    IndexSet                        &dof_set,
    const unsigned int               level)
  {
    dof_set = extract_locally_active_level_dofs(dof_handler, level);
  }
 
 
 
  template <int dim, int spacedim>
  IndexSet
  extract_locally_relevant_dofs(const DoFHandler<dim, spacedim> &dof_handler)
  {
    // collect all the locally owned dofs
    IndexSet dof_set = dof_handler.locally_owned_dofs();
 
    // now add the DoF on the adjacent ghost cells to the IndexSet
 
    // Note: For certain meshes (in particular in 3d and with many
    // processors), it is really necessary to cache intermediate data. After
    // trying several objects such as std::set, a vector that is always kept
    // sorted, and a vector that is initially unsorted and sorted once at the
    // end, the latter has been identified to provide the best performance.
    // Martin Kronbichler
    std::vector<types::global_dof_index> dof_indices;
    std::vector<types::global_dof_index> dofs_on_ghosts;
 
    for (const auto &cell : dof_handler.active_cell_iterators())
      if (cell->is_ghost())
        {
          dof_indices.resize(cell->get_fe().n_dofs_per_cell());
          cell->get_dof_indices(dof_indices);
          for (const auto dof_index : dof_indices)
            if (!dof_set.is_element(dof_index))
              dofs_on_ghosts.push_back(dof_index);
        }
 
    // sort and put into an index set
    std::sort(dofs_on_ghosts.begin(), dofs_on_ghosts.end());
    dof_set.add_indices(dofs_on_ghosts.begin(), dofs_on_ghosts.end());
    dof_set.compress();
 
    return dof_set;
  }
 
 
 
  template <int dim, int spacedim>
  void
  extract_locally_relevant_dofs(const DoFHandler<dim, spacedim> &dof_handler,
                                IndexSet                        &dof_set)
  {
    dof_set = extract_locally_relevant_dofs(dof_handler);
  }
 
 
 
  template <int dim, int spacedim>
  IndexSet
  extract_locally_relevant_level_dofs(
    const DoFHandler<dim, spacedim> &dof_handler,
    const unsigned int               level)
  {
    // collect all the locally owned dofs
    IndexSet dof_set = dof_handler.locally_owned_mg_dofs(level);
 
    // add the DoF on the adjacent ghost cells to the IndexSet
 
    // Note: For certain meshes (in particular in 3d and with many
    // processors), it is really necessary to cache intermediate data. After
    // trying several objects such as std::set, a vector that is always kept
    // sorted, and a vector that is initially unsorted and sorted once at the
    // end, the latter has been identified to provide the best performance.
    // Martin Kronbichler
    std::vector<types::global_dof_index> dof_indices;
    std::vector<types::global_dof_index> dofs_on_ghosts;
 
    for (const auto &cell : dof_handler.cell_iterators_on_level(level))
      {
        const types::subdomain_id id = cell->level_subdomain_id();
 
        // skip artificial and own cells (only look at ghost cells)
        if (id == dof_handler.get_triangulation().locally_owned_subdomain() ||
            id == numbers::artificial_subdomain_id)
          continue;
 
        dof_indices.resize(cell->get_fe().n_dofs_per_cell());
        cell->get_mg_dof_indices(dof_indices);
        for (const auto dof_index : dof_indices)
          if (!dof_set.is_element(dof_index))
            dofs_on_ghosts.push_back(dof_index);
      }
 
    // sort and fill into an index set
    std::sort(dofs_on_ghosts.begin(), dofs_on_ghosts.end());
    dof_set.add_indices(dofs_on_ghosts.begin(), dofs_on_ghosts.end());
    dof_set.compress();
 
    return dof_set;
  }
 
 
 
  template <int dim, int spacedim>
  void
  extract_locally_relevant_level_dofs(
    const DoFHandler<dim, spacedim> &dof_handler,
    const unsigned int               level,
    IndexSet                        &dof_set)
  {
    dof_set = extract_locally_relevant_level_dofs(dof_handler, level);
  }
 
 
  namespace internal
  {
    template <int dim, int spacedim>
    void
    extract_constant_modes(const DoFHandler<dim, spacedim> &dof_handler,
                           const ComponentMask             &component_mask,
                           const unsigned int               mg_level,
                           std::vector<std::vector<bool>>  &constant_modes)
    {
      const auto &locally_owned_dofs =
        (mg_level == numbers::invalid_unsigned_int) ?
          dof_handler.locally_owned_dofs() :
          dof_handler.locally_owned_mg_dofs(mg_level);
 
      // If there are no locally owned DoFs, return with an empty
      // constant_modes object:
      if (locally_owned_dofs.n_elements() == 0)
        {
          constant_modes = std::vector<std::vector<bool>>(0);
          return;
        }
 
      const unsigned int n_components = dof_handler.get_fe(0).n_components();
      Assert(component_mask.represents_n_components(n_components),
             ExcDimensionMismatch(n_components, component_mask.size()));
 
      std::vector<unsigned char> dofs_by_component(
        locally_owned_dofs.n_elements());
      internal::get_component_association(dof_handler,
                                          component_mask,
                                          dofs_by_component,
                                          mg_level);
      unsigned int n_selected_dofs = 0;
      for (unsigned int i = 0; i < n_components; ++i)
        if (component_mask[i] == true)
          n_selected_dofs +=
            std::count(dofs_by_component.begin(), dofs_by_component.end(), i);
 
      // Find local numbering within the selected components
      std::vector<unsigned int> component_numbering(
        locally_owned_dofs.n_elements(), numbers::invalid_unsigned_int);
      for (unsigned int i = 0, count = 0; i < locally_owned_dofs.n_elements();
           ++i)
        if (component_mask[dofs_by_component[i]])
          component_numbering[i] = count++;
 
      // get the element constant modes and find a translation table between
      // index in the constant modes and the components.
      //
      // TODO: We might be able to extend this also for elements which do not
      // have the same constant modes, but that is messy...
      const ::hp::FECollection<dim, spacedim> &fe_collection =
        dof_handler.get_fe_collection();
      std::vector<Table<2, bool>> element_constant_modes;
      std::vector<std::vector<std::pair<unsigned int, unsigned int>>>
        constant_mode_to_component_translation(n_components);
      {
        unsigned int n_constant_modes              = 0;
        int          first_non_empty_constant_mode = -1;
        for (unsigned int f = 0; f < fe_collection.size(); ++f)
          {
            std::pair<Table<2, bool>, std::vector<unsigned int>> data =
              fe_collection[f].get_constant_modes();
 
            // Store the index of the current element if it is the first that
            // has non-empty constant modes.
            if (first_non_empty_constant_mode < 0 && data.first.n_rows() > 0)
              {
                first_non_empty_constant_mode = f;
                // This is the first non-empty constant mode, so we figure out
                // the translation between index in the constant modes and the
                // components
                for (unsigned int i = 0; i < data.second.size(); ++i)
                  if (component_mask[data.second[i]])
                    constant_mode_to_component_translation[data.second[i]]
                      .emplace_back(n_constant_modes++, i);
              }
 
            // Add the constant modes of this element to the list and assert
            // that there are as many constant modes as for the other elements
            // (or zero constant modes).
            element_constant_modes.push_back(data.first);
            Assert(element_constant_modes.back().n_rows() == 0 ||
                     element_constant_modes.back().n_rows() ==
                       element_constant_modes[first_non_empty_constant_mode]
                         .n_rows(),
                   ExcInternalError());
          }
        AssertIndexRange(first_non_empty_constant_mode, fe_collection.size());
 
        // Now we know the number of constant modes and resize the return vector
        // accordingly
        constant_modes.clear();
        constant_modes.resize(n_constant_modes,
                              std::vector<bool>(n_selected_dofs, false));
      }
 
      // Loop over all owned cells and ask the element for the constant modes
      std::vector<types::global_dof_index> dof_indices;
 
      const auto runner = [&](const auto &task) {
        if (mg_level == numbers::invalid_unsigned_int)
          {
            for (const auto &cell : dof_handler.active_cell_iterators() |
                                      IteratorFilters::LocallyOwnedCell())
              {
                dof_indices.resize(cell->get_fe().n_dofs_per_cell());
                cell->get_dof_indices(dof_indices);
 
                task(cell);
              }
          }
        else
          {
            for (const auto &cell :
                 dof_handler.cell_iterators_on_level(mg_level))
              if (cell->is_locally_owned_on_level())
                {
                  dof_indices.resize(cell->get_fe().n_dofs_per_cell());
                  cell->get_mg_dof_indices(dof_indices);
 
                  task(cell);
                }
          }
      };
 
      runner([&](const auto &cell) {
        for (unsigned int i = 0; i < dof_indices.size(); ++i)
          if (locally_owned_dofs.is_element(dof_indices[i]))
            {
              const unsigned int loc_index =
                locally_owned_dofs.index_within_set(dof_indices[i]);
              const unsigned int comp = dofs_by_component[loc_index];
              if (component_mask[comp])
                for (auto &indices :
                     constant_mode_to_component_translation[comp])
                  constant_modes[indices
                                   .first][component_numbering[loc_index]] =
                    element_constant_modes[cell->active_fe_index()](
                      indices.second, i);
            }
      });
    }
  } // namespace internal
 
 
 
  template <int dim, int spacedim>
  void
  extract_constant_modes(const DoFHandler<dim, spacedim> &dof_handler,
                         const ComponentMask             &component_mask,
                         std::vector<std::vector<bool>>  &constant_modes)
  {
    internal::extract_constant_modes(dof_handler,
                                     component_mask,
                                     numbers::invalid_unsigned_int,
                                     constant_modes);
  }
 
 
 
  template <int dim, int spacedim>
  void
  extract_level_constant_modes(const unsigned int               level,
                               const DoFHandler<dim, spacedim> &dof_handler,
                               const ComponentMask             &component_mask,
                               std::vector<std::vector<bool>>  &constant_modes)
  {
    internal::extract_constant_modes(dof_handler,
                                     component_mask,
                                     level,
                                     constant_modes);
  }
 
 
 
  template <int dim, int spacedim>
  std::map<typename DoFHandler<dim - 1, spacedim>::active_cell_iterator,
           std::pair<typename DoFHandler<dim, spacedim>::active_cell_iterator,
                     unsigned int>>
  map_boundary_to_bulk_dof_iterators(
    const std::map<typename Triangulation<dim - 1, spacedim>::cell_iterator,
                   typename Triangulation<dim, spacedim>::face_iterator>
                                        &c1_to_c0_face,
    const DoFHandler<dim, spacedim>     &c0_dh,
    const DoFHandler<dim - 1, spacedim> &c1_dh)
  {
    // This is the returned object: a map of codimension-1 active dof cell
    // iterators to codimension-0 cells and face indices
    std::map<typename DoFHandler<dim - 1, spacedim>::active_cell_iterator,
             std::pair<typename DoFHandler<dim, spacedim>::active_cell_iterator,
                       unsigned int>>
      c1_to_c0_cell_and_face;
 
    // Shortcut if there are no faces to check
    if (c1_to_c0_face.empty())
      return c1_to_c0_cell_and_face;
 
    // This is the partial inverse of the map passed as input, for dh
    std::map<typename Triangulation<dim, spacedim>::face_iterator,
             typename DoFHandler<dim - 1, spacedim>::active_cell_iterator>
      c0_to_c1;
 
    // map volume mesh face -> codimension 1 dof cell
    for (const auto &[c1_cell, c0_cell] : c1_to_c0_face)
      if (!c1_cell->has_children())
        c0_to_c1[c0_cell] = c1_cell->as_dof_handler_iterator(c1_dh);
 
    // generate a mapping that maps codimension-1 cells
    // to codimension-0 cells and faces
    for (const auto &cell :
         c0_dh.active_cell_iterators()) // disp_dof.active_cell_iterators())
      for (const auto f : cell->face_indices())
        if (cell->face(f)->at_boundary())
          {
            const auto &it = c0_to_c1.find(cell->face(f));
            if (it != c0_to_c1.end())
              {
                const auto &c1_cell             = it->second;
                c1_to_c0_cell_and_face[c1_cell] = {cell, f};
                c0_to_c1.erase(it);
              }
          }
    // Check the dimensions: make sure all active cells we had have been mapped.
    AssertDimension(c0_to_c1.size(), 0);
    return c1_to_c0_cell_and_face;
  }
 
 
 
  template <int dim, int spacedim>
  void
  get_active_fe_indices(const DoFHandler<dim, spacedim> &dof_handler,
                        std::vector<unsigned int>       &active_fe_indices)
  {
    AssertDimension(active_fe_indices.size(),
                    dof_handler.get_triangulation().n_active_cells());
 
    std::vector<types::fe_index> indices = dof_handler.get_active_fe_indices();
 
    active_fe_indices.assign(indices.begin(), indices.end());
  }
 
  template <int dim, int spacedim>
  std::vector<IndexSet>
  locally_owned_dofs_per_subdomain(const DoFHandler<dim, spacedim> &dof_handler)
  {
    Assert(dof_handler.n_dofs() > 0,
           ExcMessage("The given DoFHandler has no DoFs."));
 
    // If the Triangulation is distributed, the only thing we can usefully
    // ask is for its locally owned subdomain
    Assert((dynamic_cast<
              const parallel::distributed::Triangulation<dim, spacedim> *>(
              &dof_handler.get_triangulation()) == nullptr),
           ExcMessage(
             "For parallel::distributed::Triangulation objects and "
             "associated DoF handler objects, asking for any information "
             "related to a subdomain other than the locally owned one does "
             "not make sense."));
 
    // The following is a random process (flip of a coin), thus should be called
    // once only.
    std::vector<::types::subdomain_id> subdomain_association(
      dof_handler.n_dofs());
    ::DoFTools::get_subdomain_association(dof_handler,
                                                subdomain_association);
 
    // Figure out how many subdomain ids there are.
    //
    // if this is a parallel triangulation, then we can just ask the
    // triangulation for this. if this is a sequential triangulation, we loop
    // over all cells and take the largest subdomain_id value we find; the
    // number of subdomains is then the largest found value plus one. (we here
    // assume that all subdomain ids up to the largest are actually used; this
    // may not be true for a sequential triangulation where these values have
    // been set by hand and not in accordance with some MPI communicator; but
    // the function returns an array indexed starting at zero, so we need to
    // collect information for each subdomain index anyway, not just for the
    // used one.)
    const unsigned int n_subdomains =
      (dynamic_cast<const parallel::TriangulationBase<dim, spacedim> *>(
         &dof_handler.get_triangulation()) == nullptr ?
         [&dof_handler]() {
           unsigned int max_subdomain_id = 0;
           for (const auto &cell : dof_handler.active_cell_iterators())
             max_subdomain_id =
               std::max(max_subdomain_id, cell->subdomain_id());
           return max_subdomain_id + 1;
         }() :
         Utilities::MPI::n_mpi_processes(
           dynamic_cast<const parallel::TriangulationBase<dim, spacedim> *>(
             &dof_handler.get_triangulation())
             ->get_communicator()));
    Assert(n_subdomains > *std::max_element(subdomain_association.begin(),
                                            subdomain_association.end()),
           ExcInternalError());
 
    std::vector<::IndexSet> index_sets(
      n_subdomains, ::IndexSet(dof_handler.n_dofs()));
 
    // loop over subdomain_association and populate IndexSet when a
    // change in subdomain ID is found
    ::types::global_dof_index i_min          = 0;
    ::types::global_dof_index this_subdomain = subdomain_association[0];
 
    for (::types::global_dof_index index = 1;
         index < subdomain_association.size();
         ++index)
      {
        // found index different from the current one
        if (subdomain_association[index] != this_subdomain)
          {
            index_sets[this_subdomain].add_range(i_min, index);
            i_min          = index;
            this_subdomain = subdomain_association[index];
          }
      }
 
    // the very last element is of different index
    if (i_min == subdomain_association.size() - 1)
      {
        index_sets[this_subdomain].add_index(i_min);
      }
 
    // otherwise there are at least two different indices
    else
      {
        index_sets[this_subdomain].add_range(i_min,
                                             subdomain_association.size());
      }
 
    for (unsigned int i = 0; i < n_subdomains; ++i)
      index_sets[i].compress();
 
    return index_sets;
  }
 
  template <int dim, int spacedim>
  std::vector<IndexSet>
  locally_relevant_dofs_per_subdomain(
    const DoFHandler<dim, spacedim> &dof_handler)
  {
    // If the Triangulation is distributed, the only thing we can usefully
    // ask is for its locally owned subdomain
    Assert((dynamic_cast<
              const parallel::distributed::Triangulation<dim, spacedim> *>(
              &dof_handler.get_triangulation()) == nullptr),
           ExcMessage(
             "For parallel::distributed::Triangulation objects and "
             "associated DoF handler objects, asking for any information "
             "related to a subdomain other than the locally owned one does "
             "not make sense."));
 
    // Collect all the locally owned DoFs
    // Note: Even though the distribution of DoFs by the
    // locally_owned_dofs_per_subdomain function is pseudo-random, we will
    // collect all the DoFs on the subdomain and its layer cell. Therefore, the
    // random nature of this function does not play a role in the extraction of
    // the locally relevant DoFs
    std::vector<IndexSet> dof_set =
      locally_owned_dofs_per_subdomain(dof_handler);
    const ::types::subdomain_id n_subdomains = dof_set.size();
 
    // Add the DoFs on the adjacent (equivalent ghost) cells to the IndexSet,
    // cache them in a set. Need to check each DoF manually because we can't
    // be sure that the DoF range of locally_owned_dofs is really contiguous.
    for (::types::subdomain_id subdomain_id = 0;
         subdomain_id < n_subdomains;
         ++subdomain_id)
      {
        // Extract the layer of cells around this subdomain
        std::function<bool(
          const typename DoFHandler<dim, spacedim>::active_cell_iterator &)>
          predicate = IteratorFilters::SubdomainEqualTo(subdomain_id);
        const std::vector<
          typename DoFHandler<dim, spacedim>::active_cell_iterator>
          active_halo_layer =
            GridTools::compute_active_cell_halo_layer(dof_handler, predicate);
 
        // Extract DoFs associated with halo layer
        std::vector<types::global_dof_index> local_dof_indices;
        std::set<types::global_dof_index>    subdomain_halo_global_dof_indices;
        for (typename std::vector<
               typename DoFHandler<dim, spacedim>::active_cell_iterator>::
               const_iterator it_cell = active_halo_layer.begin();
             it_cell != active_halo_layer.end();
             ++it_cell)
          {
            const typename DoFHandler<dim, spacedim>::active_cell_iterator
              &cell = *it_cell;
            Assert(
              cell->subdomain_id() != subdomain_id,
              ExcMessage(
                "The subdomain ID of the halo cell should not match that of the vector entry."));
 
            local_dof_indices.resize(cell->get_fe().n_dofs_per_cell());
            cell->get_dof_indices(local_dof_indices);
 
            for (const types::global_dof_index local_dof_index :
                 local_dof_indices)
              subdomain_halo_global_dof_indices.insert(local_dof_index);
          }
 
        dof_set[subdomain_id].add_indices(
          subdomain_halo_global_dof_indices.begin(),
          subdomain_halo_global_dof_indices.end());
 
        dof_set[subdomain_id].compress();
      }
 
    return dof_set;
  }
 
  template <int dim, int spacedim>
  void
  get_subdomain_association(
    const DoFHandler<dim, spacedim>  &dof_handler,
    std::vector<types::subdomain_id> &subdomain_association)
  {
    // if the Triangulation is distributed, the only thing we can usefully
    // ask is for its locally owned subdomain
    Assert((dynamic_cast<
              const parallel::distributed::Triangulation<dim, spacedim> *>(
              &dof_handler.get_triangulation()) == nullptr),
           ExcMessage(
             "For parallel::distributed::Triangulation objects and "
             "associated DoF handler objects, asking for any subdomain other "
             "than the locally owned one does not make sense."));
 
    Assert(subdomain_association.size() == dof_handler.n_dofs(),
           ExcDimensionMismatch(subdomain_association.size(),
                                dof_handler.n_dofs()));
 
    // catch an error that happened in some versions of the shared tria
    // distribute_dofs() function where we were trying to call this
    // function at a point in time when not all internal DoFHandler
    // structures were quite set up yet.
    Assert(dof_handler.n_dofs() > 0, ExcInternalError());
 
    // In case this function is executed with parallel::shared::Triangulation
    // with possibly artificial cells, we need to take "true" subdomain IDs
    // (i.e. without artificial cells). Otherwise we are good to use
    // subdomain_id as stored in cell->subdomain_id().
    std::vector<types::subdomain_id> cell_owners(
      dof_handler.get_triangulation().n_active_cells());
    if (const parallel::shared::Triangulation<dim, spacedim> *tr =
          (dynamic_cast<const parallel::shared::Triangulation<dim, spacedim> *>(
            &dof_handler.get_triangulation())))
      {
        cell_owners = tr->get_true_subdomain_ids_of_cells();
        Assert(tr->get_true_subdomain_ids_of_cells().size() ==
                 tr->n_active_cells(),
               ExcInternalError());
      }
    else
      {
        for (const auto &cell : dof_handler.active_cell_iterators() |
                                  IteratorFilters::LocallyOwnedCell())
          cell_owners[cell->active_cell_index()] = cell->subdomain_id();
      }
 
    // preset all values by an invalid value
    std::fill_n(subdomain_association.begin(),
                dof_handler.n_dofs(),
                numbers::invalid_subdomain_id);
 
    std::vector<types::global_dof_index> local_dof_indices;
    local_dof_indices.reserve(
      dof_handler.get_fe_collection().max_dofs_per_cell());
 
    // loop over all cells and record which subdomain a DoF belongs to.
    // give to the smaller subdomain_id in case it is on an interface
    for (const auto &cell : dof_handler.active_cell_iterators())
      {
        const types::subdomain_id subdomain_id =
          cell_owners[cell->active_cell_index()];
        const unsigned int dofs_per_cell = cell->get_fe().n_dofs_per_cell();
        local_dof_indices.resize(dofs_per_cell);
        cell->get_dof_indices(local_dof_indices);
 
        // set subdomain ids. if dofs already have their values set then
        // they must be on partition interfaces. in that case assign them
        // to either the previous association or the current processor
        // with the smaller subdomain id.
        for (unsigned int i = 0; i < dofs_per_cell; ++i)
          if (subdomain_association[local_dof_indices[i]] ==
              numbers::invalid_subdomain_id)
            subdomain_association[local_dof_indices[i]] = subdomain_id;
          else if (subdomain_association[local_dof_indices[i]] > subdomain_id)
            {
              subdomain_association[local_dof_indices[i]] = subdomain_id;
            }
      }
 
    Assert(std::find(subdomain_association.begin(),
                     subdomain_association.end(),
                     numbers::invalid_subdomain_id) ==
             subdomain_association.end(),
           ExcInternalError());
  }
 
 
 
  template <int dim, int spacedim>
  unsigned int
  count_dofs_with_subdomain_association(
    const DoFHandler<dim, spacedim> &dof_handler,
    const types::subdomain_id        subdomain)
  {
    std::vector<types::subdomain_id> subdomain_association(
      dof_handler.n_dofs());
    get_subdomain_association(dof_handler, subdomain_association);
 
    return std::count(subdomain_association.begin(),
                      subdomain_association.end(),
                      subdomain);
  }
 
 
 
  template <int dim, int spacedim>
  IndexSet
  dof_indices_with_subdomain_association(
    const DoFHandler<dim, spacedim> &dof_handler,
    const types::subdomain_id        subdomain)
  {
    // If we have a distributed::Triangulation only allow locally_owned
    // subdomain.
    Assert((dof_handler.get_triangulation().locally_owned_subdomain() ==
            numbers::invalid_subdomain_id) ||
             (subdomain ==
              dof_handler.get_triangulation().locally_owned_subdomain()),
           ExcMessage(
             "For parallel::distributed::Triangulation objects and "
             "associated DoF handler objects, asking for any subdomain other "
             "than the locally owned one does not make sense."));
 
    IndexSet index_set(dof_handler.n_dofs());
 
    std::vector<types::global_dof_index> local_dof_indices;
    local_dof_indices.reserve(
      dof_handler.get_fe_collection().max_dofs_per_cell());
 
    // first generate an unsorted list of all indices which we fill from
    // the back. could also insert them directly into the IndexSet, but
    // that inserts indices in the middle, which is an O(n^2) algorithm and
    // hence too expensive. Could also use std::set, but that is in general
    // more expensive than a vector
    std::vector<types::global_dof_index> subdomain_indices;
 
    for (const auto &cell : dof_handler.active_cell_iterators())
      if ((cell->is_artificial() == false) &&
          (cell->subdomain_id() == subdomain))
        {
          const unsigned int dofs_per_cell = cell->get_fe().n_dofs_per_cell();
          local_dof_indices.resize(dofs_per_cell);
          cell->get_dof_indices(local_dof_indices);
          subdomain_indices.insert(subdomain_indices.end(),
                                   local_dof_indices.begin(),
                                   local_dof_indices.end());
        }
    // sort indices and put into an index set:
    std::sort(subdomain_indices.begin(), subdomain_indices.end());
    index_set.add_indices(subdomain_indices.begin(), subdomain_indices.end());
    index_set.compress();
 
    return index_set;
  }
 
 
 
  template <int dim, int spacedim>
  void
  count_dofs_with_subdomain_association(
    const DoFHandler<dim, spacedim> &dof_handler,
    const types::subdomain_id        subdomain,
    std::vector<unsigned int>       &n_dofs_on_subdomain)
  {
    Assert(n_dofs_on_subdomain.size() == dof_handler.get_fe(0).n_components(),
           ExcDimensionMismatch(n_dofs_on_subdomain.size(),
                                dof_handler.get_fe(0).n_components()));
    std::fill(n_dofs_on_subdomain.begin(), n_dofs_on_subdomain.end(), 0);
 
    // Make sure there are at least some cells with this subdomain id
    Assert(std::any_of(
             dof_handler.begin_active(),
             typename DoFHandler<dim, spacedim>::active_cell_iterator{
               dof_handler.end()},
             [subdomain](
               const typename DoFHandler<dim, spacedim>::cell_accessor &cell) {
               return cell.subdomain_id() == subdomain;
             }),
           ExcMessage("There are no cells for the given subdomain!"));
 
    std::vector<types::subdomain_id> subdomain_association(
      dof_handler.n_dofs());
    get_subdomain_association(dof_handler, subdomain_association);
 
    std::vector<unsigned char> component_association(dof_handler.n_dofs());
    internal::get_component_association(dof_handler,
                                        ComponentMask(std::vector<bool>()),
                                        component_association);
 
    for (unsigned int c = 0; c < dof_handler.get_fe(0).n_components(); ++c)
      {
        for (types::global_dof_index i = 0; i < dof_handler.n_dofs(); ++i)
          if ((subdomain_association[i] == subdomain) &&
              (component_association[i] == static_cast<unsigned char>(c)))
            ++n_dofs_on_subdomain[c];
      }
  }
 
 
 
  namespace internal
  {
    // TODO: why is this function so complicated? It would be nice to have
    // comments that explain why we can't just loop over all components and
    // count the entries in dofs_by_component that have this component's
    // index
    template <int dim, int spacedim>
    void
    resolve_components(const FiniteElement<dim, spacedim>   &fe,
                       const std::vector<unsigned char>     &dofs_by_component,
                       const std::vector<unsigned int>      &target_component,
                       const bool                            only_once,
                       std::vector<types::global_dof_index> &dofs_per_component,
                       unsigned int                         &component)
    {
      for (unsigned int b = 0; b < fe.n_base_elements(); ++b)
        {
          const FiniteElement<dim, spacedim> &base = fe.base_element(b);
          // Dimension of base element
          unsigned int d = base.n_components();
 
          for (unsigned int m = 0; m < fe.element_multiplicity(b); ++m)
            {
              if (base.n_base_elements() > 1)
                resolve_components(base,
                                   dofs_by_component,
                                   target_component,
                                   only_once,
                                   dofs_per_component,
                                   component);
              else
                {
                  for (unsigned int dd = 0; dd < d; ++dd, ++component)
                    dofs_per_component[target_component[component]] +=
                      std::count(dofs_by_component.begin(),
                                 dofs_by_component.end(),
                                 component);
 
                  // if we have non-primitive FEs and want all components
                  // to show the number of dofs, need to copy the result to
                  // those components
                  if (!base.is_primitive() && !only_once)
                    for (unsigned int dd = 1; dd < d; ++dd)
                      dofs_per_component[target_component[component - d + dd]] =
                        dofs_per_component[target_component[component - d]];
                }
            }
        }
    }
 
 
    template <int dim, int spacedim>
    void
    resolve_components(const hp::FECollection<dim, spacedim> &fe_collection,
                       const std::vector<unsigned char>      &dofs_by_component,
                       const std::vector<unsigned int>       &target_component,
                       const bool                             only_once,
                       std::vector<types::global_dof_index> &dofs_per_component,
                       unsigned int                         &component)
    {
      // assert that all elements in the collection have the same structure
      // (base elements and multiplicity, components per base element) and
      // then simply call the function above
      for (unsigned int fe = 1; fe < fe_collection.size(); ++fe)
        {
          Assert(fe_collection[fe].n_components() ==
                   fe_collection[0].n_components(),
                 ExcNotImplemented());
          Assert(fe_collection[fe].n_base_elements() ==
                   fe_collection[0].n_base_elements(),
                 ExcNotImplemented());
          for (unsigned int b = 0; b < fe_collection[0].n_base_elements(); ++b)
            {
              Assert(fe_collection[fe].base_element(b).n_components() ==
                       fe_collection[0].base_element(b).n_components(),
                     ExcNotImplemented());
              Assert(fe_collection[fe].base_element(b).n_base_elements() ==
                       fe_collection[0].base_element(b).n_base_elements(),
                     ExcNotImplemented());
            }
        }
 
      resolve_components(fe_collection[0],
                         dofs_by_component,
                         target_component,
                         only_once,
                         dofs_per_component,
                         component);
    }
  } // namespace internal
 
 
 
  namespace internal
  {
    namespace
    {
      template <int dim, int spacedim>
      bool
      all_elements_are_primitive(const FiniteElement<dim, spacedim> &fe)
      {
        return fe.is_primitive();
      }
 
 
      template <int dim, int spacedim>
      bool
      all_elements_are_primitive(
        const ::hp::FECollection<dim, spacedim> &fe_collection)
      {
        for (unsigned int i = 0; i < fe_collection.size(); ++i)
          if (fe_collection[i].is_primitive() == false)
            return false;
 
        return true;
      }
    } // namespace
  }   // namespace internal
 
 
 
  template <int dim, int spacedim>
  std::vector<types::global_dof_index>
  count_dofs_per_fe_component(
    const DoFHandler<dim, spacedim> &dof_handler,
    const bool                       only_once,
    const std::vector<unsigned int> &target_component_)
  {
    const unsigned int n_components = dof_handler.get_fe(0).n_components();
 
    // If the empty vector was given as default argument, set up this
    // vector as identity.
    std::vector<unsigned int> target_component = target_component_;
    if (target_component.empty())
      {
        target_component.resize(n_components);
        for (unsigned int i = 0; i < n_components; ++i)
          target_component[i] = i;
      }
    else
      Assert(target_component.size() == n_components,
             ExcDimensionMismatch(target_component.size(), n_components));
 
 
    const unsigned int max_component =
      *std::max_element(target_component.begin(), target_component.end());
    const unsigned int n_target_components = max_component + 1;
 
    std::vector<types::global_dof_index> dofs_per_component(
      n_target_components, types::global_dof_index(0));
 
    // special case for only one component. treat this first since it does
    // not require any computations
    if (n_components == 1)
      {
        dofs_per_component[0] = dof_handler.n_locally_owned_dofs();
        return dofs_per_component;
      }
 
 
    // otherwise determine the number of dofs in each component separately.
    // do so in parallel
    std::vector<unsigned char> dofs_by_component(
      dof_handler.n_locally_owned_dofs());
    internal::get_component_association(dof_handler,
                                        ComponentMask(),
                                        dofs_by_component);
 
    // next count what we got
    unsigned int component = 0;
    internal::resolve_components(dof_handler.get_fe_collection(),
                                 dofs_by_component,
                                 target_component,
                                 only_once,
                                 dofs_per_component,
                                 component);
    Assert(n_components == component, ExcInternalError());
 
    // finally sanity check. this is only valid if the finite element is
    // actually primitive, so exclude other elements from this
    Assert((internal::all_elements_are_primitive(
              dof_handler.get_fe_collection()) == false) ||
             (std::accumulate(dofs_per_component.begin(),
                              dofs_per_component.end(),
                              types::global_dof_index(0)) ==
              dof_handler.n_locally_owned_dofs()),
           ExcInternalError());
 
    // reduce information from all CPUs
#ifdef DEAL_II_WITH_MPI
 
    if (const parallel::TriangulationBase<dim, spacedim> *tria =
          (dynamic_cast<const parallel::TriangulationBase<dim, spacedim> *>(
            &dof_handler.get_triangulation())))
      {
        std::vector<types::global_dof_index> local_dof_count =
          dofs_per_component;
 
        const int ierr = MPI_Allreduce(local_dof_count.data(),
                                       dofs_per_component.data(),
                                       n_target_components,
                                       DEAL_II_DOF_INDEX_MPI_TYPE,
                                       MPI_SUM,
                                       tria->get_communicator());
        AssertThrowMPI(ierr);
      }
#endif
 
    return dofs_per_component;
  }
 
 
 
  template <int dim, int spacedim>
  std::vector<types::global_dof_index>
  count_dofs_per_fe_block(const DoFHandler<dim, spacedim> &dof_handler,
                          const std::vector<unsigned int> &target_block_)
  {
    const ::hp::FECollection<dim, spacedim> &fe_collection =
      dof_handler.get_fe_collection();
    Assert(fe_collection.size() < 256, ExcNotImplemented());
 
    // If the empty vector for target_block(e.g., as default argument), then
    // set up this vector as identity. We do this set up with the first
    // element of the collection, but the whole thing can only work if
    // all elements have the same number of blocks anyway -- so check
    // that right after
    const unsigned int n_blocks = fe_collection[0].n_blocks();
 
    std::vector<unsigned int> target_block = target_block_;
    if (target_block.empty())
      {
        target_block.resize(fe_collection[0].n_blocks());
        for (unsigned int i = 0; i < n_blocks; ++i)
          target_block[i] = i;
      }
    else
      Assert(target_block.size() == n_blocks,
             ExcDimensionMismatch(target_block.size(), n_blocks));
    for (unsigned int f = 1; f < fe_collection.size(); ++f)
      Assert(fe_collection[0].n_blocks() == fe_collection[f].n_blocks(),
             ExcMessage("This function can only work if all elements in a "
                        "collection have the same number of blocks."));
 
    // special case for only one block. treat this first since it does
    // not require any computations
    if (n_blocks == 1)
      {
        std::vector<types::global_dof_index> dofs_per_block(1);
        dofs_per_block[0] = dof_handler.n_dofs();
        return dofs_per_block;
      }
 
    // Otherwise set up the right-sized object and start working
    const unsigned int max_block =
      *std::max_element(target_block.begin(), target_block.end());
    const unsigned int n_target_blocks = max_block + 1;
 
    std::vector<types::global_dof_index> dofs_per_block(n_target_blocks);
 
    // Loop over the elements of the collection, but really only consider
    // the last element (see #9271)
    for (unsigned int this_fe = fe_collection.size() - 1;
         this_fe < fe_collection.size();
         ++this_fe)
      {
        const FiniteElement<dim, spacedim> &fe = fe_collection[this_fe];
 
        std::vector<unsigned char> dofs_by_block(
          dof_handler.n_locally_owned_dofs());
        internal::get_block_association(dof_handler, dofs_by_block);
 
        // next count what we got
        for (unsigned int block = 0; block < fe.n_blocks(); ++block)
          dofs_per_block[target_block[block]] +=
            std::count(dofs_by_block.begin(), dofs_by_block.end(), block);
 
#ifdef DEAL_II_WITH_MPI
        // if we are working on a parallel mesh, we now need to collect
        // this information from all processors
        if (const parallel::TriangulationBase<dim, spacedim> *tria =
              (dynamic_cast<const parallel::TriangulationBase<dim, spacedim> *>(
                &dof_handler.get_triangulation())))
          {
            std::vector<types::global_dof_index> local_dof_count =
              dofs_per_block;
            const int ierr = MPI_Allreduce(local_dof_count.data(),
                                           dofs_per_block.data(),
                                           n_target_blocks,
                                           DEAL_II_DOF_INDEX_MPI_TYPE,
                                           MPI_SUM,
                                           tria->get_communicator());
            AssertThrowMPI(ierr);
          }
#endif
      }
 
    return dofs_per_block;
  }
 
 
 
  template <int dim, int spacedim>
  void
  map_dof_to_boundary_indices(const DoFHandler<dim, spacedim>      &dof_handler,
                              std::vector<types::global_dof_index> &mapping)
  {
    mapping.clear();
    mapping.insert(mapping.end(),
                   dof_handler.n_dofs(),
                   numbers::invalid_dof_index);
 
    std::vector<types::global_dof_index> dofs_on_face;
    dofs_on_face.reserve(dof_handler.get_fe_collection().max_dofs_per_face());
    types::global_dof_index next_boundary_index = 0;
 
    // now loop over all cells and check whether their faces are at the
    // boundary. note that we need not take special care of single lines
    // being at the boundary (using @p{cell->has_boundary_lines}), since we
    // do not support boundaries of dimension dim-2, and so every isolated
    // boundary line is also part of a boundary face which we will be
    // visiting sooner or later
    for (const auto &cell : dof_handler.active_cell_iterators())
      for (const unsigned int f : cell->face_indices())
        if (cell->at_boundary(f))
          {
            const unsigned int dofs_per_face =
              cell->get_fe().n_dofs_per_face(f);
            dofs_on_face.resize(dofs_per_face);
            cell->face(f)->get_dof_indices(dofs_on_face,
                                           cell->active_fe_index());
            for (unsigned int i = 0; i < dofs_per_face; ++i)
              if (mapping[dofs_on_face[i]] == numbers::invalid_dof_index)
                mapping[dofs_on_face[i]] = next_boundary_index++;
          }
 
    AssertDimension(next_boundary_index, dof_handler.n_boundary_dofs());
  }
 
 
 
  template <int dim, int spacedim>
  void
  map_dof_to_boundary_indices(const DoFHandler<dim, spacedim>    &dof_handler,
                              const std::set<types::boundary_id> &boundary_ids,
                              std::vector<types::global_dof_index> &mapping)
  {
    Assert(boundary_ids.find(numbers::internal_face_boundary_id) ==
             boundary_ids.end(),
           ExcInvalidBoundaryIndicator());
 
    mapping.clear();
    mapping.insert(mapping.end(),
                   dof_handler.n_dofs(),
                   numbers::invalid_dof_index);
 
    // return if there is nothing to do
    if (boundary_ids.empty())
      return;
 
    std::vector<types::global_dof_index> dofs_on_face;
    dofs_on_face.reserve(dof_handler.get_fe_collection().max_dofs_per_face());
    types::global_dof_index next_boundary_index = 0;
 
    for (const auto &cell : dof_handler.active_cell_iterators())
      for (const unsigned int f : cell->face_indices())
        if (boundary_ids.find(cell->face(f)->boundary_id()) !=
            boundary_ids.end())
          {
            const unsigned int dofs_per_face =
              cell->get_fe().n_dofs_per_face(f);
            dofs_on_face.resize(dofs_per_face);
            cell->face(f)->get_dof_indices(dofs_on_face,
                                           cell->active_fe_index());
            for (unsigned int i = 0; i < dofs_per_face; ++i)
              if (mapping[dofs_on_face[i]] == numbers::invalid_dof_index)
                mapping[dofs_on_face[i]] = next_boundary_index++;
          }
 
    AssertDimension(next_boundary_index,
                    dof_handler.n_boundary_dofs(boundary_ids));
  }
 
  namespace internal
  {
    namespace
    {
      template <int dim, int spacedim>
      std::map<types::global_dof_index, Point<spacedim>>
      map_dofs_to_support_points(
        const hp::MappingCollection<dim, spacedim> &mapping,
        const DoFHandler<dim, spacedim>            &dof_handler,
        const ComponentMask                        &in_mask)
      {
        std::map<types::global_dof_index, Point<spacedim>> support_points;
 
        const hp::FECollection<dim, spacedim> &fe_collection =
          dof_handler.get_fe_collection();
        hp::QCollection<dim> q_coll_dummy;
 
        for (unsigned int fe_index = 0; fe_index < fe_collection.size();
             ++fe_index)
          {
            // check whether every FE in the collection has support points
            Assert((fe_collection[fe_index].n_dofs_per_cell() == 0) ||
                     (fe_collection[fe_index].has_support_points()),
                   typename FiniteElement<dim>::ExcFEHasNoSupportPoints());
            q_coll_dummy.push_back(Quadrature<dim>(
              fe_collection[fe_index].get_unit_support_points()));
          }
 
        // Take care of components
        const ComponentMask mask =
          (in_mask.size() == 0 ?
             ComponentMask(fe_collection.n_components(), true) :
             in_mask);
 
        // Now loop over all cells and enquire the support points on each
        // of these. we use dummy quadrature formulas where the quadrature
        // points are located at the unit support points to enquire the
        // location of the support points in real space.
        //
        // The weights of the quadrature rule have been set to invalid
        // values by the used constructor.
        hp::FEValues<dim, spacedim> hp_fe_values(mapping,
                                                 fe_collection,
                                                 q_coll_dummy,
                                                 update_quadrature_points);
 
        std::vector<types::global_dof_index> local_dof_indices;
        for (const auto &cell : dof_handler.active_cell_iterators())
          // only work on locally relevant cells
          if (cell->is_artificial() == false)
            {
              hp_fe_values.reinit(cell);
              const FEValues<dim, spacedim> &fe_values =
                hp_fe_values.get_present_fe_values();
 
              local_dof_indices.resize(cell->get_fe().n_dofs_per_cell());
              cell->get_dof_indices(local_dof_indices);
 
              const std::vector<Point<spacedim>> &points =
                fe_values.get_quadrature_points();
              for (unsigned int i = 0; i < cell->get_fe().n_dofs_per_cell();
                   ++i)
                {
                  const unsigned int dof_comp =
                    cell->get_fe().system_to_component_index(i).first;
 
                  // insert the values into the map if it is a valid component
                  if (mask[dof_comp])
                    support_points[local_dof_indices[i]] = points[i];
                }
            }
 
        return support_points;
      }
 
 
      template <int dim, int spacedim>
      std::vector<Point<spacedim>>
      map_dofs_to_support_points_vector(
        const hp::MappingCollection<dim, spacedim> &mapping,
        const DoFHandler<dim, spacedim>            &dof_handler,
        const ComponentMask                        &mask)
      {
        std::vector<Point<spacedim>> support_points(dof_handler.n_dofs());
 
        // get the data in the form of the map as above
        const std::map<types::global_dof_index, Point<spacedim>>
          x_support_points =
            map_dofs_to_support_points(mapping, dof_handler, mask);
 
        // now convert from the map to the linear vector. make sure every
        // entry really appeared in the map
        for (types::global_dof_index i = 0; i < dof_handler.n_dofs(); ++i)
          {
            Assert(x_support_points.find(i) != x_support_points.end(),
                   ExcInternalError());
 
            support_points[i] = x_support_points.find(i)->second;
          }
 
        return support_points;
      }
    } // namespace
  }   // namespace internal
 
 
  template <int dim, int spacedim>
  void
  map_dofs_to_support_points(const Mapping<dim, spacedim>    &mapping,
                             const DoFHandler<dim, spacedim> &dof_handler,
                             std::vector<Point<spacedim>>    &support_points,
                             const ComponentMask             &mask)
  {
    AssertDimension(support_points.size(), dof_handler.n_dofs());
    Assert((dynamic_cast<
              const parallel::distributed::Triangulation<dim, spacedim> *>(
              &dof_handler.get_triangulation()) == nullptr),
           ExcMessage(
             "This function can not be used with distributed triangulations. "
             "See the documentation for more information."));
 
    // Let the internal function do all the work, just make sure that it
    // gets a MappingCollection
    const hp::MappingCollection<dim, spacedim> mapping_collection(mapping);
 
    support_points =
      internal::map_dofs_to_support_points_vector(mapping_collection,
                                                  dof_handler,
                                                  mask);
  }
 
 
  template <int dim, int spacedim>
  void
  map_dofs_to_support_points(
    const hp::MappingCollection<dim, spacedim> &mapping,
    const DoFHandler<dim, spacedim>            &dof_handler,
    std::vector<Point<spacedim>>               &support_points,
    const ComponentMask                        &mask)
  {
    AssertDimension(support_points.size(), dof_handler.n_dofs());
    Assert((dynamic_cast<
              const parallel::distributed::Triangulation<dim, spacedim> *>(
              &dof_handler.get_triangulation()) == nullptr),
           ExcMessage(
             "This function can not be used with distributed triangulations. "
             "See the documentation for more information."));
 
    // Let the internal function do all the work, just make sure that it
    // gets a MappingCollection
    support_points =
      internal::map_dofs_to_support_points_vector(mapping, dof_handler, mask);
  }
 
 
  // This function is deprecated:
  template <int dim, int spacedim>
  DEAL_II_DEPRECATED void
  map_dofs_to_support_points(
    const Mapping<dim, spacedim>                       &mapping,
    const DoFHandler<dim, spacedim>                    &dof_handler,
    std::map<types::global_dof_index, Point<spacedim>> &support_points,
    const ComponentMask                                &mask)
  {
    support_points.clear();
 
    // Let the internal function do all the work, just make sure that it
    // gets a MappingCollection
    const hp::MappingCollection<dim, spacedim> mapping_collection(mapping);
 
    support_points = internal::map_dofs_to_support_points(mapping_collection,
                                                          dof_handler,
                                                          mask);
  }
 
 
  // This function is deprecated:
  template <int dim, int spacedim>
  DEAL_II_DEPRECATED void
  map_dofs_to_support_points(
    const hp::MappingCollection<dim, spacedim>         &mapping,
    const DoFHandler<dim, spacedim>                    &dof_handler,
    std::map<types::global_dof_index, Point<spacedim>> &support_points,
    const ComponentMask                                &mask)
  {
    support_points.clear();
 
    // Let the internal function do all the work, just make sure that it
    // gets a MappingCollection
    support_points =
      internal::map_dofs_to_support_points(mapping, dof_handler, mask);
  }
 
 
  template <int dim, int spacedim>
  std::map<types::global_dof_index, Point<spacedim>>
  map_dofs_to_support_points(const Mapping<dim, spacedim>    &mapping,
                             const DoFHandler<dim, spacedim> &dof_handler,
                             const ComponentMask             &mask)
  {
    // Let the internal function do all the work, just make sure that it
    // gets a MappingCollection
    const hp::MappingCollection<dim, spacedim> mapping_collection(mapping);
 
    return internal::map_dofs_to_support_points(mapping_collection,
                                                dof_handler,
                                                mask);
  }
 
 
  template <int dim, int spacedim>
  std::map<types::global_dof_index, Point<spacedim>>
  map_dofs_to_support_points(
    const hp::MappingCollection<dim, spacedim> &mapping,
    const DoFHandler<dim, spacedim>            &dof_handler,
    const ComponentMask                        &mask)
  {
    return internal::map_dofs_to_support_points(mapping, dof_handler, mask);
  }
 
 
  template <int spacedim>
  void
  write_gnuplot_dof_support_point_info(
    std::ostream                                             &out,
    const std::map<types::global_dof_index, Point<spacedim>> &support_points)
  {
    AssertThrow(out.fail() == false, ExcIO());
 
    std::map<Point<spacedim>,
             std::vector<types::global_dof_index>,
             typename internal::ComparisonHelper<spacedim>>
      point_map;
 
    // convert to map point -> list of DoFs
    for (const auto &it : support_points)
      {
        std::vector<types::global_dof_index> &v = point_map[it.second];
        v.push_back(it.first);
      }
 
    // print the newly created map:
    for (const auto &it : point_map)
      {
        out << it.first << " \"";
        const std::vector<types::global_dof_index> &v = it.second;
        for (unsigned int i = 0; i < v.size(); ++i)
          {
            if (i > 0)
              out << ", ";
            out << v[i];
          }
 
        out << "\"\n";
      }
 
    out << std::flush;
  }
 
 
  template <int dim, int spacedim>
  void
  convert_couplings_to_blocks(const DoFHandler<dim, spacedim> &dof_handler,
                              const Table<2, Coupling>        &table,
                              std::vector<Table<2, Coupling>> &tables_by_block)
  {
    if (dof_handler.has_hp_capabilities() == false)
      {
        const FiniteElement<dim, spacedim> &fe = dof_handler.get_fe();
        const unsigned int                  nb = fe.n_blocks();
 
        tables_by_block.resize(1);
        tables_by_block[0].reinit(nb, nb);
        tables_by_block[0].fill(none);
 
        for (unsigned int i = 0; i < fe.n_components(); ++i)
          {
            const unsigned int ib = fe.component_to_block_index(i);
            for (unsigned int j = 0; j < fe.n_components(); ++j)
              {
                const unsigned int jb = fe.component_to_block_index(j);
                tables_by_block[0](ib, jb) |= table(i, j);
              }
          }
      }
    else
      {
        const hp::FECollection<dim> &fe_collection =
          dof_handler.get_fe_collection();
        tables_by_block.resize(fe_collection.size());
 
        for (unsigned int f = 0; f < fe_collection.size(); ++f)
          {
            const FiniteElement<dim, spacedim> &fe = fe_collection[f];
 
            const unsigned int nb = fe.n_blocks();
            tables_by_block[f].reinit(nb, nb);
            tables_by_block[f].fill(none);
            for (unsigned int i = 0; i < fe.n_components(); ++i)
              {
                const unsigned int ib = fe.component_to_block_index(i);
                for (unsigned int j = 0; j < fe.n_components(); ++j)
                  {
                    const unsigned int jb = fe.component_to_block_index(j);
                    tables_by_block[f](ib, jb) |= table(i, j);
                  }
              }
          }
      }
  }
 
 
 
  template <int dim, int spacedim>
  void
  make_cell_patches(SparsityPattern                 &block_list,
                    const DoFHandler<dim, spacedim> &dof_handler,
                    const unsigned int               level,
                    const std::vector<bool>         &selected_dofs,
                    const types::global_dof_index    offset)
  {
    std::vector<types::global_dof_index> indices;
 
    unsigned int i = 0;
 
    for (const auto &cell : dof_handler.cell_iterators_on_level(level))
      if (cell->is_locally_owned_on_level())
        ++i;
    block_list.reinit(i,
                      dof_handler.n_dofs(),
                      dof_handler.get_fe().n_dofs_per_cell());
    i = 0;
    for (const auto &cell : dof_handler.cell_iterators_on_level(level))
      if (cell->is_locally_owned_on_level())
        {
          indices.resize(cell->get_fe().n_dofs_per_cell());
          cell->get_mg_dof_indices(indices);
 
          if (selected_dofs.size() != 0)
            AssertDimension(indices.size(), selected_dofs.size());
 
          for (types::global_dof_index j = 0; j < indices.size(); ++j)
            {
              if (selected_dofs.empty())
                block_list.add(i, indices[j] - offset);
              else
                {
                  if (selected_dofs[j])
                    block_list.add(i, indices[j] - offset);
                }
            }
          ++i;
        }
  }
 
 
  template <int dim, int spacedim>
  void
  make_single_patch(SparsityPattern                 &block_list,
                    const DoFHandler<dim, spacedim> &dof_handler,
                    const unsigned int               level,
                    const bool                       interior_only)
  {
    const FiniteElement<dim> &fe = dof_handler.get_fe();
    block_list.reinit(1, dof_handler.n_dofs(level), dof_handler.n_dofs(level));
 
    std::vector<types::global_dof_index> indices;
    std::vector<bool>                    exclude;
 
    for (const auto &cell : dof_handler.cell_iterators_on_level(level))
      {
        indices.resize(cell->get_fe().n_dofs_per_cell());
        cell->get_mg_dof_indices(indices);
 
        if (interior_only)
          {
            // Exclude degrees of freedom on faces opposite to the vertex
            exclude.resize(fe.n_dofs_per_cell());
            std::fill(exclude.begin(), exclude.end(), false);
 
            for (const unsigned int face : cell->face_indices())
              if (cell->at_boundary(face) ||
                  cell->neighbor(face)->level() != cell->level())
                for (unsigned int i = 0; i < fe.n_dofs_per_face(face); ++i)
                  exclude[fe.face_to_cell_index(i, face)] = true;
            for (types::global_dof_index j = 0; j < indices.size(); ++j)
              if (!exclude[j])
                block_list.add(0, indices[j]);
          }
        else
          {
            for (const auto index : indices)
              block_list.add(0, index);
          }
      }
  }
 
 
  template <int dim, int spacedim>
  void
  make_child_patches(SparsityPattern                 &block_list,
                     const DoFHandler<dim, spacedim> &dof_handler,
                     const unsigned int               level,
                     const bool                       interior_dofs_only,
                     const bool                       boundary_dofs)
  {
    Assert(level > 0 && level < dof_handler.get_triangulation().n_levels(),
           ExcIndexRange(level, 1, dof_handler.get_triangulation().n_levels()));
 
    std::vector<types::global_dof_index> indices;
    std::vector<bool>                    exclude;
 
    unsigned int block = 0;
    for (const auto &pcell : dof_handler.cell_iterators_on_level(level - 1))
      {
        if (pcell->is_active())
          continue;
 
        for (unsigned int child = 0; child < pcell->n_children(); ++child)
          {
            const auto cell = pcell->child(child);
 
            // For hp, only this line here would have to be replaced.
            const FiniteElement<dim> &fe     = dof_handler.get_fe();
            const unsigned int        n_dofs = fe.n_dofs_per_cell();
            indices.resize(n_dofs);
            exclude.resize(n_dofs);
            std::fill(exclude.begin(), exclude.end(), false);
            cell->get_mg_dof_indices(indices);
 
            if (interior_dofs_only)
              {
                // Eliminate dofs on faces of the child which are on faces
                // of the parent
                for (unsigned int d = 0; d < dim; ++d)
                  {
                    const unsigned int face =
                      GeometryInfo<dim>::vertex_to_face[child][d];
                    for (unsigned int i = 0; i < fe.n_dofs_per_face(face); ++i)
                      exclude[fe.face_to_cell_index(i, face)] = true;
                  }
 
                // Now remove all degrees of freedom on the domain boundary
                // from the exclusion list
                if (boundary_dofs)
                  for (const unsigned int face :
                       GeometryInfo<dim>::face_indices())
                    if (cell->at_boundary(face))
                      for (unsigned int i = 0; i < fe.n_dofs_per_face(face);
                           ++i)
                        exclude[fe.face_to_cell_index(i, face)] = false;
              }
 
            for (unsigned int i = 0; i < n_dofs; ++i)
              if (!exclude[i])
                block_list.add(block, indices[i]);
          }
        ++block;
      }
  }
 
  template <int dim, int spacedim>
  std::vector<unsigned int>
  make_vertex_patches(SparsityPattern                 &block_list,
                      const DoFHandler<dim, spacedim> &dof_handler,
                      const unsigned int               level,
                      const bool                       interior_only,
                      const bool                       boundary_patches,
                      const bool                       level_boundary_patches,
                      const bool                       single_cell_patches,
                      const bool                       invert_vertex_mapping)
  {
    const unsigned int n_blocks     = dof_handler.get_fe().n_blocks();
    BlockMask exclude_boundary_dofs = BlockMask(n_blocks, interior_only);
    return make_vertex_patches(block_list,
                               dof_handler,
                               level,
                               exclude_boundary_dofs,
                               boundary_patches,
                               level_boundary_patches,
                               single_cell_patches,
                               invert_vertex_mapping);
  }
 
  template <int dim, int spacedim>
  std::vector<unsigned int>
  make_vertex_patches(SparsityPattern                 &block_list,
                      const DoFHandler<dim, spacedim> &dof_handler,
                      const unsigned int               level,
                      const BlockMask                 &exclude_boundary_dofs,
                      const bool                       boundary_patches,
                      const bool                       level_boundary_patches,
                      const bool                       single_cell_patches,
                      const bool                       invert_vertex_mapping)
  {
    // Vector mapping from vertex index in the triangulation to consecutive
    // block indices on this level The number of cells at a vertex
    std::vector<unsigned int> vertex_cell_count(
      dof_handler.get_triangulation().n_vertices(), 0);
 
    // Is a vertex at the boundary?
    std::vector<bool> vertex_boundary(
      dof_handler.get_triangulation().n_vertices(), false);
 
    std::vector<unsigned int> vertex_mapping(
      dof_handler.get_triangulation().n_vertices(),
      numbers::invalid_unsigned_int);
 
    // Estimate for the number of dofs at this point
    std::vector<unsigned int> vertex_dof_count(
      dof_handler.get_triangulation().n_vertices(), 0);
 
    // Identify all vertices active on this level and remember some data
    // about them
    for (const auto &cell : dof_handler.cell_iterators_on_level(level))
      for (const unsigned int v : cell->vertex_indices())
        {
          const unsigned int vg = cell->vertex_index(v);
          vertex_dof_count[vg] += cell->get_fe().n_dofs_per_cell();
          ++vertex_cell_count[vg];
          for (unsigned int d = 0; d < dim; ++d)
            {
              const unsigned int face = GeometryInfo<dim>::vertex_to_face[v][d];
              if (cell->at_boundary(face))
                vertex_boundary[vg] = true;
              else if ((!level_boundary_patches) &&
                       (cell->neighbor(face)->level() !=
                        static_cast<int>(level)))
                vertex_boundary[vg] = true;
            }
        }
    // From now on, only vertices with positive dof count are "in".
 
    // Remove vertices at boundaries or in corners
    for (unsigned int vg = 0; vg < vertex_dof_count.size(); ++vg)
      if ((!single_cell_patches && vertex_cell_count[vg] < 2) ||
          (!boundary_patches && vertex_boundary[vg]))
        vertex_dof_count[vg] = 0;
 
    // Create a mapping from all vertices to the ones used here
    unsigned int n_vertex_count = 0;
    for (unsigned int vg = 0; vg < vertex_mapping.size(); ++vg)
      if (vertex_dof_count[vg] != 0)
        vertex_mapping[vg] = n_vertex_count++;
 
    // Compactify dof count
    for (unsigned int vg = 0; vg < vertex_mapping.size(); ++vg)
      if (vertex_dof_count[vg] != 0)
        vertex_dof_count[vertex_mapping[vg]] = vertex_dof_count[vg];
 
    // Now that we have all the data, we reduce it to the part we actually
    // want
    vertex_dof_count.resize(n_vertex_count);
 
    // At this point, the list of patches is ready. Now we enter the dofs
    // into the sparsity pattern.
    block_list.reinit(vertex_dof_count.size(),
                      dof_handler.n_dofs(level),
                      vertex_dof_count);
 
    std::vector<types::global_dof_index> indices;
    std::vector<bool>                    exclude;
 
    for (const auto &cell : dof_handler.cell_iterators_on_level(level))
      {
        const FiniteElement<dim> &fe = cell->get_fe();
        indices.resize(fe.n_dofs_per_cell());
        cell->get_mg_dof_indices(indices);
 
        for (const unsigned int v : cell->vertex_indices())
          {
            const unsigned int vg    = cell->vertex_index(v);
            const unsigned int block = vertex_mapping[vg];
            if (block == numbers::invalid_unsigned_int)
              continue;
 
            // Collect excluded dofs for some block(s) if boundary dofs
            // for a block are decided to be excluded
            if (exclude_boundary_dofs.size() == 0 ||
                exclude_boundary_dofs.n_selected_blocks() != 0)
              {
                // Exclude degrees of freedom on faces opposite to the
                // vertex
                exclude.resize(fe.n_dofs_per_cell());
                std::fill(exclude.begin(), exclude.end(), false);
 
                for (unsigned int d = 0; d < dim; ++d)
                  {
                    const unsigned int a_face =
                      GeometryInfo<dim>::vertex_to_face[v][d];
                    const unsigned int face =
                      GeometryInfo<dim>::opposite_face[a_face];
                    for (unsigned int i = 0; i < fe.n_dofs_per_face(face); ++i)
                      {
                        // For each dof, get the block it is in and decide to
                        // exclude it or not
                        if (exclude_boundary_dofs[fe.system_to_block_index(
                                                      fe.face_to_cell_index(
                                                        i, face))
                                                    .first] == true)
                          exclude[fe.face_to_cell_index(i, face)] = true;
                      }
                  }
                for (unsigned int j = 0; j < indices.size(); ++j)
                  if (!exclude[j])
                    block_list.add(block, indices[j]);
              }
            else
              {
                for (const auto index : indices)
                  block_list.add(block, index);
              }
          }
      }
 
    if (invert_vertex_mapping)
      {
        // Compress vertex mapping
        unsigned int n_vertex_count = 0;
        for (unsigned int vg = 0; vg < vertex_mapping.size(); ++vg)
          if (vertex_mapping[vg] != numbers::invalid_unsigned_int)
            vertex_mapping[n_vertex_count++] = vg;
 
        // Now we reduce it to the part we actually want
        vertex_mapping.resize(n_vertex_count);
      }
 
    return vertex_mapping;
  }
 
 
  template <int dim, int spacedim>
  unsigned int
  count_dofs_on_patch(
    const std::vector<typename DoFHandler<dim, spacedim>::active_cell_iterator>
      &patch)
  {
    std::set<types::global_dof_index>    dofs_on_patch;
    std::vector<types::global_dof_index> local_dof_indices;
 
    // loop over the cells in the patch and get the DoFs on each.
    // add all of them to a std::set which automatically makes sure
    // all duplicates are ignored
    for (unsigned int i = 0; i < patch.size(); ++i)
      {
        const typename DoFHandler<dim, spacedim>::active_cell_iterator &cell =
          patch[i];
        Assert(cell->is_artificial() == false,
               ExcMessage("This function can not be called with cells that are "
                          "not either locally owned or ghost cells."));
        local_dof_indices.resize(cell->get_fe().n_dofs_per_cell());
        cell->get_dof_indices(local_dof_indices);
        dofs_on_patch.insert(local_dof_indices.begin(),
                             local_dof_indices.end());
      }
 
    // now return the number of DoFs (duplicates were ignored)
    return dofs_on_patch.size();
  }
 
 
 
  template <int dim, int spacedim>
  std::vector<types::global_dof_index>
  get_dofs_on_patch(
    const std::vector<typename DoFHandler<dim, spacedim>::active_cell_iterator>
      &patch)
  {
    std::set<types::global_dof_index>    dofs_on_patch;
    std::vector<types::global_dof_index> local_dof_indices;
 
    // loop over the cells in the patch and get the DoFs on each.
    // add all of them to a std::set which automatically makes sure
    // all duplicates are ignored
    for (unsigned int i = 0; i < patch.size(); ++i)
      {
        const typename DoFHandler<dim, spacedim>::active_cell_iterator &cell =
          patch[i];
        Assert(cell->is_artificial() == false,
               ExcMessage("This function can not be called with cells that are "
                          "not either locally owned or ghost cells."));
        local_dof_indices.resize(cell->get_fe().n_dofs_per_cell());
        cell->get_dof_indices(local_dof_indices);
        dofs_on_patch.insert(local_dof_indices.begin(),
                             local_dof_indices.end());
      }
 
    Assert((dofs_on_patch.size() == count_dofs_on_patch<dim, spacedim>(patch)),
           ExcInternalError());
 
    // return a vector with the content of the set above. copying
    // also ensures that we retain sortedness as promised in the
    // documentation and as necessary to retain the block structure
    // also on the local system
    return std::vector<types::global_dof_index>(dofs_on_patch.begin(),
                                                dofs_on_patch.end());
  }
 
 
} // end of namespace DoFTools
 
 
 
// explicit instantiations
 
#include "dof_tools.inst"
 
 
 
DEAL_II_NAMESPACE_CLOSE