sparse_ilu.h

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// -----------------------------------------------------------------------------
//
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception OR LGPL-2.1-or-later
// Copyright (C) 1999 - 2025 by the deal.II authors
//
// This file is part of the deal.II library.
//
// Detailed license information governing the source code and contributions
// can be found in LICENSE.md and CONTRIBUTING.md at the top level directory.
//
// -----------------------------------------------------------------------------
 
 
#ifndef dealii_sparse_ilu_h
#define dealii_sparse_ilu_h
 
 
#include <deal.II/base/config.h>
 
#include <deal.II/lac/exceptions.h>
#include <deal.II/lac/sparse_decomposition.h>
#include <deal.II/lac/sparse_matrix.h>
 
DEAL_II_NAMESPACE_OPEN
 
template <typename number>
class SparseILU : public SparseLUDecomposition<number>
{
public:
  using size_type = typename SparseLUDecomposition<number>::size_type;
 
  SparseILU() = default;
 
  using AdditionalData = typename SparseLUDecomposition<number>::AdditionalData;
 
  template <typename somenumber>
  void
  initialize(const SparseMatrix<somenumber> &matrix,
             const AdditionalData           &parameters = AdditionalData());
 
  template <typename somenumber>
  void
  vmult(Vector<somenumber> &dst, const Vector<somenumber> &src) const;
 
 
  template <typename somenumber>
  void
  Tvmult(Vector<somenumber> &dst, const Vector<somenumber> &src) const;
 
 
  std::size_t
  memory_consumption() const override;
 
  DeclException1(ExcInvalidStrengthening,
                 double,
                 << "The strengthening parameter " << arg1
                 << " is not greater or equal than zero!");
  DeclException1(ExcZeroPivot,
                 size_type,
                 << "While computing the ILU decomposition, the algorithm "
                    "found a zero entry on the diagonal of row "
                 << arg1
                 << ". The algorithm can not recover from this because it "
                    "wants to divide by this diagonal entry."
                    "\n\n"
                    "There are several reasons why this could be happening. "
                    "First, the matrix for which you try to compute a "
                    "decomposition might be singular. Second, the order in "
                    "which the algorithm considers might lead it to find "
                    "a zero diagonal entry even though different pivoting "
                    "strategies might not; the current implementation does "
                    "not do any pivoting (i.e., it works on rows/columns "
                    "in their natural order), and so you will trigger "
                    "this error if, for example, you have a zero in the "
                    "(0,0) entry of the matrix, even though this does "
                    "not imply that the matrix is singular."
                    "\n\n"
                    "It is possible that you can avoid the error if "
                    "you re-order degrees of freedom (using "
                    "the functions in namespace DoFRenumbering). You may "
                    "also want to consider \"strengthening the diagonal\", "
                    "using the AdditionalData::strengthen_diagonal parameter "
                    "you can set in the optional constructor argument of "
                    "this class.");
};
 
//---------------------------------------------------------------------------
 
 
DEAL_II_NAMESPACE_CLOSE
 
#endif // dealii_sparse_ilu_h