FlexibleGMRESSolver.h

// -*- tab-width: 2; indent-tabs-mode: nil; coding: utf-8-with-signature -*-
//-----------------------------------------------------------------------------
// Copyright 2000-2026 CEA (www.cea.fr) IFPEN (www.ifpenergiesnouvelles.com)
// See the top-level COPYRIGHT file for details.
// SPDX-License-Identifier: Apache-2.0
//-----------------------------------------------------------------------------
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/* solver_fgmres.h                                             (C) 2000-2026 */
/*                                                                           */
/* Flexible GMRES method solver.                                             */
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#ifndef ARCCORE_ALINA_FGMRESSOLVER_H
#define ARCCORE_ALINA_FGMRESSOLVER_H
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/*
 * This file is based on the work on AMGCL library (version march 2026)
 * which can be found at https://github.com/ddemidov/amgcl.
 *
 * Copyright (c) 2012-2022 Denis Demidov <dennis.demidov@gmail.com>
 * SPDX-License-Identifier: MIT
 */
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#include "arccore/alina/SolverUtils.h"
#include "arccore/alina/SolverBase.h"
 
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namespace Arcane::Alina
{
 
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struct FlexibleGMRESSolverParams
{
  using params = FlexibleGMRESSolverParams;

  Int32 M = 30;

  Int32 maxiter = 100;

  double tol = 1.0e-8;

  double abstol = std::numeric_limits<double>::min();

  bool ns_search = false;

  bool verbose = false;
 
  FlexibleGMRESSolverParams() = default;
 
  FlexibleGMRESSolverParams(const PropertyTree& p)
  : ARCCORE_ALINA_PARAMS_IMPORT_VALUE(p, M)
  , ARCCORE_ALINA_PARAMS_IMPORT_VALUE(p, maxiter)
  , ARCCORE_ALINA_PARAMS_IMPORT_VALUE(p, tol)
  , ARCCORE_ALINA_PARAMS_IMPORT_VALUE(p, abstol)
  , ARCCORE_ALINA_PARAMS_IMPORT_VALUE(p, ns_search)
  , ARCCORE_ALINA_PARAMS_IMPORT_VALUE(p, verbose)
  {
    p.check_params({ "M", "maxiter", "tol", "abstol", "ns_search", "verbose" });
  }
 
  void get(PropertyTree& p, const std::string& path) const
  {
    ARCCORE_ALINA_PARAMS_EXPORT_VALUE(p, path, M);
    ARCCORE_ALINA_PARAMS_EXPORT_VALUE(p, path, maxiter);
    ARCCORE_ALINA_PARAMS_EXPORT_VALUE(p, path, tol);
    ARCCORE_ALINA_PARAMS_EXPORT_VALUE(p, path, abstol);
    ARCCORE_ALINA_PARAMS_EXPORT_VALUE(p, path, ns_search);
    ARCCORE_ALINA_PARAMS_EXPORT_VALUE(p, path, verbose);
  }
};
 
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template <class Backend, class InnerProduct = detail::default_inner_product>
class FlexibleGMRESSolver
: public SolverBase
{
 public:
 
  typedef Backend backend_type;
 
  typedef typename Backend::vector vector;
  typedef typename Backend::value_type value_type;
  typedef typename Backend::params backend_params;
 
  typedef typename math::scalar_of<value_type>::type scalar_type;
  typedef typename math::rhs_of<value_type>::type rhs_type;
  typedef typename math::inner_product_impl<rhs_type>::return_type coef_type;
 
  using params = FlexibleGMRESSolverParams;

  FlexibleGMRESSolver(const AlinaContext& context, size_t n,
                      const params& prm = params(),
                      const backend_params& bprm = backend_params(),
                      const InnerProduct& inner_product = InnerProduct())
  : SolverBase(context)
  , prm(prm)
  , n(n)
  , H(prm.M + 1, prm.M)
  , s(prm.M + 1)
  , cs(prm.M + 1)
  , sn(prm.M + 1)
  , r(Backend::create_vector(n, bprm))
  , inner_product(inner_product)
  {
    v.reserve(prm.M + 1);
    for (unsigned i = 0; i <= prm.M; ++i)
      v.push_back(Backend::create_vector(n, bprm));
 
    z.reserve(prm.M);
    for (unsigned i = 0; i < prm.M; ++i)
      z.push_back(Backend::create_vector(n, bprm));
  }

  template <class Matrix, class Precond, class Vec1, class Vec2>
  SolverResult operator()(Matrix const& A, Precond const& P, Vec1 const& rhs, Vec2& x) const
  {
    ScopedStreamModifier ss(std::cout);
 
    scalar_type norm_rhs = norm(rhs);
    if (norm_rhs < Alina::detail::eps<scalar_type>(1)) {
      if (prm.ns_search) {
        norm_rhs = math::identity<scalar_type>();
      }
      else {
        backend::clear(x);
        return SolverResult(0, norm_rhs);
      }
    }
 
    scalar_type eps = std::max(prm.tol * norm_rhs, prm.abstol);
    scalar_type norm_r = math::zero<scalar_type>();
 
    unsigned iter = 0;
    while (true) {
      backend::residual(rhs, A, x, *v[0]);
 
      // -- Check stopping condition
      if ((norm_r = norm(*v[0])) < eps || iter >= prm.maxiter)
        break;
 
      // -- Inner GMRES iteration
      std::fill(s.begin(), s.end(), 0);
      s[0] = norm_r;
 
      backend::axpby(math::inverse(norm_r), *v[0], math::zero<scalar_type>(), *v[0]);
 
      unsigned j = 0;
      while (true) {
        // -- Arnoldi process
        //
        // Build an orthonormal basis V and matrix H such that
        //     A V_{i-1} = V_{i} H
 
        vector& v_new = *v[j + 1];
 
        P.apply(*v[j], *z[j]);
        backend::spmv(math::identity<scalar_type>(), A, *z[j],
                      math::zero<scalar_type>(), v_new);
 
        for (unsigned k = 0; k <= j; ++k) {
          H(k, j) = inner_product(v_new, *v[k]);
          backend::axpby(-H(k, j), *v[k], math::identity<scalar_type>(), v_new);
        }
        H(j + 1, j) = norm(v_new);
 
        backend::axpby(math::inverse(H(j + 1, j)), v_new, math::zero<scalar_type>(), v_new);
 
        for (unsigned k = 0; k < j; ++k)
          detail::apply_plane_rotation(H(k, j), H(k + 1, j), cs[k], sn[k]);
 
        detail::generate_plane_rotation(H(j, j), H(j + 1, j), cs[j], sn[j]);
        detail::apply_plane_rotation(H(j, j), H(j + 1, j), cs[j], sn[j]);
        detail::apply_plane_rotation(s[j], s[j + 1], cs[j], sn[j]);
 
        scalar_type inner_res = std::abs(s[j + 1]);
 
        if (prm.verbose && iter % 5 == 0)
          std::cout << iter << "\t" << std::scientific << inner_res / norm_rhs << std::endl;
 
        // Check for termination
        ++j, ++iter;
        if (iter >= prm.maxiter || j >= prm.M || inner_res <= eps)
          break;
      }
 
      // -- GMRES terminated: eval solution
      for (unsigned i = j; i-- > 0;) {
        s[i] /= H(i, i);
        for (unsigned k = 0; k < i; ++k)
          s[k] -= H(k, i) * s[i];
      }
 
      backend::lin_comb(j, s, z, math::identity<scalar_type>(), x);
    }
 
    return SolverResult(iter, norm_r / norm_rhs);
  }

  template <class Precond, class Vec1, class Vec2>
  SolverResult operator()(Precond const& P, Vec1 const& rhs, Vec2& x) const
  {
    return (*this)(P.system_matrix(), P, rhs, x);
  }
 
  size_t bytes() const
  {
    size_t b = 0;
 
    b += H.size() * sizeof(coef_type);
    b += backend::bytes(s);
    b += backend::bytes(cs);
    b += backend::bytes(sn);
    b += backend::bytes(*r);
 
    for (const auto& x : v)
      b += backend::bytes(*x);
    for (const auto& x : z)
      b += backend::bytes(*x);
 
    return b;
  }
 
  friend std::ostream& operator<<(std::ostream& os, const FlexibleGMRESSolver& s)
  {
    return os << "Type:             FGMRES(" << s.prm.M << ")"
              << "\nUnknowns:         " << s.n
              << "\nMemory footprint: " << human_readable_memory(s.bytes())
              << "\n";
  }
 
 public:
 
  params prm;
 
 private:
 
  size_t n;
 
  mutable multi_array<coef_type, 2> H;
  mutable std::vector<coef_type> s, cs, sn;
  std::shared_ptr<vector> r;
  std::vector<std::shared_ptr<vector>> v;
  std::vector<std::shared_ptr<vector>> z;
 
  InnerProduct inner_product;
 
  template <class Vec>
  scalar_type norm(const Vec& x) const
  {
    return std::abs(sqrt(inner_product(x, x)));
  }
};
 
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} // namespace Arcane::Alina
 
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#endif