IDRSSolver.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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/* IDDRSolver.h                                                (C) 2000-2026 */
/*                                                                           */
/* IDR(s) (Induced Dimension Reduction) method.                              */
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#ifndef ARCCORE_ALINA_IDRSSOLVER_H
#define ARCCORE_ALINA_IDRSSOLVER_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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/*
 * The code is ported from Matlab code published at
 * http://ta.twi.tudelft.nl/nw/users/gijzen/IDR.html.
 *
 * This is a very stable and efficient IDR(s) variant (implemented in the MATLAB
 * code idrs.m given above) as described in: Martin B. van Gijzen and Peter
 * Sonneveld, Algorithm 913: An Elegant IDR(s) Variant that Efficiently Exploits
 * Bi-orthogonality Properties. ACM Transactions on Mathematical Software, Vol.
 * 38, No. 1, pp. 5:1-5:19, 2011 (copyright ACM).
 */
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#include "arccore/alina/SolverUtils.h"
#include "arccore/alina/AlinaUtils.h"
 
#include <random>
 
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namespace Arcane::Alina
{
 
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struct IDRSSolverParams
{
  using params = IDRSSolverParams;

  Int32 s = 4;

  double omega = 0.7;

  bool smoothing = false;

  bool replacement = false;

  Int32 maxiter = 100;

  double tol = 1e-8;

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

  bool ns_search = false;

  bool verbose = false;
 
  IDRSSolverParams() = default;
 
  IDRSSolverParams(const PropertyTree& p)
  : ARCCORE_ALINA_PARAMS_IMPORT_VALUE(p, s)
  , ARCCORE_ALINA_PARAMS_IMPORT_VALUE(p, omega)
  , ARCCORE_ALINA_PARAMS_IMPORT_VALUE(p, smoothing)
  , ARCCORE_ALINA_PARAMS_IMPORT_VALUE(p, replacement)
  , 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({ "s", "omega", "smoothing", "replacement", "maxiter", "tol", "abstol", "ns_search", "verbose" });
  }
 
  void get(PropertyTree& p, const std::string& path) const
  {
    ARCCORE_ALINA_PARAMS_EXPORT_VALUE(p, path, s);
    ARCCORE_ALINA_PARAMS_EXPORT_VALUE(p, path, omega);
    ARCCORE_ALINA_PARAMS_EXPORT_VALUE(p, path, smoothing);
    ARCCORE_ALINA_PARAMS_EXPORT_VALUE(p, path, replacement);
    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 IDRSSolver
: public SolverBase
{
 public:
 
  using backend_type = Backend;
  using BackendType = Backend;
 
  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<typename math::rhs_of<value_type>::type>::return_type coef_type;
 
  using params = IDRSSolverParams;

  IDRSSolver(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)
  , inner_product(inner_product)
  , M(prm.s, prm.s)
  , f(prm.s)
  , c(prm.s)
  , r(Backend::create_vector(n, bprm))
  , v(Backend::create_vector(n, bprm))
  , t(Backend::create_vector(n, bprm))
  {
    static const scalar_type one = math::identity<scalar_type>();
    static const scalar_type zero = math::zero<scalar_type>();
 
    if (prm.smoothing) {
      x_s = Backend::create_vector(n, bprm);
      r_s = Backend::create_vector(n, bprm);
    }
 
    G.reserve(prm.s);
    U.reserve(prm.s);
    for (Int32 i = 0; i < prm.s; ++i) {
      G.push_back(Backend::create_vector(n, bprm));
      U.push_back(Backend::create_vector(n, bprm));
    }
 
    // Initialize P.
    P.reserve(prm.s);
    {
      std::vector<rhs_type> p(n);
 
      int pid = inner_product.rank();
      const int nt = 1;
 
      const int tid = 0;
      std::mt19937 rng(pid * nt + tid);
      std::uniform_real_distribution<scalar_type> rnd(-1, 1);
 
      for (unsigned j = 0; j < prm.s; ++j) {
        for (ptrdiff_t i = 0; i < n; ++i) {
          p[i] = math::constant<rhs_type>(rnd(rng));
        }
        P.push_back(Backend::copy_vector(p, bprm));
      }
 
      for (unsigned j = 0; j < prm.s; ++j) {
        for (unsigned k = 0; k < j; ++k) {
          coef_type alpha = inner_product(*P[k], *P[j]);
          backend::axpby(-alpha, *P[k], one, *P[j]);
        }
        scalar_type norm_pj = norm(*P[j]);
        backend::axpby(math::inverse(norm_pj), *P[j], zero, *P[j]);
      }
    }
  }

  template <class Matrix, class Precond, class Vec1, class Vec2>
  SolverResult operator()(Matrix const& A, Precond const& Prec, Vec1 const& rhs, Vec2& x) const
  {
    static const scalar_type one = math::identity<scalar_type>();
    static const scalar_type zero = math::zero<scalar_type>();
 
    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);
 
    // Compute initial residual:
    backend::residual(rhs, A, x, *r);
 
    scalar_type res_norm = norm(*r);
    if (res_norm <= eps) {
      // Initial guess is a good enough solution.
      return SolverResult(0, res_norm / norm_rhs);
    }
 
    if (prm.smoothing) {
      backend::copy(x, *x_s);
      backend::copy(*r, *r_s);
    }
 
    // Initialization.
    coef_type om = math::identity<coef_type>();
 
    for (unsigned i = 0; i < prm.s; ++i) {
      backend::clear(*G[i]);
      backend::clear(*U[i]);
 
      for (unsigned j = 0; j < prm.s; ++j)
        M(i, j) = (i == j);
    }
 
    // Main iteration loop, build G-spaces:
    size_t iter = 0;
    while (iter < prm.maxiter && res_norm > eps) {
      // New righ-hand size for small system:
      for (unsigned i = 0; i < prm.s; ++i)
        f[i] = inner_product(*r, *P[i]);
 
      for (unsigned k = 0; k < prm.s; ++k) {
        // Compute new v
        backend::copy(*r, *v);
 
        // Solve small system (Note: M is lower triangular)
        // and make v orthogonal to P:
        for (unsigned i = k; i < prm.s; ++i) {
          c[i] = f[i];
          for (unsigned j = k; j < i; ++j)
            c[i] -= M(i, j) * c[j];
          c[i] = math::inverse(M(i, i)) * c[i];
 
          backend::axpby(-c[i], *G[i], one, *v);
        }
 
        Prec.apply(*v, *t);
 
        // Compute new U[k]
        backend::axpby(om, *t, c[k], *U[k]);
        for (unsigned i = k + 1; i < prm.s; ++i)
          backend::axpby(c[i], *U[i], one, *U[k]);
 
        // Compute new G[k], G[k] is in space G_j
        backend::spmv(one, A, *U[k], zero, *G[k]);
 
        // Bi-Orthogonalise the new basis vectors:
        for (unsigned i = 0; i < k; ++i) {
          coef_type alpha = inner_product(*G[k], *P[i]) / M(i, i);
 
          backend::axpby(-alpha, *G[i], one, *G[k]);
          backend::axpby(-alpha, *U[i], one, *U[k]);
        }
 
        // New column of M = P'*G  (first k-1 entries are zero)
        for (unsigned i = k; i < prm.s; ++i)
          M(i, k) = inner_product(*G[k], *P[i]);
 
        precondition(!math::is_zero(M(k, k)), "IDR(s) breakdown: zero M[k,k]");
 
        // Make r orthogonal to q_i, i = [0..k)
        coef_type beta = math::inverse(M(k, k)) * f[k];
        backend::axpby(-beta, *G[k], one, *r);
        backend::axpby(beta, *U[k], one, x);
 
        res_norm = norm(*r);
 
        // Smoothing
        if (prm.smoothing) {
          backend::axpbypcz(one, *r_s, -one, *r, zero, *t);
          coef_type gamma = inner_product(*t, *r_s) / inner_product(*t, *t);
          backend::axpby(-gamma, *t, one, *r_s);
          backend::axpbypcz(-gamma, *x_s, gamma, x, one, *x_s);
          res_norm = norm(*r_s);
        }
 
        if (prm.verbose && iter % 5 == 0)
          std::cout << iter << "\t" << std::scientific << res_norm / norm_rhs << std::endl;
        if (res_norm <= eps || ++iter >= prm.maxiter)
          break;
 
        // New f = P'*r (first k  components are zero)
        for (unsigned i = k + 1; i < prm.s; ++i)
          f[i] -= beta * M(i, k);
      }
 
      if (res_norm <= eps || iter >= prm.maxiter)
        break;
 
      // Now we have sufficient vectors in G_j to compute residual in G_j+1
      // Note: r is already perpendicular to P so v = r
 
      Prec.apply(*r, *v);
      backend::spmv(one, A, *v, zero, *t);
 
      // Computation of a new omega
      om = omega(*t, *r);
      precondition(!math::is_zero(om), "IDR(s) breakdown: zero omega");
 
      backend::axpby(-om, *t, one, *r);
      backend::axpby(om, *v, one, x);
 
      if (prm.replacement) {
        backend::residual(rhs, A, x, *r);
      }
      res_norm = norm(*r);
 
      // Smoothing.
      if (prm.smoothing) {
        backend::axpbypcz(one, *r_s, -one, *r, zero, *t);
        coef_type gamma = inner_product(*t, *r_s) / inner_product(*t, *t);
        backend::axpby(-gamma, *t, one, *r_s);
        backend::axpbypcz(-gamma, *x_s, gamma, x, one, *x_s);
        res_norm = norm(*r_s);
      }
 
      ++iter;
    }
 
    if (prm.smoothing)
      backend::copy(*x_s, x);
 
    return SolverResult(iter, res_norm / 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 += M.size() * sizeof(coef_type);
 
    b += backend::bytes(f);
    b += backend::bytes(c);
 
    b += backend::bytes(*r);
    b += backend::bytes(*v);
    b += backend::bytes(*t);
 
    if (x_s)
      b += backend::bytes(*x_s);
    if (r_s)
      b += backend::bytes(*r_s);
 
    for (const auto& v : P)
      b += backend::bytes(*v);
    for (const auto& v : G)
      b += backend::bytes(*v);
    for (const auto& v : U)
      b += backend::bytes(*v);
 
    return b;
  }
 
  friend std::ostream& operator<<(std::ostream& os, const IDRSSolver& s)
  {
    return os << "Type:             IDR(" << s.prm.s << ")"
              << "\nUnknowns:         " << s.n
              << "\nMemory footprint: " << human_readable_memory(s.bytes())
              << std::endl;
  }
 
 public:
 
  params prm;
 
 private:
 
  size_t n;
 
  InnerProduct inner_product;
 
  mutable multi_array<coef_type, 2> M;
  mutable std::vector<coef_type> f, c;
 
  std::shared_ptr<vector> r, v, t;
  std::shared_ptr<vector> x_s;
  std::shared_ptr<vector> r_s;
 
  std::vector<std::shared_ptr<vector>> P, G, U;
 
  template <class Vec>
  scalar_type norm(const Vec& x) const
  {
    return std::abs(sqrt(inner_product(x, x)));
  }
 
  template <class Vector1, class Vector2>
  coef_type omega(const Vector1& t, const Vector2& s) const
  {
    scalar_type norm_t = norm(t);
    scalar_type norm_s = norm(s);
 
    coef_type ts = inner_product(t, s);
    scalar_type rho = math::norm(ts / (norm_t * norm_s));
    coef_type om = ts / (norm_t * norm_t);
 
    if (rho < prm.omega)
      om *= prm.omega / rho;
 
    return om;
  }
};
 
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} // namespace Arcane::Alina
 
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#endif