PicardSolver.H

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/* Copyright 2024 Justin Angus
 *
 * This file is part of WarpX.
 *
 * License: BSD-3-Clause-LBNL
 */
#ifndef PICARD_SOLVER_H_
#define PICARD_SOLVER_H_
 
#include "NonlinearSolver.H"
 
#include <AMReX_ParmParse.H>
#include "Utils/TextMsg.H"
 
#include <vector>
#include <istream>
#include <filesystem>

 
template<class Vec, class Ops>
class PicardSolver : public NonlinearSolver<Vec,Ops>
{
public:
 
    PicardSolver() = default;
 
    ~PicardSolver() override = default;
 
    // Prohibit Move and Copy operations
    PicardSolver(const PicardSolver&) = delete;
    PicardSolver& operator=(const PicardSolver&) = delete;
    PicardSolver(PicardSolver&&) noexcept = delete;
    PicardSolver& operator=(PicardSolver&&) noexcept = delete;
 
    void Define (const Vec&  a_U,
                       Ops*  a_ops) override;
 
    void Solve (Vec&   a_U,
          const Vec&   a_b,
          amrex::Real  a_time,
          amrex::Real  a_dt,
          int          a_step) const override;
 
    void GetSolverParams (amrex::Real&  a_rtol,
                          amrex::Real&  a_atol,
                          int&          a_maxits) override
    {
        a_rtol = m_rtol;
        a_atol = m_atol;
        a_maxits = m_maxits;
    }
 
    void PrintParams () const override
    {
        amrex::Print() << "Picard max iterations:      " << m_maxits << "\n";
        amrex::Print() << "Picard relative tolerance:  " << m_rtol << "\n";
        amrex::Print() << "Picard absolute tolerance:  " << m_atol << "\n";
        amrex::Print() << "Picard require convergence: " << (m_require_convergence?"true":"false") << "\n";
    }
 
private:

    mutable Vec m_Usave, m_R;

    Ops* m_ops = nullptr;

    bool m_require_convergence = true;

    amrex::Real m_rtol = 1.0e-6;

    amrex::Real m_atol = 0.;

    int m_maxits = 100;

    mutable int m_total_iters = 0;
 
    void ParseParameters( );
 
};
 
template <class Vec, class Ops>
void PicardSolver<Vec,Ops>::Define (const Vec&  a_U,
                                    Ops*        a_ops)
{
    WARPX_ALWAYS_ASSERT_WITH_MESSAGE(
        !this->m_is_defined,
        "Picard nonlinear solver object is already defined!");
 
    ParseParameters();
 
    m_Usave.Define(a_U);
    m_R.Define(a_U);
 
    m_ops = a_ops;
 
    this->m_is_defined = true;
 
    // Create diagnostic file and write header
    if (amrex::ParallelDescriptor::IOProcessor()
        && !this->m_diagnostic_file.empty()
        && !amrex::FileExists(this->m_diagnostic_file)) {
 
        std::filesystem::path const diagnostic_path(this->m_diagnostic_file);
        std::filesystem::path const diagnostic_dir = diagnostic_path.parent_path();
        if (!diagnostic_dir.empty()) {
            std::filesystem::create_directories(diagnostic_dir);
        }
 
        std::ofstream diagnostic_file{this->m_diagnostic_file, std::ofstream::out | std::ofstream::trunc};
        int c = 0;
        diagnostic_file << "#";
        diagnostic_file << "[" << c++ << "]step()";
        diagnostic_file << " ";
        diagnostic_file << "[" << c++ << "]time(s)";
        diagnostic_file << " ";
        diagnostic_file << "[" << c++ << "]iters";
        diagnostic_file << " ";
        diagnostic_file << "[" << c++ << "]total_iters";
        diagnostic_file << " ";
        diagnostic_file << "[" << c++ << "]norm_abs";
        diagnostic_file << " ";
        diagnostic_file << "[" << c++ << "]norm_rel";
        diagnostic_file << "\n";
        diagnostic_file.close();
    }
 
}
 
template <class Vec, class Ops>
void PicardSolver<Vec,Ops>::ParseParameters ()
{
    const amrex::ParmParse pp_picard("picard");
    pp_picard.query("verbose",             this->m_verbose);
    pp_picard.query("absolute_tolerance",  m_atol);
    pp_picard.query("relative_tolerance",  m_rtol);
    pp_picard.query("max_iterations",      m_maxits);
    pp_picard.query("require_convergence", m_require_convergence);
    pp_picard.query("diagnostic_file",     this->m_diagnostic_file);
    pp_picard.query("diagnostic_interval", this->m_diagnostic_interval);
}
 
template <class Vec, class Ops>
void PicardSolver<Vec,Ops>::Solve (Vec&         a_U,
                             const Vec&         a_b,
                                   amrex::Real  a_time,
                                   amrex::Real  a_dt,
                                   int          a_step) const
{
    BL_PROFILE("PicardSolver::Solve()");
    WARPX_ALWAYS_ASSERT_WITH_MESSAGE(
        this->m_is_defined,
        "PicardSolver::Solve() called on undefined object");
    using namespace amrex::literals;
 
    //
    // Picard fixed-point iteration method to solve nonlinear
    // equation of form: U = b + R(U)
    //
 
    amrex::Real norm_abs = 0.;
    amrex::Real norm0 = 1._rt;
    amrex::Real norm_rel = 0.;
 
    int iter;
    for (iter = 0; iter < m_maxits;) {
 
        // Save previous state for norm calculation
        m_Usave.Copy(a_U);
 
        // Update the solver state (a_U = a_b + m_R)
        m_ops->ComputeRHS( m_R, a_U, a_time, iter, false );
        a_U.Copy(a_b);
        a_U += m_R;
 
        // Compute the step norm and update iter
        m_Usave -= a_U;
        norm_abs = m_Usave.norm2();
        if (iter == 0) {
            if (norm_abs > 0.) { norm0 = norm_abs; }
            else { norm0 = 1._rt; }
        }
        norm_rel = norm_abs/norm0;
        iter++;
 
        // Check for convergence criteria
        if (this->m_verbose || iter == m_maxits) {
            amrex::Print() << "Picard: iter = " << std::setw(3) << iter <<  ", norm = "
                           << std::scientific << std::setprecision(5) << norm_abs << " (abs.), "
                           << std::scientific << std::setprecision(5) << norm_rel << " (rel.)" << "\n";
        }
 
        if (norm_abs < m_atol) {
            this->m_exit_status = 2;
            if (this->m_verbose) {
                amrex::Print() << "Picard: exiting at iter = " << std::setw(3) << iter
                               << ". Satisfied absolute tolerance " << m_atol << "\n";
            }
            break;
        }
 
        if (norm_rel < m_rtol) {
            this->m_exit_status = 3;
            if (this->m_verbose) {
                amrex::Print() << "Picard: exiting at iter = " << std::setw(3) << iter
                               << ". Satisfied relative tolerance " << m_rtol << "\n";
            }
            break;
        }
 
        if (iter >= m_maxits) {
            if (this->m_verbose) {
                amrex::Print() << "Picard: exiting at iter = " << std::setw(3) << iter
                               << ". Maximum iteration reached: iter = " << m_maxits << "\n";
            }
            break;
        }
 
    }
 
    // Update total iteration count
    m_total_iters += iter;
 
    if (m_rtol > 0. && iter == m_maxits) {
        std::stringstream convergenceMsg;
        convergenceMsg << "Picard solver failed to converge after " << iter <<
                          " iterations. Relative norm is " << norm_rel <<
                          " and the relative tolerance is " << m_rtol <<
                          ". Absolute norm is " << norm_abs <<
                          " and the absolute tolerance is " << m_atol;
        if (this->m_verbose) { amrex::Print() << convergenceMsg.str() << "\n"; }
        ablastr::warn_manager::WMRecordWarning("PicardSolver", convergenceMsg.str());
 
        if (m_require_convergence) { this->m_exit_status = -5; }
        else { this->m_exit_status = 0; }
    }
 
    if (!this->m_diagnostic_file.empty() && amrex::ParallelDescriptor::IOProcessor() &&
        ((a_step+1)%this->m_diagnostic_interval==0 || a_step==0)) {
        std::ofstream diagnostic_file{this->m_diagnostic_file, std::ofstream::out | std::ofstream::app};
        diagnostic_file << std::setprecision(14);
        diagnostic_file << a_step+1;
        diagnostic_file << " ";
        diagnostic_file << a_time + a_dt;
        diagnostic_file << " ";
        diagnostic_file << iter;
        diagnostic_file << " ";
        diagnostic_file << m_total_iters;
        diagnostic_file << " ";
        diagnostic_file << norm_abs;
        diagnostic_file << " ";
        diagnostic_file << norm_rel;
        diagnostic_file << "\n";
        diagnostic_file.close();
    }
 
}
 
#endif