NativeGmres.hpp

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// SPDX-License-Identifier: Apache-2.0
// Copyright 2026 Caleb Buahin

 
#pragma once
 
#include <cstddef>
#include <functional>
#include <vector>
 
namespace openswmm::fv {

class NativeGmres {
public:
    using Apply = std::function<bool(const double*, double*)>;
    struct Controls {
        int restart = 40, max_iterations = 200;
        int stagnant_restarts = 3;
        double absolute_tolerance = 1.e-12, relative_tolerance = 1.e-10;
        std::size_t workspace_limit_bytes = 64 * 1024 * 1024;
    };
    enum class Status {
        Converged,
        InvalidInput,
        MemoryLimit,
        NonFinite,
        OperatorFailure,
        PreconditionerFailure,
        Breakdown,
        Stagnated,
        IterationLimit
    };
    struct Result {
        Status status = Status::InvalidInput;
        int iterations = 0, restarts = 0, matvecs = 0;
        int preconditioner_applications = 0, reorthogonalizations = 0;
        std::size_t workspace_bytes = 0;
        double true_residual = 0.0, estimated_residual = 0.0, tolerance = 0.0;
        bool happy_breakdown = false;
        int workspace_growths = 0;     // vector capacity increases in this call
        bool reused_workspace = false; // every required vector reused without growth
        bool converged() const { return status == Status::Converged; }
    };

    Result solve(const std::vector<double>& rhs, const Apply& matrix, const Apply& inverse,
                 const Controls& controls, std::vector<double>& output);
    std::size_t workspaceBytes() const { return bytes(); }
    void clearWorkspace() { release(); }
 
private:
    std::vector<double> basis_, hessenberg_, cosine_, sine_, g_, y_;
    std::vector<double> x_, r_, w_, z_, update_, image_;
    std::size_t bytes() const;
    void release();
};
} // namespace openswmm::fv