SparseRegularInverse.h

// Copyright (C) 2017-2025 Yixuan Qiu <yixuan.qiu@cos.name>
//
// This Source Code Form is subject to the terms of the Mozilla
// Public License v. 2.0. If a copy of the MPL was not distributed
// with this file, You can obtain one at https://mozilla.org/MPL/2.0/.
 
#ifndef SPECTRA_SPARSE_REGULAR_INVERSE_H
#define SPECTRA_SPARSE_REGULAR_INVERSE_H
 
#include <Eigen/Core>
#include <Eigen/SparseCore>
#include <Eigen/IterativeLinearSolvers>
#include <stdexcept>
 
namespace Spectra {

template <typename Scalar_, int Uplo = Eigen::Lower, int Flags = Eigen::ColMajor, typename StorageIndex = int>
class SparseRegularInverse
{
public:
    using Scalar = Scalar_;
 
private:
    using Index = Eigen::Index;
    using Vector = Eigen::Matrix<Scalar, Eigen::Dynamic, 1>;
    using MapConstVec = Eigen::Map<const Vector>;
    using MapVec = Eigen::Map<Vector>;
    using SparseMatrix = Eigen::SparseMatrix<Scalar, Flags, StorageIndex>;
    using ConstGenericSparseMatrix = const Eigen::Ref<const SparseMatrix>;
 
    ConstGenericSparseMatrix m_mat;
    const Index m_n;
    Eigen::ConjugateGradient<SparseMatrix> m_cg;
    mutable CompInfo m_info;
 
public:
    template <typename Derived>
    SparseRegularInverse(const Eigen::SparseMatrixBase<Derived>& mat) :
        m_mat(mat), m_n(mat.rows())
    {
        static_assert(
            static_cast<int>(Derived::PlainObject::IsRowMajor) == static_cast<int>(SparseMatrix::IsRowMajor),
            "SparseRegularInverse: the \"Flags\" template parameter does not match the input matrix (Eigen::ColMajor/Eigen::RowMajor)");
 
        if (mat.rows() != mat.cols())
            throw std::invalid_argument("SparseRegularInverse: matrix must be square");
 
        m_cg.compute(mat);
        m_info = (m_cg.info() == Eigen::Success) ?
            CompInfo::Successful :
            CompInfo::NumericalIssue;
    }

    Index rows() const { return m_n; }
    Index cols() const { return m_n; }

    CompInfo info() const { return m_info; }

    // y_out = inv(B) * x_in
    void solve(const Scalar* x_in, Scalar* y_out) const
    {
        MapConstVec x(x_in, m_n);
        MapVec y(y_out, m_n);
        y.noalias() = m_cg.solve(x);
 
        m_info = (m_cg.info() == Eigen::Success) ?
            CompInfo::Successful :
            CompInfo::NotConverging;
        if (m_info != CompInfo::Successful)
            throw std::runtime_error("SparseRegularInverse: CG solver does not converge");
    }

    // y_out = B * x_in
    void perform_op(const Scalar* x_in, Scalar* y_out) const
    {
        MapConstVec x(x_in, m_n);
        MapVec y(y_out, m_n);
        y.noalias() = m_mat.template selfadjointView<Uplo>() * x;
    }
};
 
}  // namespace Spectra
 
#endif  // SPECTRA_SPARSE_REGULAR_INVERSE_H