DenseGenRealShiftSolve.h

// Copyright (C) 2016-2025 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_DENSE_GEN_REAL_SHIFT_SOLVE_H
#define SPECTRA_DENSE_GEN_REAL_SHIFT_SOLVE_H
 
#include <Eigen/Core>
#include <Eigen/LU>
#include <stdexcept>
 
namespace Spectra {

template <typename Scalar_, int Flags = Eigen::ColMajor>
class DenseGenRealShiftSolve
{
public:
    using Scalar = Scalar_;
 
private:
    using Index = Eigen::Index;
    using Matrix = Eigen::Matrix<Scalar, Eigen::Dynamic, Eigen::Dynamic, Flags>;
    using Vector = Eigen::Matrix<Scalar, Eigen::Dynamic, 1>;
    using MapConstVec = Eigen::Map<const Vector>;
    using MapVec = Eigen::Map<Vector>;
    using ConstGenericMatrix = const Eigen::Ref<const Matrix>;
 
    ConstGenericMatrix m_mat;
    const Index m_n;
    Eigen::PartialPivLU<Matrix> m_solver;
 
public:
    template <typename Derived>
    DenseGenRealShiftSolve(const Eigen::MatrixBase<Derived>& mat) :
        m_mat(mat), m_n(mat.rows())
    {
        static_assert(
            static_cast<int>(Derived::PlainObject::IsRowMajor) == static_cast<int>(Matrix::IsRowMajor),
            "DenseGenRealShiftSolve: the \"Flags\" template parameter does not match the input matrix (Eigen::ColMajor/Eigen::RowMajor)");
 
        if (mat.rows() != mat.cols())
            throw std::invalid_argument("DenseGenRealShiftSolve: matrix must be square");
    }

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

    void set_shift(const Scalar& sigma)
    {
        m_solver.compute(m_mat - sigma * Matrix::Identity(m_n, m_n));
    }

    // y_out = inv(A - sigma * I) * 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_solver.solve(x);
    }
};
 
}  // namespace Spectra
 
#endif  // SPECTRA_DENSE_GEN_REAL_SHIFT_SOLVE_H