text: // Copyright (C) 2016-2020 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_SYM_MAT_PROD_H
  #define SPECTRA_SPARSE_SYM_MAT_PROD_H
  
  #include <Eigen/Core>
  #include <Eigen/SparseCore>
  
  namespace Spectra {
  
  ///
  /// \ingroup MatOp
  ///
  /// This class defines the matrix-vector multiplication operation on a
  /// sparse real symmetric matrix \f$A\f$, i.e., calculating \f$y=Ax\f$ for any vector
  /// \f$x\f$. It is mainly used in the SymEigsSolver eigen solver.
  ///
  template <typename Scalar_, int Uplo = Eigen::Lower, int Flags = Eigen::ColMajor, typename StorageIndex = int>
  class SparseSymMatProd
  {
  private:
      using Scalar = Scalar_;
      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;
  
  public:
      ///
      /// Constructor to create the matrix operation object.
      ///
      /// \param mat An **Eigen** sparse matrix object, whose type can be
      /// `Eigen::SparseMatrix<Scalar, ...>` or its mapped version
      /// `Eigen::Map<Eigen::SparseMatrix<Scalar, ...> >`.
      ///
      SparseSymMatProd(ConstGenericSparseMatrix& mat) :
          m_mat(mat)
      {}
  
      ///
      /// Return the number of rows of the underlying matrix.
      ///
      Index rows() const { return m_mat.rows(); }
      ///
      /// Return the number of columns of the underlying matrix.
      ///
      Index cols() const { return m_mat.cols(); }
  
      ///
      /// Perform the matrix-vector multiplication operation \f$y=Ax\f$.
      ///
      /// \param x_in  Pointer to the \f$x\f$ vector.
      /// \param y_out Pointer to the \f$y\f$ vector.
      ///
      // y_out = A * x_in
      void perform_op(const Scalar* x_in, Scalar* y_out) const
      {
          MapConstVec x(x_in, m_mat.cols());
          MapVec y(y_out, m_mat.rows());
          y.noalias() = m_mat.template selfadjointView<Uplo>() * x;
      }
  
      ///
      /// Perform the matrix-matrix multiplication operation \f$y=Ax\f$.
      ///
      SparseMatrix operator*(const SparseMatrix& mat_in) const
      {
          SparseMatrix result;
          result = m_mat.template selfadjointView<Uplo>() * mat_in;
          return result;
      }
  
      ///
      /// Extract (i,j) element of the underlying matrix.
      ///
      Scalar operator()(Index i, Index j) const
      {
          return m_mat.coeff(i, j);
      }
  };
  }  // namespace Spectra
  
  #endif  // SPECTRA_SPARSE_SYM_MAT_PROD_H
  