EigenBlockMatrixView.h

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#pragma once
 
#include <Eigen/Core>
 
namespace MathLib
{
template <int D, typename M>
class EigenBlockMatrixViewFunctor
{
public:
    static constexpr int rows = M::RowsAtCompileTime == Eigen::Dynamic
                                    ? Eigen::Dynamic
                                    : D * M::RowsAtCompileTime;
    static constexpr int cols = M::ColsAtCompileTime == Eigen::Dynamic
                                    ? Eigen::Dynamic
                                    : D * M::ColsAtCompileTime;
    using Scalar = typename M::Scalar;
    using Matrix = Eigen::Matrix<Scalar, rows, cols, Eigen::ColMajor>;
 
    constexpr explicit EigenBlockMatrixViewFunctor(const M& matrix)
        : matrix_(matrix){};
 
    constexpr const Scalar operator()(Eigen::Index row, Eigen::Index col) const
    {
        if (row / matrix_.rows() != col / matrix_.cols())
        {
            return 0;
        }
        return matrix_(row % matrix_.rows(), col % matrix_.cols());
    }
 
private:
    const typename M::Nested& matrix_;
};
 
template <int D, typename M>
constexpr Eigen::CwiseNullaryOp<
    EigenBlockMatrixViewFunctor<D, M>,
    typename EigenBlockMatrixViewFunctor<D, M>::Matrix>
eigenBlockMatrixView(const Eigen::MatrixBase<M>& matrix)
{
    using Matrix = typename EigenBlockMatrixViewFunctor<D, M>::Matrix;
    return Matrix::NullaryExpr(
        D * matrix.rows(), D * matrix.cols(),
        EigenBlockMatrixViewFunctor<D, M>(matrix.derived()));
}
}  // namespace MathLib