Fast coefficient lookup / reference for Eigen3 arrays

eigen_helper.hpp

// Fast coefficient reference for rowmajor array with fixed number of columns
template <class Scalar_, int Rows_, int Cols_, int Flags_, int MaxRows_, int MaxCols_,
          std::enable_if_t<(Flags_ & Eigen::RowMajor) && (Cols_ != Eigen::Dynamic), bool> = true>
auto CO(Eigen::Array<Scalar_, Rows_, Cols_, Flags_, MaxRows_, MaxCols_> &arr,
               Eigen::Index row, Eigen::Index col) -> Scalar_ & {
  return *(arr.data() + Cols_ * row + col);
}

// Fast coefficient const reference for rowmajor array with fixed number of columns
template <class Scalar_, int Rows_, int Cols_, int Flags_, int MaxRows_, int MaxCols_,
          std::enable_if_t<(Flags_ & Eigen::RowMajor) && (Cols_ != Eigen::Dynamic), bool> = true>
auto CO(Eigen::Array<Scalar_, Rows_, Cols_, Flags_, MaxRows_, MaxCols_> const &arr,
               Eigen::Index row, Eigen::Index col) -> Scalar_ const & {
  return *(arr.data() + Cols_ * row + col);
}

// Fast coefficient reference for colmajor array with fixed number of rows
template <class Scalar_, int Rows_, int Cols_, int Flags_, int MaxRows_, int MaxCols_,
          std::enable_if_t<!(Flags_ & Eigen::RowMajor) && (Rows_ != Eigen::Dynamic), bool> = true>
auto CO(Eigen::Array<Scalar_, Rows_, Cols_, Flags_, MaxRows_, MaxCols_> &arr,
               Eigen::Index row, Eigen::Index col) -> Scalar_ & {
  return *(arr.data() + row + Rows_ * col);
}

// Fast coefficient const reference for colmajor array with fixed number of rows
template <class Scalar_, int Rows_, int Cols_, int Flags_, int MaxRows_, int MaxCols_,
          std::enable_if_t<!(Flags_ & Eigen::RowMajor) && (Rows_ != Eigen::Dynamic), bool> = true>
auto CO(Eigen::Array<Scalar_, Rows_, Cols_, Flags_, MaxRows_, MaxCols_> const &arr,
               Eigen::Index row, Eigen::Index col) -> Scalar_ const & {
  return *(arr.data() + row + Rows_ * col + row);
}

// Fast coefficient reference for 1d arrays
template <class Scalar_, int Rows_, int Cols_, int Flags_, int MaxRows_, int MaxCols_,
          std::enable_if_t<(Rows_ == 1) || (Cols_ == 1), bool> = true>
auto CO(Eigen::Array<Scalar_, Rows_, Cols_, Flags_, MaxRows_, MaxCols_> &arr,
               Eigen::Index idx) -> Scalar_ & {
  return *(arr.data() + idx);
}

// Fast coefficient const reference for 1d arrays
template <class Scalar_, int Rows_, int Cols_, int Flags_, int MaxRows_, int MaxCols_,
          std::enable_if_t<(Rows_ == 1) || (Cols_ == 1), bool> = true>
auto CO(Eigen::Array<Scalar_, Rows_, Cols_, Flags_, MaxRows_, MaxCols_> const &arr,
               Eigen::Index idx) -> Scalar_ const & {
  return *(arr.data() + idx);
}