tcrossprod.hpp

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#ifndef STAN_MATH_PRIM_FUN_TCROSSPROD_HPP
#define STAN_MATH_PRIM_FUN_TCROSSPROD_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/fun/Eigen.hpp>
#include <stan/math/prim/fun/to_ref.hpp>
#include <stan/math/prim/fun/typedefs.hpp>
 
namespace stan {
namespace math {
 
template <typename T, require_eigen_vt<std::is_arithmetic, T>* = nullptr>
inline Eigen::Matrix<value_type_t<T>, T::RowsAtCompileTime,
                     T::RowsAtCompileTime>
tcrossprod(const T& M) {
  if (M.rows() == 0) {
    return {};
  }
  const auto& M_ref = to_ref(M);
  if (M.rows() == 1) {
    return M_ref * M_ref.transpose();
  }
  Eigen::Matrix<value_type_t<T>, T::RowsAtCompileTime, T::RowsAtCompileTime>
      result(M.rows(), M.rows());
  return result.setZero().template selfadjointView<Eigen::Upper>().rankUpdate(
      M_ref);
}
 
}  // namespace math
}  // namespace stan
 
#endif