eigendecompose_sym.hpp

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#ifndef STAN_MATH_PRIM_FUN_EIGENDECOMPOSE_SYM_HPP
#define STAN_MATH_PRIM_FUN_EIGENDECOMPOSE_SYM_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/err.hpp>
#include <stan/math/prim/fun/Eigen.hpp>
 
namespace stan {
namespace math {
 
template <typename EigMat, require_eigen_t<EigMat>* = nullptr,
          require_not_st_var<EigMat>* = nullptr>
std::tuple<Eigen::Matrix<value_type_t<EigMat>, -1, -1>,
           Eigen::Matrix<value_type_t<EigMat>, -1, 1>>
eigendecompose_sym(EigMat&& m) {
  if (unlikely(m.size() == 0)) {
    return std::make_tuple(Eigen::Matrix<value_type_t<EigMat>, -1, -1>(0, 0),
                           Eigen::Matrix<value_type_t<EigMat>, -1, 1>(0, 1));
  }
  using PlainMat = plain_type_t<EigMat>;
  decltype(auto) m_ref = to_ref(std::forward<EigMat>(m));
  check_symmetric("eigendecompose_sym", "m", m_ref);
 
  Eigen::SelfAdjointEigenSolver<PlainMat> solver(
      std::forward<decltype(m_ref)>(m_ref));
  return std::make_tuple(std::move(solver.eigenvectors()),
                         std::move(solver.eigenvalues()));
}
 
}  // namespace math
}  // namespace stan
#endif