unit_vector_constrain.hpp

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#ifndef STAN_MATH_PRIM_CONSTRAINT_UNIT_VECTOR_CONSTRAIN_HPP
#define STAN_MATH_PRIM_CONSTRAINT_UNIT_VECTOR_CONSTRAIN_HPP
 
#include <stan/math/prim/err.hpp>
#include <stan/math/prim/fun/Eigen.hpp>
#include <stan/math/prim/fun/dot_self.hpp>
#include <stan/math/prim/fun/sqrt.hpp>
#include <cmath>
 
namespace stan {
namespace math {
 
template <typename T, require_eigen_col_vector_t<T>* = nullptr,
          require_not_vt_autodiff<T>* = nullptr>
inline plain_type_t<T> unit_vector_constrain(const T& y) {
  using std::sqrt;
  check_nonzero_size("unit_vector_constrain", "y", y);
  auto&& y_ref = to_ref(y);
  value_type_t<T> SN = dot_self(y_ref);
  check_positive_finite("unit_vector_constrain", "norm", SN);
  return y_ref.array() / sqrt(SN);
}
 
template <typename T1, typename T2, require_eigen_col_vector_t<T1>* = nullptr,
          require_all_not_vt_autodiff<T1, T2>* = nullptr>
inline plain_type_t<T1> unit_vector_constrain(const T1& y, T2& lp) {
  using std::sqrt;
  check_nonzero_size("unit_vector_constrain", "y", y);
  auto&& y_ref = to_ref(y);
  value_type_t<T1> SN = dot_self(y_ref);
  check_positive_finite("unit_vector_constrain", "norm", SN);
  lp -= 0.5 * SN;
  return y_ref.array() / sqrt(SN);
}
 
template <bool Jacobian, typename T, require_not_std_vector_t<T>* = nullptr>
inline auto unit_vector_constrain(const T& y, return_type_t<T>& lp) {
  if (Jacobian) {
    return unit_vector_constrain(y, lp);
  } else {
    return unit_vector_constrain(y);
  }
}
 
template <bool Jacobian, typename T, require_std_vector_t<T>* = nullptr>
inline auto unit_vector_constrain(const T& y, return_type_t<T>& lp) {
  return apply_vector_unary<T>::apply(
      y, [&lp](auto&& v) { return unit_vector_constrain<Jacobian>(v, lp); });
}
 
}  // namespace math
}  // namespace stan
 
#endif