ordered_constrain.hpp

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#ifndef STAN_MATH_PRIM_CONSTRAINT_ORDERED_CONSTRAIN_HPP
#define STAN_MATH_PRIM_CONSTRAINT_ORDERED_CONSTRAIN_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/fun/Eigen.hpp>
#include <stan/math/prim/fun/exp.hpp>
#include <stan/math/prim/fun/sum.hpp>
#include <stan/math/prim/fun/to_ref.hpp>
#include <cmath>
 
namespace stan {
namespace math {
 
template <typename EigVec, require_eigen_col_vector_t<EigVec>* = nullptr,
          require_not_st_var<EigVec>* = nullptr>
inline plain_type_t<EigVec> ordered_constrain(const EigVec& x) {
  using std::exp;
  Eigen::Index k = x.size();
  plain_type_t<EigVec> y(k);
  const auto& x_ref = to_ref(x);
  if (unlikely(k == 0)) {
    return y;
  }
  y[0] = x_ref[0];
  for (Eigen::Index i = 1; i < k; ++i) {
    y.coeffRef(i) = y.coeff(i - 1) + exp(x_ref.coeff(i));
  }
  return y;
}
 
template <typename EigVec, require_eigen_col_vector_t<EigVec>* = nullptr>
inline auto ordered_constrain(const EigVec& x, value_type_t<EigVec>& lp) {
  const auto& x_ref = to_ref(x);
  if (likely(x.size() > 1)) {
    lp += sum(x_ref.tail(x.size() - 1));
  }
  return ordered_constrain(x_ref);
}
 
template <bool Jacobian, typename T, require_not_std_vector_t<T>* = nullptr>
inline auto ordered_constrain(const T& x, return_type_t<T>& lp) {
  if (Jacobian) {
    return ordered_constrain(x, lp);
  } else {
    return ordered_constrain(x);
  }
}
 
template <bool Jacobian, typename T, require_std_vector_t<T>* = nullptr>
inline auto ordered_constrain(const T& x, return_type_t<T>& lp) {
  return apply_vector_unary<T>::apply(
      x, [&lp](auto&& v) { return ordered_constrain<Jacobian>(v, lp); });
}
 
}  // namespace math
}  // namespace stan
 
#endif