rank.hpp

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#ifndef STAN_MATH_PRIM_FUN_RANK_HPP
#define STAN_MATH_PRIM_FUN_RANK_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/err.hpp>
 
namespace stan {
namespace math {
 
template <typename C, require_container_t<C>* = nullptr>
inline int rank(C&& v, int s) {
  check_range("rank", "v", v.size(), s);
  --s;  // adjust for indexing by one
  return apply_vector_unary<C>::reduce(std::forward<C>(v), [s](auto&& vec) {
    auto&& vec_ref = to_ref(std::forward<decltype(vec)>(vec));
    return (vec_ref.array() < vec_ref.coeff(s)).template cast<int>().sum();
  });
}
 
}  // namespace math
}  // namespace stan
 
#endif