sort_indices.hpp

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#ifndef STAN_MATH_PRIM_FUN_SORT_INDICES_HPP
#define STAN_MATH_PRIM_FUN_SORT_INDICES_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/fun/Eigen.hpp>
#include <stan/math/prim/fun/to_ref.hpp>
#include <algorithm>
#include <vector>
 
namespace stan {
namespace math {
 
namespace internal {
template <bool ascending, typename C>
class index_comparator {
  const C& xs_;
 
 public:
  explicit index_comparator(const C& xs) : xs_(xs) {}
 
  bool operator()(int i, int j) const {
    if (ascending) {
      return xs_[i - 1] < xs_[j - 1];
    } else {
      return xs_[i - 1] > xs_[j - 1];
    }
  }
};
 
}  // namespace internal
 
template <bool ascending, typename C>
std::vector<int> sort_indices(const C& xs) {
  using idx_t = index_type_t<C>;
  idx_t xs_size = xs.size();
  std::vector<int> idxs;
  idxs.resize(xs_size);
  for (idx_t i = 0; i < xs_size; ++i) {
    idxs[i] = i + 1;
  }
  internal::index_comparator<ascending, ref_type_t<C>> comparator(to_ref(xs));
  std::sort(idxs.begin(), idxs.end(), comparator);
  return idxs;
}
 
}  // namespace math
}  // namespace stan
 
#endif