#pragma once

#include <cstddef>
#include <functional>

namespace prometheus {

  namespace detail {

    /// \brief Combine a hash value with nothing.
    /// It's the boundary condition of this serial functions.
    ///
    /// \param seed Not effect.
    inline void hash_combine(std::size_t *) {}

    /// \brief Combine the given hash value with another obeject.
    ///
    /// \param seed The given hash value. It's a input/output parameter.
    /// \param value The object that will be combined with the given hash value.
    template <typename T>
    inline void hash_combine(std::size_t *seed, const T &value) {
      *seed ^= std::hash<T>{}(value) + 0x9e3779b9 + (*seed << 6) + (*seed >> 2);
    }

    /// \brief Combine the given hash value with another objects. It's a recursion。
    ///
    /// \param seed The give hash value. It's a input/output parameter.
    /// \param value The object that will be combined with the given hash value.
    /// \param args The objects that will be combined with the given hash value.
    template <typename T, typename... Types>
    inline void hash_combine(std::size_t *seed, const T &value,
                             const Types &...args) {
      hash_combine(seed, value);
      hash_combine(seed, args...);
    }

    /// \brief Compute a hash value of the given args.
    ///
    /// \param args The arguments that will be computed hash value.
    /// \return The hash value of the given args.
    template <typename... Types>
    inline std::size_t hash_value(const Types &...args) {
      std::size_t seed = 0;
      hash_combine(&seed, args...);
      return seed;
    }

  }  // namespace detail

}  // namespace prometheus