text: #ifndef ENTT_CORE_MEMORY_HPP
  #define ENTT_CORE_MEMORY_HPP
  
  #include <cstddef>
  #include <limits>
  #include <memory>
  #include <tuple>
  #include <type_traits>
  #include <utility>
  #include "../config/config.h"
  
  namespace entt {
  
  /**
   * @brief Unwraps fancy pointers, does nothing otherwise (waiting for C++20).
   * @tparam Type Pointer type.
   * @param ptr Fancy or raw pointer.
   * @return A raw pointer that represents the address of the original pointer.
   */
  template<typename Type>
  [[nodiscard]] constexpr auto to_address(Type &&ptr) ENTT_NOEXCEPT {
      if constexpr(std::is_pointer_v<std::remove_cv_t<std::remove_reference_t<Type>>>) {
          return ptr;
      } else {
          return to_address(std::forward<Type>(ptr).operator->());
      }
  }
  
  /**
   * @brief Utility function to design allocation-aware containers.
   * @tparam Allocator Type of allocator.
   * @param lhs A valid allocator.
   * @param rhs Another valid allocator.
   */
  template<typename Allocator>
  constexpr void propagate_on_container_copy_assignment([[maybe_unused]] Allocator &lhs, [[maybe_unused]] Allocator &rhs) ENTT_NOEXCEPT {
      if constexpr(std::allocator_traits<Allocator>::propagate_on_container_copy_assignment::value) {
          lhs = rhs;
      }
  }
  
  /**
   * @brief Utility function to design allocation-aware containers.
   * @tparam Allocator Type of allocator.
   * @param lhs A valid allocator.
   * @param rhs Another valid allocator.
   */
  template<typename Allocator>
  constexpr void propagate_on_container_move_assignment([[maybe_unused]] Allocator &lhs, [[maybe_unused]] Allocator &rhs) ENTT_NOEXCEPT {
      if constexpr(std::allocator_traits<Allocator>::propagate_on_container_move_assignment::value) {
          lhs = std::move(rhs);
      }
  }
  
  /**
   * @brief Utility function to design allocation-aware containers.
   * @tparam Allocator Type of allocator.
   * @param lhs A valid allocator.
   * @param rhs Another valid allocator.
   */
  template<typename Allocator>
  constexpr void propagate_on_container_swap([[maybe_unused]] Allocator &lhs, [[maybe_unused]] Allocator &rhs) ENTT_NOEXCEPT {
      ENTT_ASSERT(std::allocator_traits<Allocator>::propagate_on_container_swap::value || lhs == rhs, "Cannot swap the containers");
  
      if constexpr(std::allocator_traits<Allocator>::propagate_on_container_swap::value) {
          using std::swap;
          swap(lhs, rhs);
      }
  }
  
  /**
   * @brief Checks whether a value is a power of two or not.
   * @param value A value that may or may not be a power of two.
   * @return True if the value is a power of two, false otherwise.
   */
  [[nodiscard]] inline constexpr bool is_power_of_two(const std::size_t value) ENTT_NOEXCEPT {
      return value && ((value & (value - 1)) == 0);
  }
  
  /**
   * @brief Computes the smallest power of two greater than or equal to a value.
   * @param value The value to use.
   * @return The smallest power of two greater than or equal to the given value.
   */
  [[nodiscard]] inline constexpr std::size_t next_power_of_two(const std::size_t value) ENTT_NOEXCEPT {
      ENTT_ASSERT(value < (std::size_t{1u} << (std::numeric_limits<std::size_t>::digits - 1)), "Numeric limits exceeded");
      std::size_t curr = value - (value != 0u);
  
      for(int next = 1; next < std::numeric_limits<std::size_t>::digits; next = next * 2) {
          curr |= curr >> next;
      }
  
      return ++curr;
  }
  
  /**
   * @brief Fast module utility function (powers of two only).
   * @param value A value for which to calculate the modulus.
   * @param mod _Modulus_, it must be a power of two.
   * @return The common remainder.
   */
  [[nodiscard]] inline constexpr std::size_t fast_mod(const std::size_t value, const std::size_t mod) ENTT_NOEXCEPT {
      ENTT_ASSERT(is_power_of_two(mod), "Value must be a power of two");
      return value & (mod - 1u);
  }
  
  /**
   * @brief Deleter for allocator-aware unique pointers (waiting for C++20).
   * @tparam Args Types of arguments to use to construct the object.
   */
  template<typename Allocator>
  struct allocation_deleter: private Allocator {
      /*! @brief Allocator type. */
      using allocator_type = Allocator;
      /*! @brief Pointer type. */
      using pointer = typename std::allocator_traits<Allocator>::pointer;
  
      /**
       * @brief Inherited constructors.
       * @param alloc The allocator to use.
       */
      allocation_deleter(const allocator_type &alloc)
          : Allocator{alloc} {}
  
      /**
       * @brief Destroys the pointed object and deallocates its memory.
       * @param ptr A valid pointer to an object of the given type.
       */
      void operator()(pointer ptr) {
          using alloc_traits = typename std::allocator_traits<Allocator>;
          alloc_traits::destroy(*this, to_address(ptr));
          alloc_traits::deallocate(*this, ptr, 1u);
      }
  };
  
  /**
   * @brief Allows `std::unique_ptr` to use allocators (waiting for C++20).
   * @tparam Type Type of object to allocate for and to construct.
   * @tparam Allocator Type of allocator used to manage memory and elements.
   * @tparam Args Types of arguments to use to construct the object.
   * @param allocator The allocator to use.
   * @param args Parameters to use to construct the object.
   * @return A properly initialized unique pointer with a custom deleter.
   */
  template<typename Type, typename Allocator, typename... Args>
  auto allocate_unique(Allocator &allocator, Args &&...args) {
      static_assert(!std::is_array_v<Type>, "Array types are not supported");
  
      using alloc_traits = typename std::allocator_traits<Allocator>::template rebind_traits<Type>;
      using allocator_type = typename alloc_traits::allocator_type;
  
      allocator_type alloc{allocator};
      auto ptr = alloc_traits::allocate(alloc, 1u);
  
      ENTT_TRY {
          alloc_traits::construct(alloc, to_address(ptr), std::forward<Args>(args)...);
      }
      ENTT_CATCH {
          alloc_traits::deallocate(alloc, ptr, 1u);
          ENTT_THROW;
      }
  
      return std::unique_ptr<Type, allocation_deleter<allocator_type>>{ptr, alloc};
  }
  
  /**
   * @cond TURN_OFF_DOXYGEN
   * Internal details not to be documented.
   */
  
  namespace internal {
  
  template<typename Type>
  struct uses_allocator_construction {
      template<typename Allocator, typename... Params>
      static constexpr auto args([[maybe_unused]] const Allocator &allocator, Params &&...params) ENTT_NOEXCEPT {
          if constexpr(!std::uses_allocator_v<Type, Allocator> && std::is_constructible_v<Type, Params...>) {
              return std::forward_as_tuple(std::forward<Params>(params)...);
          } else {
              static_assert(std::uses_allocator_v<Type, Allocator>, "Ill-formed request");
  
              if constexpr(std::is_constructible_v<Type, std::allocator_arg_t, const Allocator &, Params...>) {
                  return std::tuple<std::allocator_arg_t, const Allocator &, Params &&...>(std::allocator_arg, allocator, std::forward<Params>(params)...);
              } else {
                  static_assert(std::is_constructible_v<Type, Params..., const Allocator &>, "Ill-formed request");
                  return std::forward_as_tuple(std::forward<Params>(params)..., allocator);
              }
          }
      }
  };
  
  template<typename Type, typename Other>
  struct uses_allocator_construction<std::pair<Type, Other>> {
      using type = std::pair<Type, Other>;
  
      template<typename Allocator, typename First, typename Second>
      static constexpr auto args(const Allocator &allocator, std::piecewise_construct_t, First &&first, Second &&second) ENTT_NOEXCEPT {
          return std::make_tuple(
              std::piecewise_construct,
              std::apply([&allocator](auto &&...curr) { return uses_allocator_construction<Type>::args(allocator, std::forward<decltype(curr)>(curr)...); }, std::forward<First>(first)),
              std::apply([&allocator](auto &&...curr) { return uses_allocator_construction<Other>::args(allocator, std::forward<decltype(curr)>(curr)...); }, std::forward<Second>(second)));
      }
  
      template<typename Allocator>
      static constexpr auto args(const Allocator &allocator) ENTT_NOEXCEPT {
          return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::tuple<>{}, std::tuple<>{});
      }
  
      template<typename Allocator, typename First, typename Second>
      static constexpr auto args(const Allocator &allocator, First &&first, Second &&second) ENTT_NOEXCEPT {
          return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::forward_as_tuple(std::forward<First>(first)), std::forward_as_tuple(std::forward<Second>(second)));
      }
  
      template<typename Allocator, typename First, typename Second>
      static constexpr auto args(const Allocator &allocator, const std::pair<First, Second> &value) ENTT_NOEXCEPT {
          return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::forward_as_tuple(value.first), std::forward_as_tuple(value.second));
      }
  
      template<typename Allocator, typename First, typename Second>
      static constexpr auto args(const Allocator &allocator, std::pair<First, Second> &&value) ENTT_NOEXCEPT {
          return uses_allocator_construction<type>::args(allocator, std::piecewise_construct, std::forward_as_tuple(std::move(value.first)), std::forward_as_tuple(std::move(value.second)));
      }
  };
  
  } // namespace internal
  
  /**
   * Internal details not to be documented.
   * @endcond
   */
  
  /**
   * @brief Uses-allocator construction utility (waiting for C++20).
   *
   * Primarily intended for internal use. Prepares the argument list needed to
   * create an object of a given type by means of uses-allocator construction.
   *
   * @tparam Type Type to return arguments for.
   * @tparam Allocator Type of allocator used to manage memory and elements.
   * @tparam Args Types of arguments to use to construct the object.
   * @param allocator The allocator to use.
   * @param args Parameters to use to construct the object.
   * @return The arguments needed to create an object of the given type.
   */
  template<typename Type, typename Allocator, typename... Args>
  constexpr auto uses_allocator_construction_args(const Allocator &allocator, Args &&...args) ENTT_NOEXCEPT {
      return internal::uses_allocator_construction<Type>::args(allocator, std::forward<Args>(args)...);
  }
  
  /**
   * @brief Uses-allocator construction utility (waiting for C++20).
   *
   * Primarily intended for internal use. Creates an object of a given type by
   * means of uses-allocator construction.
   *
   * @tparam Type Type of object to create.
   * @tparam Allocator Type of allocator used to manage memory and elements.
   * @tparam Args Types of arguments to use to construct the object.
   * @param allocator The allocator to use.
   * @param args Parameters to use to construct the object.
   * @return A newly created object of the given type.
   */
  template<typename Type, typename Allocator, typename... Args>
  constexpr Type make_obj_using_allocator(const Allocator &allocator, Args &&...args) {
      return std::make_from_tuple<Type>(internal::uses_allocator_construction<Type>::args(allocator, std::forward<Args>(args)...));
  }
  
  /**
   * @brief Uses-allocator construction utility (waiting for C++20).
   *
   * Primarily intended for internal use. Creates an object of a given type by
   * means of uses-allocator construction at an uninitialized memory location.
   *
   * @tparam Type Type of object to create.
   * @tparam Allocator Type of allocator used to manage memory and elements.
   * @tparam Args Types of arguments to use to construct the object.
   * @param value Memory location in which to place the object.
   * @param allocator The allocator to use.
   * @param args Parameters to use to construct the object.
   * @return A pointer to the newly created object of the given type.
   */
  template<typename Type, typename Allocator, typename... Args>
  constexpr Type *uninitialized_construct_using_allocator(Type *value, const Allocator &allocator, Args &&...args) {
      return std::apply([&](auto &&...curr) { return new(value) Type(std::forward<decltype(curr)>(curr)...); }, internal::uses_allocator_construction<Type>::args(allocator, std::forward<Args>(args)...));
  }
  
  } // namespace entt
  
  #endif
  