    // Copyright (C) 2016  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_UTILITY_Hh_
    #define DLIB_UTILITY_Hh_

    #include <cstddef>
    #include <utility>

    /*
        This header contains back-ports of C++14/17 functions and type traits
        found in <utility> header of the standard library.
    */

    namespace dlib
    {
    #ifdef __cpp_lib_integer_sequence
        template<std::size_t... Ints>
        using index_sequence = std::index_sequence<Ints...>;

        template<std::size_t N>
        using make_index_sequence = std::make_index_sequence<N>;

        template<class... T>
        using index_sequence_for = std::index_sequence_for<T...>;
    #else
        // ---------------------------------------------------------------------
        template<std::size_t... Ints>
        struct index_sequence
        {
            using type = index_sequence;
            using value_type = std::size_t;
            static constexpr std::size_t size() noexcept {return sizeof...(Ints);}
        };

        template<class Sequence1, class Sequence2>
        struct merge_and_renumber;

        template<std::size_t... I1, std::size_t... I2>
        struct merge_and_renumber<index_sequence<I1...>, index_sequence<I2...>>
                : index_sequence < I1..., (sizeof...(I1) + I2)... > {};

        template<std::size_t N>
        struct make_index_sequence
                : merge_and_renumber < typename make_index_sequence < N / 2 >::type,
                        typename make_index_sequence < N - N / 2 >::type > {};

        template<> struct make_index_sequence<0> : index_sequence<> {};
        template<> struct make_index_sequence<1> : index_sequence<0> {};

        template<typename... Ts>
        using index_sequence_for = make_index_sequence<sizeof...(Ts)>;
        // ---------------------------------------------------------------------
    #endif

        // ---------------------------------------------------------------------

        template<class Sequence>
        struct pop_front {};

        template<std::size_t I, std::size_t... Ints>
        struct pop_front<index_sequence<I, Ints...>>
        {
            using type = index_sequence<Ints...>;
        };

        template<class Sequence>
        using pop_front_t = typename pop_front<Sequence>::type;

        // ---------------------------------------------------------------------

        struct in_place_t
        {
            explicit in_place_t() = default;
        };

        static constexpr in_place_t in_place{};

        // ---------------------------------------------------------------------
    }

    #endif //DLIB_UTILITY_Hh_