    // Copyright (C) 2010  Davis E. King (davis@dlib.net)
    // License: Boost Software License   See LICENSE.txt for the full license.
    #ifndef DLIB_UNORDERED_PAiR_H__
    #define DLIB_UNORDERED_PAiR_H__

    #include "serialize.h"

    namespace dlib
    {

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

        template <
            typename T
            >
        struct unordered_pair
        {
            /*!
                REQUIREMENTS ON T
                    T must be default constructable, copyable, and comparable using
                    operator < and ==

                WHAT THIS OBJECT REPRESENTS
                    This object is very similar to the std::pair struct except unordered_pair
                    is only capable of representing an unordered set of two items rather than
                    an ordered list of two items like std::pair.

                    This is best illustrated by example.  Suppose we have the following
                    five variables:
                        std::pair<int,int> p1(1, 5), p2(5,1);
                        unordered_pair<int> up1(1,5), up2(5,1), up3(6,7);

                    Then it is the case that:
                        up1 == up2
                        up1 != up3
                        p1 != p2

                   So the unordered_pair doesn't care about the order of the arguments.
                   In this case, up1 and up2 are both equivalent.

            !*/

            typedef T type;
            typedef T first_type;
            typedef T second_type;

            const T first;
            const T second;

            unordered_pair() : first(), second()
            /*!
                ensures
                    - #first and #second are default initialized
            !*/ {}

            unordered_pair(
                const T& a,
                const T& b
            ) :
                first( a < b ? a : b),
                second(a < b ? b : a)
            /*!
                ensures
                    - #first <= #second
                    - #first and #second contain copies of the items a and b.
            !*/ {}

            template <typename U>
            unordered_pair (
                const unordered_pair <U>& p
            ) :
                first(p.first),
                second(p.second)
            /*!
                ensures
                    - #*this is a copy of p
            !*/ {}

            unordered_pair& operator= (
                const unordered_pair& item
            )
            /*!
                ensures
                    - #*this == item
            !*/
            {
                const_cast<T&>(first) = item.first;
                const_cast<T&>(second) = item.second;
                return *this;
            }
        };

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

        template <typename T>
        bool operator==(const unordered_pair<T>& a, const unordered_pair <T>& b)
        {
            return a.first == b.first && a.second == b.second;
        }

        template <typename T>
        bool operator!=(const unordered_pair<T>& a, const unordered_pair <T>& b)
        {
            return !(a == b);
        }

        template <typename T>
        bool operator<(const unordered_pair<T>& a, const unordered_pair<T>& b)
        {
            return (a.first < b.first || (!(b.first < a.first) && a.second < b.second));
        }

        template <typename T>
        bool operator>(const unordered_pair<T>& a, const unordered_pair <T>& b)
        {
            return b < a;
        }

        template <typename T>
        bool operator<=(const unordered_pair<T>& a, const unordered_pair <T>& b)
        {
            return !(b < a);
        }

        template <typename T>
        bool operator>=(const unordered_pair<T>& a, const unordered_pair <T>& b)
        {
            return !(a < b);
        }

        template <typename T>
        unordered_pair<T> make_unordered_pair (const T& a, const T& b)
        {
            return unordered_pair<T>(a,b);
        }

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

        template <typename T>
        void serialize (
            const unordered_pair<T>& item,
            std::ostream& out
        )
        {
            try
            {
                serialize(item.first,out);
                serialize(item.second,out);
            }
            catch (serialization_error& e)
            { throw serialization_error(e.info + "\n   while serializing object of type unordered_pair"); }
        }

        template <typename T>
        void deserialize (
            unordered_pair<T>& item,
            std::istream& in
        )
        {
            try
            {
                T a, b;
                deserialize(a,in);
                deserialize(b,in);
                item = make_unordered_pair(a,b);
            }
            catch (serialization_error& e)
            { throw serialization_error(e.info + "\n   while deserializing object of type unordered_pair"); }
        }

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

    }

    #endif // DLIB_UNORDERED_PAiR_H__