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Filename : heap_merge.hpp
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// boost heap: heap merge algorithms
//
// Copyright (C) 2011 Tim Blechmann
//
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)

#ifndef BOOST_HEAP_MERGE_HPP
#define BOOST_HEAP_MERGE_HPP

#include <algorithm>

#include <boost/concept/assert.hpp>
#include <boost/heap/heap_concepts.hpp>
#include <boost/type_traits/conditional.hpp>
#include <boost/type_traits/is_same.hpp>

#ifdef BOOST_HAS_PRAGMA_ONCE
#pragma once
#endif


namespace boost  {
namespace heap   {
namespace detail {

template <typename Heap1, typename Heap2>
struct heap_merge_emulate
{
    struct dummy_reserver
    {
        static void reserve (Heap1 & lhs, std::size_t required_size)
        {}
    };

    struct reserver
    {
        static void reserve (Heap1 & lhs, std::size_t required_size)
        {
            lhs.reserve(required_size);
        }
    };

    typedef typename boost::conditional<Heap1::has_reserve,
                                      reserver,
                                      dummy_reserver>::type space_reserver;

    static void merge(Heap1 & lhs, Heap2 & rhs)
    {
        if (Heap1::constant_time_size && Heap2::constant_time_size) {
            if (Heap1::has_reserve) {
                std::size_t required_size = lhs.size() + rhs.size();
                space_reserver::reserve(lhs, required_size);
            }
        }

        // FIXME: container adaptors could benefit from first appending all elements and then restoring the heap order
        // FIXME: optimize: if we have ordered iterators and we can efficiently insert keys with a below the lowest key in the heap
        //                  d-ary, b and fibonacci heaps fall into this category

        while (!rhs.empty()) {
            lhs.push(rhs.top());
            rhs.pop();
        }

        lhs.set_stability_count((std::max)(lhs.get_stability_count(),
                                           rhs.get_stability_count()));
        rhs.set_stability_count(0);
    }

};


template <typename Heap>
struct heap_merge_same_mergable
{
    static void merge(Heap & lhs, Heap & rhs)
    {
        lhs.merge(rhs);
    }
};


template <typename Heap>
struct heap_merge_same
{
    static const bool is_mergable = Heap::is_mergable;
    typedef typename boost::conditional<is_mergable,
                                      heap_merge_same_mergable<Heap>,
                                      heap_merge_emulate<Heap, Heap>
                                     >::type heap_merger;

    static void merge(Heap & lhs, Heap & rhs)
    {
        heap_merger::merge(lhs, rhs);
    }
};

} /* namespace detail */


/** merge rhs into lhs
 *
 *  \b Effect: lhs contains all elements that have been part of rhs, rhs is empty.
 *
 * */
template <typename Heap1,
          typename Heap2
         >
void heap_merge(Heap1 & lhs, Heap2 & rhs)
{
    BOOST_CONCEPT_ASSERT((boost::heap::PriorityQueue<Heap1>));
    BOOST_CONCEPT_ASSERT((boost::heap::PriorityQueue<Heap2>));

    // if this assertion is triggered, the value_compare types are incompatible
    BOOST_STATIC_ASSERT((boost::is_same<typename Heap1::value_compare, typename Heap2::value_compare>::value));

    const bool same_heaps = boost::is_same<Heap1, Heap2>::value;

    typedef typename boost::conditional<same_heaps,
                                      detail::heap_merge_same<Heap1>,
                                      detail::heap_merge_emulate<Heap1, Heap2>
                                     >::type heap_merger;

    heap_merger::merge(lhs, rhs);
}


} /* namespace heap */
} /* namespace boost */

#endif /* BOOST_HEAP_MERGE_HPP */