introsort.detail.h

// Copyright (c) 2013 Craig Henderson
// https://github.com/cdmh/sorting_algorithms

#include "quicksort.detail.h"
#include "heap_sort.h"

namespace cdmh {

namespace detail {

// the heap sort uses std::make_heap and std::sort_heap, both of which only work with random access iterators.
// this functor allows us to use heap_sort or not depending on the iterator type
template<typename It, bool IsRandomIterator=std::is_same<typename std::iterator_traits<It>::iterator_category, std::random_access_iterator_tag>::value>
struct heap_sort_if_we_can
{
    template<typename... Args>
    bool operator()(Args...)
    {
        return false;
    }
};

template<typename It>
struct heap_sort_if_we_can<It, true>
{
    template<typename Pred>
    bool operator()(It begin, It end, Pred pred)
    {
        heap_sort(begin, end, pred);
        return true;
    }
};


// detail introsort implementation takes a depth count
template<typename It, typename Pred>
inline void introsort(It begin, It end, Pred pred, size_t depth)
{
    auto const size = std::distance(begin, end);
    if (size <= 1)
        return;

    // if we've reached the depth limit, then attempt a heap sort.
    // if heap sort is not available to us, then continue with quicksort algorithm
    if (--depth == 0  &&  heap_sort_if_we_can<It>()(begin, end, pred))
        return;

    auto splits = detail::quicksort_splits(begin, end, pred);
    introsort(begin, splits.first, pred, depth);
    introsort(splits.second, end, pred, depth);
}

}   // namespace detail

}   // namespace cdmh

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