cocktail_sort.h

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

#include "sort.h"

namespace cdmh {

// Cocktail Sort
// a bidirectional bubble sort, with the same (slow) performance characteristics
//     Worst case performance       O(n^2)
//     Best case performance        O(n)
//     Average case performance     O(n^2)
//     Wost case space complexity   O(1)
// http://en.wikipedia.org/wiki/Cocktail_sort

template<typename It, typename Pred=std::less<typename std::iterator_traits<It>::value_type>>
inline void cocktail_sort(It begin, It end, Pred pred=Pred())
{
    if (std::distance(begin, end) <= 1)
        return;

    bool finished = false;
    while (!finished)
    {
        // bubble sort forward iteration
        finished = true;
        std::advance(end, -1);
        for (auto it=begin; it!=end; ++it)
        {
            auto next = detail::advance(it, 1);
            if (pred(*next, *it))
            {
                std::swap(*it, *next);
                finished = false;
            }
        }

        if (!finished)
        {
            // bubble sort backward iteration
            finished = true;
            std::advance(begin, 1);
            for (auto it =std::reverse_iterator<It>(end),
                      ite=std::reverse_iterator<It>(begin);
                 it!=ite; ++it)
            {
                auto next = detail::advance(it, 1);
                if (pred(*it, *next))
                {
                    std::swap(*it, *next);
                    finished = false;
                }
            }
        }
    }
}

}   // namespace cdmh

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