libcruft-util/algo/sort.hpp

/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * Copyright:
 *      2017, Danny Robson <danny@nerdcruft.net>
 */

#ifndef CRUFT_UTIL_ALGO_SORT_HPP
#define CRUFT_UTIL_ALGO_SORT_HPP

#include "../debug.hpp"

#include <iterator>
#include <algorithm>
#include <numeric>
#include <vector>


namespace cruft::util::sort {
    namespace detail {
        template <typename IndexA, typename IndexB, typename RandomIt>
        void
        index_swap (IndexA a, IndexB b, RandomIt value)
        {
            static_assert(
                std::is_base_of<
                    std::random_access_iterator_tag,
                    typename std::iterator_traits<RandomIt>::iterator_category
                >::value
            );

            std::swap (value[a], value[b]);
        }

        template <typename IndexA, typename IndexB, typename RandomIt, typename ...Tail>
        void
        index_swap (IndexA a, IndexB b, RandomIt value, Tail ...tail)
        {
            index_swap (a, b, value);
            index_swap (a, b, tail...);
        };
    }

    ///////////////////////////////////////////////////////////////////////////
    // rearrange the values in the arrays specified by the iterators value and
    // ...tail by moving values to the positions described by the mapping of
    // offset-to-dest_index in idx_first:idx_last.
    //
    // ie, the indices { 2, 1, 0 } will reverse a series of arrays of three
    // elements
    //
    // all operations occur in-place, and the indices may be rearranged during
    // the operation.
    template <typename IndexIt, typename ValueIt, typename ...OtherIt>
    void
    reorder (IndexIt idx_first, IndexIt idx_last, ValueIt value, OtherIt ...tail)
    {
        static_assert (
            std::is_base_of<
                std::random_access_iterator_tag,
                typename std::iterator_traits<IndexIt>::iterator_category
            >::value
        );

        static_assert (
            std::is_base_of<
                std::random_access_iterator_tag,
                typename std::iterator_traits<ValueIt>::iterator_category
            >::value
        );

        // Bail early on zero size arrays, partly so we can simplify some
        // debugging checks
        auto size = std::distance (idx_first, idx_last);
        if (!size)
            return;

        CHECK_LT (*std::max_element (idx_first, idx_last), size);

        for (decltype(size) i = 0; i < size - 1; ++i) {
            while (i != (decltype(size))idx_first[i]) {
                auto j = idx_first[i];

                detail::index_swap (i, j, value, tail..., idx_first);
            }
        }
    }

    ///////////////////////////////////////////////////////////////////////////
    // sort an array specified by the iterators key_first:key_last using a
    // comparator, and optionally a series of additional value iterators
    // specified by ...values.
    //
    // sorting is performed in-place and will invariably allocate memory.
    template <typename RandomIt, class Comparator, class ...Args>
    void
    soa (RandomIt key_first, RandomIt key_last, Comparator comp, Args ...values)
    {
        static_assert (
            std::is_base_of<
                std::random_access_iterator_tag,
                typename std::iterator_traits<RandomIt>::iterator_category
            >::value
        );

        // bail early on guaranteed sorted or degenerate cases. we can make some
        // assumptions about minimum array sizes and non-wrapping indices later on
        // this way.
        if (std::distance (key_first, key_last) <= 1)
        return;

        // generate a list of indices into the key array and sort them so we have,
        // in effect, a sorted list of pointers.
        auto size = std::distance (key_first, key_last);
        std::vector<decltype(size)> indices (size);
        std::iota (std::begin (indices), std::end (indices), 0);

        std::sort (std::begin (indices),
            std::end   (indices),
        [&] (const auto &cruft_util_sort_soa_a, const auto &cruft_util_sort_soa_b)
        {
            // GCC: we use the ridiculous parameter names to avoid a name aliasing
            // bug/warning under gcc 6.3.0; if the client passes a comparator
            // lambda that uses the same parameter names then a warning will be
            // generated. given 'a' and 'b' aren't unlikely names we try to avoid
            // them here.
            return comp (
                key_first[cruft_util_sort_soa_a],
                key_first[cruft_util_sort_soa_b]
            );
        });

        // convert from a sorted list of pointers to a mapping of pointers to
        // desired final offsets. this is done so we can palm it off to the
        // reorder function.
        // TODO: avoid the need for this inverse array.
        decltype (indices) dest (indices.size ());
        for (decltype(size) i = 0; i < (decltype(size))dest.size (); ++i) {
        dest[indices[i]] = i;
        }

        // reorder all the arrays using the mapping we have discovered.
        reorder (std::begin (dest), std::end (dest), key_first, values...);
    }
}

#endif