libcruft-util/cruft/util/algo/sort.hpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright:
 *      2017, Danny Robson <danny@nerdcruft.net>
 */

#ifndef CRUFT_UTIL_ALGO_SORT_HPP
#define CRUFT_UTIL_ALGO_SORT_HPP

#include "../debug/assert.hpp"
#include "../cast.hpp"

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


namespace cruft::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);
            }
        }
    }


    ///////////////////////////////////////////////////////////////////////////
    /// Fill a supplied index buffer with a sorted range of indices over an
    /// identically size collection of data elements addressed by the
    /// RandomIterator `data`.
    ///
    /// Does not modify any data element pointed to by `data`.
    template <
        typename IndexT,
        typename RandomT,
        typename ComparatorT = std::less<>
    >
    void
    indices (
        IndexT idx_first,
        IndexT idx_last,
        RandomT data,
        ComparatorT &&cmp
    ) {
        // initialise a monotonic sequence of indices
        std::iota (idx_first, idx_last, 0);

        // sort using the indices
        std::sort (idx_first, idx_last, [&] (auto a, auto b) {
            return cmp (
                *(data + a),
                *(data + b)
            );
        });
    }


    ///////////////////////////////////////////////////////////////////////////
    // 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 < ::cruft::cast::sign<ssize_t> (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