path: root/libstdc++-v3/include/parallel/multiway_mergesort.h

// -*- C++ -*-

// Copyright (C) 2007, 2008 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the terms
// of the GNU General Public License as published by the Free Software
// Foundation; either version 2, or (at your option) any later
// version.

// This library is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this library; see the file COPYING.  If not, write to
// the Free Software Foundation, 59 Temple Place - Suite 330, Boston,
// MA 02111-1307, USA.

// As a special exception, you may use this file as part of a free
// software library without restriction.  Specifically, if other files
// instantiate templates or use macros or inline functions from this
// file, or you compile this file and link it with other files to
// produce an executable, this file does not by itself cause the
// resulting executable to be covered by the GNU General Public
// License.  This exception does not however invalidate any other
// reasons why the executable file might be covered by the GNU General
// Public License.

/** @file parallel/multiway_mergesort.h
 *  @brief Parallel multiway merge sort.
 *  This file is a GNU parallel extension to the Standard C++ Library.
 */

// Written by Johannes Singler.

#ifndef _GLIBCXX_PARALLEL_MERGESORT_H
#define _GLIBCXX_PARALLEL_MERGESORT_H 1

#include <vector>

#include <parallel/basic_iterator.h>
#include <bits/stl_algo.h>
#include <parallel/parallel.h>
#include <parallel/multiway_merge.h>

namespace __gnu_parallel
{

/** @brief Subsequence description. */
template<typename _DifferenceTp>
  struct Piece
  {
    typedef _DifferenceTp difference_type;

    /** @brief Begin of subsequence. */
    difference_type begin;

    /** @brief End of subsequence. */
    difference_type end;
  };

/** @brief Data accessed by all threads.
  *
  *  PMWMS = parallel multiway mergesort */
template<typename RandomAccessIterator>
  struct PMWMSSortingData
  {
    typedef std::iterator_traits<RandomAccessIterator> traits_type;
    typedef typename traits_type::value_type value_type;
    typedef typename traits_type::difference_type difference_type;

    /** @brief Number of threads involved. */
    thread_index_t num_threads;

    /** @brief Input begin. */
    RandomAccessIterator source;

    /** @brief Start indices, per thread. */
    difference_type* starts;

    /** @brief Temporary arrays for each thread.
     *
     *  Indirection Allows using the temporary storage in different
     *  ways, without code duplication.
     *  @see _GLIBCXX_MULTIWAY_MERGESORT_COPY_LAST */
    value_type** temporaries;

#if _GLIBCXX_MULTIWAY_MERGESORT_COPY_LAST
    /** @brief Storage in which to sort. */
    RandomAccessIterator* sorting_places;

    /** @brief Storage into which to merge. */
    value_type** merging_places;
#else
    /** @brief Storage in which to sort. */
    value_type** sorting_places;

    /** @brief Storage into which to merge. */
    RandomAccessIterator* merging_places;
#endif
    /** @brief Samples. */
    value_type* samples;

    /** @brief Offsets to add to the found positions. */
    difference_type* offsets;

    /** @brief Pieces of data to merge @c [thread][sequence] */
    std::vector<Piece<difference_type> >* pieces;

    /** @brief Stable sorting desired. */
    bool stable;
};

/**
  *  @brief Select samples from a sequence.
  *  @param sd Pointer to algorithm data. Result will be placed in
  *  @c sd->samples.
  *  @param num_samples Number of samples to select.
  */
template<typename RandomAccessIterator, typename _DifferenceTp>
  void 
  determine_samples(PMWMSSortingData<RandomAccessIterator>* sd,
                    _DifferenceTp& num_samples)
  {
    typedef std::iterator_traits<RandomAccessIterator> traits_type;
    typedef typename traits_type::value_type value_type;
    typedef _DifferenceTp difference_type;

    thread_index_t iam = omp_get_thread_num();

    num_samples = _Settings::get().sort_mwms_oversampling * sd->num_threads - 1;

    difference_type* es = new difference_type[num_samples + 2];

    equally_split(sd->starts[iam + 1] - sd->starts[iam], 
                  num_samples + 1, es);

    for (difference_type i = 0; i < num_samples; ++i)
      ::new(&(sd->samples[iam * num_samples + i]))
	  value_type(sd->source[sd->starts[iam] + es[i + 1]]);

    delete[] es;
  }

/** @brief PMWMS code executed by each thread.
  *  @param sd Pointer to algorithm data.
  *  @param comp Comparator.
  */
template<typename RandomAccessIterator, typename Comparator>
  void 
  parallel_sort_mwms_pu(PMWMSSortingData<RandomAccessIterator>* sd,
                        Comparator& comp)
  {
    typedef std::iterator_traits<RandomAccessIterator> traits_type;
    typedef typename traits_type::value_type value_type;
    typedef typename traits_type::difference_type difference_type;

    thread_index_t iam = omp_get_thread_num();

    // Length of this thread's chunk, before merging.
    difference_type length_local = sd->starts[iam + 1] - sd->starts[iam];

#if _GLIBCXX_MULTIWAY_MERGESORT_COPY_LAST
    typedef RandomAccessIterator SortingPlacesIterator;

    // Sort in input storage.
    sd->sorting_places[iam] = sd->source + sd->starts[iam];
#else
    typedef value_type* SortingPlacesIterator;

    // Sort in temporary storage, leave space for sentinel.
    sd->sorting_places[iam] = sd->temporaries[iam] = 
        static_cast<value_type*>(
        ::operator new(sizeof(value_type) * (length_local + 1)));

    // Copy there.
    std::uninitialized_copy(sd->source + sd->starts[iam],
                            sd->source + sd->starts[iam] + length_local,
                            sd->sorting_places[iam]);
#endif

    // Sort locally.
    if (sd->stable)
      __gnu_sequential::stable_sort(sd->sorting_places[iam],
                                    sd->sorting_places[iam] + length_local,
                                    comp);
    else
      __gnu_sequential::sort(sd->sorting_places[iam],
                             sd->sorting_places[iam] + length_local,
                             comp);

    // Invariant: locally sorted subsequence in sd->sorting_places[iam],
    // sd->sorting_places[iam] + length_local.
    const _Settings& __s = _Settings::get();
    if (__s.sort_splitting == SAMPLING)
      {
        difference_type num_samples;
        determine_samples(sd, num_samples);

#       pragma omp barrier

#       pragma omp single
        __gnu_sequential::sort(sd->samples,
                               sd->samples + (num_samples * sd->num_threads),
                               comp);

#       pragma omp barrier

        for (int s = 0; s < sd->num_threads; ++s)
          {
            // For each sequence.
              if (num_samples * iam > 0)
                sd->pieces[iam][s].begin = 
                    std::lower_bound(sd->sorting_places[s],
                        sd->sorting_places[s]
                            + (sd->starts[s + 1] - sd->starts[s]),
                        sd->samples[num_samples * iam],
                        comp)
                    - sd->sorting_places[s];
            else
              // Absolute beginning.
              sd->pieces[iam][s].begin = 0;

            if ((num_samples * (iam + 1)) < (num_samples * sd->num_threads))
              sd->pieces[iam][s].end =
                  std::lower_bound(sd->sorting_places[s],
                          sd->sorting_places[s]
                              + (sd->starts[s + 1] - sd->starts[s]),
                          sd->samples[num_samples * (iam + 1)],
                          comp)
                  - sd->sorting_places[s];
            else
              // Absolute end.
              sd->pieces[iam][s].end = sd->starts[s + 1] - sd->starts[s];
            }
      }
    else if (__s.sort_splitting == EXACT)
      {
#       pragma omp barrier

        std::vector<std::pair<SortingPlacesIterator, SortingPlacesIterator> >
            seqs(sd->num_threads);
        for (int s = 0; s < sd->num_threads; ++s)
          seqs[s] = std::make_pair(sd->sorting_places[s],
                                   sd->sorting_places[s]
                                       + (sd->starts[s + 1] - sd->starts[s]));

        std::vector<SortingPlacesIterator> offsets(sd->num_threads);

        // if not last thread
        if (iam < sd->num_threads - 1)
          multiseq_partition(seqs.begin(), seqs.end(),
                             sd->starts[iam + 1], offsets.begin(), comp);

        for (int seq = 0; seq < sd->num_threads; ++seq)
          {
            // for each sequence
            if (iam < (sd->num_threads - 1))
              sd->pieces[iam][seq].end = offsets[seq] - seqs[seq].first;
            else
              // very end of this sequence
              sd->pieces[iam][seq].end = (sd->starts[seq + 1]
					  - sd->starts[seq]);
          }

#       pragma omp barrier

        for (int seq = 0; seq < sd->num_threads; ++seq)
          {
            // For each sequence.
            if (iam > 0)
              sd->pieces[iam][seq].begin = sd->pieces[iam - 1][seq].end;
            else
              // Absolute beginning.
              sd->pieces[iam][seq].begin = 0;
          }
      }

    // Offset from target begin, length after merging.
    difference_type offset = 0, length_am = 0;
    for (int s = 0; s < sd->num_threads; ++s)
      {
        length_am += sd->pieces[iam][s].end - sd->pieces[iam][s].begin;
        offset += sd->pieces[iam][s].begin;
      }

#if _GLIBCXX_MULTIWAY_MERGESORT_COPY_LAST
    // Merge to temporary storage, uninitialized creation not possible
    // since there is no multiway_merge calling the placement new
    // instead of the assignment operator.
    // XXX incorrect (de)construction
    sd->merging_places[iam] = sd->temporaries[iam] =
        static_cast<value_type*>(::operator new(sizeof(value_type)
						* length_am));
#else
    // Merge directly to target.
    sd->merging_places[iam] = sd->source + offset;
#endif
    std::vector<std::pair<SortingPlacesIterator, SortingPlacesIterator> >
        seqs(sd->num_threads);

    for (int s = 0; s < sd->num_threads; ++s)
      {
        seqs[s] =
	  std::make_pair(sd->sorting_places[s] + sd->pieces[iam][s].begin,
			 sd->sorting_places[s] + sd->pieces[iam][s].end);
      }

    multiway_merge(seqs.begin(), seqs.end(), sd->merging_places[iam], comp,
                   length_am, sd->stable, false, sequential_tag());

#   pragma omp barrier

#if _GLIBCXX_MULTIWAY_MERGESORT_COPY_LAST
    // Write back.
    std::copy(sd->merging_places[iam],
              sd->merging_places[iam] + length_am,
              sd->source + offset);
#endif

    ::operator delete(sd->temporaries[iam]);
  }

/** @brief PMWMS main call.
  *  @param begin Begin iterator of sequence.
  *  @param end End iterator of sequence.
  *  @param comp Comparator.
  *  @param n Length of sequence.
  *  @param num_threads Number of threads to use.
  *  @param stable Stable sorting.
  */
template<typename RandomAccessIterator, typename Comparator>
  void
  parallel_sort_mwms(RandomAccessIterator begin, RandomAccessIterator end,
                     Comparator comp, typename
		     std::iterator_traits<RandomAccessIterator>::
		     difference_type n, int num_threads, bool stable)
  {
    _GLIBCXX_CALL(n)

    typedef std::iterator_traits<RandomAccessIterator> traits_type;
    typedef typename traits_type::value_type value_type;
    typedef typename traits_type::difference_type difference_type;

    if (n <= 1)
      return;

    // at least one element per thread
    if (num_threads > n)
      num_threads = static_cast<thread_index_t>(n);

    // shared variables
    PMWMSSortingData<RandomAccessIterator> sd;
    difference_type* starts;
    const _Settings& __s = _Settings::get();

#   pragma omp parallel num_threads(num_threads)
      {
        num_threads = omp_get_num_threads();  //no more threads than requested

#       pragma omp single
          {
            sd.num_threads = num_threads;
            sd.source = begin;
            sd.temporaries = new value_type*[num_threads];

#if _GLIBCXX_MULTIWAY_MERGESORT_COPY_LAST
            sd.sorting_places = new RandomAccessIterator[num_threads];
            sd.merging_places = new value_type*[num_threads];
#else
            sd.sorting_places = new value_type*[num_threads];
            sd.merging_places = new RandomAccessIterator[num_threads];
#endif

            if (__s.sort_splitting == SAMPLING)
              {
                unsigned int size = 
                    (__s.sort_mwms_oversampling * num_threads - 1)
                        * num_threads;
                sd.samples = static_cast<value_type*>(
		  ::operator new(size * sizeof(value_type)));
              }
            else
              sd.samples = NULL;

            sd.offsets = new difference_type[num_threads - 1];
            sd.pieces = new std::vector<Piece<difference_type> >[num_threads];
            for (int s = 0; s < num_threads; ++s)
              sd.pieces[s].resize(num_threads);
            starts = sd.starts = new difference_type[num_threads + 1];
            sd.stable = stable;

            difference_type chunk_length = n / num_threads;
            difference_type split = n % num_threads;
            difference_type pos = 0;
            for (int i = 0; i < num_threads; ++i)
              {
                starts[i] = pos;
                pos += (i < split) ? (chunk_length + 1) : chunk_length;
              }
            starts[num_threads] = pos;
          }

        // Now sort in parallel.
        parallel_sort_mwms_pu(&sd, comp);
      } //parallel

    delete[] starts;
    delete[] sd.temporaries;
    delete[] sd.sorting_places;
    delete[] sd.merging_places;

    if (__s.sort_splitting == SAMPLING)
      ::operator delete(sd.samples);

    delete[] sd.offsets;
    delete[] sd.pieces;
  }
} //namespace __gnu_parallel

#endif