Merged with mainline at revision 128810.

[official-gcc.git] / libstdc++-v3 / include / parallel / partition.h

// -*- C++ -*-

// Copyright (C) 2007 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/partition.h
 *  @brief Parallel implementation of std::partition(),
 *  std::nth_element(), and std::partial_sort().
 *  This file is a GNU parallel extension to the Standard C++ Library.
 */

// Written by Johannes Singler and Felix Putze.

#ifndef _GLIBCXX_PARALLEL_PARTITION_H
#define _GLIBCXX_PARALLEL_PARTITION_H 1

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

/** @brief Decide whether to declare certain variable volatile in this file. */
#define _GLIBCXX_VOLATILE volatile

namespace __gnu_parallel
{
  /** @brief Parallel implementation of std::partition.
   *  @param begin Begin iterator of input sequence to split.
   *  @param end End iterator of input sequence to split.
   *  @param pred Partition predicate, possibly including some kind of pivot.
   *  @param max_num_threads Maximum number of threads to use for this task.
   *  @return Number of elements not fulfilling the predicate. */
  template<typename RandomAccessIterator, typename Predicate>
  inline typename std::iterator_traits<RandomAccessIterator>::difference_type
  parallel_partition(RandomAccessIterator begin, RandomAccessIterator end,
                     Predicate pred, thread_index_t max_num_threads)
  {
    typedef std::iterator_traits<RandomAccessIterator> traits_type;
    typedef typename traits_type::value_type value_type;
    typedef typename traits_type::difference_type difference_type;

    difference_type n = end - begin;

    _GLIBCXX_CALL(n)

    // Shared.
    _GLIBCXX_VOLATILE difference_type left = 0, right = n - 1;
    _GLIBCXX_VOLATILE difference_type leftover_left, leftover_right, leftnew, rightnew;
    bool* reserved_left, * reserved_right;

    reserved_left = new bool[max_num_threads];
    reserved_right = new bool[max_num_threads];

    difference_type chunk_size;
    if (Settings::partition_chunk_share > 0.0)
      chunk_size = std::max((difference_type)Settings::partition_chunk_size, (difference_type)((double)n * Settings::partition_chunk_share / (double)max_num_threads));
    else
      chunk_size = Settings::partition_chunk_size;

    // At least good for two processors.
    while (right - left + 1 >= 2 * max_num_threads * chunk_size)
      {
        difference_type num_chunks = (right - left + 1) / chunk_size;
        thread_index_t num_threads = (int)std::min((difference_type)max_num_threads, num_chunks / 2);

        for (int r = 0; r < num_threads; r++)
          {
            reserved_left[r] = false;
            reserved_right[r] = false;
          }
        leftover_left = 0;
        leftover_right = 0;

#pragma omp parallel num_threads(num_threads)
        {
          // Private.
          difference_type thread_left, thread_left_border, thread_right, thread_right_border;
          thread_left = left + 1;

          // Just to satisfy the condition below.
          thread_left_border = thread_left - 1;
          thread_right = n - 1;
          thread_right_border = thread_right + 1;

          bool iam_finished = false;
          while (!iam_finished)
            {
              if (thread_left > thread_left_border)
#pragma omp critical
                {
                  if (left + (chunk_size - 1) > right)
                    iam_finished = true;
                  else
                    {
                      thread_left = left;
                      thread_left_border = left + (chunk_size - 1);
                      left += chunk_size;
                    }
                }

              if (thread_right < thread_right_border)
#pragma omp critical
                {
                  if (left > right - (chunk_size - 1))
                    iam_finished = true;
                  else
                    {
                      thread_right = right;
                      thread_right_border = right - (chunk_size - 1);
                      right -= chunk_size;
                    }
                }

              if (iam_finished)
                break;

              // Swap as usual.
              while (thread_left < thread_right)
                {
                  while (pred(begin[thread_left]) && thread_left <= thread_left_border)
                    thread_left++;
                  while (!pred(begin[thread_right]) && thread_right >= thread_right_border)
                    thread_right--;

                  if (thread_left > thread_left_border || thread_right < thread_right_border)
                    // Fetch new chunk(s).
                    break;

                  std::swap(begin[thread_left], begin[thread_right]);
                  thread_left++;
                  thread_right--;
                }
            }

          // Now swap the leftover chunks to the right places.
          if (thread_left <= thread_left_border)
#pragma omp atomic
            leftover_left++;
          if (thread_right >= thread_right_border)
#pragma omp atomic
            leftover_right++;

#pragma omp barrier

#pragma omp single
          {
            leftnew = left - leftover_left * chunk_size;
            rightnew = right + leftover_right * chunk_size;
          }

#pragma omp barrier

          // <=> thread_left_border + (chunk_size - 1) >= leftnew
          if (thread_left <= thread_left_border
              && thread_left_border >= leftnew)
            {
              // Chunk already in place, reserve spot.
              reserved_left[(left - (thread_left_border + 1)) / chunk_size] = true;
            }

          // <=> thread_right_border - (chunk_size - 1) <= rightnew
          if (thread_right >= thread_right_border
              && thread_right_border <= rightnew)
            {
              // Chunk already in place, reserve spot.
              reserved_right[((thread_right_border - 1) - right) / chunk_size] = true;
            }

#pragma omp barrier

          if (thread_left <= thread_left_border && thread_left_border < leftnew)
            {
              // Find spot and swap.
              difference_type swapstart = -1;
#pragma omp critical
              {
                for (int r = 0; r < leftover_left; r++)
                  if (!reserved_left[r])
                    {
                      reserved_left[r] = true;
                      swapstart = left - (r + 1) * chunk_size;
                      break;
                    }
              }

#if _GLIBCXX_ASSERTIONS
              _GLIBCXX_PARALLEL_ASSERT(swapstart != -1);
#endif

              std::swap_ranges(begin + thread_left_border - (chunk_size - 1), begin + thread_left_border + 1, begin + swapstart);
            }

          if (thread_right >= thread_right_border
              && thread_right_border > rightnew)
            {
              // Find spot and swap
              difference_type swapstart = -1;
#pragma omp critical
              {
                for (int r = 0; r < leftover_right; r++)
                  if (!reserved_right[r])
                    {
                      reserved_right[r] = true;
                      swapstart = right + r * chunk_size + 1;
                      break;
                    }
              }

#if _GLIBCXX_ASSERTIONS
              _GLIBCXX_PARALLEL_ASSERT(swapstart != -1);
#endif

              std::swap_ranges(begin + thread_right_border, begin + thread_right_border + chunk_size, begin + swapstart);
            }
#if _GLIBCXX_ASSERTIONS
#pragma omp barrier

#pragma omp single
          {
            for (int r = 0; r < leftover_left; r++)
              _GLIBCXX_PARALLEL_ASSERT(reserved_left[r]);
            for (int r = 0; r < leftover_right; r++)
              _GLIBCXX_PARALLEL_ASSERT(reserved_right[r]);
          }

#pragma omp barrier
#endif

#pragma omp barrier
          left = leftnew;
          right = rightnew;
        }
      } // end "recursion"

    difference_type final_left = left, final_right = right;

    while (final_left < final_right)
      {
        // Go right until key is geq than pivot.
        while (pred(begin[final_left]) && final_left < final_right)
          final_left++;

        // Go left until key is less than pivot.
        while (!pred(begin[final_right]) && final_left < final_right)
          final_right--;

        if (final_left == final_right)
          break;
        std::swap(begin[final_left], begin[final_right]);
        final_left++;
        final_right--;
      }

    // All elements on the left side are < piv, all elements on the
    // right are >= piv
    delete[] reserved_left;
    delete[] reserved_right;

    // Element "between" final_left and final_right might not have
    // been regarded yet
    if (final_left < n && !pred(begin[final_left]))
      // Really swapped.
      return final_left;
    else
      return final_left + 1;
  }

  /** 
   *  @brief Parallel implementation of std::nth_element().
   *  @param begin Begin iterator of input sequence.
   *  @param nth Iterator of element that must be in position afterwards.
   *  @param end End iterator of input sequence.
   *  @param comp Comparator. 
   */
  template<typename RandomAccessIterator, typename Comparator>
  void 
  parallel_nth_element(RandomAccessIterator begin, RandomAccessIterator nth, RandomAccessIterator end, 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;

    _GLIBCXX_CALL(end - begin)

    RandomAccessIterator split;
    value_type pivot;
    random_number rng;

    difference_type minimum_length = std::max<difference_type>(2, Settings::partition_minimal_n);

    // Break if input range to small.
    while (static_cast<sequence_index_t>(end - begin) >= minimum_length)
      {
        difference_type n = end - begin;

        RandomAccessIterator pivot_pos = begin +  rng(n);

        // Swap pivot_pos value to end.
        if (pivot_pos != (end - 1))
          std::swap(*pivot_pos, *(end - 1));
        pivot_pos = end - 1;

        // XXX Comparator must have first_value_type, second_value_type, result_type
        // Comparator == __gnu_parallel::lexicographic<S, int, __gnu_parallel::less<S, S> > 
        // pivot_pos == std::pair<S, int>*
        // XXX binder2nd only for RandomAccessIterators??
        __gnu_parallel::binder2nd<Comparator, value_type, value_type, bool> pred(comp, *pivot_pos);

        // Divide, leave pivot unchanged in last place.
        RandomAccessIterator split_pos1, split_pos2;
        split_pos1 = begin + parallel_partition(begin, end - 1, pred, get_max_threads());

        // Left side: < pivot_pos; right side: >= pivot_pos

        // Swap pivot back to middle.
        if (split_pos1 != pivot_pos)
          std::swap(*split_pos1, *pivot_pos);
        pivot_pos = split_pos1;

        // In case all elements are equal, split_pos1 == 0
        if ((split_pos1 + 1 - begin) < (n >> 7) || (end - split_pos1) < (n >> 7))
          {
            // Very unequal split, one part smaller than one 128th
            // elements not stricly larger than the pivot.
            __gnu_parallel::unary_negate<__gnu_parallel::binder1st<Comparator, value_type, value_type, bool>, value_type> pred(__gnu_parallel::binder1st<Comparator, value_type, value_type, bool>(comp, *pivot_pos));

            // Find other end of pivot-equal range.
            split_pos2 = __gnu_sequential::partition(split_pos1 + 1, end, pred);
          }
        else
          // Only skip the pivot.
          split_pos2 = split_pos1 + 1;

        // Compare iterators.
        if (split_pos2 <= nth)
          begin = split_pos2;
        else if (nth < split_pos1)
          end = split_pos1;
        else
          break;
      }

    // Only at most Settings::partition_minimal_n elements left.
    __gnu_sequential::sort(begin, end, comp);
  }

  /** @brief Parallel implementation of std::partial_sort().
   *  @param begin Begin iterator of input sequence.
   *  @param middle Sort until this position.
   *  @param end End iterator of input sequence.
   *  @param comp Comparator. */
  template<typename RandomAccessIterator, typename Comparator>
  void
  parallel_partial_sort(RandomAccessIterator begin, RandomAccessIterator middle, RandomAccessIterator end, Comparator comp)
  {
    parallel_nth_element(begin, middle, end, comp);
    std::sort(begin, middle, comp);
  }

}       //namespace __gnu_parallel

#undef _GLIBCXX_VOLATILE

#endif