gcc/libstdc++-v3/include/parallel/quicksort.h

// -*- C++ -*-

// Copyright (C) 2007 Free Software Foundation, Inc.
//
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/** @file parallel/quicksort.h
 *  @brief Implementation of a unbalanced parallel quicksort (in-place).
 *  This file is a GNU parallel extension to the Standard C++ Library.
 */

// Written by Johannes Singler.

#ifndef _GLIBCXX_PARALLEL_QUICKSORT_H
#define _GLIBCXX_PARALLEL_QUICKSORT_H 1

#include <parallel/parallel.h>
#include <parallel/partition.h>

namespace __gnu_parallel
{
  /** @brief Unbalanced quicksort divide step.
   *  @param begin Begin iterator of subsequence.
   *  @param end End iterator of subsequence.
   *  @param comp Comparator.
   *  @param pivot_rank Desired rank of the pivot.
   *  @param num_samples Choose pivot from that many samples.
   *  @param num_threads Number of threads that are allowed to work on
   *  this part.
   */
  template<typename RandomAccessIterator, typename Comparator>
  inline
  typename std::iterator_traits<RandomAccessIterator>::difference_type
  parallel_sort_qs_divide(
      RandomAccessIterator begin,
      RandomAccessIterator end,
      Comparator comp,
      typename std::iterator_traits<RandomAccessIterator>::difference_type
          pivot_rank,
      typename std::iterator_traits<RandomAccessIterator>::difference_type
          num_samples,
      thread_index_t 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;
    num_samples = std::min(num_samples, n);

    // Allocate uninitialized, to avoid default constructor.
    value_type* samples = static_cast<value_type*>(
      ::operator new(num_samples * sizeof(value_type)));

    for (difference_type s = 0; s < num_samples; ++s)
      {
        const unsigned long long index = static_cast<unsigned long long>(s)
                        * n / num_samples;
        new(&(samples[s])) value_type(begin[index]);
      }

    __gnu_sequential::sort(samples, samples + num_samples, comp);

    value_type& pivot = samples[pivot_rank * num_samples / n];

    __gnu_parallel::binder2nd<Comparator, value_type, value_type, bool>
        pred(comp, pivot);
    difference_type split = parallel_partition(begin, end, pred, num_threads);

    delete[] samples;

    return split;
  }

  /** @brief Unbalanced quicksort conquer step.
   *  @param begin Begin iterator of subsequence.
   *  @param end End iterator of subsequence.
   *  @param comp Comparator.
   *  @param num_threads Number of threads that are allowed to work on
   *  this part.
   */
  template<typename RandomAccessIterator, typename Comparator>
  inline void
  parallel_sort_qs_conquer(RandomAccessIterator begin,
                           RandomAccessIterator end,
                           Comparator comp,
                           thread_index_t 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;

    if (num_threads <= 1)
      {
        __gnu_sequential::sort(begin, end, comp);
        return;
      }

    difference_type n = end - begin, pivot_rank;

    if (n <= 1)
      return;

    thread_index_t num_threads_left;

    if ((num_threads % 2) == 1)
      num_threads_left = num_threads / 2 + 1;
    else
      num_threads_left = num_threads / 2;

    pivot_rank = n * num_threads_left / num_threads;

    difference_type split = parallel_sort_qs_divide(
        begin, end, comp, pivot_rank,
        Settings::sort_qs_num_samples_preset, num_threads);

#pragma omp parallel sections
    {
#pragma omp section
      parallel_sort_qs_conquer(begin, begin + split,
                               comp, num_threads_left);
#pragma omp section
      parallel_sort_qs_conquer(begin + split, end,
                               comp, num_threads - num_threads_left);
    }
  }



  /** @brief Unbalanced quicksort main call.
   *  @param begin Begin iterator of input sequence.
   *  @param end End iterator input sequence, ignored.
   *  @param comp Comparator.
   *  @param n Length of input sequence.
   *  @param num_threads Number of threads that are allowed to work on
   *  this part.
   */
  template<typename RandomAccessIterator, typename Comparator>
  inline void
  parallel_sort_qs(
      RandomAccessIterator begin,
      RandomAccessIterator end,
      Comparator comp,
      typename std::iterator_traits<RandomAccessIterator>::difference_type n,
      int num_threads)
  {
    _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 == 0)
      return;

    // At least one element per processor.
    if (num_threads > n)
      num_threads = static_cast<thread_index_t>(n);

    Settings::sort_qs_num_samples_preset = 100;

    // Hard to avoid.
    omp_set_num_threads(num_threads);

    parallel_sort_qs_conquer(begin, begin + n, comp, num_threads);
  }

} //namespace __gnu_parallel

#endif