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// -*- 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/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>
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);
::operator 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>
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::get().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>
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);
// Hard to avoid.
omp_set_num_threads(num_threads);
parallel_sort_qs_conquer(begin, begin + n, comp, num_threads);
}
} //namespace __gnu_parallel
#endif
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