partition.h

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// -*- C++ -*-

// Copyright (C) 2007-2019 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 3, 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.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @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 <parallel/random_number.h>
#include <bits/stl_algo.h>
#include <parallel/parallel.h>

/** @brief Decide whether to declare certain variables volatile. */
#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 __num_threads Maximum number of threads to use for this task.
    *  @return Number of elements not fulfilling the predicate. */
  template<typename _RAIter, typename _Predicate>
    typename std::iterator_traits<_RAIter>::difference_type
    __parallel_partition(_RAIter __begin, _RAIter __end,
                         _Predicate __pred, _ThreadIndex __num_threads)
    {
      typedef std::iterator_traits<_RAIter> _TraitsType;
      typedef typename _TraitsType::value_type _ValueType;
      typedef typename _TraitsType::difference_type _DifferenceType;

      _DifferenceType __n = __end - __begin;

      _GLIBCXX_CALL(__n)

      const _Settings& __s = _Settings::get();

      // shared
      _GLIBCXX_VOLATILE _DifferenceType __left = 0, __right = __n - 1,
                                        __dist = __n,
                                        __leftover_left, __leftover_right,
                                        __leftnew, __rightnew;

      // just 0 or 1, but int to allow atomic operations
      int* __reserved_left = 0, * __reserved_right = 0;

      _DifferenceType __chunk_size = __s.partition_chunk_size;

      //at least two chunks per thread
      if (__dist >= 2 * __num_threads * __chunk_size)
#       pragma omp parallel num_threads(__num_threads)
        {
#         pragma omp single
          {
            __num_threads = omp_get_num_threads();
            __reserved_left = new int[__num_threads];
            __reserved_right = new int[__num_threads];

            if (__s.partition_chunk_share > 0.0)
              __chunk_size = std::max<_DifferenceType>
                (__s.partition_chunk_size, (double)__n 
                 * __s.partition_chunk_share / (double)__num_threads);
            else
              __chunk_size = __s.partition_chunk_size;
          }

          while (__dist >= 2 * __num_threads * __chunk_size)
            {
#             pragma omp single
              {
                _DifferenceType __num_chunks = __dist / __chunk_size;

                for (_ThreadIndex __r = 0; __r < __num_threads; ++__r)
                  {
                    __reserved_left [__r] = 0; // false
                    __reserved_right[__r] = 0; // false
                  }
                __leftover_left = 0;
                __leftover_right = 0;
              } //implicit barrier

              // Private.
              _DifferenceType __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;
             // Just to satisfy the condition below.
              __thread_right_border = __thread_right + 1;

              bool __iam_finished = false;
              while (!__iam_finished)
                {
                  if (__thread_left > __thread_left_border)
                    {
                      _DifferenceType __former_dist =
                              __fetch_and_add(&__dist, -__chunk_size);
                      if (__former_dist < __chunk_size)
                        {
                          __fetch_and_add(&__dist, __chunk_size);
                          __iam_finished = true;
                          break;
                        }
                      else
                        {
                          __thread_left =
                                  __fetch_and_add(&__left, __chunk_size);
                          __thread_left_border =
                                  __thread_left + (__chunk_size - 1);
                        }
                    }

                  if (__thread_right < __thread_right_border)
                    {
                      _DifferenceType __former_dist =
                              __fetch_and_add(&__dist, -__chunk_size);
                      if (__former_dist < __chunk_size)
                        {
                          __fetch_and_add(&__dist, __chunk_size);
                          __iam_finished = true;
                          break;
                        }
                      else
                        {
                          __thread_right =
                                  __fetch_and_add(&__right, -__chunk_size);
                          __thread_right_border =
                                  __thread_right - (__chunk_size - 1);
                        }
                    }

                  // 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::iter_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

              _DifferenceType
                    __leftold = __left,
                    __leftnew = __left - __leftover_left * __chunk_size,
                    __rightold = __right,
                    __rightnew = __right + __leftover_right * __chunk_size;

              // <=> __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] = 1;
                }

              // <=> __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] = 1;
                }

#             pragma omp barrier

              if (__thread_left <= __thread_left_border
                  && __thread_left_border < __leftnew)
                {
                  // Find spot and swap.
                  _DifferenceType __swapstart = -1;
                  for (int __r = 0; __r < __leftover_left; ++__r)
                    if (__reserved_left[__r] == 0
                        && __compare_and_swap(&(__reserved_left[__r]), 0, 1))
                      {
                        __swapstart = __leftold - (__r + 1) * __chunk_size;
                        break;
                      }

#if _GLIBCXX_PARALLEL_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
                  _DifferenceType __swapstart = -1;
                  for (int __r = 0; __r < __leftover_right; ++__r)
                    if (__reserved_right[__r] == 0
                        && __compare_and_swap(&(__reserved_right[__r]), 0, 1))
                      {
                        __swapstart = __rightold + __r * __chunk_size + 1;
                        break;
                      }

#if _GLIBCXX_PARALLEL_ASSERTIONS
                  _GLIBCXX_PARALLEL_ASSERT(__swapstart != -1);
#endif

                  std::swap_ranges(__begin + __thread_right_border,
                                   __begin + __thread_right_border
                                   + __chunk_size, __begin + __swapstart);
              }
#if _GLIBCXX_PARALLEL_ASSERTIONS
#             pragma omp barrier

#             pragma omp single
              {
                for (_DifferenceType __r = 0; __r < __leftover_left; ++__r)
                  _GLIBCXX_PARALLEL_ASSERT(__reserved_left[__r] == 1);
                for (_DifferenceType __r = 0; __r < __leftover_right; ++__r)
                  _GLIBCXX_PARALLEL_ASSERT(__reserved_right[__r] == 1);
              }
#endif

              __left = __leftnew;
              __right = __rightnew;
              __dist = __right - __left + 1;
            }

#           pragma omp flush(__left, __right)
        } // end "recursion" //parallel

        _DifferenceType __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::iter_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 _RAIter, typename _Compare>
    void 
    __parallel_nth_element(_RAIter __begin, _RAIter __nth, 
                           _RAIter __end, _Compare __comp)
    {
      typedef std::iterator_traits<_RAIter> _TraitsType;
      typedef typename _TraitsType::value_type _ValueType;
      typedef typename _TraitsType::difference_type _DifferenceType;

      _GLIBCXX_CALL(__end - __begin)

      _RAIter __split;
      _RandomNumber __rng;

      const _Settings& __s = _Settings::get();
      _DifferenceType __minimum_length = std::max<_DifferenceType>(2,
        std::max(__s.nth_element_minimal_n, __s.partition_minimal_n));

      // Break if input range to small.
      while (static_cast<_SequenceIndex>(__end - __begin) >= __minimum_length)
        {
          _DifferenceType __n = __end - __begin;

          _RAIter __pivot_pos = __begin + __rng(__n);

          // Swap __pivot_pos value to end.
          if (__pivot_pos != (__end - 1))
            std::iter_swap(__pivot_pos, __end - 1);
          __pivot_pos = __end - 1;

          // _Compare must have first_value_type, second_value_type,
          // result_type
          // _Compare ==
          // __gnu_parallel::_Lexicographic<S, int,
          //                                __gnu_parallel::_Less<S, S> >
          // __pivot_pos == std::pair<S, int>*
          __gnu_parallel::__binder2nd<_Compare, _ValueType, _ValueType, bool>
            __pred(__comp, *__pivot_pos);

          // Divide, leave pivot unchanged in last place.
          _RAIter __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::iter_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 strictly larger than the pivot.
              __gnu_parallel::__unary_negate<__gnu_parallel::
                __binder1st<_Compare, _ValueType,
                            _ValueType, bool>, _ValueType>
                __pred(__gnu_parallel::__binder1st<_Compare, _ValueType,
                       _ValueType, 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::nth_element(__begin, __nth, __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 _RAIter, typename _Compare>
    void
    __parallel_partial_sort(_RAIter __begin,
                            _RAIter __middle,
                            _RAIter __end, _Compare __comp)
    {
      __parallel_nth_element(__begin, __middle, __end, __comp);
      std::sort(__begin, __middle, __comp);
    }

} //namespace __gnu_parallel

#undef _GLIBCXX_VOLATILE

#endif /* _GLIBCXX_PARALLEL_PARTITION_H */