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

// -*- C++ -*-

// Copyright (C) 2007 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
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// of the GNU General Public License as published by the Free Software
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/** @file parallel/merge.h
 *  @brief Parallel implementation of std::merge().
 *  This file is a GNU parallel extension to the Standard C++ Library.
 */

// Written by Johannes Singler.

#ifndef _GLIBCXX_PARALLEL_MERGE_H
#define _GLIBCXX_PARALLEL_MERGE_H 1

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

namespace __gnu_parallel
{
  /** @brief Merge routine being able to merge only the @c max_length
   * smallest elements.
   *
   * The @c begin iterators are advanced accordingly, they might not
   * reach @c end, in contrast to the usual variant.
   * @param begin1 Begin iterator of first sequence.
   * @param end1 End iterator of first sequence.
   * @param begin2 Begin iterator of second sequence.
   * @param end2 End iterator of second sequence.
   * @param target Target begin iterator.
   * @param max_length Maximum number of elements to merge.
   * @param comp Comparator.
   * @return Output end iterator. */
  template<typename RandomAccessIterator1, typename RandomAccessIterator2, typename OutputIterator, typename _DifferenceTp, typename Comparator>
  OutputIterator
  merge_advance_usual(RandomAccessIterator1& begin1, RandomAccessIterator1 end1, RandomAccessIterator2& begin2, RandomAccessIterator2 end2, OutputIterator target, _DifferenceTp max_length, Comparator comp)
  {
    typedef _DifferenceTp difference_type;
    while (begin1 != end1 && begin2 != end2 && max_length > 0)
      {
	// array1[i1] < array0[i0]
	if (comp(*begin2, *begin1))
	  *target++ = *begin2++;
	else
	  *target++ = *begin1++;
	max_length--;
      }

    if (begin1 != end1)
      {
	target = std::copy(begin1, begin1 + max_length, target);
	begin1 += max_length;
      }
    else
      {
	target = std::copy(begin2, begin2 + max_length, target);
	begin2 += max_length;
      }
    return target;
  }

  /** @brief Merge routine being able to merge only the @c max_length
   * smallest elements.
   *
   * The @c begin iterators are advanced accordingly, they might not
   * reach @c end, in contrast to the usual variant.
   * Specially designed code should allow the compiler to generate
   * conditional moves instead of branches.
   * @param begin1 Begin iterator of first sequence.
   * @param end1 End iterator of first sequence.
   * @param begin2 Begin iterator of second sequence.
   * @param end2 End iterator of second sequence.
   * @param target Target begin iterator.
   * @param max_length Maximum number of elements to merge.
   * @param comp Comparator.
   * @return Output end iterator. */
  template<typename RandomAccessIterator1, typename RandomAccessIterator2, typename OutputIterator, typename _DifferenceTp, typename Comparator>
  OutputIterator
  merge_advance_movc(RandomAccessIterator1& begin1, RandomAccessIterator1 end1, RandomAccessIterator2& begin2, RandomAccessIterator2 end2, OutputIterator target, _DifferenceTp max_length, Comparator comp)
  {
    typedef _DifferenceTp difference_type;
    typedef typename std::iterator_traits<RandomAccessIterator1>::value_type value_type1;
    typedef typename std::iterator_traits<RandomAccessIterator2>::value_type value_type2;

#if _GLIBCXX_ASSERTIONS
    _GLIBCXX_PARALLEL_ASSERT(max_length >= 0);
#endif

    while (begin1 != end1 && begin2 != end2 && max_length > 0)
      {
	value_type1 element1;
	value_type2 element2;
	RandomAccessIterator1 next1;
	RandomAccessIterator2 next2;

	next1 = begin1 + 1;
	next2 = begin2 + 1;
	element1 = *begin1;
	element2 = *begin2;

	if (comp(element2, element1))
	  {
	    element1 = element2;
	    begin2 = next2;
	  }
	else
	  {
	    begin1 = next1;
	  }

	*target = element1;

	target++;
	max_length--;
      }
    if (begin1 != end1)
      {
	target = std::copy(begin1, begin1 + max_length, target);
	begin1 += max_length;
      }
    else
      {
	target = std::copy(begin2, begin2 + max_length, target);
	begin2 += max_length;
      }
    return target;
  }

  /** @brief Merge routine being able to merge only the @c max_length
   * smallest elements.
   *
   *  The @c begin iterators are advanced accordingly, they might not
   *  reach @c end, in contrast to the usual variant.
   *  Static switch on whether to use the conditional-move variant.
   *  @param begin1 Begin iterator of first sequence.
   *  @param end1 End iterator of first sequence.
   *  @param begin2 Begin iterator of second sequence.
   *  @param end2 End iterator of second sequence.
   *  @param target Target begin iterator.
   *  @param max_length Maximum number of elements to merge.
   *  @param comp Comparator.
   *  @return Output end iterator. */
  template<typename RandomAccessIterator1, typename RandomAccessIterator2, typename OutputIterator, typename _DifferenceTp, typename Comparator>
  inline OutputIterator
  merge_advance(RandomAccessIterator1& begin1, RandomAccessIterator1 end1, RandomAccessIterator2& begin2, RandomAccessIterator2 end2, OutputIterator target, _DifferenceTp max_length, Comparator comp)
  {
    _GLIBCXX_CALL(max_length)

      return merge_advance_movc(begin1, end1, begin2, end2, target, max_length, comp);
  }

  /** @brief Merge routine fallback to sequential in case the
      iterators of the two input sequences are of different type.
      *  @param begin1 Begin iterator of first sequence.
      *  @param end1 End iterator of first sequence.
      *  @param begin2 Begin iterator of second sequence.
      *  @param end2 End iterator of second sequence.
      *  @param target Target begin iterator.
      *  @param max_length Maximum number of elements to merge.
      *  @param comp Comparator.
      *  @return Output end iterator. */
  template<typename RandomAccessIterator1, typename RandomAccessIterator2, typename RandomAccessIterator3, typename Comparator>
  inline RandomAccessIterator3
  parallel_merge_advance(RandomAccessIterator1& begin1, RandomAccessIterator1 end1,
			 RandomAccessIterator2& begin2, RandomAccessIterator2 end2,	//different iterators, parallel implementation not available
			 RandomAccessIterator3 target,
			 typename std::iterator_traits<RandomAccessIterator1>::difference_type max_length, Comparator comp)
  {
    return merge_advance(begin1, end1, begin2, end2, target, max_length, comp);
  }

  /** @brief Parallel merge routine being able to merge only the @c
   * max_length smallest elements.
   *
   *  The @c begin iterators are advanced accordingly, they might not
   *  reach @c end, in contrast to the usual variant.
   *  The functionality is projected onto parallel_multiway_merge.
   *  @param begin1 Begin iterator of first sequence.
   *  @param end1 End iterator of first sequence.
   *  @param begin2 Begin iterator of second sequence.
   *  @param end2 End iterator of second sequence.
   *  @param target Target begin iterator.
   *  @param max_length Maximum number of elements to merge.
   *  @param comp Comparator.
   *  @return Output end iterator.
   */
  template<typename RandomAccessIterator1, typename RandomAccessIterator3, typename Comparator>
  inline RandomAccessIterator3
  parallel_merge_advance(RandomAccessIterator1& begin1, RandomAccessIterator1 end1, RandomAccessIterator1& begin2, RandomAccessIterator1 end2, RandomAccessIterator3 target, typename std::iterator_traits<RandomAccessIterator1>::difference_type max_length, Comparator comp)
  {
    typedef typename std::iterator_traits<RandomAccessIterator1>::value_type
      value_type;
    typedef typename std::iterator_traits<RandomAccessIterator1>::difference_type
      difference_type1 /* == difference_type2 */;
    typedef typename std::iterator_traits<RandomAccessIterator3>::difference_type
      difference_type3;

    std::pair<RandomAccessIterator1, RandomAccessIterator1> seqs[2] = { std::make_pair(begin1, end1), std::make_pair(begin2, end2) };
    RandomAccessIterator3 target_end = parallel_multiway_merge(seqs, seqs + 2, target, comp, max_length, true, false);

    return target_end;
  }
}	//namespace __gnu_parallel

#endif