1  // -*- C++ -*-
       2  
       3  // Copyright (C) 2007-2023 Free Software Foundation, Inc.
       4  //
       5  // This file is part of the GNU ISO C++ Library.  This library is free
       6  // software; you can redistribute it and/or modify it under the terms
       7  // of the GNU General Public License as published by the Free Software
       8  // Foundation; either version 3, or (at your option) any later
       9  // version.
      10  
      11  // This library is distributed in the hope that it will be useful, but
      12  // WITHOUT ANY WARRANTY; without even the implied warranty of
      13  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
      14  // General Public License for more details.
      15  
      16  // Under Section 7 of GPL version 3, you are granted additional
      17  // permissions described in the GCC Runtime Library Exception, version
      18  // 3.1, as published by the Free Software Foundation.
      19  
      20  // You should have received a copy of the GNU General Public License and
      21  // a copy of the GCC Runtime Library Exception along with this program;
      22  // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
      23  // <http://www.gnu.org/licenses/>.
      24  
      25  /** @file parallel/equally_split.h
      26   *  This file is a GNU parallel extension to the Standard C++ Library.
      27   */
      28  
      29  // Written by Johannes Singler.
      30  
      31  #ifndef _GLIBCXX_PARALLEL_EQUALLY_SPLIT_H
      32  #define _GLIBCXX_PARALLEL_EQUALLY_SPLIT_H 1
      33  
      34  namespace __gnu_parallel
      35  {
      36    /** @brief function to split a sequence into parts of almost equal size.
      37     *
      38     *  The resulting sequence __s of length __num_threads+1 contains the
      39     *  splitting positions when splitting the range [0,__n) into parts of
      40     *  almost equal size (plus minus 1).  The first entry is 0, the last
      41     *  one n. There may result empty parts.
      42     *  @param __n Number of elements
      43     *  @param __num_threads Number of parts
      44     *  @param __s Splitters
      45     *  @returns End of __splitter sequence, i.e. @c __s+__num_threads+1 */
      46    template<typename _DifferenceType, typename _OutputIterator>
      47      _OutputIterator
      48      __equally_split(_DifferenceType __n, _ThreadIndex __num_threads,
      49  		    _OutputIterator __s)
      50      {
      51        _DifferenceType __chunk_length = __n / __num_threads;
      52        _DifferenceType __num_longer_chunks = __n % __num_threads;
      53        _DifferenceType __pos = 0;
      54        for (_ThreadIndex __i = 0; __i < __num_threads; ++__i)
      55  	{
      56  	  *__s++ = __pos;
      57  	  __pos += ((__i < __num_longer_chunks)
      58  		    ? (__chunk_length + 1) : __chunk_length);
      59  	}
      60        *__s++ = __n;
      61        return __s;
      62      }
      63  
      64    /** @brief function to split a sequence into parts of almost equal size.
      65     *
      66     *  Returns the position of the splitting point between
      67     *  thread number __thread_no (included) and
      68     *  thread number __thread_no+1 (excluded).
      69     *  @param __n Number of elements
      70     *  @param __num_threads Number of parts
      71     *  @param __thread_no Number of threads
      72     *  @returns splitting point */
      73    template<typename _DifferenceType>
      74      _DifferenceType
      75      __equally_split_point(_DifferenceType __n,
      76  			  _ThreadIndex __num_threads,
      77  			  _ThreadIndex __thread_no)
      78      {
      79        _DifferenceType __chunk_length = __n / __num_threads;
      80        _DifferenceType __num_longer_chunks = __n % __num_threads;
      81        if (__thread_no < __num_longer_chunks)
      82  	return __thread_no * (__chunk_length + 1);
      83        else
      84  	return __num_longer_chunks * (__chunk_length + 1)
      85            + (__thread_no - __num_longer_chunks) * __chunk_length;
      86      }
      87  }
      88  
      89  #endif /* _GLIBCXX_PARALLEL_EQUALLY_SPLIT_H */