libstdc++-v3/include/parallel/equally_split.h

   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 */