libstdc++
equally_split.h
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00001 // -*- C++ -*-
00002 
00003 // Copyright (C) 2007, 2008, 2009, 2010, 2011 Free Software Foundation, Inc.
00004 //
00005 // This file is part of the GNU ISO C++ Library.  This library is free
00006 // software; you can redistribute it and/or modify it under the terms
00007 // of the GNU General Public License as published by the Free Software
00008 // Foundation; either version 3, or (at your option) any later
00009 // version.
00010 
00011 // This library is distributed in the hope that it will be useful, but
00012 // WITHOUT ANY WARRANTY; without even the implied warranty of
00013 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
00014 // General Public License for more details.
00015 
00016 // Under Section 7 of GPL version 3, you are granted additional
00017 // permissions described in the GCC Runtime Library Exception, version
00018 // 3.1, as published by the Free Software Foundation.
00019 
00020 // You should have received a copy of the GNU General Public License and
00021 // a copy of the GCC Runtime Library Exception along with this program;
00022 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
00023 // <http://www.gnu.org/licenses/>.
00024 
00025 /** @file parallel/equally_split.h
00026  *  This file is a GNU parallel extension to the Standard C++ Library.
00027  */
00028 
00029 // Written by Johannes Singler.
00030 
00031 #ifndef _GLIBCXX_PARALLEL_EQUALLY_SPLIT_H
00032 #define _GLIBCXX_PARALLEL_EQUALLY_SPLIT_H 1
00033 
00034 namespace __gnu_parallel
00035 {
00036   /** @brief function to split a sequence into parts of almost equal size.
00037    *
00038    *  The resulting sequence __s of length __num_threads+1 contains the
00039    *  splitting positions when splitting the range [0,__n) into parts of
00040    *  almost equal size (plus minus 1).  The first entry is 0, the last
00041    *  one n. There may result empty parts.
00042    *  @param __n Number of elements
00043    *  @param __num_threads Number of parts
00044    *  @param __s Splitters
00045    *  @returns End of __splitter sequence, i.e. @c __s+__num_threads+1 */
00046   template<typename _DifferenceType, typename _OutputIterator>
00047     _OutputIterator
00048     __equally_split(_DifferenceType __n, _ThreadIndex __num_threads,
00049             _OutputIterator __s)
00050     {
00051       _DifferenceType __chunk_length = __n / __num_threads;
00052       _DifferenceType __num_longer_chunks = __n % __num_threads;
00053       _DifferenceType __pos = 0;
00054       for (_ThreadIndex __i = 0; __i < __num_threads; ++__i)
00055     {
00056       *__s++ = __pos;
00057       __pos += ((__i < __num_longer_chunks)
00058             ? (__chunk_length + 1) : __chunk_length);
00059     }
00060       *__s++ = __n;
00061       return __s;
00062     }
00063 
00064   /** @brief function to split a sequence into parts of almost equal size.
00065    *
00066    *  Returns the position of the splitting point between
00067    *  thread number __thread_no (included) and
00068    *  thread number __thread_no+1 (excluded).
00069    *  @param __n Number of elements
00070    *  @param __num_threads Number of parts
00071    *  @param __thread_no Number of threads
00072    *  @returns splitting point */
00073   template<typename _DifferenceType>
00074     _DifferenceType
00075     __equally_split_point(_DifferenceType __n,
00076               _ThreadIndex __num_threads,
00077               _ThreadIndex __thread_no)
00078     {
00079       _DifferenceType __chunk_length = __n / __num_threads;
00080       _DifferenceType __num_longer_chunks = __n % __num_threads;
00081       if (__thread_no < __num_longer_chunks)
00082     return __thread_no * (__chunk_length + 1);
00083       else
00084     return __num_longer_chunks * (__chunk_length + 1)
00085           + (__thread_no - __num_longer_chunks) * __chunk_length;
00086     }
00087 }
00088 
00089 #endif /* _GLIBCXX_PARALLEL_EQUALLY_SPLIT_H */