libstdc++
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00001 // -*- C++ -*- 00002 00003 // Copyright (C) 2007, 2008, 2009 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/list_partition.h 00026 * @brief Functionality to split sequence referenced by only input 00027 * iterators. 00028 * This file is a GNU parallel extension to the Standard C++ Library. 00029 */ 00030 00031 // Written by Leonor Frias Moya and Johannes Singler. 00032 00033 #ifndef _GLIBCXX_PARALLEL_LIST_PARTITION_H 00034 #define _GLIBCXX_PARALLEL_LIST_PARTITION_H 1 00035 00036 #include <parallel/parallel.h> 00037 #include <vector> 00038 00039 namespace __gnu_parallel 00040 { 00041 /** @brief Shrinks and doubles the ranges. 00042 * @param os_starts Start positions worked on (oversampled). 00043 * @param count_to_two Counts up to 2. 00044 * @param range_length Current length of a chunk. 00045 * @param make_twice Whether the @c os_starts is allowed to be 00046 * grown or not 00047 */ 00048 template<typename InputIterator> 00049 void 00050 shrink_and_double(std::vector<InputIterator>& os_starts, 00051 size_t& count_to_two, size_t& range_length, 00052 const bool make_twice) 00053 { 00054 ++count_to_two; 00055 if (not make_twice or count_to_two < 2) 00056 shrink(os_starts, count_to_two, range_length); 00057 else 00058 { 00059 os_starts.resize((os_starts.size() - 1) * 2 + 1); 00060 count_to_two = 0; 00061 } 00062 } 00063 00064 /** @brief Combines two ranges into one and thus halves the number of ranges. 00065 * @param os_starts Start positions worked on (oversampled). 00066 * @param count_to_two Counts up to 2. 00067 * @param range_length Current length of a chunk. */ 00068 template<typename InputIterator> 00069 void 00070 shrink(std::vector<InputIterator>& os_starts, size_t& count_to_two, 00071 size_t& range_length) 00072 { 00073 for (typename std::vector<InputIterator>::size_type i = 0; 00074 i <= (os_starts.size() / 2); ++i) 00075 os_starts[i] = os_starts[i * 2]; 00076 range_length *= 2; 00077 } 00078 00079 /** @brief Splits a sequence given by input iterators into parts of 00080 * almost equal size 00081 * 00082 * The function needs only one pass over the sequence. 00083 * @param begin Begin iterator of input sequence. 00084 * @param end End iterator of input sequence. 00085 * @param starts Start iterators for the resulting parts, dimension 00086 * @c num_parts+1. For convenience, @c starts @c [num_parts] 00087 * contains the end iterator of the sequence. 00088 * @param lengths Length of the resulting parts. 00089 * @param num_parts Number of parts to split the sequence into. 00090 * @param f Functor to be applied to each element by traversing it 00091 * @param oversampling Oversampling factor. If 0, then the 00092 * partitions will differ in at most @f$ \sqrt{\mathrm{end} - 00093 * \mathrm{begin}} @f$ elements. Otherwise, the ratio between the 00094 * longest and the shortest part is bounded by @f$ 00095 * 1/(\mathrm{oversampling} \cdot \mathrm{num\_parts}) @f$. 00096 * @return Length of the whole sequence. 00097 */ 00098 template<typename InputIterator, typename FunctorType> 00099 size_t 00100 list_partition(const InputIterator begin, const InputIterator end, 00101 InputIterator* starts, size_t* lengths, const int num_parts, 00102 FunctorType& f, int oversampling = 0) 00103 { 00104 bool make_twice = false; 00105 00106 // The resizing algorithm is chosen according to the oversampling factor. 00107 if (oversampling == 0) 00108 { 00109 make_twice = true; 00110 oversampling = 1; 00111 } 00112 00113 std::vector<InputIterator> os_starts(2 * oversampling * num_parts + 1); 00114 00115 os_starts[0]= begin; 00116 InputIterator prev = begin, it = begin; 00117 size_t dist_limit = 0, dist = 0; 00118 size_t cur = 1, next = 1; 00119 size_t range_length = 1; 00120 size_t count_to_two = 0; 00121 while (it != end) 00122 { 00123 cur = next; 00124 for (; cur < os_starts.size() and it != end; ++cur) 00125 { 00126 for (dist_limit += range_length; 00127 dist < dist_limit and it != end; ++dist) 00128 { 00129 f(it); 00130 ++it; 00131 } 00132 os_starts[cur] = it; 00133 } 00134 00135 // Must compare for end and not cur < os_starts.size() , because 00136 // cur could be == os_starts.size() as well 00137 if (it == end) 00138 break; 00139 00140 shrink_and_double(os_starts, count_to_two, range_length, make_twice); 00141 next = os_starts.size() / 2 + 1; 00142 } 00143 00144 // Calculation of the parts (one must be extracted from current 00145 // because the partition beginning at end, consists only of 00146 // itself). 00147 size_t size_part = (cur - 1) / num_parts; 00148 int size_greater = static_cast<int>((cur - 1) % num_parts); 00149 starts[0] = os_starts[0]; 00150 00151 size_t index = 0; 00152 00153 // Smallest partitions. 00154 for (int i = 1; i < (num_parts + 1 - size_greater); ++i) 00155 { 00156 lengths[i - 1] = size_part * range_length; 00157 index += size_part; 00158 starts[i] = os_starts[index]; 00159 } 00160 00161 // Biggest partitions. 00162 for (int i = num_parts + 1 - size_greater; i <= num_parts; ++i) 00163 { 00164 lengths[i - 1] = (size_part+1) * range_length; 00165 index += (size_part+1); 00166 starts[i] = os_starts[index]; 00167 } 00168 00169 // Correction of the end size (the end iteration has not finished). 00170 lengths[num_parts - 1] -= (dist_limit - dist); 00171 00172 return dist; 00173 } 00174 } 00175 00176 #endif /* _GLIBCXX_PARALLEL_LIST_PARTITION_H */