3 // Copyright (C) 2007, 2008, 2009 Free Software Foundation, Inc.
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
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12 // WITHOUT ANY WARRANTY; without even the implied warranty of
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14 // General Public License for more details.
16 // Under Section 7 of GPL version 3, you are granted additional
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22 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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25 /** @file parallel/list_partition.h
26 * @brief Functionality to split sequence referenced by only input
28 * This file is a GNU parallel extension to the Standard C++ Library.
31 // Written by Leonor Frias Moya and Johannes Singler.
33 #ifndef _GLIBCXX_PARALLEL_LIST_PARTITION_H
34 #define _GLIBCXX_PARALLEL_LIST_PARTITION_H 1
36 #include <parallel/parallel.h>
39 namespace __gnu_parallel
41 /** @brief Shrinks and doubles the ranges.
42 * @param os_starts Start positions worked on (oversampled).
43 * @param count_to_two Counts up to 2.
44 * @param range_length Current length of a chunk.
45 * @param make_twice Whether the @c os_starts is allowed to be
48 template<typename InputIterator>
50 shrink_and_double(std::vector<InputIterator>& os_starts,
51 size_t& count_to_two, size_t& range_length,
52 const bool make_twice)
55 if (not make_twice or count_to_two < 2)
56 shrink(os_starts, count_to_two, range_length);
59 os_starts.resize((os_starts.size() - 1) * 2 + 1);
64 /** @brief Combines two ranges into one and thus halves the number of ranges.
65 * @param os_starts Start positions worked on (oversampled).
66 * @param count_to_two Counts up to 2.
67 * @param range_length Current length of a chunk. */
68 template<typename InputIterator>
70 shrink(std::vector<InputIterator>& os_starts, size_t& count_to_two,
73 for (typename std::vector<InputIterator>::size_type i = 0;
74 i <= (os_starts.size() / 2); ++i)
75 os_starts[i] = os_starts[i * 2];
79 /** @brief Splits a sequence given by input iterators into parts of
82 * The function needs only one pass over the sequence.
83 * @param begin Begin iterator of input sequence.
84 * @param end End iterator of input sequence.
85 * @param starts Start iterators for the resulting parts, dimension
86 * @c num_parts+1. For convenience, @c starts @c [num_parts]
87 * contains the end iterator of the sequence.
88 * @param lengths Length of the resulting parts.
89 * @param num_parts Number of parts to split the sequence into.
90 * @param f Functor to be applied to each element by traversing it
91 * @param oversampling Oversampling factor. If 0, then the
92 * partitions will differ in at most @f$ \sqrt{\mathrm{end} -
93 * \mathrm{begin}} @f$ elements. Otherwise, the ratio between the
94 * longest and the shortest part is bounded by @f$
95 * 1/(\mathrm{oversampling} \cdot \mathrm{num\_parts}) @f$.
96 * @return Length of the whole sequence.
98 template<typename InputIterator, typename FunctorType>
100 list_partition(const InputIterator begin, const InputIterator end,
101 InputIterator* starts, size_t* lengths, const int num_parts,
102 FunctorType& f, int oversampling = 0)
104 bool make_twice = false;
106 // The resizing algorithm is chosen according to the oversampling factor.
107 if (oversampling == 0)
113 std::vector<InputIterator> os_starts(2 * oversampling * num_parts + 1);
116 InputIterator prev = begin, it = begin;
117 size_t dist_limit = 0, dist = 0;
118 size_t cur = 1, next = 1;
119 size_t range_length = 1;
120 size_t count_to_two = 0;
124 for (; cur < os_starts.size() and it != end; ++cur)
126 for (dist_limit += range_length;
127 dist < dist_limit and it != end; ++dist)
135 // Must compare for end and not cur < os_starts.size() , because
136 // cur could be == os_starts.size() as well
140 shrink_and_double(os_starts, count_to_two, range_length, make_twice);
141 next = os_starts.size() / 2 + 1;
144 // Calculation of the parts (one must be extracted from current
145 // because the partition beginning at end, consists only of
147 size_t size_part = (cur - 1) / num_parts;
148 int size_greater = static_cast<int>((cur - 1) % num_parts);
149 starts[0] = os_starts[0];
153 // Smallest partitions.
154 for (int i = 1; i < (num_parts + 1 - size_greater); ++i)
156 lengths[i - 1] = size_part * range_length;
158 starts[i] = os_starts[index];
161 // Biggest partitions.
162 for (int i = num_parts + 1 - size_greater; i <= num_parts; ++i)
164 lengths[i - 1] = (size_part+1) * range_length;
165 index += (size_part+1);
166 starts[i] = os_starts[index];
169 // Correction of the end size (the end iteration has not finished).
170 lengths[num_parts - 1] -= (dist_limit - dist);
176 #endif /* _GLIBCXX_PARALLEL_LIST_PARTITION_H */