Filename: //usr/include/c++/8/parallel/list_partition.h

// -*- C++ -*- // Copyright (C) 2007-2018 Free Software Foundation, Inc. // // This file is part of the GNU ISO C++ Library. This library is free // software; you can redistribute __it and/or modify __it under the terms // of the GNU General Public License as published by the Free Software // Foundation; either version 3, or (at your option) any later // version. // This library is distributed in the hope that __it will be useful, but // WITHOUT ANY WARRANTY; without even the implied warranty of // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU // General Public License for more details. // Under Section 7 of GPL version 3, you are granted additional // permissions described in the GCC Runtime Library Exception, version // 3.1, as published by the Free Software Foundation. // You should have received a copy of the GNU General Public License and // a copy of the GCC Runtime Library Exception along with this program; // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see // <http://www.gnu.org/licenses/>. /** @file parallel/list_partition.h * @brief _Functionality to split __sequence referenced by only input * iterators. * This file is a GNU parallel extension to the Standard C++ Library. */ // Written by Leonor Frias Moya and Johannes Singler. #ifndef _GLIBCXX_PARALLEL_LIST_PARTITION_H #define _GLIBCXX_PARALLEL_LIST_PARTITION_H 1 #include <parallel/parallel.h> #include <vector> namespace __gnu_parallel { /** @brief Shrinks and doubles the ranges. * @param __os_starts Start positions worked on (oversampled). * @param __count_to_two Counts up to 2. * @param __range_length Current length of a chunk. * @param __make_twice Whether the @c __os_starts is allowed to be * grown or not */ template<typename _IIter> void __shrink_and_double(std::vector<_IIter>& __os_starts, size_t& __count_to_two, size_t& __range_length, const bool __make_twice) { ++__count_to_two; if (!__make_twice || __count_to_two < 2) __shrink(__os_starts, __count_to_two, __range_length); else { __os_starts.resize((__os_starts.size() - 1) * 2 + 1); __count_to_two = 0; } } /** @brief Combines two ranges into one and thus halves the number of ranges. * @param __os_starts Start positions worked on (oversampled). * @param __count_to_two Counts up to 2. * @param __range_length Current length of a chunk. */ template<typename _IIter> void __shrink(std::vector<_IIter>& __os_starts, size_t& __count_to_two, size_t& __range_length) { for (typename std::vector<_IIter>::size_type __i = 0; __i <= (__os_starts.size() / 2); ++__i) __os_starts[__i] = __os_starts[__i * 2]; __range_length *= 2; } /** @brief Splits a sequence given by input iterators into parts of * almost equal size * * The function needs only one pass over the sequence. * @param __begin Begin iterator of input sequence. * @param __end End iterator of input sequence. * @param __starts Start iterators for the resulting parts, dimension * @c __num_parts+1. For convenience, @c __starts @c [__num_parts] * contains the end iterator of the sequence. * @param __lengths Length of the resulting parts. * @param __num_parts Number of parts to split the sequence into. * @param __f Functor to be applied to each element by traversing __it * @param __oversampling Oversampling factor. If 0, then the * partitions will differ in at most * \f$\sqrt{\mathrm{end} - \mathrm{begin}}\f$ * elements. Otherwise, the ratio between the * longest and the shortest part is bounded by * \f$1/(\mathrm{oversampling} \cdot \mathrm{num\_parts})\f$ * @return Length of the whole sequence. */ template<typename _IIter, typename _FunctorType> size_t list_partition(const _IIter __begin, const _IIter __end, _IIter* __starts, size_t* __lengths, const int __num_parts, _FunctorType& __f, int __oversampling = 0) { bool __make_twice = false; // The resizing algorithm is chosen according to the oversampling factor. if (__oversampling == 0) { __make_twice = true; __oversampling = 1; } std::vector<_IIter> __os_starts(2 * __oversampling * __num_parts + 1); __os_starts[0] = __begin; _IIter __prev = __begin, __it = __begin; size_t __dist_limit = 0, __dist = 0; size_t __cur = 1, __next = 1; size_t __range_length = 1; size_t __count_to_two = 0; while (__it != __end) { __cur = __next; for (; __cur < __os_starts.size() and __it != __end; ++__cur) { for (__dist_limit += __range_length; __dist < __dist_limit and __it != __end; ++__dist) { __f(__it); ++__it; } __os_starts[__cur] = __it; } // Must compare for end and not __cur < __os_starts.size() , because // __cur could be == __os_starts.size() as well if (__it == __end) break; __shrink_and_double(__os_starts, __count_to_two, __range_length, __make_twice); __next = __os_starts.size() / 2 + 1; } // Calculation of the parts (one must be extracted from __current // because the partition beginning at end, consists only of // itself). size_t __size_part = (__cur - 1) / __num_parts; int __size_greater = static_cast<int>((__cur - 1) % __num_parts); __starts[0] = __os_starts[0]; size_t __index = 0; // Smallest partitions. for (int __i = 1; __i < (__num_parts + 1 - __size_greater); ++__i) { __lengths[__i - 1] = __size_part * __range_length; __index += __size_part; __starts[__i] = __os_starts[__index]; } // Biggest partitions. for (int __i = __num_parts + 1 - __size_greater; __i <= __num_parts; ++__i) { __lengths[__i - 1] = (__size_part+1) * __range_length; __index += (__size_part+1); __starts[__i] = __os_starts[__index]; } // Correction of the end size (the end iteration has not finished). __lengths[__num_parts - 1] -= (__dist_limit - __dist); return __dist; } } #endif /* _GLIBCXX_PARALLEL_LIST_PARTITION_H */

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