libstdc++
list_partition.h
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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 _IIter>
00049     void
00050     __shrink_and_double(std::vector<_IIter>& __os_starts,
00051             size_t& __count_to_two, size_t& __range_length,
00052             const bool __make_twice)
00053     {
00054       ++__count_to_two;
00055       if (!__make_twice || __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 _IIter>
00069     void
00070     __shrink(std::vector<_IIter>& __os_starts, size_t& __count_to_two,
00071          size_t& __range_length)
00072     {
00073       for (typename std::vector<_IIter>::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 
00093    *  \sqrt{\mathrm{__end} - \mathrm{__begin}} 
00094    *  __elements. Otherwise, the ratio between the
00095    *  longest and the shortest part is bounded by
00096    *  1/(\mathrm{__oversampling} \cdot \mathrm{num\_parts})
00097    *  @return Length of the whole sequence.
00098    */
00099   template<typename _IIter, typename _FunctorType>
00100     size_t
00101     list_partition(const _IIter __begin, const _IIter __end,
00102                    _IIter* __starts, size_t* __lengths, const int __num_parts,
00103                    _FunctorType& __f, int __oversampling = 0)
00104     {
00105       bool __make_twice = false;
00106 
00107       // The resizing algorithm is chosen according to the oversampling factor.
00108       if (__oversampling == 0)
00109         {
00110           __make_twice = true;
00111           __oversampling = 1;
00112         }
00113 
00114       std::vector<_IIter> __os_starts(2 * __oversampling * __num_parts + 1);
00115 
00116       __os_starts[0] = __begin;
00117       _IIter __prev  = __begin, __it = __begin;
00118       size_t __dist_limit = 0, __dist = 0;
00119       size_t __cur = 1, __next = 1;
00120       size_t __range_length = 1;
00121       size_t __count_to_two = 0;
00122       while (__it != __end)
00123         {
00124           __cur = __next;
00125           for (; __cur < __os_starts.size() and __it != __end; ++__cur)
00126             {
00127               for (__dist_limit += __range_length;
00128                    __dist < __dist_limit and __it != __end; ++__dist)
00129                 {
00130                   __f(__it);
00131                   ++__it;
00132                 }
00133               __os_starts[__cur] = __it;
00134             }
00135 
00136           // Must compare for end and not __cur < __os_starts.size() , because
00137           // __cur could be == __os_starts.size() as well
00138           if (__it == __end)
00139             break;
00140 
00141           __shrink_and_double(__os_starts, __count_to_two, __range_length,
00142                               __make_twice);
00143           __next = __os_starts.size() / 2 + 1;
00144         }
00145 
00146       // Calculation of the parts (one must be extracted from __current
00147       // because the partition beginning at end, consists only of
00148       // itself).
00149       size_t __size_part = (__cur - 1) / __num_parts;
00150       int __size_greater = static_cast<int>((__cur - 1) % __num_parts);
00151       __starts[0] = __os_starts[0];
00152 
00153       size_t __index = 0;
00154 
00155       // Smallest partitions.
00156       for (int __i = 1; __i < (__num_parts + 1 - __size_greater); ++__i)
00157         {
00158           __lengths[__i - 1] =  __size_part * __range_length;
00159           __index += __size_part;
00160           __starts[__i] = __os_starts[__index];
00161         }
00162 
00163       // Biggest partitions.
00164       for (int __i = __num_parts + 1 - __size_greater; __i <= __num_parts;
00165            ++__i)
00166         {
00167           __lengths[__i - 1] =  (__size_part+1) * __range_length;
00168           __index += (__size_part+1);
00169           __starts[__i] = __os_starts[__index];
00170         }
00171 
00172       // Correction of the end size (the end iteration has not finished).
00173       __lengths[__num_parts - 1] -= (__dist_limit - __dist);
00174 
00175       return __dist;
00176     }
00177 }
00178 
00179 #endif /* _GLIBCXX_PARALLEL_LIST_PARTITION_H */