list_partition.h
Go to the documentation of this file.
// -*- C++ -*-
// Copyright (C) 2007-2013 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
* \sqrt{\mathrm{__end} - \mathrm{__begin}}
* __elements. Otherwise, the ratio between the
* longest and the shortest part is bounded by
* 1/(\mathrm{__oversampling} \cdot \mathrm{num\_parts})
* @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 */