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// -*- C++ -*-
// Copyright (C) 2007, 2008, 2009 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 InputIterator>
void
shrink_and_double(std::vector<InputIterator>& os_starts,
size_t& count_to_two, size_t& range_length,
const bool make_twice)
{
++count_to_two;
if (not make_twice or 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 InputIterator>
void
shrink(std::vector<InputIterator>& os_starts, size_t& count_to_two,
size_t& range_length)
{
for (typename std::vector<InputIterator>::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 InputIterator, typename FunctorType>
size_t
list_partition(const InputIterator begin, const InputIterator end,
InputIterator* 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<InputIterator> os_starts(2 * oversampling * num_parts + 1);
os_starts[0]= begin;
InputIterator 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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