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/*
* SPDX-FileCopyrightText: 2012 Peter Penz <[email protected]>
* SPDX-FileCopyrightText: 2012 Emmanuel Pescosta <[email protected]>
* SPDX-FileCopyrightText: 2013 Frank Reininghaus <[email protected]>
*
* SPDX-License-Identifier: GPL-2.0-or-later
*/
#ifndef KFILEITEMMODELSORTALGORITHM_H
#define KFILEITEMMODELSORTALGORITHM_H
#include <QtConcurrentRun>
#include <algorithm>
/**
* Sorts the items using the merge sort algorithm is used to assure a
* worst-case of O(n * log(n)) and to keep the number of comparisons low.
*
* The implementation is based on qStableSortHelper() from qalgorithms.h
* SPDX-FileCopyrightText: 2011 Nokia Corporation and/or its subsidiary(-ies).
*/
template <typename RandomAccessIterator, typename LessThan>
static void mergeSort(RandomAccessIterator begin,
RandomAccessIterator end,
const LessThan& lessThan)
{
// The implementation is based on qStableSortHelper() from qalgorithms.h
// SPDX-FileCopyrightText: 2011 Nokia Corporation and/or its subsidiary(-ies).
const int span = end - begin;
if (span < 2) {
return;
}
const RandomAccessIterator middle = begin + span / 2;
mergeSort(begin, middle, lessThan);
mergeSort(middle, end, lessThan);
merge(begin, middle, end, lessThan);
}
/**
* Uses up to \a numberOfThreads threads to sort the items between
* \a begin and \a end. Only item ranges longer than
* \a parallelMergeSortingThreshold are split to be sorted by two different
* threads.
*
* The comparison function \a lessThan must be reentrant.
*/
template <typename RandomAccessIterator, typename LessThan>
static void parallelMergeSort(RandomAccessIterator begin,
RandomAccessIterator end,
LessThan lessThan,
int numberOfThreads,
int parallelMergeSortingThreshold = 100)
{
const int span = end - begin;
if (numberOfThreads > 1 && span > parallelMergeSortingThreshold) {
const int newNumberOfThreads = numberOfThreads / 2;
const RandomAccessIterator middle = begin + span / 2;
QFuture<void> future = QtConcurrent::run(parallelMergeSort<RandomAccessIterator, LessThan>, begin, middle, lessThan, newNumberOfThreads, parallelMergeSortingThreshold);
parallelMergeSort(middle, end, lessThan, newNumberOfThreads, parallelMergeSortingThreshold);
future.waitForFinished();
merge(begin, middle, end, lessThan);
} else {
mergeSort(begin, end, lessThan);
}
}
/**
* Merges the sorted item ranges between \a begin and \a pivot and
* between \a pivot and \a end into a single sorted range between
* \a begin and \a end.
*
* The implementation is based on qMerge() from qalgorithms.h
* SPDX-FileCopyrightText: 2011 Nokia Corporation and/or its subsidiary(-ies).
*/
template <typename RandomAccessIterator, typename LessThan>
static void merge(RandomAccessIterator begin,
RandomAccessIterator pivot,
RandomAccessIterator end,
const LessThan& lessThan)
{
// The implementation is based on qMerge() from qalgorithms.h
// SPDX-FileCopyrightText: 2011 Nokia Corporation and/or its subsidiary(-ies).
const int len1 = pivot - begin;
const int len2 = end - pivot;
if (len1 == 0 || len2 == 0) {
return;
}
if (len1 + len2 == 2) {
if (lessThan(*(begin + 1), *(begin))) {
qSwap(*begin, *(begin + 1));
}
return;
}
RandomAccessIterator firstCut;
RandomAccessIterator secondCut;
int len2Half;
if (len1 > len2) {
const int len1Half = len1 / 2;
firstCut = begin + len1Half;
secondCut = std::lower_bound<RandomAccessIterator,
decltype(*firstCut), const LessThan&>(pivot, end, *firstCut, lessThan);
len2Half = secondCut - pivot;
} else {
len2Half = len2 / 2;
secondCut = pivot + len2Half;
firstCut = std::upper_bound<RandomAccessIterator,
decltype(*secondCut), const LessThan&>(begin, pivot, *secondCut, lessThan);
}
std::rotate(firstCut, pivot, secondCut);
RandomAccessIterator newPivot = firstCut + len2Half;
merge(begin, firstCut, newPivot, lessThan);
merge(newPivot, secondCut, end, lessThan);
}
#endif
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