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/*
* Copyright 2002 Adrian Thurston <thurston@cs.queensu.ca>
*/
/* This file is part of Aapl.
*
* Aapl is free software; you can redistribute it and/or modify it under the
* terms of the GNU Lesser General Public License as published by the Free
* Software Foundation; either version 2.1 of the License, or (at your option)
* any later version.
*
* Aapl 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 Lesser General Public License for
* more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with Aapl; if not, write to the Free Software Foundation, Inc., 59
* Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
#ifndef _AAPL_QUICKSORT_H
#define _AAPL_QUICKSORT_H
#include "insertsort.h"
#ifdef AAPL_NAMESPACE
namespace Aapl {
#endif
/**
* \addtogroup sort
* @{
*/
/**
* \class QuickSort
* \brief Quick sort an array of data.
*
* QuickSort can be used to sort any array of objects of type T provided a
* compare class is given. QuickSort is in-place. It does not require any
* temporary storage.
*
* Objects are not made aware that they are being moved around in memory.
* Assignment operators, constructors and destructors are never invoked by the
* sort.
*
* QuickSort runs in O(n*log(n)) time in the average case. It is faster than
* mergsort in the average case because it does less moving of data. The
* performance of quicksort depends mostly on the choice of pivot. This
* implementation picks the pivot as the median of first, middle, last. This
* choice of pivot avoids the O(n^2) worst case for input already sorted, but
* it is still possible to encounter the O(n^2) worst case. For example an
* array of identical elements will run in O(n^2)
*
* QuickSort is not a stable sort. Elements with the same key will not have
* their relative ordering preserved. QuickSort switches to an InsertSort
* when the size of the array being sorted is small. This happens when
* directly sorting a small array or when QuickSort calls iteself recursively
* on a small portion of a larger array.
*/
/*@}*/
/* QuickSort. */
template <class T, class Compare> class QuickSort :
public InsertSort<T, Compare>
{
public:
/* Sorting interface routine. */
void sort(T *data, long len);
private:
/* Recursive worker. */
void doSort(T *start, T *end);
T *partition(T *start, T *end);
inline T *median(T *start, T *end);
};
#define _QS_INSERTION_THRESH 16
/* Finds the median of start, middle, end. */
template <class T, class Compare> T *QuickSort<T,Compare>::
median(T *start, T *end)
{
T *pivot, *mid = start + (end-start)/2;
/* CChoose the pivot. */
if ( compare(*start, *mid) < 0 ) {
if ( compare(*mid, *end) < 0 )
pivot = mid;
else if ( compare(*start, *end) < 0 )
pivot = end;
else
pivot = start;
}
else if ( compare(*start, *end) < 0 )
pivot = start;
else if ( compare(*mid, *end) < 0 )
pivot = end;
else
pivot = mid;
return pivot;
}
template <class T, class Compare> T *QuickSort<T,Compare>::
partition(T *start, T *end)
{
/* Use the median of start, middle, end as the pivot. First save
* it off then move the last element to the free spot. */
char pcPivot[sizeof(T)];
T *pivot = median(start, end);
memcpy( pcPivot, pivot, sizeof(T) );
if ( pivot != end )
memcpy( pivot, end, sizeof(T) );
T *first = start-1;
T *last = end;
pivot = (T*) pcPivot;
/* Shuffle element to the correct side of the pivot, ending
* up with the free spot where the pivot will go. */
while ( true ) {
/* Throw one element ahead to the free spot at last. */
while ( true ) {
first += 1;
if ( first == last )
goto done;
if ( compare( *first, *pivot ) > 0 ) {
memcpy(last, first, sizeof(T));
break;
}
}
/* Throw one element back to the free spot at first. */
while ( true ) {
last -= 1;
if ( last == first )
goto done;
if ( compare( *last, *pivot ) < 0 ) {
memcpy(first, last, sizeof(T));
break;
}
}
}
done:
/* Put the pivot into the middle spot for it. */
memcpy( first, pivot, sizeof(T) );
return first;
}
template< class T, class Compare> void QuickSort<T,Compare>::
doSort(T *start, T *end)
{
long len = end - start + 1;
if ( len > _QS_INSERTION_THRESH ) {
/* Use quicksort. */
T *pivot = partition( start, end );
doSort(start, pivot-1);
doSort(pivot+1, end);
}
else if ( len > 1 ) {
/* Array is small, use insertion sort. */
InsertSort<T, Compare>::sort( start, len );
}
}
/**
* \brief Quick sort an array of data.
*/
template< class T, class Compare>
void QuickSort<T,Compare>::sort(T *data, long len)
{
/* Call recursive worker. */
doSort(data, data+len-1);
}
#ifdef AAPL_NAMESPACE
}
#endif
#endif /* _AAPL_QUICKSORT_H */
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