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Diffstat (limited to 'aapl/quicksort.h')
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diff --git a/aapl/quicksort.h b/aapl/quicksort.h new file mode 100644 index 0000000..bb6941e --- /dev/null +++ b/aapl/quicksort.h @@ -0,0 +1,185 @@ +/* + * Copyright 2002 Adrian Thurston <thurston@complang.org> + */ + +/* 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 */ |