Q Sort Algorithm

by Vijayaprasad 2010-01-28 16:07:46

/*
* @(#)QSortAlgorithm.java 1.6f 95/01/31 James Gosling
*
* Copyright (c) 1994-1995 Sun Microsystems, Inc. All Rights Reserved.
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* THE SOFTWARE, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
* TO THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
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*
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* CONTROL EQUIPMENT IN HAZARDOUS ENVIRONMENTS REQUIRING FAIL-SAFE
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/**
* A quick sort demonstration algorithm
* SortAlgorithm.java, Thu Oct 27 10:32:35 1994
*
* @author James Gosling
* @version 1.6f, 31 Jan 1995
*/
/**
* 19 Feb 1996: Fixed to avoid infinite loop discoved by Paul Haeberli.
* Misbehaviour expressed when the pivot element was not unique.
* -Jason Harrison
*
* 21 Jun 1996: Modified code based on comments from Paul Haeberli, and
* Peter Schweizer (Peter.Schweizer@mni.fh-giessen.de).
* Used Daeron Meyer's (daeron@geom.umn.edu) code for the
* new pivoting code. - Jason Harrison
*
* 09 Jan 1998: Another set of bug fixes by Thomas Everth (everth@wave.co.nz)
* and John Brzustowski (jbrzusto@gpu.srv.ualberta.ca).
*/

class QSortAlgorithm extends SortAlgorithm {
void sort(int a[], int lo0, int hi0) throws Exception {
int lo = lo0;
int hi = hi0;
pause(lo, hi);
if (lo >= hi) {
return;
}
else if( lo == hi - 1 ) {
/*
* sort a two element list by swapping if necessary
*/
if (a[lo] > a[hi]) {
int T = a[lo];
a[lo] = a[hi];
a[hi] = T;
}
return;
}


/*
* Pick a pivot and move it out of the way
*/
int pivot = a[(lo + hi) / 2];
a[(lo + hi) / 2] = a[hi];
a[hi] = pivot;

while( lo < hi ) {
/*
* Search forward from a[lo] until an element is found that
* is greater than the pivot or lo >= hi
*/
while (a[lo] <= pivot && lo < hi) {
lo++;
}

/*
* Search backward from a[hi] until element is found that
* is less than the pivot, or lo >= hi
*/
while (pivot <= a[hi] && lo < hi ) {
hi--;
}

/*
* Swap elements a[lo] and a[hi]
*/
if( lo < hi ) {
int T = a[lo];
a[lo] = a[hi];
a[hi] = T;
pause();
}

if (stopRequested) {
return;
}
}

/*
* Put the median in the "center" of the list
*/
a[hi0] = a[hi];
a[hi] = pivot;

/*
* Recursive calls, elements a[lo0] to a[lo-1] are less than or
* equal to pivot, elements a[hi+1] to a[hi0] are greater than
* pivot.
*/
sort(a, lo0, lo-1);
sort(a, hi+1, hi0);
}

void sort(int a[]) throws Exception {
sort(a, 0, a.length-1);
}
}

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