qsort.java

A Convex Hull is the smallest convex polygon that contains every point of the set S. A polygon P is

class qsort{
    void sort(Point 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 ) {
             
            
            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);
    }
}