#define	SWAP(x,n)	do {t=x[n];x[n]=x[n+1];x[n+1]=t;} while(0)
#define	SSWAP(x,n)	do {if (x[n] > x[n+1]) SWAP(x,n);} while(0)

/* nelt >= 3 */
static void	median_to_front(int	*base,
				int	nelt)
{
	int	t;
	SSWAP(base,0); SSWAP(base,1); SSWAP(base,0);
	SWAP(base,0); /* cheap version -- not quite median value */
}

/* 0 <= nelt < INT_MAX, base[0..nelt-1] valid */
void	quicksort(int	*base,
		  int	nelt)
{
	int pivot, t, hi, lo;

	switch (nelt) { /* base cases */
	case 2: SSWAP(base,0); /* fall through */
	case 0: case 1: return;
	}
	/* 3 <= nelt */
	median_to_front(base, nelt); pivot = base[0];
	lo = 1; hi = nelt-1;
	/* lo < hi */
	/* base[0..lo-1] <= pivot < base[hi+1..nelt-1] */
	while (lo <= hi) {
		while (lo < nelt && base[lo] <= pivot)
			lo++;
		/* 0 <= lo == nelt || base[lo] > pivot */
		while (0 <= hi && pivot < base[hi])
			--hi;
		/* -1 == hi < nelt || base[hi] <= pivot */
		if (lo < hi) {
			t = base[lo]; base[lo] = base[hi]; base[hi] = t;
		}
	}
	/* hi < lo */
	/* base[0..lo-1] = base[0..hi] <= pivot < base[hi+1..nelt-1]
	 *                                      = base[lo..nelt-1]
	 * 0 < lo = hi+1; lo=nelt possible if all entries identical */
	/* t = base[0]; */ base[0] = base[lo-1]; base[lo-1] = pivot;
	/* lo-1 < nelt */
	while (0 <= hi && base[hi] == pivot)
		hi--;
	/* hi < 0 || (0 < hi && base[hi] < pivot) */
	if (0 < hi) quicksort(base,hi+1);
	if (lo < nelt)
		quicksort(base+lo,nelt-lo); /* nelt-1 - (hi+1) + 1 */
}