快速排序的一种高雅写法

最近看源码，发现了一段快速排序的代码，挺有意思，一起来看看：

 
 
static void
_pqsort(void *a, size_t n, size_t es,
    int (*cmp) (const void *, const void *), void *lrange, void *rrange)
{
	char *pa, *pb, *pc, *pd, *pl, *pm, *pn;
	size_t d, r;
	int swaptype, cmp_result;
 
loop:	SWAPINIT(a, es);
	if (n < 7) {
		for (pm = (char *) a + es; pm < (char *) a + n * es; pm += es)
			for (pl = pm; pl > (char *) a && cmp(pl - es, pl) > 0;
			     pl -= es)
				swap(pl, pl - es);
		return;
	}
	pm = (char *) a + (n / 2) * es;
	if (n > 7) {
		pl = (char *) a;
		pn = (char *) a + (n - 1) * es;
		if (n > 40) {
			d = (n / 8) * es;
			pl = med3(pl, pl + d, pl + 2 * d, cmp);
			pm = med3(pm - d, pm, pm + d, cmp);
			pn = med3(pn - 2 * d, pn - d, pn, cmp);
		}
		pm = med3(pl, pm, pn, cmp);
	}
	swap(a, pm);
	pa = pb = (char *) a + es;
 
	pc = pd = (char *) a + (n - 1) * es;
	for (;;) {
		while (pb <= pc && (cmp_result = cmp(pb, a)) <= 0) {
			if (cmp_result == 0) {
				swap(pa, pb);
				pa += es;
			}
			pb += es;
		}
		while (pb <= pc && (cmp_result = cmp(pc, a)) >= 0) {
			if (cmp_result == 0) {
				swap(pc, pd);
				pd -= es;
			}
			pc -= es;
		}
		if (pb > pc)
			break;
		swap(pb, pc);
		pb += es;
		pc -= es;
	}
 
	pn = (char *) a + n * es;
	r = min(pa - (char *) a, pb - pa);
	vecswap(a, pb - r, r);
	r = min((size_t)(pd - pc), pn - pd - es);
	vecswap(pb, pn - r, r);
	if ((r = pb - pa) > es) {
                void *_l = a, *_r = ((unsigned char*)a)+r-1;
                if (!((lrange < _l && rrange < _l) ||
                    (lrange > _r && rrange > _r)))
		    _pqsort(a, r / es, es, cmp, lrange, rrange);
        }
	if ((r = pd - pc) > es) {
                void *_l, *_r;
 
		/* Iterate rather than recurse to save stack space */
		a = pn - r;
		n = r / es;
 
                _l = a;
                _r = ((unsigned char*)a)+r-1;
                if (!((lrange < _l && rrange < _l) ||
                    (lrange > _r && rrange > _r)))
		    goto loop;
	}
/*		qsort(pn - r, r / es, es, cmp);*/
}
 
void
pqsort(void *a, size_t n, size_t es,
    int (*cmp) (const void *, const void *), size_t lrange, size_t rrange)
{
    _pqsort(a,n,es,cmp,((unsigned char*)a)+(lrange*es),
                       ((unsigned char*)a)+((rrange+1)*es)-1);
}

快速排序很常见，在笔试面试中也很重要。