sdbm_hash.c

来自「apache 安装教程 apache 安装教程」· C语言 代码 · 共 49 行

/*
 * sdbm - ndbm work-alike hashed database library
 * based on Per-Aake Larson's Dynamic Hashing algorithms. BIT 18 (1978).
 * author: [e-mail]
 * status: public domain. keep it that way.
 *
 * hashing routine
 */

#include "sdbm.h"
/*
 * polynomial conversion ignoring overflows
 * [this seems to work remarkably well, in fact better
 * then the ndbm hash function. Replace at your own risk]
 * use: 65599	nice.
 *      65587   even better. 
 */
long
sdbm_hash(str, len)
register char *str;
register int len;
{
	register unsigned long n = 0;

#define DUFF	/* go ahead and use the loop-unrolled version */
#ifdef DUFF

#define HASHC	n = *str++ + 65599 * n

	if (len > 0) {
		register int loop = (len + 8 - 1) >> 3;

		switch(len & (8 - 1)) {
		case 0:	do {
			HASHC;	case 7:	HASHC;
		case 6:	HASHC;	case 5:	HASHC;
		case 4:	HASHC;	case 3:	HASHC;
		case 2:	HASHC;	case 1:	HASHC;
			} while (--loop);
		}

	}
#else
	while (len--)
		n = *str++ + 65599 * n;
#endif
	return n;
}