murmurhash.c revision 1.6 
1/*	$NetBSD: murmurhash.c,v 1.6 2013/10/26 21:06:38 rmind Exp $	*/
2
3/*
4 * MurmurHash2 -- from the original code:
5 *
6 * "MurmurHash2 was written by Austin Appleby, and is placed in the public
7 * domain. The author hereby disclaims copyright to this source code."
8 *
9 * References:
10 *	http://code.google.com/p/smhasher/
11 *	https://sites.google.com/site/murmurhash/
12 */
13
14#include <sys/cdefs.h>
15
16#if defined(_KERNEL) || defined(_STANDALONE)
17__KERNEL_RCSID(0, "$NetBSD: murmurhash.c,v 1.6 2013/10/26 21:06:38 rmind Exp $");
18
19#else
20
21#if defined(LIBC_SCCS) && !defined(lint)
22__RCSID("$NetBSD: murmurhash.c,v 1.6 2013/10/26 21:06:38 rmind Exp $");
23#endif /* LIBC_SCCS and not lint */
24
25#include "namespace.h"
26#endif
27
28#include <sys/types.h>
29#include <sys/param.h>
30#include <sys/hash.h>
31
32#if !defined(_KERNEL) && !defined(_STANDALONE)
33#ifdef __weak_alias
34__weak_alias(murmurhash2,_murmurhash2)
35#endif
36#endif
37
38uint32_t
39murmurhash2(const void *key, size_t len, uint32_t seed)
40{
41	/*
42	 * Note: 'm' and 'r' are mixing constants generated offline.
43	 * They're not really 'magic', they just happen to work well.
44	 * Initialize the hash to a 'random' value.
45	 */
46	const uint32_t m = 0x5bd1e995;
47	const int r = 24;
48
49	const uint8_t *data = key;
50	uint32_t h = seed ^ (uint32_t)len;
51
52	if (__predict_true(ALIGNED_POINTER(key, uint32_t))) {
53		while (len >= sizeof(uint32_t)) {
54			uint32_t k = *(const uint32_t *)data;
55
56			k *= m;
57			k ^= k >> r;
58			k *= m;
59
60			h *= m;
61			h ^= k;
62
63			data += sizeof(uint32_t);
64			len -= sizeof(uint32_t);
65		}
66	} else {
67		while (len >= sizeof(uint32_t)) {
68			uint32_t k;
69
70			k  = data[0];
71			k |= data[1] << 8;
72			k |= data[2] << 16;
73			k |= data[3] << 24;
74
75			k *= m;
76			k ^= k >> r;
77			k *= m;
78
79			h *= m;
80			h ^= k;
81
82			data += sizeof(uint32_t);
83			len -= sizeof(uint32_t);
84		}
85	}
86
87	/* Handle the last few bytes of the input array. */
88	switch (len) {
89	case 3:
90		h ^= data[2] << 16;
91		/* FALLTHROUGH */
92	case 2:
93		h ^= data[1] << 8;
94		/* FALLTHROUGH */
95	case 1:
96		h ^= data[0];
97		h *= m;
98	}
99
100	/*
101	 * Do a few final mixes of the hash to ensure the last few
102	 * bytes are well-incorporated.
103	 */
104	h ^= h >> 13;
105	h *= m;
106	h ^= h >> 15;
107
108	return h;
109}
110