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[m6w6/libmemcached] / libmemcached / hsieh_hash.c
1 /* By Paul Hsieh (C) 2004, 2005. Covered under the Paul Hsieh
2 * derivative license.
3 * See: http://www.azillionmonkeys.com/qed/weblicense.html for license
4 * details.
5 * http://www.azillionmonkeys.com/qed/hash.html
6 */
7
8 #include "common.h"
9
10 #undef get16bits
11 #if (defined(__GNUC__) && defined(__i386__))
12 #define get16bits(d) (*((const uint16_t *) (d)))
13 #endif
14
15 #if !defined (get16bits)
16 #define get16bits(d) ((((uint32_t)(((const uint8_t *)(d))[1])) << 8)\
17 +(uint32_t)(((const uint8_t *)(d))[0]) )
18 #endif
19
20 uint32_t hsieh_hash(char *key, size_t key_length)
21 {
22 uint32_t hash = 0, tmp;
23 int rem;
24
25 if (key_length <= 0 || key == NULL) return 0;
26
27 rem = key_length & 3;
28 key_length >>= 2;
29
30 /* Main loop */
31 for (;key_length > 0; key_length--) {
32 hash += get16bits (key);
33 tmp = (get16bits (key+2) << 11) ^ hash;
34 hash = (hash << 16) ^ tmp;
35 key += 2*sizeof (uint16_t);
36 hash += hash >> 11;
37 }
38
39 /* Handle end cases */
40 switch (rem) {
41 case 3: hash += get16bits (key);
42 hash ^= hash << 16;
43 hash ^= key[sizeof (uint16_t)] << 18;
44 hash += hash >> 11;
45 break;
46 case 2: hash += get16bits (key);
47 hash ^= hash << 11;
48 hash += hash >> 17;
49 break;
50 case 1: hash += *key;
51 hash ^= hash << 10;
52 hash += hash >> 1;
53 }
54
55 /* Force "avalanching" of final 127 bits */
56 hash ^= hash << 3;
57 hash += hash >> 5;
58 hash ^= hash << 4;
59 hash += hash >> 17;
60 hash ^= hash << 25;
61 hash += hash >> 6;
62
63 return hash;
64 }
65