hash.cc revision 5821806d5e7f356e8fa4b058a389a808ea183019 
1// From http://www.azillionmonkeys.com/qed/hash.html
2
3#include "base/hash.h"
4
5typedef uint32 uint32_t;
6typedef uint16 uint16_t;
7
8namespace base {
9
10#undef get16bits
11#if (defined(__GNUC__) && defined(__i386__)) || defined(__WATCOMC__) \
12  || defined(_MSC_VER) || defined (__BORLANDC__) || defined (__TURBOC__)
13#define get16bits(d) (*((const uint16_t *) (d)))
14#endif
15
16#if !defined (get16bits)
17#define get16bits(d) ((((uint32_t)(((const uint8_t *)(d))[1])) << 8)\
18                       +(uint32_t)(((const uint8_t *)(d))[0]) )
19#endif
20
21uint32 SuperFastHash(const char * data, int len) {
22  uint32_t hash = len, tmp;
23  int rem;
24
25  if (len <= 0 || data == NULL)
26    return 0;
27
28  rem = len & 3;
29  len >>= 2;
30
31  /* Main loop */
32  for (; len > 0; len--) {
33    hash  += get16bits(data);
34    tmp    = (get16bits(data + 2) << 11) ^ hash;
35    hash   = (hash << 16) ^ tmp;
36    data  += 2 * sizeof(uint16_t);
37    hash  += hash >> 11;
38  }
39
40  /* Handle end cases */
41  switch (rem) {
42    case 3:
43      hash += get16bits(data);
44      hash ^= hash << 16;
45
46      // Treat the final character as signed. This ensures all platforms behave
47      // consistently with the original x86 code.
48      hash ^= static_cast<signed char>(data[sizeof(uint16_t)]) << 18;
49      hash += hash >> 11;
50      break;
51    case 2:
52      hash += get16bits(data);
53      hash ^= hash << 11;
54      hash += hash >> 17;
55      break;
56    case 1:
57      hash += static_cast<signed char>(*data);
58      hash ^= hash << 10;
59      hash += hash >> 1;
60  }
61
62  /* Force "avalanching" of final 127 bits */
63  hash ^= hash << 3;
64  hash += hash >> 5;
65  hash ^= hash << 4;
66  hash += hash >> 17;
67  hash ^= hash << 25;
68  hash += hash >> 6;
69
70  return hash;
71}
72
73}  // namespace base
74