1 //-----------------------------------------------------------------------------
2 // MurmurHash2 was written by Austin Appleby, and is placed in the public
3 // domain. The author hereby disclaims copyright to this source code.
4
5 // Note - This code makes a few assumptions about how your machine behaves -
6
7 // 1. We can read a 4-byte value from any address without crashing
8 // 2. sizeof(int) == 4
9
10 // And it has a few limitations -
11
12 // 1. It will not work incrementally.
13 // 2. It will not produce the same results on little-endian and big-endian
14 // machines.
15
16 #include "MurmurHash2.h"
17
18 //-----------------------------------------------------------------------------
19 // Platform-specific functions and macros
20
21 // Microsoft Visual Studio
22
23 #if defined(_MSC_VER)
24
25 #define BIG_CONSTANT(x) (x)
26
27 // Other compilers
28
29 #else // defined(_MSC_VER)
30
31 #define BIG_CONSTANT(x) (x##LLU)
32
33 #endif // !defined(_MSC_VER)
34
35 //-----------------------------------------------------------------------------
36
MurmurHash2(const void * key,int len,uint32_t seed)37 uint32_t MurmurHash2 ( const void * key, int len, uint32_t seed )
38 {
39 // 'm' and 'r' are mixing constants generated offline.
40 // They're not really 'magic', they just happen to work well.
41
42 const uint32_t m = 0x5bd1e995;
43 const int r = 24;
44
45 // Initialize the hash to a 'random' value
46
47 uint32_t h = seed ^ len;
48
49 // Mix 4 bytes at a time into the hash
50
51 const unsigned char * data = (const unsigned char *)key;
52
53 while(len >= 4)
54 {
55 uint32_t k = *(uint32_t*)data;
56
57 k *= m;
58 k ^= k >> r;
59 k *= m;
60
61 h *= m;
62 h ^= k;
63
64 data += 4;
65 len -= 4;
66 }
67
68 // Handle the last few bytes of the input array
69
70 switch(len)
71 {
72 case 3: h ^= data[2] << 16;
73 case 2: h ^= data[1] << 8;
74 case 1: h ^= data[0];
75 h *= m;
76 };
77
78 // Do a few final mixes of the hash to ensure the last few
79 // bytes are well-incorporated.
80
81 h ^= h >> 13;
82 h *= m;
83 h ^= h >> 15;
84
85 return h;
86 }
87