1 //-----------------------------------------------------------------------------

2 // MurmurHash2, by Austin Appleby

3

4 // Note - This code makes a few assumptions about how your machine behaves -

5

6 // 1. We can read a 4-byte value from any address without crashing

7 // 2. sizeof(int) == 4

8

9 // And it has a few limitations -

10

11 // 1. It will not work incrementally.

12 // 2. It will not produce the same results on little-endian and big-endian

13 // machines.

14

15 unsigned int MurmurHash2 ( const void * key, int len, unsigned int seed )

16 {

17 // 'm' and 'r' are mixing constants generated offline.

18 // They're not really 'magic', they just happen to work well.

19

20 const unsigned int m = 0x5bd1e995;

21 const int r = 24;

22

23 // Initialize the hash to a 'random' value

24

25 unsigned int h = seed ^ len;

26

27 // Mix 4 bytes at a time into the hash

28

29 const unsigned char * data = (const unsigned char *)key;

30

31 while(len >= 4)

32 {

33 unsigned int k = *(unsigned int *)data;

34

35 k *= m;

36 k ^= k >> r;

37 k *= m;

38

39 h *= m;

40 h ^= k;

41

42 data += 4;

43 len -= 4;

44 }

45

46 // Handle the last few bytes of the input array

47

48 switch(len)

49 {

50 case 3: h ^= data[2] << 16;

51 case 2: h ^= data[1] << 8;

52 case 1: h ^= data[0];

53 h *= m;

54 };

55

56 // Do a few final mixes of the hash to ensure the last few

57 // bytes are well-incorporated.

58

59 h ^= h >> 13;

60 h *= m;

61 h ^= h >> 15;

62

63 return h;

64 }