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annotate MurmurHash2.c @ 28:18a72865dd78

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marked TODO: doubleTableSize corrupts mem
author Nina Engelhardt <nengel@mailbox.tu-berlin.de>
date Wed, 06 Mar 2013 14:35:01 +0100
parents 1218b245530c
children
rev   line source
Me@14 1
Me@14 2 /*This file is sample code pulled off the web -- NOT part of the compiled code
Me@14 3 * make sure it doesn't get included in makefile 'cause it doesn't compile
Me@14 4 */
Me@14 5
Me@14 6 //-----------------------------------------------------------------------------
Me@14 7 // MurmurHash2, by Austin Appleby
Me@14 8
Me@14 9 // Note - This code makes a few assumptions about how your machine behaves -
Me@14 10
Me@14 11 // 1. We can read a 4-byte value from any address without crashing
Me@14 12 // 2. sizeof(int) == 4
Me@14 13
Me@14 14 // And it has a few limitations -
Me@14 15
Me@14 16 // 1. It will not work incrementally.
Me@14 17 // 2. It will not produce the same results on little-endian and big-endian
Me@14 18 // machines.
Me@14 19
Me@14 20 unsigned int MurmurHash2 ( const void * key, int len, unsigned int seed )
Me@14 21 {
Me@14 22 // 'm' and 'r' are mixing constants generated offline.
Me@14 23 // They're not really 'magic', they just happen to work well.
Me@14 24
Me@14 25 const unsigned int m = 0x5bd1e995;
Me@14 26 const int r = 24;
Me@14 27
Me@14 28 // Initialize the hash to a 'random' value
Me@14 29
Me@14 30 unsigned int h = seed ^ len;
Me@14 31
Me@14 32 // Mix 4 bytes at a time into the hash
Me@14 33
Me@14 34 const unsigned char * data = (const unsigned char *)key;
Me@14 35
Me@14 36 while(len >= 4)
Me@14 37 {
Me@14 38 unsigned int k = *(unsigned int *)data;
Me@14 39
Me@14 40 k *= m;
Me@14 41 k ^= k >> r;
Me@14 42 k *= m;
Me@14 43
Me@14 44 h *= m;
Me@14 45 h ^= k;
Me@14 46
Me@14 47 data += 4;
Me@14 48 len -= 4;
Me@14 49 }
Me@14 50
Me@14 51 // Handle the last few bytes of the input array
Me@14 52
Me@14 53 switch(len)
Me@14 54 {
Me@14 55 case 3: h ^= data[2] << 16;
Me@14 56 case 2: h ^= data[1] << 8;
Me@14 57 case 1: h ^= data[0];
Me@14 58 h *= m;
Me@14 59 };
Me@14 60
Me@14 61 // Do a few final mixes of the hash to ensure the last few
Me@14 62 // bytes are well-incorporated.
Me@14 63
Me@14 64 h ^= h >> 13;
Me@14 65 h *= m;
Me@14 66 h ^= h >> 15;
Me@14 67
Me@14 68 return h;
Me@14 69 }