MYUtilities / MurmurHash.c

/*
 *  MurmurHash.c
 *  MYUtilities
 *
 *  This file created by Jens Alfke on 3/17/08.
 *  Algorithm & source code by Austin Appleby, released to public domain.
 *  <http://murmurhash.googlepages.com/>
 *  Downloaded 3/16/2008.
 *  Modified slightly by Jens Alfke (use standard uint32_t and size_t types;
 *  change 'm' and 'r' to #defines for better C compatibility.)
 *
 */

#include "MurmurHash.h"


//-----------------------------------------------------------------------------
// MurmurHash2, by Austin Appleby

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

// 1. We can read a 4-byte value from any address without crashing
// 2. sizeof(int) == 4      **Jens: I fixed this by changing 'unsigned int' to 'uint32_t'**

// And it has a few limitations -

// 1. It will not work incrementally.
// 2. It will not produce the same results on little-endian and big-endian
//    machines.

uint32_t MurmurHash2 ( const void * key, size_t len, uint32_t seed )
{
    // 'm' and 'r' are mixing constants generated offline.
    // They're not really 'magic', they just happen to work well.
    
    #define m 0x5bd1e995
    #define r 24
    
    // Initialize the hash to a 'random' value
    
    uint32_t h = seed ^ len;
    
    // Mix 4 bytes at a time into the hash
    
    const unsigned char * data = (const unsigned char *)key;
    
    while(len >= 4)
    {
        uint32_t k = *(uint32_t *)data;
        
        k *= m; 
        k ^= k >> r; 
        k *= m; 
        
        h *= m; 
        h ^= k;
        
        data += 4;
        len -= 4;
    }
    
    // Handle the last few bytes of the input array
    
    switch(len)
    {
	case 3: h ^= data[2] << 16;
	case 2: h ^= data[1] << 8;
	case 1: h ^= data[0];
                h *= m;
    };
    
    // Do a few final mixes of the hash to ensure the last few
    // bytes are well-incorporated.
    
    h ^= h >> 13;
    h *= m;
    h ^= h >> 15;
    
    return h;
} 
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