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murmurhash2.c


#ifndef MURMURHASH2_C
#define MURMURHASH2_C

#define     MURMURHASH2_DIGEST_LENGTH   4

//-----------------------------------------------------------------------------
// 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

// 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.

unsigned int MurmurHash2 ( const void * key, int len, unsigned int seed )
{
      // 'm' and 'r' are mixing constants generated offline.
      // They're not really 'magic', they just happen to work well.

      const unsigned int m = 0x5bd1e995;
      const int r = 24;

      // Initialize the hash to a 'random' value

      unsigned int h = seed ^ len;

      // Mix 4 bytes at a time into the hash

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

      while(len >= 4)
      {
            unsigned int k = *(unsigned int *)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;
}

#endif

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