75 lines
1.6 KiB
C++
75 lines
1.6 KiB
C++
#ifndef __CXXMPH_MURMUR_HASH2__
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#define __CXXMPH_MURMUR_HASH2__
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//-----------------------------------------------------------------------------
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// MurmurHash2, by Austin Appleby
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// Note - This code makes a few assumptions about how your machine behaves -
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// 1. We can read a 4-byte value from any address without crashing
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// 2. sizeof(int) == 4
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// And it has a few limitations -
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// 1. It will not work incrementally.
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// 2. It will not produce the same results on little-endian and big-endian
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// machines.
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namespace cxxmph {
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inline // not measured, for making compilation easier only
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unsigned int MurmurHash2 ( const void * key, int len, unsigned int seed )
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{
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// 'm' and 'r' are mixing constants generated offline.
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// They're not really 'magic', they just happen to work well.
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const unsigned int m = 0x5bd1e995;
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const int r = 24;
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// Initialize the hash to a 'random' value
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unsigned int h = seed ^ len;
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// Mix 4 bytes at a time into the hash
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const unsigned char * data = (const unsigned char *)key;
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while(len >= 4)
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{
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unsigned int k = *(unsigned int *)data;
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k *= m;
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k ^= k >> r;
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k *= m;
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h *= m;
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h ^= k;
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data += 4;
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len -= 4;
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}
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// Handle the last few bytes of the input array
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switch(len)
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{
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case 3: h ^= data[2] << 16;
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case 2: h ^= data[1] << 8;
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case 1: h ^= data[0];
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h *= m;
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};
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// Do a few final mixes of the hash to ensure the last few
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// bytes are well-incorporated.
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h ^= h >> 13;
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h *= m;
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h ^= h >> 15;
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return h;
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}
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} // namespace cxxmph
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#endif // __CXXMPH_MURMUR_HASH2__
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