MurmurHash is a non-cryptographic hash function created by Austin Appleby. [1]
The hash
MurmurHash 2.0 is a relatively simple algorithm, which compiles down to about 52 instruction on the x86. It is noted[2] for being exceptionally fast, often two to four times faster than comparable[3] aglorithms such as FNV[4], Jenkins' lookup3[5][6] and Hsieh's SuperFastHash[7], with excellent distribution [8], avalanche behavior [9] and overall collision resistance [10]. A survey of hash functions[11] concluded that "Murmur2 is the only of the complex hash functions that provides good performance for all kinds of keys. It can be recommended as a general-purpose hashing function."
Variations
The first version was MurmurHash 1.0, but it was superceded by 2.0, which Appleby says is "much faster & mixes better." [12]. All have been released into the public domain.
The fastest implementation[13] reads four bytes at a time and generates canonical values only on a processor with Intel [little-endian] byte. For processors that require aligned reads[14] or have Motorola big-endian byte order[15], there are versions that work correctly but at approximately half speed. The algorithm generates a 32-bit hash value and is optimized for processors with 32-bit or larger registers.
Implementation
The algorithm was originally expressed in C++, and has been ported to a number of popular languages, including Python[16], C# and Java[17].
//-----------------------------------------------------------------------------
// 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;
}