This is a port of the Murmur3 hash function. Murmur3 is a non-cryptographic hash, designed to be fast and excellent-quality for making things like hash tables or bloom filters. This is a port of the original C++ code, designed for Visual Studio, into standard C that gcc can compile efficiently.

Just compile and link your program with murmur3.c, and be sure to include murmur3.h to get the function prototypes. There are three hash functions:

void MurmurHash3_x86_32 (const void *key, int len, uint32_t seed, void *out);

void MurmurHash3_x86_128(const void *key, int len, uint32_t seed, void *out);

void MurmurHash3_x64_128(const void *key, int len, uint32_t seed, void *out);

All three of these functions have the same interface: you give them key, a pointer to the data you wish to hash; len, the length in bytes; seed, an arbitrary seed number which you can use to tweak the hash, and out, a pointer to a buffer big enough to hold the hash's output value.

The hash functions differ in both their internal mechanisms and in their outputs. They are specialized for different use cases:

MurmurHash3_x86_32 has the lowest throughput, but also the lowest latency. If you're making a hash table that usually has small keys, this is probably the one you want to use on 32-bit machines. It has a 32-bit output.

MurmurHash3_x86_128 is also designed for 32-bit systems, but produces a 128-bit output, and has about 30% higher throughput than the previous hash. Be warned, though, that its latency for a single 16-byte key is about 86% longer!

MurmurHash3_x64_128 is the best of the lot, if you're using a 64-bit machine. Its throughput is 250% higher than MurmurHash3_x86_32, but it has roughly the same latency. It has a 128-bit output.

The hash functions are designed to work efficiently on x86 processors; in particular, they make some assumptions about the endianness of the processor, and about the speed of unaligned reads. If you have problems running this code on non-x86 architectures, it should be possible to modify it to work correctly and efficiently -- I just don't have access to those machines for testing. The code in murmur3.c is pretty straightforward, and shouldn't be too hard to alter.

There is an example program, example.c, which you can look at and play with. You can build it with the makefile.

All this code is in the public domain. Murmur3 was created by Austin Appleby, and the C port and general tidying up was done by Peter Scott. If you'd like to contribute something, I would love to add your name to this list.