Simple MurmurHash3 Implementation (from Demetri Spanos)

Murmur3.cpp

// Minimal Murmur3 implementation shared by Demetri Spanos on Handmade Network Discord
//
// Code is deliberately terse and simplistic
// Intended to be the first hash function you reach for e.g. a simple hash table
// *** NB THIS IS NOT A CRYPTOGRAPHIC HASH ***
//
// @demetrispanos:
// "yes let me reiterate the moral of this story
// there is never any reason to use a dumb made up hash function
// use murmur3 or jenkins-one-at-a-time for a 0-effort version
// both are quite good, though not quite as good as modern state of the art
// but modern state of the art ones aren't as simple (i.e. not 20 lines long)
// banish "TODO: better hash function" from your life
// just use my 20 line murmur3"

static inline uint32_t rotl32 (uint32_t x, int8_t r) { return (x << r) | (x >> (32 - r)); }

static inline uint32_t fmix (uint32_t h ) {
	h ^= h >> 16; h *= 0x85ebca6b;
	h ^= h >> 13; h *= 0xc2b2ae35;
	return h ^= h >> 16;
}

uint32_t m32 (const void *key, uint32_t len, uint32_t h1) {
	const uint8_t *tail = (const uint8_t*)key + (len/4)*4; // handle this separately

	uint32_t c1 = 0xcc9e2d51, c2 = 0x1b873593;

	// body (full 32-bit blocks) handled uniformly
	for (uint32_t *p = (uint32_t*) key; p < (const uint32_t*) tail; p++) {
	uint32_t k1 = *p; k1 *= c1; k1 = rotl32(k1,15); k1 *= c2; // MUR1
	h1 ^= k1; h1 = rotl32(h1,13); h1 = h1*5+0xe6546b64; // MUR2
	}

	uint32_t t = 0; // handle up to 3 tail bytes
	switch(len & 3) {
	case 3: t ^= tail[2] << 16;
	case 2: t ^= tail[1] << 8;
	case 1: {t ^= tail[0]; t *= c1; t = rotl32(t,15); t *= c2; h1 ^= t;};
	}
	return fmix(h1 ^ len);
}