erl_hash

#include "murmur2.h"

/*
 * Same implementation as in: https://github.com/Netflix/dynomite/blob/dev/src/hashkit/dyn_murmur.c
 *
 * "Murmur" hash provided by Austin, tanjent@gmail.com
 * http://murmurhash.googlepages.com/
 *
 * 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.
 *
 *  Updated to murmur2 hash - BP
 */

uint32_t hash_murmur2(const unsigned char *key, size_t length)
{
    /*
     * '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 uint32_t seed = (0xdeadbeef * (uint32_t)length);
    const int r = 24;

    /* Initialize the hash to a 'random' value */

    uint32_t h = seed ^ (uint32_t)length;

    /* Mix 4 bytes at a time into the hash */

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

    while (length >= 4) {
        unsigned int k = *(unsigned int *)data;

        k *= m;
        k ^= k >> r;
        k *= m;

        h *= m;
        h ^= k;

        data += 4;
        length -= 4;
    }

    /* Handle the last few bytes of the input array */

    switch (length) {
        case 3:
            h ^= ((uint32_t)data[2]) << 16;

        case 2:
            h ^= ((uint32_t)data[1]) << 8;

        case 1:
            h ^= data[0];
            h *= m;

        default:
            break;
    };

    /*
    * 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;
}