libvirt/src/util/virhashcode.c

/*
 * virhashcode.c: hash code generation
 *
 * Copyright (C) 2012 Red Hat, Inc.
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library.  If not, see
 * <http://www.gnu.org/licenses/>.
 *
 * The hash code generation is based on the public domain MurmurHash3 from Austin Appleby:
 * http://code.google.com/p/smhasher/source/browse/trunk/MurmurHash3.cpp
 *
 * We use only the 32 bit variant because the 2 produce different results while
 * we need to produce the same result regardless of the architecture as
 * clients can be both 64 or 32 bit at the same time.
 */

#include <config.h>

#include "virhashcode.h"
#include "bitrotate.h"

/* slower than original but handles platforms that do only aligned reads */
static inline uint32_t getblock(const uint8_t *p, int i)
{
    uint32_t r;
    size_t size = sizeof(r);

    memcpy(&r, &p[i * size], size);

    return r;
}

/*
 * Finalization mix - force all bits of a hash block to avalanche
 */
static inline uint32_t fmix(uint32_t h)
{
    h ^= h >> 16;
    h *= 0x85ebca6b;
    h ^= h >> 13;
    h *= 0xc2b2ae35;
    h ^= h >> 16;

    return h;
}


uint32_t virHashCodeGen(const void *key, size_t len, uint32_t seed)
{
    const uint8_t *blocks;
    const uint8_t *tail;
    size_t nblocks;
    uint32_t h1;
    uint32_t k1;
    uint32_t c1;
    uint32_t c2;
    size_t i;

    blocks = (const uint8_t *)key;
    nblocks = len / 4;
    h1 = seed;
    c1 = 0xcc9e2d51;
    c2 = 0x1b873593;

    /* body */

    for (i = 0; i < nblocks; i++) {

        k1 = getblock(blocks, i);

        k1 *= c1;
        k1 = rotl32(k1, 15);
        k1 *= c2;

        h1 ^= k1;
        h1 = rotl32(h1, 13);
        h1 = h1 * 5 + 0xe6546b64;
    }

    /* tail */

    tail = (const uint8_t *)key + nblocks * 4;

    k1 = 0;

    switch (len & 3) {
    case 3:
        k1 ^= tail[2] << 16;
        /* fallthrough */
    case 2:
        k1 ^= tail[1] << 8;
        /* fallthrough */
    case 1:
        k1 ^= tail[0];
        k1 *= c1;
        k1 = rotl32(k1, 15);
        k1 *= c2;
        h1 ^= k1;
        /* fallthrough */
    default:
        break;
    }

    /* finalization */

    h1 ^= len;
    h1 = fmix(h1);

    return h1;
}