murmurhash.h

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/*  murmurhash.h
 *
 *  Copyright (C) 2023 abxh
 *
 *  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.
 *  See the file LICENSE included with this distribution for more
 *  information. */
 
#pragma once
 
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
 
static inline uint32_t internal_murmur_32_scramble(uint32_t k)
{
    k *= 0xcc9e2d51;
    k = (k << 15) | (k >> 17);
    k *= 0x1b873593;
    return k;
}
 
static inline uint32_t murmur3_32(const uint8_t *key_ptr, const uint32_t len, const uint32_t seed)
{
    uint32_t h = seed;
    uint32_t k;
 
    /* Read in groups of 4. */
    for (size_t i = len >> 2; i; i--) {
        // Here is a source of differing results across endiannesses.
        // A swap here has no effects on hash properties though.
        memcpy(&k, key_ptr, sizeof(uint32_t));
        key_ptr += sizeof(uint32_t);
        h ^= internal_murmur_32_scramble(k);
        h = (h << 13) | (h >> 19);
        h = h * 5 + 0xe6546b64;
    }
 
    /* Read the rest. */
    k = 0;
    for (size_t i = len & 3; i; i--) {
        k <<= 8;
        k |= key_ptr[i - 1];
    }
 
    // A swap is *not* necessary here because the preceding loop already
    // places the low bytes in the low places according to whatever
    // endianness we use. Swaps only apply when the memory is copied in a
    // chunk.
    h ^= internal_murmur_32_scramble(k);
 
    /* Finalize. */
    h ^= len;
    h ^= h >> 16;
    h *= 0x85ebca6b;
    h ^= h >> 13;
    h *= 0xc2b2ae35;
    h ^= h >> 16;
    return h;
}
 
// vim: ft=c