Redis 字典用到的哈希函数

Redis 字典用到的哈希函数

Hash Functions

http://www.cse.yorku.ca/~oz/hash.html

/* MurmurHash2, by Austin Appleby
 * 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.
 */
unsigned int dictGenHashFunction(const void *key, int len) {
    /* 'm' and 'r' are mixing constants generated offline.
     They're not really 'magic', they just happen to work well.  */
    uint32_t seed = dict_hash_function_seed;
    const uint32_t m = 0x5bd1e995;
    const int r = 24;


    /* Initialize the hash to a 'random' value */
    uint32_t h = seed ^ len;


    /* Mix 4 bytes at a time into the hash */
    const unsigned char *data = (const unsigned char *)key;


    while(len >= 4) {
        uint32_t k = *(uint32_t*)data;


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


        h *= m;
        h ^= k;


        data += 4;
        len -= 4;
    }


    /* Handle the last few bytes of the input array  */
    switch(len) {
    case 3: h ^= data[2] << 16;
    case 2: h ^= data[1] << 8;
    case 1: h ^= data[0]; h *= m;
    };


    /* 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 (unsigned int)h;
}


/* And a case insensitive hash function (based on djb hash) */
unsigned int dictGenCaseHashFunction(const unsigned char *buf, int len) {
    unsigned int hash = (unsigned int)dict_hash_function_seed;


    while (len--)
        hash = ((hash << 5) + hash) + (tolower(*buf++)); /* hash * 33 + c */
    return hash;
}

参考下面说明：

A comprehensive collection of hash functions, a hash visualiser and some test results [see Mckenzie et al. Selecting a Hashing Algorithm, SP&E 20(2):209-224, Feb 1990] will be available someday. If you just want to have a good hash function, and cannot wait, djb2 is one of the best string hash functions i know. it has excellent distribution and speed on many different sets of keys and table sizes. you are not likely to do better with one of the "well known" functions such as PJW, K&R[1], etc. Also see tpop pp. 126 for graphing hash functions.

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djb2

this algorithm (k=33) was first reported by dan bernstein many years ago in comp.lang.c. another version of this algorithm (now favored by bernstein) uses xor: hash(i) = hash(i - 1) * 33 ^ str[i]; the magic of number 33 (why it works better than many other constants, prime or not) has never been adequately explained.

    unsigned long    hash(unsigned char *str)    {        unsigned long hash = 5381;        int c;        while (c = *str++)            hash = ((hash << 5) + hash) + c; /* hash * 33 + c */        return hash;    }

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sdbm

this algorithm was created for sdbm (a public-domain reimplementation of ndbm) database library. it was found to do well in scrambling bits, causing better distribution of the keys and fewer splits. it also happens to be a good general hashing function with good distribution. the actual function is hash(i) = hash(i - 1) * 65599 + str[i]; what is included below is the faster version used in gawk. [there is even a faster, duff-device version] the magic constant 65599 was picked out of thin air while experimenting with different constants, and turns out to be a prime. this is one of the algorithms used in berkeley db (see sleepycat) and elsewhere.

    static unsigned long    sdbm(str)    unsigned char *str;    {        unsigned long hash = 0;        int c;        while (c = *str++)            hash = c + (hash << 6) + (hash << 16) - hash;        return hash;    }

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lose lose

This hash function appeared in K&R (1st ed) but at least the reader was warned: "This is not the best possible algorithm, but it has the merit of extreme simplicity." This is an understatement; It is a terrible hashing algorithm, and it could have been much better without sacrificing its "extreme simplicity." [see the second edition!] Many C programmers use this function without actually testing it, or checking something like Knuth's Sorting and Searching, so it stuck. It is now found mixed with otherwise respectable code, eg. cnews. sigh. [see also: tpop]

    unsigned long    hash(unsigned char *str)    {unsigned int hash = 0;int c;while (c = *str++)    hash += c;return hash;    }

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