REDIS源码中一些值得学习的技术细节02

1. Redis中散列函数的实现：

Redis针对整数key和字符串key，采用了不同的散列函数

对于整数key，redis使用了 Thomas Wang的 32 bit Mix Function，实现了dict.c/dictIntHashFunction函数：

 1 /* Thomas Wang‘s 32 bit Mix Function */
 2 unsigned int dictIntHashFunction(unsigned int key)
 3 {
 4     key += ~(key << 15);
 5     key ^=  (key >> 10);
 6     key +=  (key << 3);
 7     key ^=  (key >> 6);
 8     key += ~(key << 11);
 9     key ^=  (key >> 16);
10     return key;
11 }

这段代码的妙处我还没来得及仔细研究，等研究好了会在这里补上，不过找到了两个初看还不错的链接：

首先是Thomas Wang大神本人的链接:

http://web.archive.org/web/20071223173210/http://www.concentric.net/~Ttwang/tech/inthash.htm

再者是他人根据上面链接和其他资料写的总结

http://blog.csdn.net/jasper_xulei/article/details/18364313

对于字符串形式的key，redis使用了MurmurHash2算法和djb算法:

MurmurHash2算法对于key是大小写敏感的，而且在大端机器和小端机器上生成结果不一致

redis的dict.c/dictGenHashFunction是MurmurHash2算法的C语言实现：

 1 unsigned int dictGenHashFunction(const void *key, int len) {
 2     /* ‘m‘ and ‘r‘ are mixing constants generated offline.
 3      They‘re not really ‘magic‘, they just happen to work well.  */
 4     uint32_t seed = dict_hash_function_seed;
 5     const uint32_t m = 0x5bd1e995;
 6     const int r = 24;
 7
 8     /* Initialize the hash to a ‘random‘ value */
 9     uint32_t h = seed ^ len;
10
11     /* Mix 4 bytes at a time into the hash */
12     const unsigned char *data = (const unsigned char *)key;
13
14     while(len >= 4) {
15         uint32_t k = *(uint32_t*)data;
16
17         k *= m;
18         k ^= k >> r;
19         k *= m;
20
21         h *= m;
22         h ^= k;
23
24         data += 4;
25         len -= 4;
26     }
27
28     /* Handle the last few bytes of the input array  */
29     switch(len) {
30     case 3: h ^= data[2] << 16;
31     case 2: h ^= data[1] << 8;
32     case 1: h ^= data[0]; h *= m;
33     };
34
35     /* Do a few final mixes of the hash to ensure the last few
36      * bytes are well-incorporated. */
37     h ^= h >> 13;
38     h *= m;
39     h ^= h >> 15;
40
41     return (unsigned int)h;
42 }

而redis则借助djb函数实现了不区分大小写的散列函数dict.c/dictGenCaseHashFunction：

1 unsigned int dictGenCaseHashFunction(const unsigned char *buf, int len) {
2     unsigned int hash = (unsigned int)dict_hash_function_seed;
3
4     while (len--)
5         hash = ((hash << 5) + hash) + (tolower(*buf++)); /* hash * 33 + c */
6     return hash;
7 }

以上三个散列函数(dictIntHashFunction, dictIntHashFunction, dictGenCaseHashFunction)分别用在了redis的不同地方，用以实现了不同场合下的散列需求，接下来将会详细介绍。

2.Redis中不同场合下几种不同散列函数的使用