REDIS源码中一些值得学习的技术细节02

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：

 /* Thomas Wang's 32 bit Mix Function */
 unsigned int dictIntHashFunction(unsigned int key)
 {
     key += ~(key << );
     key ^=  (key >> );
     key +=  (key << );
     key ^=  (key >> );
     key += ~(key << );
     key ^=  (key >> );
     return key;
 }

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：

 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 = ;

     /* 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 >= ) {
         uint32_t k = *(uint32_t*)data;

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

         h *= m;
         h ^= k;

         data += ;
         len -= ;
     }

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

     /* Do a few final mixes of the hash to ensure the last few
      * bytes are well-incorporated. */
     h ^= h >> ;
     h *= m;
     h ^= h >> ;

     return (unsigned int)h;
 }

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：

 unsigned int dictGenCaseHashFunction(const unsigned char *buf, int len) {
     unsigned int hash = (unsigned int)dict_hash_function_seed;

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

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，接下来的文章将会详细介绍。