Bob Hash

BobHash散列算法测评

最新推荐文章于 2021-05-18 13:08:41 发布

weixin_30509393

最新推荐文章于 2021-05-18 13:08:41 发布

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CC 4.0 BY-SA版权

文章标签： java

原文链接：http://www.cnblogs.com/napoleon_liu/archive/2010/12/22/1913832.html

本文对比了BobHash与time33两种散列算法。BobHash在5万多数据集上未出现冲突，展现出良好的散列特性，尽管其速度较time33慢约3倍。通过代码实现展示了BobHash的具体操作过程。

Bob 研究了很多哈希，并实现了自己的高效hash。我测试一下了，5w多的数据，time33出现了6条重复，bobhash没有一条重复。不过time33的速度是比bobhash快的。

http://burtleburtle.net/bob/hash/doobs.html

源码

typedef unsigned int uint32_t;

#define hashsize(n) ((uint32_t)1<<(n))
#define hashmask(n) (hashsize(n)-1)

#define mix(a,b,c) \
{ \
a -= b; a -= c; a ^= (c>>13); \
b -= c; b -= a; b ^= (a<<8); \
c -= a; c -= b; c ^= (b>>13); \
a -= b; a -= c; a ^= (c>>12); \
b -= c; b -= a; b ^= (a<<16); \
c -= a; c -= b; c ^= (b>>5); \
a -= b; a -= c; a ^= (c>>3); \
b -= c; b -= a; b ^= (a<<10); \
c -= a; c -= b; c ^= (b>>15); \
}

/* same, but slower, works on systems that might have 8 byte ub4's */
#define mix2(a,b,c) \
{ \
a -= b; a -= c; a ^= (c>>13); \
b -= c; b -= a; b ^= (a<< 8); \
c -= a; c -= b; c ^= ((b&0xffffffff)>>13); \
a -= b; a -= c; a ^= ((c&0xffffffff)>>12); \
b -= c; b -= a; b = (b ^ (a<<16)) & 0xffffffff; \
c -= a; c -= b; c = (c ^ (b>> 5)) & 0xffffffff; \
a -= b; a -= c; a = (a ^ (c>> 3)) & 0xffffffff; \
b -= c; b -= a; b = (b ^ (a<<10)) & 0xffffffff; \
c -= a; c -= b; c = (c ^ (b>>15)) & 0xffffffff; \
}

inline
uint32_t bob_hash(char const *k, int length, uint32_t initval)
{
   uint32_t a,b,c,len;

   /* Set up the internal state */
   len = length;
   a = b = 0x9e3779b9; /* the golden ratio; an arbitrary value */
   c = initval;           /* the previous hash value */

   /*---------------------------------------- handle most of the key */
   while (len >= 3)
   {
      a += k[0];
      b += k[1];
      c += k[2];
      mix(a,b,c);
      k += 3; len -= 3;
   }

   /*-------------------------------------- handle the last 2 ub4's */
   c += (length<<2);   /* <<2 to produce the same results as hash() */
   switch(len)              /* all the case statements fall through */
   {
     /* c is reserved for the length */
   case 2 : b+=k[1];
   case 1 : a+=k[0];
     /* case 0: nothing left to add */
   }
   mix(a,b,c);
   /*-------------------------------------------- report the result */
   return c;
}