time33 / DJBX33A (Daniel J. Bernstein, Times 33 with Addition) APR


0x00 介绍

  这是很出名的times33哈希算法,也成为DJBX33A (Daniel J. Bernstein, Times 33 with Addition),此算法被perl语言采用并在Berkeley DB中出现。它是已知的最好的哈希算法之一,在处理以字符串为键值的哈希时,有着极快的计算效率和很好哈希分布,而且分类非常好(冲突小,分布均匀)。

0x01 算法代码

uint32_t time33(char const *str, int len) 
{ 
    unsigned long  hash = 0; 
    for (int i = 0; i < len; i++) { 
        hash = hash *33 + (unsigned long) str[i]; 
    } 
    return hash; 
}

位操作实现

unsigned long time33(char const *str, int len) 
{ 
    unsigned long  hash = 0; 
    for (int i = 0; i < len; i++) { 
        hash = ((hash <<5) + hash) + (unsigned long) str[i]; 
    } 
    return hash; 
}

0x02 为什么是33 ?

  DJBX33A (Daniel J. Bernstein, Times 33 with Addition)
 
  This is Daniel J. Bernstein's popular `times 33' hash function as
  posted by him years ago on comp.lang.c. It basically uses a function
  like ``hash(i) = hash(i-1) * 33 + str[i]''. This is one of the best
  known hash functions for strings. Because it is both computed very
  fast and distributes very well.
 
  The magic of number 33, i.e. why it works better than many other
  constants, prime or not, has never been adequately explained by
  anyone. So I try an explanation: if one experimentally tests all
  multipliers between 1 and 256 (as RSE did now) one detects that even
  numbers are not useable at all. The remaining 128 odd numbers
  (except for the number 1) work more or less all equally well. They
  all distribute in an acceptable way and this way fill a hash table
  with an average percent of approx. 86%.
 
  If one compares the Chi^2 values of the variants, the number 33 not
  even has the best value. But the number 33 and a few other equally
  good numbers like 17, 31, 63, 127 and 129 have nevertheless a great
  advantage to the remaining numbers in the large set of possible
  multipliers: their multiply operation can be replaced by a faster
  operation based on just one shift plus either a single addition
  or subtraction operation. And because a hash function has to both
  distribute good _and_ has to be very fast to compute, those few
  numbers should be preferred and seems to be the reason why Daniel J.
  Bernstein also preferred it.
 
                   -- Ralf S. Engelschall <rse@engelschall.com>

标签: time33, djbx33a, djbx33a apr