Wang/Jenkins Hash算法在网上提到的也甚多,但是很少有人或有文章能系统地能将该算法的来龙去脉说明白。于是,我就充当了该苦工,幸好还是找到了一些东西,尝试着将该算法简单道来。

最早,Bob Jenkins提出了多个基于字符串通用Hash算法(搜Jenkins Hash就知道了),而Thomas Wang在Jenkins的基础上,针对固定整数输入做了相应的Hash算法。因而,其名字也就成了Wang/Jenkins Hash,其64位版本的 Hash算法如下:

uint64_t hash(uint64_t key) {
key = (~key) + (key << 21); // key = (key << 21) - key - 1;
key = key ^ (key >> 24);
key = (key + (key << 3)) + (key << 8); // key * 265
key = key ^ (key >> 14);
key = (key + (key << 2)) + (key << 4); // key * 21
key = key ^ (key >> 28);
key = key + (key << 31);
return key;
}

其关键特性是:

  • 1.雪崩性(更改输入参数的任何一位,就将引起输出有一半以上的位发生变化)
  • 2.可逆性(input ==> hash ==> inverse_hash ==> input)

其逆Hash函数为:

uint64_t inverse_hash(uint64_t key) {
uint64_t tmp;

// Invert key = key + (key << 31)
tmp = key-(key<<31);
key = key-(tmp<<31);

// Invert key = key ^ (key >> 28)
tmp = key^key>>28;
key = key^tmp>>28;

// Invert key *= 21
key *= 14933078535860113213u;

// Invert key = key ^ (key >> 14)
tmp = key^key>>14;
tmp = key^tmp>>14;
tmp = key^tmp>>14;
key = key^tmp>>14;

// Invert key *= 265
key *= 15244667743933553977u;

// Invert key = key ^ (key >> 24)
tmp = key^key>>24;
key = key^tmp>>24;

// Invert key = (~key) + (key << 21)
tmp = ~key;
tmp = ~(key-(tmp<<21));
tmp = ~(key-(tmp<<21));
key = ~(key-(tmp<<21));

return key;
}

由上述的算法实现可知,原始的hash算法过程是非常快的,而其逆Hash算法则比较慢一些。

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