Jenkins hash
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。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
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。
特征:0.
1.jenkins 32位Hash算法:http://burtleburtle.net/bob/hash/integer.html
2.Geoffrey Irving's blog: http://naml.us/blog/tag/thomas-wang