PHP的hash函数使用的是DJBX33A(也叫time33),算法如下:
static inline unsigned long hash_func(char *arKey, unsigned int nKeyLength)
{
register long hash = 5381;
while (nKeyLength) {
hash = ((hash << 5) + hash) + *arKey++;
nKeyLength--;
}
return hash;
}
其中arKey为键值, nKeyLength为键值的长度.
算法基本原理为, hash值初始值为5381, 再重复nKeyLength次操作((hash << 5) + hash) + *arKey++. 而PHP在此基础上进行了改进以提升性能.
static inline unsigned long zend_hash_func(char *arKey, unsigned int nKeyLength)
{
register long hash = 5381;
for (; nKeyLength >= 8; nKeyLength -= 8) {
hash = ((hash << 5) + hash) + *arKey++;
hash = ((hash << 5) + hash) + *arKey++;
hash = ((hash << 5) + hash) + *arKey++;
hash = ((hash << 5) + hash) + *arKey++;
hash = ((hash << 5) + hash) + *arKey++;
hash = ((hash << 5) + hash) + *arKey++;
hash = ((hash << 5) + hash) + *arKey++;
hash = ((hash << 5) + hash) + *arKey++;
}
switch(nKeyLength) {
case 8: hash = ((hash << 5) + hash) + *arKey++;
case 7: hash = ((hash << 5) + hash) + *arKey++;
case 6: hash = ((hash << 5) + hash) + *arKey++;
case 5: hash = ((hash << 5) + hash) + *arKey++;
case 4: hash = ((hash << 5) + hash) + *arKey++;
case 3: hash = ((hash << 5) + hash) + *arKey++;
case 2: hash = ((hash << 5) + hash) + *arKey++;
case 1: hash = ((hash << 5) + hash) + *arKey++; break;
case 0: break;
}
return hash;
}
即, nKeyLength大于8时,每八次计算一次. 小于8时,计算nKeyLength次.