PHP内核分析－HASH函数

2024年5月8日 59次阅读

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.

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