php-5.6.26源代码 - hash存储结构 - hash算法

 

// zend_inline_hash_func 实现在文件“php-5.6.26\Zend\zend_hash.h”
    h = zend_inline_hash_func(arKey, nKeyLength); 
    {
        // zend_hash_add 定义在文件“php-5.6.26\Zend\zend_hash.h”
        /*
         * 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>
         */
        
        static inline ulong zend_inline_hash_func(const char *arKey, uint nKeyLength)
        {
            register ulong hash = 5381;
        
            /* variant with the hash unrolled eight times */
            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 7: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
                case 6: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
                case 5: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
                case 4: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
                case 3: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
                case 2: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
                case 1: hash = ((hash << 5) + hash) + *arKey++; break;
                case 0: break;
        EMPTY_SWITCH_DEFAULT_CASE()
            }
            return hash;
        }
    }