php hashtable的hash算法

<pre name="code" class="cpp">/*
 * 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 <[email protected]>
*/
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;
}

在php源代碼Zend/zend_hash.h文件裏定義了這個名叫DJBX33A的hash算法

hash(x) = hash(x-1) *33 + string[x];

初始值爲5381;