djb2 hash algorithm.

HISTORY This algorithm (k=33) was first reported by Dan Bernstein many years ago in comp.lang.c. Another version of this algorithm (now favored by bernstein) uses XOR:

 hash(i) = hash(i - 1) * 33 ^ str[i];

The magic of number 33 (why it works better than many other constants, prime or not) has never been adequately explained.

Parameters

buf	input buffer to hash
len	length of buffer

Returns: 32 bit sized hash