dtstrhash.c revision 3e14f97f673e8a630f076077de35afdd43dc1587
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin/***********************************************************************
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin* This software is part of the ast package *
3e14f97f673e8a630f076077de35afdd43dc1587Roger A. Faulkner* Copyright (c) 1985-2010 AT&T Intellectual Property *
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin* and is licensed under the *
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin* Common Public License, Version 1.0 *
7c2fbfb345896881c631598ee3852ce9ce33fb07April Chin* by AT&T Intellectual Property *
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin* A copy of the License is available at *
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin* (with md5 checksum 059e8cd6165cb4c31e351f2b69388fd9) *
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin* Information and Software Systems Research *
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin* AT&T Research *
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin* Florham Park NJ *
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin* Glenn Fowler <gsf@research.att.com> *
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin* David Korn <dgk@research.att.com> *
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin* Phong Vo <kpv@research.att.com> *
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin***********************************************************************/
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin/* Hashing a string into an unsigned integer.
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin** The basic method is to continuingly accumulate bytes and multiply
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin** with some given prime. The length n of the string is added last.
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin** The recurrent equation is like this:
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin** h[k] = (h[k-1] + bytes)*prime for 0 <= k < n
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin** h[n] = (h[n-1] + n)*prime
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin** The prime is chosen to have a good distribution of 1-bits so that
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin** the multiplication will distribute the bits in the accumulator well.
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin** The below code accumulates 2 bytes at a time for speed.
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin** Written by Kiem-Phong Vo (02/28/03)
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin if(n <= 0)
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin n = s - (unsigned char*)args;
da2e3ebdc1edfbc5028edf1354e7dd2fa69a7968chin return (h+n)*DT_PRIME;