usr/src/lib/libast/common/cdt/dtstrhash.c

   1 /***********************************************************************
   2 *                                                                      *
   3 *               This software is part of the ast package               *
   4 *          Copyright (c) 1985-2010 AT&T Intellectual Property          *
   5 *                      and is licensed under the                       *
   6 *                  Common Public License, Version 1.0                  *
   7 *                    by AT&T Intellectual Property                     *
   8 *                                                                      *
   9 *                A copy of the License is available at                 *
  10 *            http://www.opensource.org/licenses/cpl1.0.txt             *
  11 *         (with md5 checksum 059e8cd6165cb4c31e351f2b69388fd9)         *
  12 *                                                                      *
  13 *              Information and Software Systems Research               *
  14 *                            AT&T Research                             *
  15 *                           Florham Park NJ                            *
  16 *                                                                      *
  17 *                 Glenn Fowler <gsf@research.att.com>                  *
  18 *                  David Korn <dgk@research.att.com>                   *
  19 *                   Phong Vo <kpv@research.att.com>                    *
  20 *                                                                      *
  21 ***********************************************************************/
  22 #include        "dthdr.h"
  23
  24 /* Hashing a string into an unsigned integer.
  25 ** The basic method is to continuingly accumulate bytes and multiply
  26 ** with some given prime. The length n of the string is added last.
  27 ** The recurrent equation is like this:
  28 **      h[k] = (h[k-1] + bytes)*prime   for 0 <= k < n
  29 **      h[n] = (h[n-1] + n)*prime
  30 ** The prime is chosen to have a good distribution of 1-bits so that
  31 ** the multiplication will distribute the bits in the accumulator well.
  32 ** The below code accumulates 2 bytes at a time for speed.
  33 **
  34 ** Written by Kiem-Phong Vo (02/28/03)
  35 */
  36
  37 #if __STD_C
  38 uint dtstrhash(reg uint h, Void_t* args, reg int n)
  39 #else
  40 uint dtstrhash(h,args,n)
  41 reg uint        h;
  42 Void_t*         args;
  43 reg int         n;
  44 #endif
  45 {
  46         reg unsigned char*      s = (unsigned char*)args;
  47
  48         if(n <= 0)
  49         {       for(; *s != 0; s += s[1] ? 2 : 1)
  50                         h = (h + (s[0]<<8) + s[1])*DT_PRIME;
  51                 n = s - (unsigned char*)args;
  52         }
  53         else
  54         {       reg unsigned char*      ends;
  55                 for(ends = s+n-1; s < ends; s += 2)
  56                         h = (h + (s[0]<<8) + s[1])*DT_PRIME;
  57                 if(s <= ends)
  58                         h = (h + (s[0]<<8))*DT_PRIME;
  59         }
  60         return (h+n)*DT_PRIME;
  61 }