locale/hashval.h

   1 /* Implement simple hashing table with string based keys.
   2    Copyright (C) 1994-1997, 2000, 2001, 2002 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4    Written by Ulrich Drepper <drepper@gnu.ai.mit.edu>, October 1994.
   5
   6    The GNU C Library is free software; you can redistribute it and/or
   7    modify it under the terms of the GNU Lesser General Public
   8    License as published by the Free Software Foundation; either
   9    version 2.1 of the License, or (at your option) any later version.
  10
  11    The GNU C Library is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14    Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser General Public
  17    License along with the GNU C Library; if not, write to the Free
  18    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  19    02111-1307 USA.  */
  20
  21 #ifndef LONGBITS
  22 # include <limits.h>
  23 # define LONGBITS (sizeof (long int) * CHAR_BIT)
  24 #endif
  25
  26 unsigned long int compute_hashval (const void *key, size_t keylen);
  27
  28 unsigned long int
  29 compute_hashval (key, keylen)
  30      const void *key;
  31      size_t keylen;
  32 {
  33   size_t cnt;
  34   unsigned long int hval;
  35
  36   /* Compute the hash value for the given string.  The algorithm
  37      is taken from [Aho,Sethi,Ullman], modified to reduce the number of
  38      collisions for short strings with very varied bit patterns.
  39      See http://www.clisp.org/haible/hashfunc.html.  */
  40   cnt = 0;
  41   hval = keylen;
  42   while (cnt < keylen)
  43     {
  44       hval = (hval << 9) | (hval >> (LONGBITS - 9));
  45       hval += (unsigned long int) *(((char *) key) + cnt++);
  46     }
  47   return hval != 0 ? hval : ~((unsigned long int) 0);
  48 }