locale/hashval.h

   1 /* Implement simple hashing table with string based keys.
   2    Copyright (C) 1994-2024 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #ifndef hashval_t
  20 # define hashval_t unsigned long int
  21 #endif
  22 #include <limits.h>             /* For CHAR_BIT.  */
  23
  24 hashval_t
  25 compute_hashval (const void *key, size_t keylen)
  26 {
  27   size_t cnt;
  28   hashval_t hval;
  29
  30   /* Compute the hash value for the given string.  The algorithm
  31      is taken from [Aho,Sethi,Ullman], modified to reduce the number of
  32      collisions for short strings with very varied bit patterns.
  33      See http://www.clisp.org/haible/hashfunc.html.  */
  34   cnt = 0;
  35   hval = keylen;
  36   while (cnt < keylen)
  37     {
  38       hval = (hval << 9) | (hval >> (sizeof hval * CHAR_BIT - 9));
  39       hval += (hashval_t) ((const unsigned char *) key)[cnt++];
  40     }
  41   return hval != 0 ? hval : ~((hashval_t) 0);
  42 }