locale/hashval.h

   1 /* Implement simple hashing table with string based keys.
   2    Copyright (C) 1994-2021 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, see
  18    <https://www.gnu.org/licenses/>.  */
  19
  20 #ifndef hashval_t
  21 # define hashval_t unsigned long int
  22 #endif
  23 #include <limits.h>             /* For CHAR_BIT.  */
  24
  25 hashval_t
  26 compute_hashval (const void *key, size_t keylen)
  27 {
  28   size_t cnt;
  29   hashval_t hval;
  30
  31   /* Compute the hash value for the given string.  The algorithm
  32      is taken from [Aho,Sethi,Ullman], modified to reduce the number of
  33      collisions for short strings with very varied bit patterns.
  34      See http://www.clisp.org/haible/hashfunc.html.  */
  35   cnt = 0;
  36   hval = keylen;
  37   while (cnt < keylen)
  38     {
  39       hval = (hval << 9) | (hval >> (sizeof hval * CHAR_BIT - 9));
  40       hval += (hashval_t) ((const unsigned char *) key)[cnt++];
  41     }
  42   return hval != 0 ? hval : ~((hashval_t) 0);
  43 }