1 /* Implement simple hashing table with string based keys.
2 Copyright (C) 1994-2022 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
compute_hashval(const void * key,size_t keylen)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 }
43