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a7b65cdc | 1 | /* Implement simple hashing table with string based keys. |
688903eb | 2 | Copyright (C) 1994-2018 Free Software Foundation, Inc. |
a7b65cdc UD |
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 | |
59ba27a6 PE |
17 | License along with the GNU C Library; if not, see |
18 | <http://www.gnu.org/licenses/>. */ | |
a7b65cdc | 19 | |
a3f9038c RM |
20 | #ifndef hashval_t |
21 | # define hashval_t unsigned long int | |
405550bf | 22 | #endif |
a3f9038c | 23 | #include <limits.h> /* For CHAR_BIT. */ |
405550bf | 24 | |
a3f9038c RM |
25 | hashval_t |
26 | compute_hashval (const void *key, size_t keylen) | |
a7b65cdc UD |
27 | { |
28 | size_t cnt; | |
a3f9038c | 29 | hashval_t hval; |
a7b65cdc UD |
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 | { | |
a3f9038c | 39 | hval = (hval << 9) | (hval >> (sizeof hval * CHAR_BIT - 9)); |
a3a6c312 | 40 | hval += (hashval_t) ((const unsigned char *) key)[cnt++]; |
a7b65cdc | 41 | } |
a3f9038c | 42 | return hval != 0 ? hval : ~((hashval_t) 0); |
a7b65cdc | 43 | } |