1 /* Copyright (C) 1993-2023 Free Software Foundation, Inc.
2 This file is part of the GNU C Library.
3
4 The GNU C Library is free software; you can redistribute it and/or
5 modify it under the terms of the GNU Lesser General Public
6 License as published by the Free Software Foundation; either
7 version 2.1 of the License, or (at your option) any later version.
8
9 The GNU C Library is distributed in the hope that it will be useful,
10 but WITHOUT ANY WARRANTY; without even the implied warranty of
11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 Lesser General Public License for more details.
13
14 You should have received a copy of the GNU Lesser General Public
15 License along with the GNU C Library; if not, see
16 <https://www.gnu.org/licenses/>. */
17
18 #include <errno.h>
19 #include <malloc.h>
20 #include <string.h>
21 #include <stdint.h>
22 #include <search.h>
23 #include <limits.h>
24
25 /* [Aho,Sethi,Ullman] Compilers: Principles, Techniques and Tools, 1986
26 [Knuth] The Art of Computer Programming, part 3 (6.4) */
27
28
29 /* The reentrant version has no static variables to maintain the state.
30 Instead the interface of all functions is extended to take an argument
31 which describes the current status. */
32 typedef struct _ENTRY
33 {
34 unsigned int used;
35 ENTRY entry;
36 }
37 _ENTRY;
38
39
40 /* For the used double hash method the table size has to be a prime. To
41 correct the user given table size we need a prime test. This trivial
42 algorithm is adequate because
43 a) the code is (most probably) called a few times per program run and
44 b) the number is small because the table must fit in the core */
45 static int
46 isprime (unsigned int number)
47 {
48 /* no even number will be passed */
49 for (unsigned int div = 3; div <= number / div; div += 2)
50 if (number % div == 0)
51 return 0;
52 return 1;
53 }
54
55 /* Before using the hash table we must allocate memory for it.
56 Test for an existing table are done. We allocate one element
57 more as the found prime number says. This is done for more effective
58 indexing as explained in the comment for the hsearch function.
59 The contents of the table is zeroed, especially the field used
60 becomes zero. */
61 int
62 __hcreate_r (size_t nel, struct hsearch_data *htab)
63 {
64 /* Test for correct arguments. */
65 if (htab == NULL)
66 {
67 __set_errno (EINVAL);
68 return 0;
69 }
70
71 /* There is still another table active. Return with error. */
72 if (htab->table != NULL)
73 return 0;
74
75 /* We need a size of at least 3. Otherwise the hash functions we
76 use will not work. */
77 if (nel < 3)
78 nel = 3;
79
80 /* Change nel to the first prime number in the range [nel, UINT_MAX - 2],
81 The '- 2' means 'nel += 2' cannot overflow. */
82 for (nel |= 1; ; nel += 2)
83 {
84 if (UINT_MAX - 2 < nel)
85 {
86 __set_errno (ENOMEM);
87 return 0;
88 }
89 if (isprime (nel))
90 break;
91 }
92
93 htab->size = nel;
94 htab->filled = 0;
95
96 /* allocate memory and zero out */
97 htab->table = (_ENTRY *) calloc (htab->size + 1, sizeof (_ENTRY));
98 if (htab->table == NULL)
99 return 0;
100
101 /* everything went alright */
102 return 1;
103 }
104 libc_hidden_def (__hcreate_r)
105 weak_alias (__hcreate_r, hcreate_r)
106
107
108 /* After using the hash table it has to be destroyed. The used memory can
109 be freed and the local static variable can be marked as not used. */
110 void
111 __hdestroy_r (struct hsearch_data *htab)
112 {
113 /* Test for correct arguments. */
114 if (htab == NULL)
115 {
116 __set_errno (EINVAL);
117 return;
118 }
119
120 /* Free used memory. */
121 free (htab->table);
122
123 /* the sign for an existing table is an value != NULL in htable */
124 htab->table = NULL;
125 }
126 libc_hidden_def (__hdestroy_r)
127 weak_alias (__hdestroy_r, hdestroy_r)
128
129
130 /* This is the search function. It uses double hashing with open addressing.
131 The argument item.key has to be a pointer to an zero terminated, most
132 probably strings of chars. The function for generating a number of the
133 strings is simple but fast. It can be replaced by a more complex function
134 like ajw (see [Aho,Sethi,Ullman]) if the needs are shown.
135
136 We use an trick to speed up the lookup. The table is created by hcreate
137 with one more element available. This enables us to use the index zero
138 special. This index will never be used because we store the first hash
139 index in the field used where zero means not used. Every other value
140 means used. The used field can be used as a first fast comparison for
141 equality of the stored and the parameter value. This helps to prevent
142 unnecessary expensive calls of strcmp. */
143 int
144 __hsearch_r (ENTRY item, ACTION action, ENTRY **retval,
145 struct hsearch_data *htab)
146 {
147 unsigned int hval;
148 unsigned int count;
149 unsigned int len = strlen (item.key);
150 unsigned int idx;
151
152 /* Compute an value for the given string. Perhaps use a better method. */
153 hval = len;
154 count = len;
155 while (count-- > 0)
156 {
157 hval <<= 4;
158 hval += item.key[count];
159 }
160 if (hval == 0)
161 ++hval;
162
163 /* First hash function: simply take the modulo but prevent zero. */
164 idx = hval % htab->size + 1;
165
166 if (htab->table[idx].used)
167 {
168 /* Further action might be required according to the action value. */
169 if (htab->table[idx].used == hval
170 && strcmp (item.key, htab->table[idx].entry.key) == 0)
171 {
172 *retval = &htab->table[idx].entry;
173 return 1;
174 }
175
176 /* Second hash function, as suggested in [Knuth] */
177 unsigned int hval2 = 1 + hval % (htab->size - 2);
178 unsigned int first_idx = idx;
179
180 do
181 {
182 /* Because SIZE is prime this guarantees to step through all
183 available indices. */
184 if (idx <= hval2)
185 idx = htab->size + idx - hval2;
186 else
187 idx -= hval2;
188
189 /* If we visited all entries leave the loop unsuccessfully. */
190 if (idx == first_idx)
191 break;
192
193 /* If entry is found use it. */
194 if (htab->table[idx].used == hval
195 && strcmp (item.key, htab->table[idx].entry.key) == 0)
196 {
197 *retval = &htab->table[idx].entry;
198 return 1;
199 }
200 }
201 while (htab->table[idx].used);
202 }
203
204 /* An empty bucket has been found. */
205 if (action == ENTER)
206 {
207 /* If table is full and another entry should be entered return
208 with error. */
209 if (htab->filled == htab->size)
210 {
211 __set_errno (ENOMEM);
212 *retval = NULL;
213 return 0;
214 }
215
216 htab->table[idx].used = hval;
217 htab->table[idx].entry = item;
218
219 ++htab->filled;
220
221 *retval = &htab->table[idx].entry;
222 return 1;
223 }
224
225 __set_errno (ESRCH);
226 *retval = NULL;
227 return 0;
228 }
229 libc_hidden_def (__hsearch_r)
230 weak_alias (__hsearch_r, hsearch_r)