2 Copyright (C) 2000, 2001 Free Software Foundation, Inc.
4 This file is part of Wget.
6 This program is free software; you can redistribute it and/or modify
7 it under the terms of the GNU General Public License as published by
8 the Free Software Foundation; either version 2 of the License, or (at
9 your option) any later version.
11 This program 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
14 GNU General Public License for more details.
16 You should have received a copy of the GNU General Public License
17 along with this program; if not, write to the Free Software
18 Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */
28 #endif /* HAVE_STRING_H */
42 # define xmalloc malloc
43 # define xrealloc realloc
49 Hash tables are an implementation technique used to implement
50 mapping between objects. Provided a good hashing function is used,
51 they guarantee constant-time access and storing of information.
52 Duplicate keys are not allowed.
54 The basics are all covered. hash_table_new creates a hash table,
55 and hash_table_destroy deletes it. hash_table_put establishes a
56 mapping between a key and a value. hash_table_get retrieves the
57 value that corresponds to a key. hash_table_exists queries whether
58 a key is stored in a table at all. hash_table_remove removes a
59 mapping that corresponds to a key. hash_table_map allows you to
60 map through all the entries in a hash table. hash_table_clear
61 clears all the entries from the hash table.
63 The number of mappings in a table is not limited, except by the
64 amount of memory. As you add new elements to a table, it regrows
65 as necessary. If you have an idea about how many elements you will
66 store, you can provide a hint to hash_table_new().
68 The hashing and equality functions are normally provided by the
69 user. For the special (and frequent) case of hashing strings, you
70 can use the pre-canned make_string_hash_table(), which provides the
71 string hashing function from the Dragon Book, and a string equality
72 wrapper around strcmp().
74 When specifying your own hash and test functions, make sure the
77 - The test function returns non-zero for keys that are considered
78 "equal", zero otherwise.
80 - The hash function returns a number that represents the
81 "distinctness" of the object. In more precise terms, it means
82 that for any two objects that test "equal" under the test
83 function, the hash function MUST produce the same result.
85 This does not mean that each distinct object must produce a
86 distinct value, only that non-distinct objects must produce the
87 same values! For instance, a hash function that returns 0 for
88 any given object is a perfectly valid (albeit extremely bad) hash
89 function. A hash function that hashes a string by adding up all
90 its characters is another example of a valid (but quite bad) hash
93 The above stated rule is quite easy to enforce. For example, if
94 your testing function compares strings case-insensitively, all
95 your function needs to do is lower-case the string characters
96 before calculating a hash. That way you have easily guaranteed
97 that case differences will not result in a different hash.
99 - (optional) Choose the hash function to get as good "spreading" as
100 possible. A good hash function will react to even a small change
101 in its input with a completely different resulting hash.
102 Finally, don't make your hash function extremely slow, because
103 you're then defeating the purpose of hashing.
105 Note that neither keys nor values are copied when inserted into the
106 hash table, so they must exist for the lifetime of the table. This
107 means that e.g. the use of static strings is OK, but objects with a
108 shorter life-time need to be copied (with strdup() or the like in
109 the case of strings) before being inserted. */
113 All the hash mappings (key-value pairs of pointers) are stored in a
114 contiguous array. The position of each mapping is determined by
115 applying the hash function to the key: location = hash(key) % size.
116 If two different keys end up on the same position, the collision is
117 resolved by placing the second mapping at the next empty place in
118 the array following the occupied place. This method of collision
119 resolution is called "linear probing".
121 There are more advanced collision resolution mechanisms (quadratic
122 probing, double hashing), but we don't use them because they
123 involve more non-sequential access to the array, and therefore
124 worse cache behavior. Linear probing works well as long as the
125 fullness/size ratio is kept below 75%. We make sure to regrow or
126 rehash the hash table whenever this threshold is exceeded.
128 Collisions make deletion tricky because finding collisions again
129 relies on new empty spots not being created. That's why
130 hash_table_remove is careful to rehash the mappings that follow the
139 unsigned long (*hash_function) (const void *);
140 int (*test_function) (const void *, const void *);
142 int size; /* size of the array */
143 int count; /* number of non-empty, non-deleted
146 struct mapping *mappings;
149 #define EMPTY_MAPPING_P(mp) ((mp)->key == NULL)
150 #define NEXT_MAPPING(mp, mappings, size) (mp == mappings + (size - 1) \
153 #define LOOP_NON_EMPTY(mp, mappings, size) \
154 for (; !EMPTY_MAPPING_P (mp); mp = NEXT_MAPPING (mp, mappings, size))
156 #define HASH_POSITION(ht, key) (ht->hash_function (key) % ht->size)
158 /* Find a prime near, but greather than or equal to SIZE. */
161 prime_size (int size)
163 static const unsigned long primes [] = {
164 19, 29, 41, 59, 79, 107, 149, 197, 263, 347, 457, 599, 787, 1031,
165 1361, 1777, 2333, 3037, 3967, 5167, 6719, 8737, 11369, 14783,
166 19219, 24989, 32491, 42257, 54941, 71429, 92861, 120721, 156941,
167 204047, 265271, 344857, 448321, 582821, 757693, 985003, 1280519,
168 1664681, 2164111, 2813353, 3657361, 4754591, 6180989, 8035301,
169 10445899, 13579681, 17653589, 22949669, 29834603, 38784989,
170 50420551, 65546729, 85210757, 110774011, 144006217, 187208107,
171 243370577, 316381771, 411296309, 534685237, 695090819, 903618083,
172 1174703521, 1527114613, 1985248999, 2580823717UL, 3355070839UL
175 for (i = 0; i < ARRAY_SIZE (primes); i++)
176 if (primes[i] >= size)
182 /* Create a hash table of INITIAL_SIZE with hash function
183 HASH_FUNCTION and test function TEST_FUNCTION. If you wish to
184 start out with a "small" table which will be regrown as needed,
185 specify 0 as INITIAL_SIZE. */
188 hash_table_new (int initial_size,
189 unsigned long (*hash_function) (const void *),
190 int (*test_function) (const void *, const void *))
192 struct hash_table *ht
193 = (struct hash_table *)xmalloc (sizeof (struct hash_table));
194 ht->hash_function = hash_function;
195 ht->test_function = test_function;
196 ht->size = prime_size (initial_size);
198 ht->mappings = xmalloc (ht->size * sizeof (struct mapping));
199 memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
203 /* Free the data associated with hash table HT. */
206 hash_table_destroy (struct hash_table *ht)
208 xfree (ht->mappings);
212 /* The heart of almost all functions in this file -- find the mapping
213 whose KEY is equal to key, using a linear probing loop. Returns
214 the offset of the mapping in ht->mappings. */
216 static inline struct mapping *
217 find_mapping (struct hash_table *ht, const void *key)
219 struct mapping *mappings = ht->mappings;
221 struct mapping *mp = mappings + HASH_POSITION (ht, key);
222 int (*equals) (const void *, const void *) = ht->test_function;
224 LOOP_NON_EMPTY (mp, mappings, size)
225 if (equals (key, mp->key))
230 /* Get the value that corresponds to the key KEY in the hash table HT.
231 If no value is found, return NULL. Note that NULL is a legal value
232 for value; if you are storing NULLs in your hash table, you can use
233 hash_table_exists to be sure that a (possibly NULL) value exists in
234 the table. Or, you can use hash_table_get_pair instead of this
238 hash_table_get (struct hash_table *ht, const void *key)
240 struct mapping *mp = find_mapping (ht, key);
247 /* Like hash_table_get, but writes out the pointers to both key and
248 value. Returns non-zero on success. */
251 hash_table_get_pair (struct hash_table *ht, const void *lookup_key,
252 void *orig_key, void *value)
254 struct mapping *mp = find_mapping (ht, lookup_key);
259 *(void **)orig_key = mp->key;
261 *(void **)value = mp->value;
268 /* Return 1 if KEY exists in HT, 0 otherwise. */
271 hash_table_exists (struct hash_table *ht, const void *key)
273 return find_mapping (ht, key) != NULL;
276 #define MAX(i, j) (((i) >= (j)) ? (i) : (j))
278 /* Grow hash table HT as necessary, and rehash all the key-value
282 grow_hash_table (struct hash_table *ht)
284 struct mapping *old_mappings = ht->mappings;
285 struct mapping *old_end = ht->mappings + ht->size;
287 int old_count = ht->count; /* for assert() below */
290 printf ("growing from %d to %d\n", ht->size, prime_size (ht->size * 2));
293 ht->size = prime_size (ht->size * 2);
295 ht->mappings = xmalloc (ht->size * sizeof (struct mapping));
296 memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
298 /* Need to reset this; hash_table_put will reinitialize it. */
301 for (mp = old_mappings; mp < old_end; mp++)
302 if (!EMPTY_MAPPING_P (mp))
303 hash_table_put (ht, mp->key, mp->value);
305 assert (ht->count == old_count);
306 xfree (old_mappings);
309 /* Put VALUE in the hash table HT under the key KEY. This regrows the
310 table if necessary. */
313 hash_table_put (struct hash_table *ht, const void *key, void *value)
315 struct mapping *mappings = ht->mappings;
317 int (*equals) (const void *, const void *) = ht->test_function;
319 struct mapping *mp = mappings + HASH_POSITION (ht, key);
321 LOOP_NON_EMPTY (mp, mappings, size)
322 if (equals (key, mp->key))
324 mp->key = (void *)key; /* const? */
330 mp->key = (void *)key; /* const? */
333 if (ht->count > ht->size * 3 / 4)
334 /* When table is 75% full, regrow it. */
335 grow_hash_table (ht);
338 /* Remove a mapping that matches KEY from HT. Return 0 if there was
339 no such entry; return 1 if an entry was removed. */
342 hash_table_remove (struct hash_table *ht, const void *key)
344 struct mapping *mp = find_mapping (ht, key);
350 struct mapping *mappings = ht->mappings;
355 /* Rehash all the entries following MP. The alternative
356 approach is to mark entry as deleted, but that leaves a lot
357 of garbage. More importantly, this method makes
358 hash_table_get and hash_table_put measurably faster. */
360 mp = NEXT_MAPPING (mp, mappings, size);
361 LOOP_NON_EMPTY (mp, mappings, size)
363 const void *key2 = mp->key;
364 struct mapping *mp_new = mappings + HASH_POSITION (ht, key2);
366 /* Find the new location for the key. */
368 LOOP_NON_EMPTY (mp_new, mappings, size)
369 if (key2 == mp_new->key)
370 /* The mapping MP (key2) is already where we want it (in
371 MP_NEW's "chain" of keys.) */
384 /* Clear HT of all entries. After calling this function, the count
385 and the fullness of the hash table will be zero. The size will
389 hash_table_clear (struct hash_table *ht)
391 memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
395 /* Map MAPFUN over all the mappings in hash table HT. MAPFUN is
396 called with three arguments: the key, the value, and the CLOSURE.
398 It is undefined what happens if you add or remove entries in the
399 hash table while hash_table_map is running. The exception is the
400 entry you're currently mapping over; you may remove or change that
404 hash_table_map (struct hash_table *ht,
405 int (*mapfun) (void *, void *, void *),
408 struct mapping *mp = ht->mappings;
409 struct mapping *end = ht->mappings + ht->size;
411 for (; mp < end; mp++)
412 if (!EMPTY_MAPPING_P (mp))
417 if (mapfun (key, mp->value, closure))
419 if (mp->key != key && !EMPTY_MAPPING_P (mp))
424 /* Return the number of elements in the hash table. This is not the
425 same as the physical size of the hash table, which is always
426 greater than the number of elements. */
428 hash_table_count (struct hash_table *ht)
433 /* Support for hash tables whose keys are strings. */
435 /* 31 bit hash function. Taken from Gnome's glib. This seems to
436 perform much better than the above. */
438 string_hash (const void *key)
444 for (p += 1; *p != '\0'; p++)
445 h = (h << 5) - h + *p;
451 /* If I ever need it: hashing of integers. */
454 inthash (unsigned int key)
469 string_cmp (const void *s1, const void *s2)
471 return !strcmp ((const char *)s1, (const char *)s2);
474 /* Return a hash table of initial size INITIAL_SIZE suitable to use
478 make_string_hash_table (int initial_size)
480 return hash_table_new (initial_size, string_hash, string_cmp);
490 print_hash_table_mapper (void *key, void *value, void *count)
493 printf ("%s: %s\n", (const char *)key, (char *)value);
498 print_hash (struct hash_table *sht)
501 hash_table_map (sht, print_hash_table_mapper, &debug_count);
502 assert (debug_count == sht->count);
508 struct hash_table *ht = make_string_hash_table (0);
510 while ((fgets (line, sizeof (line), stdin)))
512 int len = strlen (line);
516 if (!hash_table_exists (ht, line))
517 hash_table_put (ht, strdup (line), "here I am!");
522 if (hash_table_get_pair (ht, line, &line_copy, NULL))
524 hash_table_remove (ht, line);
534 printf ("%d %d\n", ht->count, ht->size);