2 Copyright (C) 2000, 2001 Free Software Foundation, Inc.
4 This file is part of GNU Wget.
6 GNU Wget 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 GNU Wget 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 Wget; 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_contains queries
58 whether a key is stored in a table at all. hash_table_remove
59 removes a mapping that corresponds to a key. hash_table_map allows
60 you to map through all the entries in a hash table.
61 hash_table_clear 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 an
71 efficient string hashing function, and a string equality wrapper
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 int resize_threshold; /* after size exceeds this number of
147 entries, resize the table. */
149 struct mapping *mappings;
152 #define EMPTY_MAPPING_P(mp) ((mp)->key == NULL)
153 #define NEXT_MAPPING(mp, mappings, size) (mp == mappings + (size - 1) \
156 #define LOOP_NON_EMPTY(mp, mappings, size) \
157 for (; !EMPTY_MAPPING_P (mp); mp = NEXT_MAPPING (mp, mappings, size))
159 /* #### We might want to multiply with the "golden ratio" here to get
160 better randomness for keys that do not result from a good hash
161 function. This is currently not a problem in Wget because we only
162 use the string hash tables. */
164 #define HASH_POSITION(ht, key) (ht->hash_function (key) % ht->size)
166 /* Find a prime near, but greather than or equal to SIZE. */
169 prime_size (int size)
171 static const unsigned long primes [] = {
172 19, 29, 41, 59, 79, 107, 149, 197, 263, 347, 457, 599, 787, 1031,
173 1361, 1777, 2333, 3037, 3967, 5167, 6719, 8737, 11369, 14783,
174 19219, 24989, 32491, 42257, 54941, 71429, 92861, 120721, 156941,
175 204047, 265271, 344857, 448321, 582821, 757693, 985003, 1280519,
176 1664681, 2164111, 2813353, 3657361, 4754591, 6180989, 8035301,
177 10445899, 13579681, 17653589, 22949669, 29834603, 38784989,
178 50420551, 65546729, 85210757, 110774011, 144006217, 187208107,
179 243370577, 316381771, 411296309, 534685237, 695090819, 903618083,
180 1174703521, 1527114613, 1985248999, 2580823717UL, 3355070839UL
183 for (i = 0; i < ARRAY_SIZE (primes); i++)
184 if (primes[i] >= size)
190 /* Create a hash table of INITIAL_SIZE with hash function
191 HASH_FUNCTION and test function TEST_FUNCTION. INITIAL_SIZE will
192 be rounded to the next prime, so you don't have to worry about it
193 being a prime number.
195 Consequently, if you wish to start out with a "small" table which
196 will be regrown as needed, specify INITIAL_SIZE 0. */
199 hash_table_new (int initial_size,
200 unsigned long (*hash_function) (const void *),
201 int (*test_function) (const void *, const void *))
203 struct hash_table *ht
204 = (struct hash_table *)xmalloc (sizeof (struct hash_table));
206 ht->hash_function = hash_function;
207 ht->test_function = test_function;
209 ht->size = prime_size (initial_size);
210 ht->resize_threshold = ht->size * 3 / 4;
214 ht->mappings = xmalloc (ht->size * sizeof (struct mapping));
215 memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
220 /* Free the data associated with hash table HT. */
223 hash_table_destroy (struct hash_table *ht)
225 xfree (ht->mappings);
229 /* The heart of almost all functions in this file -- find the mapping
230 whose KEY is equal to key, using linear probing. Returns the
231 mapping that matches KEY, or NULL if none matches. */
233 static inline struct mapping *
234 find_mapping (struct hash_table *ht, const void *key)
236 struct mapping *mappings = ht->mappings;
238 struct mapping *mp = mappings + HASH_POSITION (ht, key);
239 int (*equals) (const void *, const void *) = ht->test_function;
241 LOOP_NON_EMPTY (mp, mappings, size)
242 if (equals (key, mp->key))
247 /* Get the value that corresponds to the key KEY in the hash table HT.
248 If no value is found, return NULL. Note that NULL is a legal value
249 for value; if you are storing NULLs in your hash table, you can use
250 hash_table_contains to be sure that a (possibly NULL) value exists
251 in the table. Or, you can use hash_table_get_pair instead of this
255 hash_table_get (struct hash_table *ht, const void *key)
257 struct mapping *mp = find_mapping (ht, key);
264 /* Like hash_table_get, but writes out the pointers to both key and
265 value. Returns non-zero on success. */
268 hash_table_get_pair (struct hash_table *ht, const void *lookup_key,
269 void *orig_key, void *value)
271 struct mapping *mp = find_mapping (ht, lookup_key);
276 *(void **)orig_key = mp->key;
278 *(void **)value = mp->value;
285 /* Return 1 if HT contains KEY, 0 otherwise. */
288 hash_table_contains (struct hash_table *ht, const void *key)
290 return find_mapping (ht, key) != NULL;
293 /* Grow hash table HT as necessary, and rehash all the key-value
297 grow_hash_table (struct hash_table *ht)
299 struct mapping *old_mappings = ht->mappings;
300 struct mapping *old_end = ht->mappings + ht->size;
301 struct mapping *mp, *mappings;
304 newsize = prime_size (ht->size * 2);
306 printf ("growing from %d to %d\n", ht->size, newsize);
310 ht->resize_threshold = newsize * 3 / 4;
312 mappings = xmalloc (ht->size * sizeof (struct mapping));
313 memset (mappings, '\0', ht->size * sizeof (struct mapping));
314 ht->mappings = mappings;
316 for (mp = old_mappings; mp < old_end; mp++)
317 if (!EMPTY_MAPPING_P (mp))
319 struct mapping *new_mp = mappings + HASH_POSITION (ht, mp->key);
320 /* We don't need to call test function and worry about
321 collisions because all the keys come from the hash table
322 and are therefore guaranteed to be unique. */
323 LOOP_NON_EMPTY (new_mp, mappings, newsize)
328 xfree (old_mappings);
331 /* Put VALUE in the hash table HT under the key KEY. This regrows the
332 table if necessary. */
335 hash_table_put (struct hash_table *ht, const void *key, void *value)
337 struct mapping *mappings = ht->mappings;
339 int (*equals) (const void *, const void *) = ht->test_function;
341 struct mapping *mp = mappings + HASH_POSITION (ht, key);
343 LOOP_NON_EMPTY (mp, mappings, size)
344 if (equals (key, mp->key))
346 mp->key = (void *)key; /* const? */
352 mp->key = (void *)key; /* const? */
355 if (ht->count > ht->resize_threshold)
356 /* When table is 75% full, regrow it. */
357 grow_hash_table (ht);
360 /* Remove a mapping that matches KEY from HT. Return 0 if there was
361 no such entry; return 1 if an entry was removed. */
364 hash_table_remove (struct hash_table *ht, const void *key)
366 struct mapping *mp = find_mapping (ht, key);
372 struct mapping *mappings = ht->mappings;
377 /* Rehash all the entries following MP. The alternative
378 approach is to mark the entry as deleted, i.e. create a
379 "tombstone". That makes remove faster, but leaves a lot of
380 garbage and slows down hash_table_get and hash_table_put. */
382 mp = NEXT_MAPPING (mp, mappings, size);
383 LOOP_NON_EMPTY (mp, mappings, size)
385 const void *key2 = mp->key;
386 struct mapping *mp_new = mappings + HASH_POSITION (ht, key2);
388 /* Find the new location for the key. */
390 LOOP_NON_EMPTY (mp_new, mappings, size)
391 if (key2 == mp_new->key)
392 /* The mapping MP (key2) is already where we want it (in
393 MP_NEW's "chain" of keys.) */
406 /* Clear HT of all entries. After calling this function, the count
407 and the fullness of the hash table will be zero. The size will
411 hash_table_clear (struct hash_table *ht)
413 memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
417 /* Map MAPFUN over all the mappings in hash table HT. MAPFUN is
418 called with three arguments: the key, the value, and the CLOSURE.
420 It is undefined what happens if you add or remove entries in the
421 hash table while hash_table_map is running. The exception is the
422 entry you're currently mapping over; you may remove or change that
426 hash_table_map (struct hash_table *ht,
427 int (*mapfun) (void *, void *, void *),
430 struct mapping *mp = ht->mappings;
431 struct mapping *end = ht->mappings + ht->size;
433 for (; mp < end; mp++)
434 if (!EMPTY_MAPPING_P (mp))
439 if (mapfun (key, mp->value, closure))
441 /* hash_table_remove might have moved the adjacent
443 if (mp->key != key && !EMPTY_MAPPING_P (mp))
448 /* Return the number of elements in the hash table. This is not the
449 same as the physical size of the hash table, which is always
450 greater than the number of elements. */
453 hash_table_count (struct hash_table *ht)
458 /* Functions from this point onward are meant for convenience and
459 don't strictly belong to this file. However, this is as good a
460 place for them as any. */
463 Support for hash tables whose keys are strings.
466 /* 31 bit hash function. Taken from Gnome's glib, modified to use
469 We used to use the popular hash function from the Dragon Book, but
470 this one seems to perform much better. */
473 string_hash (const void *key)
479 for (p += 1; *p != '\0'; p++)
480 h = (h << 5) - h + *p;
485 /* Frontend for strcmp usable for hash tables. */
488 string_cmp (const void *s1, const void *s2)
490 return !strcmp ((const char *)s1, (const char *)s2);
493 /* Return a hash table of initial size INITIAL_SIZE suitable to use
497 make_string_hash_table (int initial_size)
499 return hash_table_new (initial_size, string_hash, string_cmp);
503 Support for hash tables whose keys are strings, but which are
504 compared case-insensitively.
507 /* Like string_hash, but produce the same hash regardless of the case. */
510 string_hash_nocase (const void *key)
513 unsigned int h = TOLOWER (*p);
516 for (p += 1; *p != '\0'; p++)
517 h = (h << 5) - h + TOLOWER (*p);
522 /* Like string_cmp, but doing case-insensitive compareison. */
525 string_cmp_nocase (const void *s1, const void *s2)
527 return !strcasecmp ((const char *)s1, (const char *)s2);
530 /* Like make_string_hash_table, but uses string_hash_nocase and
531 string_cmp_nocase. */
534 make_nocase_string_hash_table (int initial_size)
536 return hash_table_new (initial_size, string_hash_nocase, string_cmp_nocase);
540 /* If I ever need it: hashing of integers. */
543 inthash (unsigned int key)
563 print_hash_table_mapper (void *key, void *value, void *count)
566 printf ("%s: %s\n", (const char *)key, (char *)value);
571 print_hash (struct hash_table *sht)
574 hash_table_map (sht, print_hash_table_mapper, &debug_count);
575 assert (debug_count == sht->count);
581 struct hash_table *ht = make_string_hash_table (0);
583 while ((fgets (line, sizeof (line), stdin)))
585 int len = strlen (line);
589 if (!hash_table_contains (ht, line))
590 hash_table_put (ht, strdup (line), "here I am!");
595 if (hash_table_get_pair (ht, line, &line_copy, NULL))
597 hash_table_remove (ht, line);
607 printf ("%d %d\n", ht->count, ht->size);