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.
20 In addition, as a special exception, the Free Software Foundation
21 gives permission to link the code of its release of Wget with the
22 OpenSSL project's "OpenSSL" library (or with modified versions of it
23 that use the same license as the "OpenSSL" library), and distribute
24 the linked executables. You must obey the GNU General Public License
25 in all respects for all of the code used other than "OpenSSL". If you
26 modify this file, you may extend this exception to your version of the
27 file, but you are not obligated to do so. If you do not wish to do
28 so, delete this exception statement from your version. */
38 #endif /* HAVE_STRING_H */
52 # define xmalloc malloc
53 # define xrealloc realloc
57 # define TOLOWER(x) ('A' <= (x) && (x) <= 'Z' ? (x) - 32 : (x))
62 Hash tables are an implementation technique used to implement
63 mapping between objects. Assuming a good hashing function is used,
64 they provide near-constant-time access and storing of information.
65 Duplicate keys are not allowed.
67 This file defines the following entry points: hash_table_new
68 creates a hash table, and hash_table_destroy deletes it.
69 hash_table_put establishes a mapping between a key and a value.
70 hash_table_get retrieves the value that corresponds to a key.
71 hash_table_contains queries whether a key is stored in a table at
72 all. hash_table_remove removes a mapping that corresponds to a
73 key. hash_table_map allows you to map through all the entries in a
74 hash table. hash_table_clear clears all the entries from the hash
77 The number of mappings in a table is not limited, except by the
78 amount of memory. As you add new elements to a table, it regrows
79 as necessary. If you have an idea about how many elements you will
80 store, you can provide a hint to hash_table_new().
82 The hashing and equality functions depend on the type of key and
83 are normally provided by the user. For the special (and frequent)
84 case of using string keys, you can use the pre-canned
85 make_string_hash_table(), which provides an efficient string
86 hashing function, and a string equality wrapper around strcmp().
88 When specifying your hash and test functions, make sure the
91 - The test function returns non-zero for keys that are considered
92 "equal", zero otherwise.
94 - The hash function returns a number that represents the
95 "distinctness" of the object. In more precise terms, it means
96 that for any two objects that test "equal" under the test
97 function, the hash function MUST produce the same result.
99 This does not mean that each distinct object must produce a
100 distinct value, only that non-distinct objects must produce the
101 same values! For instance, a hash function that returns 0 for
102 any given object is a perfectly valid (albeit extremely bad) hash
103 function. A hash function that hashes a string by adding up all
104 its characters is another example of a valid (but quite bad) hash
107 The above stated rule is quite easy to enforce. For example, if
108 your testing function compares strings case-insensitively, all
109 your function needs to do is lower-case the string characters
110 before calculating a hash. That way you have easily guaranteed
111 that case differences will not result in a different hash.
113 - If you care about performance, choose a hash function with as
114 good "spreading" as possible. A good hash function will react to
115 even a small change in its input with a completely different
116 resulting hash. Finally, don't make the hash function itself
117 overly slow, because you'll be incurring a non-negligible
118 overhead to reads and writes to the hash table.
120 Note that neither keys nor values are copied when inserted into the
121 hash table, so they must exist for the lifetime of the table. This
122 means that e.g. the use of static strings is OK, but objects with a
123 shorter life-time need to be copied (with strdup() or the like in
124 the case of strings) before being inserted. */
128 All the hash mappings (key-value pairs of pointers) are stored in a
129 contiguous array. The position of each mapping is determined by
130 the hash value of its key and the size of the table: location :=
131 hash(key) % size. If two different keys end up on the same
132 position (hash collision), the one that came second is placed at
133 the next empty position following the occupied place. This
134 collision resolution technique is called "linear probing".
136 There are more advanced collision resolution mechanisms (quadratic
137 probing, double hashing), but we don't use them because they incur
138 more non-sequential access to the array, which results in worse
139 cache behavior. Linear probing works well as long as the
140 fullness/size ratio is kept below 75%. We make sure to regrow or
141 rehash the hash table whenever this threshold is exceeded.
143 Collisions make deletion tricky because finding collisions again
144 relies on new empty spots not being created. That's why
145 hash_table_remove is careful to rehash the mappings that follow the
148 /* Maximum allowed fullness: when hash table's fullness exceeds this
149 value, the table is resized. */
150 #define HASH_MAX_FULLNESS 0.75
152 /* The hash table size is multiplied by this factor (and then rounded
153 to the next prime) with each resize. This guarantees infrequent
155 #define HASH_RESIZE_FACTOR 2
163 unsigned long (*hash_function) PARAMS ((const void *));
164 int (*test_function) PARAMS ((const void *, const void *));
166 int size; /* size of the array */
167 int count; /* number of non-empty, non-deleted
170 int resize_threshold; /* after size exceeds this number of
171 entries, resize the table. */
172 int prime_offset; /* the offset of the current prime in
175 struct mapping *mappings; /* the array of mapping pairs. */
178 /* We use all-bit-set marker to mean that a mapping is empty. It is
179 (hopefully) illegal as a pointer, and it allows the users to use
180 NULL (as well as any non-negative integer) as key. */
181 #define NON_EMPTY(mp) (mp->key != (void *)~(unsigned long)0)
183 /* "Next" mapping is the mapping after MP, but wrapping back to
184 MAPPINGS when MP would reach MAPPINGS+SIZE. */
185 #define NEXT_MAPPING(mp, mappings, size) (mp != mappings + (size - 1) \
188 /* Loop over non-empty mappings starting at MP. */
189 #define LOOP_NON_EMPTY(mp, mappings, size) \
190 for (; NON_EMPTY (mp); mp = NEXT_MAPPING (mp, mappings, size))
192 /* #### We might want to multiply with the "golden ratio" here to get
193 better randomness for keys that do not result from a good hash
194 function. This is currently not a problem in Wget because we only
195 use the string hash tables. */
197 #define HASH_POSITION(ht, key) (ht->hash_function (key) % ht->size)
199 /* Find a prime near, but greather than or equal to SIZE. Of course,
200 the primes are not calculated, but looked up from a table. The
201 table does not contain all primes in range, just a selection useful
204 PRIME_OFFSET is a minor optimization: if specified, it starts the
205 search for the prime number beginning with the specific offset in
206 the prime number table. The final offset is stored in the same
210 prime_size (int size, int *prime_offset)
212 static const unsigned long primes [] = {
213 13, 19, 29, 41, 59, 79, 107, 149, 197, 263, 347, 457, 599, 787, 1031,
214 1361, 1777, 2333, 3037, 3967, 5167, 6719, 8737, 11369, 14783,
215 19219, 24989, 32491, 42257, 54941, 71429, 92861, 120721, 156941,
216 204047, 265271, 344857, 448321, 582821, 757693, 985003, 1280519,
217 1664681, 2164111, 2813353, 3657361, 4754591, 6180989, 8035301,
218 10445899, 13579681, 17653589, 22949669, 29834603, 38784989,
219 50420551, 65546729, 85210757, 110774011, 144006217, 187208107,
220 243370577, 316381771, 411296309, 534685237, 695090819, 903618083,
221 1174703521, 1527114613, 1985248999,
222 (unsigned long)0x99d43ea5, (unsigned long)0xc7fa5177
224 int i = *prime_offset;
226 for (; i < countof (primes); i++)
227 if (primes[i] >= size)
229 /* Set the offset to the next prime. That is safe because,
230 next time we are called, it will be with a larger SIZE,
231 which means we could never return the same prime anyway.
232 (If that is not the case, the caller can simply reset
234 *prime_offset = i + 1;
242 static unsigned long ptrhash PARAMS ((const void *));
243 static int ptrcmp PARAMS ((const void *, const void *));
245 /* Create a hash table with hash function HASH_FUNCTION and test
246 function TEST_FUNCTION. The table is empty (its count is 0), but
247 pre-allocated to store at least ITEMS items.
249 ITEMS is the number of items that the table can accept without
250 needing to resize. It is useful when creating a table that is to
251 be immediately filled with a known number of items. In that case,
252 the regrows are a waste of time, and specifying ITEMS correctly
253 will avoid them altogether.
255 Note that hash tables grow dynamically regardless of ITEMS. The
256 only use of ITEMS is to preallocate the table and avoid unnecessary
257 dynamic regrows. Don't bother making ITEMS prime because it's not
258 used as size unchanged. To start with a small table that grows as
259 needed, simply specify zero ITEMS.
261 If HASH_FUNCTION is not provided, identity table is assumed,
262 i.e. key pointers are compared as keys. If you want strings with
263 equal contents to hash the same, use make_string_hash_table. */
266 hash_table_new (int items,
267 unsigned long (*hash_function) (const void *),
268 int (*test_function) (const void *, const void *))
271 struct hash_table *ht = xnew (struct hash_table);
273 ht->hash_function = hash_function ? hash_function : ptrhash;
274 ht->test_function = test_function ? test_function : ptrcmp;
276 /* If the size of struct hash_table ever becomes a concern, this
277 field can go. (Wget doesn't create many hashes.) */
278 ht->prime_offset = 0;
280 /* Calculate the size that ensures that the table will store at
281 least ITEMS keys without the need to resize. */
282 size = 1 + items / HASH_MAX_FULLNESS;
283 size = prime_size (size, &ht->prime_offset);
285 ht->resize_threshold = size * HASH_MAX_FULLNESS;
286 /*assert (ht->resize_threshold >= items);*/
288 ht->mappings = xnew_array (struct mapping, ht->size);
289 /* Mark mappings as empty. We use 0xff rather than 0 to mark empty
290 keys because it allows us to store NULL keys to the table. */
291 memset (ht->mappings, 255, size * sizeof (struct mapping));
298 /* Free the data associated with hash table HT. */
301 hash_table_destroy (struct hash_table *ht)
303 xfree (ht->mappings);
307 /* The heart of most functions in this file -- find the mapping whose
308 KEY is equal to key, using linear probing. Returns the mapping
309 that matches KEY, or the first empty mapping if none matches. */
311 static inline struct mapping *
312 find_mapping (const struct hash_table *ht, const void *key)
314 struct mapping *mappings = ht->mappings;
316 struct mapping *mp = mappings + HASH_POSITION (ht, key);
317 int (*equals) PARAMS ((const void *, const void *)) = ht->test_function;
319 LOOP_NON_EMPTY (mp, mappings, size)
320 if (equals (key, mp->key))
325 /* Get the value that corresponds to the key KEY in the hash table HT.
326 If no value is found, return NULL. Note that NULL is a legal value
327 for value; if you are storing NULLs in your hash table, you can use
328 hash_table_contains to be sure that a (possibly NULL) value exists
329 in the table. Or, you can use hash_table_get_pair instead of this
333 hash_table_get (const struct hash_table *ht, const void *key)
335 struct mapping *mp = find_mapping (ht, key);
342 /* Like hash_table_get, but writes out the pointers to both key and
343 value. Returns non-zero on success. */
346 hash_table_get_pair (const struct hash_table *ht, const void *lookup_key,
347 void *orig_key, void *value)
349 struct mapping *mp = find_mapping (ht, lookup_key);
353 *(void **)orig_key = mp->key;
355 *(void **)value = mp->value;
362 /* Return 1 if HT contains KEY, 0 otherwise. */
365 hash_table_contains (const struct hash_table *ht, const void *key)
367 struct mapping *mp = find_mapping (ht, key);
368 return NON_EMPTY (mp);
371 /* Grow hash table HT as necessary, and rehash all the key-value
375 grow_hash_table (struct hash_table *ht)
377 struct mapping *old_mappings = ht->mappings;
378 struct mapping *old_end = ht->mappings + ht->size;
379 struct mapping *mp, *mappings;
382 newsize = prime_size (ht->size * HASH_RESIZE_FACTOR, &ht->prime_offset);
384 printf ("growing from %d to %d; fullness %.2f%% to %.2f%%\n",
386 100.0 * ht->count / ht->size,
387 100.0 * ht->count / newsize);
391 ht->resize_threshold = newsize * HASH_MAX_FULLNESS;
393 mappings = xnew_array (struct mapping, newsize);
394 memset (mappings, 255, newsize * sizeof (struct mapping));
395 ht->mappings = mappings;
397 for (mp = old_mappings; mp < old_end; mp++)
400 struct mapping *new_mp = mappings + HASH_POSITION (ht, mp->key);
401 /* We don't need to test for uniqueness of keys because they
402 come from the hash table and are therefore known to be
404 LOOP_NON_EMPTY (new_mp, mappings, newsize)
409 xfree (old_mappings);
412 /* Put VALUE in the hash table HT under the key KEY. This regrows the
413 table if necessary. */
416 hash_table_put (struct hash_table *ht, const void *key, void *value)
418 struct mapping *mp = find_mapping (ht, key);
421 /* update existing item */
422 mp->key = (void *)key; /* const? */
427 /* If adding the item would make the table exceed max. fullness,
428 grow the table first. */
429 if (ht->count >= ht->resize_threshold)
431 grow_hash_table (ht);
432 mp = find_mapping (ht, key);
437 mp->key = (void *)key; /* const? */
441 /* Remove a mapping that matches KEY from HT. Return 0 if there was
442 no such entry; return 1 if an entry was removed. */
445 hash_table_remove (struct hash_table *ht, const void *key)
447 struct mapping *mp = find_mapping (ht, key);
453 struct mapping *mappings = ht->mappings;
458 /* Rehash all the entries following MP. The alternative
459 approach is to mark the entry as deleted, i.e. create a
460 "tombstone". That makes remove faster, but leaves a lot of
461 garbage and slows down hash_table_get and hash_table_put. */
463 mp = NEXT_MAPPING (mp, mappings, size);
464 LOOP_NON_EMPTY (mp, mappings, size)
466 const void *key2 = mp->key;
467 struct mapping *mp_new = mappings + HASH_POSITION (ht, key2);
469 /* Find the new location for the key. */
471 LOOP_NON_EMPTY (mp_new, mappings, size)
472 if (key2 == mp_new->key)
473 /* The mapping MP (key2) is already where we want it (in
474 MP_NEW's "chain" of keys.) */
487 /* Clear HT of all entries. After calling this function, the count
488 and the fullness of the hash table will be zero. The size will
492 hash_table_clear (struct hash_table *ht)
494 memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
498 /* Map MAPFUN over all the mappings in hash table HT. MAPFUN is
499 called with three arguments: the key, the value, and MAPARG.
501 It is undefined what happens if you add or remove entries in the
502 hash table while hash_table_map is running. The exception is the
503 entry you're currently mapping over; you may remove or change that
507 hash_table_map (struct hash_table *ht,
508 int (*mapfun) (void *, void *, void *),
511 struct mapping *mp = ht->mappings;
512 struct mapping *end = ht->mappings + ht->size;
514 for (; mp < end; mp++)
520 if (mapfun (key, mp->value, maparg))
522 /* hash_table_remove might have moved the adjacent
524 if (mp->key != key && NON_EMPTY (mp))
529 /* Return the number of elements in the hash table. This is not the
530 same as the physical size of the hash table, which is always
531 greater than the number of elements. */
534 hash_table_count (const struct hash_table *ht)
539 /* Functions from this point onward are meant for convenience and
540 don't strictly belong to this file. However, this is as good a
541 place for them as any. */
544 * Support for hash tables whose keys are strings.
548 /* 31 bit hash function. Taken from Gnome's glib, modified to use
551 We used to use the popular hash function from the Dragon Book, but
552 this one seems to perform much better. */
555 string_hash (const void *key)
561 for (p += 1; *p != '\0'; p++)
562 h = (h << 5) - h + *p;
567 /* Frontend for strcmp usable for hash tables. */
570 string_cmp (const void *s1, const void *s2)
572 return !strcmp ((const char *)s1, (const char *)s2);
575 /* Return a hash table of preallocated to store at least ITEMS items
576 suitable to use strings as keys. */
579 make_string_hash_table (int items)
581 return hash_table_new (items, string_hash, string_cmp);
585 * Support for hash tables whose keys are strings, but which are
586 * compared case-insensitively.
590 /* Like string_hash, but produce the same hash regardless of the case. */
593 string_hash_nocase (const void *key)
596 unsigned int h = TOLOWER (*p);
599 for (p += 1; *p != '\0'; p++)
600 h = (h << 5) - h + TOLOWER (*p);
605 /* Like string_cmp, but doing case-insensitive compareison. */
608 string_cmp_nocase (const void *s1, const void *s2)
610 return !strcasecmp ((const char *)s1, (const char *)s2);
613 /* Like make_string_hash_table, but uses string_hash_nocase and
614 string_cmp_nocase. */
617 make_nocase_string_hash_table (int items)
619 return hash_table_new (items, string_hash_nocase, string_cmp_nocase);
622 /* Hashing of pointers. Used for hash tables that are keyed by
623 pointer identity. (Common Lisp calls them EQ hash tables, and Java
624 calls them IdentityHashMaps.) */
627 ptrhash (const void *ptr)
629 unsigned long key = (unsigned long)ptr;
652 ptrcmp (const void *ptr1, const void *ptr2)
663 print_hash_table_mapper (void *key, void *value, void *count)
666 printf ("%s: %s\n", (const char *)key, (char *)value);
671 print_hash (struct hash_table *sht)
674 hash_table_map (sht, print_hash_table_mapper, &debug_count);
675 assert (debug_count == sht->count);
681 struct hash_table *ht = make_string_hash_table (0);
683 while ((fgets (line, sizeof (line), stdin)))
685 int len = strlen (line);
689 if (!hash_table_contains (ht, line))
690 hash_table_put (ht, strdup (line), "here I am!");
695 if (hash_table_get_pair (ht, line, &line_copy, NULL))
697 hash_table_remove (ht, line);
707 printf ("%d %d\n", ht->count, ht->size);