2 Copyright (C) 2000 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. */
37 # define xmalloc malloc
38 # define xrealloc realloc
44 Hash tables are an implementation technique used to implement
45 mapping between objects. Provided a good hashing function is used,
46 they guarantee constant-time access and storing of information.
47 Duplicate keys are not allowed.
49 The basics are all covered. hash_table_new creates a hash table,
50 and hash_table_destroy deletes it. hash_table_put establishes a
51 mapping between a key and a value. hash_table_get retrieves the
52 value that corresponds to a key. hash_table_exists queries whether
53 a key is stored in a table at all. hash_table_remove removes a
54 mapping that corresponds to a key. hash_table_map allows you to
55 map through all the entries in a hash table. hash_table_clear
56 clears all the entries from the hash table.
58 The number of mappings in a table is not limited, except by the
59 amount of memory. As you add new elements to a table, it regrows
60 as necessary. If you have an idea about how many elements you will
61 store, you can provide a hint to hash_table_new().
63 The hashing and equality functions are normally provided by the
64 user. For the special (and frequent) case of hashing strings, you
65 can use the pre-canned make_string_hash_table(), which provides the
66 string hashing function from the Dragon Book, and a string equality
67 wrapper around strcmp().
69 When specifying your own hash and test functions, make sure the
72 - The test function returns non-zero for keys that are considered
73 "equal", zero otherwise.
75 - The hash function returns a number that represents the
76 "distinctness" of the object. In more precise terms, it means
77 that for any two objects that test "equal" under the test
78 function, the hash function MUST produce the same result.
80 This does not mean that each distinct object must produce a
81 distinct value, only that non-distinct objects must produce the
82 same values! For instance, a hash function that returns 0 for
83 any given object is a perfectly valid (albeit extremely bad) hash
86 The above stated rule is quite easy to enforce. For example, if
87 your testing function compares strings case-insensitively, all
88 your function needs to do is lower-case the string characters
89 before calculating a hash. That way you have easily guaranteed
90 that changes in case will not result in a different hash.
92 - (optional) Choose the hash function to get as good "spreading" as
93 possible. A good hash function will react to even a small change
94 in its input with a completely different resulting hash.
95 Finally, don't make your hash function extremely slow, because
96 you're then defeating the purpose of hashing.
98 Note that neither keys nor values are copied when inserted into the
99 hash table, so they must exist for the lifetime of the table. This
100 means that e.g. the use of static strings is OK, but objects with a
101 shorter life-time need to be copied (with strdup() or the like in
102 the case of strings) before being inserted. */
106 All the hash mappings (key-value pairs of pointers) are stored in a
107 contiguous array. The position of each mapping is determined by
108 applying the hash function to the key: location = hash(key) % size.
109 If two different keys end up on the same position, the collision is
110 resolved by placing the second mapping at the next empty place in
111 the array following the occupied place. This method of collision
112 resolution is called "linear probing".
114 There are more advanced collision resolution mechanisms (quadratic
115 probing, double hashing), but we don't use them because they
116 involve more non-sequential access to the array, and therefore
117 worse cache behavior. Linear probing works well as long as the
118 fullness/size ratio is kept below 75%. We make sure to regrow or
119 rehash the hash table whenever this threshold is exceeded.
121 Collisions make deletion tricky because finding collisions again
122 relies on new empty spots not being created. That's why
123 hash_table_remove only marks the spot as deleted rather than really
132 unsigned long (*hash_function) (const void *);
133 int (*test_function) (const void *, const void *);
135 int size; /* size of the array */
136 int fullness; /* number of non-empty fields */
137 int count; /* number of non-empty, non-deleted
140 struct mapping *mappings;
143 #define ENTRY_DELETED ((void *)0xdeadbeef)
144 #define ENTRY_EMPTY NULL
146 #define DELETED_ENTRY_P(ptr) ((ptr) == ENTRY_DELETED)
147 #define EMPTY_ENTRY_P(ptr) ((ptr) == ENTRY_EMPTY)
149 /* Find a prime near, but greather than or equal to SIZE. */
152 prime_size (int size)
154 static const unsigned long primes [] = {
155 19, 29, 41, 59, 79, 107, 149, 197, 263, 347, 457, 599, 787, 1031,
156 1361, 1777, 2333, 3037, 3967, 5167, 6719, 8737, 11369, 14783,
157 19219, 24989, 32491, 42257, 54941, 71429, 92861, 120721, 156941,
158 204047, 265271, 344857, 448321, 582821, 757693, 985003, 1280519,
159 1664681, 2164111, 2813353, 3657361, 4754591, 6180989, 8035301,
160 10445899, 13579681, 17653589, 22949669, 29834603, 38784989,
161 50420551, 65546729, 85210757, 110774011, 144006217, 187208107,
162 243370577, 316381771, 411296309, 534685237, 695090819, 903618083,
163 1174703521, 1527114613, 1985248999, 2580823717UL, 3355070839UL
166 for (i = 0; i < ARRAY_SIZE (primes); i++)
167 if (primes[i] >= size)
173 /* Create a hash table of INITIAL_SIZE with hash function
174 HASH_FUNCTION and test function TEST_FUNCTION. If you wish to
175 start out with a "small" table which will be regrown as needed,
176 specify 0 as INITIAL_SIZE. */
179 hash_table_new (int initial_size,
180 unsigned long (*hash_function) (const void *),
181 int (*test_function) (const void *, const void *))
183 struct hash_table *ht
184 = (struct hash_table *)xmalloc (sizeof (struct hash_table));
185 ht->hash_function = hash_function;
186 ht->test_function = test_function;
187 ht->size = prime_size (initial_size);
190 ht->mappings = xmalloc (ht->size * sizeof (struct mapping));
191 memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
195 /* Free the data associated with hash table HT. */
198 hash_table_destroy (struct hash_table *ht)
200 xfree (ht->mappings);
204 /* The heart of almost all functions in this file -- find the mapping
205 whose KEY is equal to key, using a linear probing loop. Returns
206 the offset of the mapping in ht->mappings. This should probably be
210 find_mapping (struct hash_table *ht, const void *key)
212 struct mapping *mappings = ht->mappings;
214 int location = ht->hash_function (key) % size;
217 struct mapping *mp = mappings + location;
218 void *mp_key = mp->key;
220 if (EMPTY_ENTRY_P (mp_key))
222 else if (DELETED_ENTRY_P (mp_key)
223 || !ht->test_function (key, mp_key))
225 if (++location == size)
233 /* Get the value that corresponds to the key KEY in the hash table HT.
234 If no value is found, return NULL. Note that NULL is a legal value
235 for value; if you are storing NULLs in your hash table, you can use
236 hash_table_exists to be sure that a (possibly NULL) value exists in
237 the table. Or, you can use hash_table_get_pair instead of this
241 hash_table_get (struct hash_table *ht, const void *key)
243 int location = find_mapping (ht, key);
247 return ht->mappings[location].value;
250 /* Like hash_table_get, but writes out the pointers to both key and
251 value. Returns non-zero on success. */
254 hash_table_get_pair (struct hash_table *ht, const void *lookup_key,
255 void *orig_key, void *value)
257 int location = find_mapping (ht, lookup_key);
262 struct mapping *mp = ht->mappings + location;
264 *(void **)orig_key = mp->key;
266 *(void **)value = mp->value;
271 /* Return 1 if KEY exists in HT, 0 otherwise. */
274 hash_table_exists (struct hash_table *ht, const void *key)
276 return find_mapping (ht, key) >= 0;
279 #define MAX(i, j) (((i) >= (j)) ? (i) : (j))
281 /* Grow hash table HT as necessary, and rehash all the key-value
285 grow_hash_table (struct hash_table *ht)
288 struct mapping *old_mappings = ht->mappings;
289 int old_count = ht->count; /* for assert() below */
290 int old_size = ht->size;
292 /* To minimize the number of regrowth, we'd like to resize the hash
293 table exponentially. Normally, this would be done by doubling
294 ht->size (and round it to next prime) on each regrow:
296 ht->size = prime_size (ht->size * 2);
298 But it is possible that the table has large fullness because of
299 the many deleted entries. If that is the case, we don't want to
300 blindly grow the table; we just want to rehash it. For that
301 reason, we use ht->count as the relevant parameter. MAX is used
302 only because we don't want to actually shrink the table. (But
303 maybe that's wrong.) */
305 int needed_size = prime_size (ht->count * 3);
306 ht->size = MAX (old_size, needed_size);
309 printf ("growing from %d to %d\n", old_size, ht->size);
312 ht->mappings = xmalloc (ht->size * sizeof (struct mapping));
313 memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
315 /* Need to reset these two; hash_table_put will reinitialize them. */
318 for (i = 0; i < old_size; i++)
320 struct mapping *mp = old_mappings + i;
321 void *mp_key = mp->key;
323 if (!EMPTY_ENTRY_P (mp_key)
324 && !DELETED_ENTRY_P (mp_key))
325 hash_table_put (ht, mp_key, mp->value);
327 assert (ht->count == old_count);
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 /* Cannot use find_mapping here because we're actually looking for
340 struct mapping *mappings = ht->mappings;
342 int location = ht->hash_function (key) % size;
345 struct mapping *mp = mappings + location;
346 void *mp_key = mp->key;
348 if (EMPTY_ENTRY_P (mp_key))
353 mp->key = (void *)key; /* const? */
357 else if (DELETED_ENTRY_P (mp_key)
358 || !ht->test_function (key, mp_key))
360 if (++location == size)
363 else /* equal to key and not deleted */
365 /* We're replacing an existing entry, so ht->count and
366 ht->fullness remain unchanged. */
370 if (ht->fullness * 4 > ht->size * 3)
371 /* When fullness exceeds 75% of size, regrow the table. */
372 grow_hash_table (ht);
375 /* Remove KEY from HT. */
378 hash_table_remove (struct hash_table *ht, const void *key)
380 int location = find_mapping (ht, key);
385 struct mapping *mappings = ht->mappings;
386 struct mapping *mp = mappings + location;
387 /* We don't really remove an entry from the hash table: we just
388 mark it as deleted. This is because there may be other
389 entries located after this entry whose hash points to a
390 location before this entry. (Example: keys A, B and C have
391 the same hash. If you were to really *delete* B from the
392 table, C could no longer be found.) */
394 /* Optimization addendum: if the mapping that follows LOCATION
395 is already empty, that is a sure sign that nobody depends on
396 LOCATION being non-empty. (This is because we're using
397 linear probing. This would not be the case with double
398 hashing.) In that case, we may safely delete the mapping. */
400 /* This could be generalized so that the all the non-empty
401 locations following LOCATION are simply shifted leftward. It
402 would make deletion a bit slower, but it would remove the
403 ugly DELETED_ENTRY_P checks from all the rest of the code,
404 making the whole thing faster. */
405 int location_after = (location + 1) == ht->size ? 0 : location + 1;
406 struct mapping *mp_after = mappings + location_after;
408 if (EMPTY_ENTRY_P (mp_after->key))
410 mp->key = ENTRY_EMPTY;
414 mp->key = ENTRY_DELETED;
421 /* Clear HT of all entries. After calling this function, the count
422 and the fullness of the hash table will be zero. The size will
426 hash_table_clear (struct hash_table *ht)
428 memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
433 /* Map MAPFUN over all the mappings in hash table HT. MAPFUN is
434 called with three arguments: the key, the value, and the CLOSURE.
435 Don't add or remove entries from HT while hash_table_map is being
436 called, or strange things may happen. */
439 hash_table_map (struct hash_table *ht,
440 int (*mapfun) (void *, void *, void *),
443 struct mapping *mappings = ht->mappings;
445 for (i = 0; i < ht->size; i++)
447 struct mapping *mp = mappings + i;
448 void *mp_key = mp->key;
450 if (!EMPTY_ENTRY_P (mp_key)
451 && !DELETED_ENTRY_P (mp_key))
452 if (mapfun (mp_key, mp->value, closure))
457 /* Support for hash tables whose keys are strings. */
459 /* supposedly from the Dragon Book P436. */
461 string_hash (const void *sv)
464 unsigned const char *x = (unsigned const char *) sv;
470 if ((g = h & 0xf0000000) != 0)
471 h = (h ^ (g >> 24)) ^ g;
478 /* If I ever need it: hashing of integers. */
481 inthash (unsigned int key)
496 string_cmp (const void *s1, const void *s2)
498 return !strcmp ((const char *)s1, (const char *)s2);
501 /* Return a hash table of initial size INITIAL_SIZE suitable to use
505 make_string_hash_table (int initial_size)
507 return hash_table_new (initial_size, string_hash, string_cmp);
517 print_hash_table_mapper (void *key, void *value, void *count)
520 printf ("%s: %s\n", (const char *)key, (char *)value);
525 print_hash (struct hash_table *sht)
528 hash_table_map (sht, print_hash_table_mapper, &debug_count);
529 assert (debug_count == sht->count);
535 struct hash_table *ht = make_string_hash_table (0);
537 while ((fgets (line, sizeof (line), stdin)))
539 int len = strlen (line);
543 if (!hash_table_exists (ht, line))
544 hash_table_put (ht, strdup (line), "here I am!");
549 if (hash_table_get_pair (ht, line, &line_copy, NULL))
551 hash_table_remove (ht, line);
561 printf ("%d %d %d\n", ht->count, ht->fullness, ht->size);