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. */
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
91 The above stated rule is quite easy to enforce. For example, if
92 your testing function compares strings case-insensitively, all
93 your function needs to do is lower-case the string characters
94 before calculating a hash. That way you have easily guaranteed
95 that changes in case will not result in a different hash.
97 - (optional) Choose the hash function to get as good "spreading" as
98 possible. A good hash function will react to even a small change
99 in its input with a completely different resulting hash.
100 Finally, don't make your hash function extremely slow, because
101 you're then defeating the purpose of hashing.
103 Note that neither keys nor values are copied when inserted into the
104 hash table, so they must exist for the lifetime of the table. This
105 means that e.g. the use of static strings is OK, but objects with a
106 shorter life-time need to be copied (with strdup() or the like in
107 the case of strings) before being inserted. */
111 All the hash mappings (key-value pairs of pointers) are stored in a
112 contiguous array. The position of each mapping is determined by
113 applying the hash function to the key: location = hash(key) % size.
114 If two different keys end up on the same position, the collision is
115 resolved by placing the second mapping at the next empty place in
116 the array following the occupied place. This method of collision
117 resolution is called "linear probing".
119 There are more advanced collision resolution mechanisms (quadratic
120 probing, double hashing), but we don't use them because they
121 involve more non-sequential access to the array, and therefore
122 worse cache behavior. Linear probing works well as long as the
123 fullness/size ratio is kept below 75%. We make sure to regrow or
124 rehash the hash table whenever this threshold is exceeded.
126 Collisions make deletion tricky because finding collisions again
127 relies on new empty spots not being created. That's why
128 hash_table_remove only marks the spot as deleted rather than really
137 unsigned long (*hash_function) (const void *);
138 int (*test_function) (const void *, const void *);
140 int size; /* size of the array */
141 int fullness; /* number of non-empty fields */
142 int count; /* number of non-empty, non-deleted
145 struct mapping *mappings;
148 #define ENTRY_DELETED ((void *)0xdeadbeef)
149 #define ENTRY_EMPTY NULL
151 #define DELETED_ENTRY_P(ptr) ((ptr) == ENTRY_DELETED)
152 #define EMPTY_ENTRY_P(ptr) ((ptr) == ENTRY_EMPTY)
154 /* Find a prime near, but greather than or equal to SIZE. */
157 prime_size (int size)
159 static const unsigned long primes [] = {
160 19, 29, 41, 59, 79, 107, 149, 197, 263, 347, 457, 599, 787, 1031,
161 1361, 1777, 2333, 3037, 3967, 5167, 6719, 8737, 11369, 14783,
162 19219, 24989, 32491, 42257, 54941, 71429, 92861, 120721, 156941,
163 204047, 265271, 344857, 448321, 582821, 757693, 985003, 1280519,
164 1664681, 2164111, 2813353, 3657361, 4754591, 6180989, 8035301,
165 10445899, 13579681, 17653589, 22949669, 29834603, 38784989,
166 50420551, 65546729, 85210757, 110774011, 144006217, 187208107,
167 243370577, 316381771, 411296309, 534685237, 695090819, 903618083,
168 1174703521, 1527114613, 1985248999, 2580823717UL, 3355070839UL
171 for (i = 0; i < ARRAY_SIZE (primes); i++)
172 if (primes[i] >= size)
178 /* Create a hash table of INITIAL_SIZE with hash function
179 HASH_FUNCTION and test function TEST_FUNCTION. If you wish to
180 start out with a "small" table which will be regrown as needed,
181 specify 0 as INITIAL_SIZE. */
184 hash_table_new (int initial_size,
185 unsigned long (*hash_function) (const void *),
186 int (*test_function) (const void *, const void *))
188 struct hash_table *ht
189 = (struct hash_table *)xmalloc (sizeof (struct hash_table));
190 ht->hash_function = hash_function;
191 ht->test_function = test_function;
192 ht->size = prime_size (initial_size);
195 ht->mappings = xmalloc (ht->size * sizeof (struct mapping));
196 memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
200 /* Free the data associated with hash table HT. */
203 hash_table_destroy (struct hash_table *ht)
205 xfree (ht->mappings);
209 /* The heart of almost all functions in this file -- find the mapping
210 whose KEY is equal to key, using a linear probing loop. Returns
211 the offset of the mapping in ht->mappings. This should probably be
215 find_mapping (struct hash_table *ht, const void *key)
217 struct mapping *mappings = ht->mappings;
219 int location = ht->hash_function (key) % size;
222 struct mapping *mp = mappings + location;
223 void *mp_key = mp->key;
225 if (EMPTY_ENTRY_P (mp_key))
227 else if (DELETED_ENTRY_P (mp_key)
228 || !ht->test_function (key, mp_key))
230 if (++location == size)
238 /* Get the value that corresponds to the key KEY in the hash table HT.
239 If no value is found, return NULL. Note that NULL is a legal value
240 for value; if you are storing NULLs in your hash table, you can use
241 hash_table_exists to be sure that a (possibly NULL) value exists in
242 the table. Or, you can use hash_table_get_pair instead of this
246 hash_table_get (struct hash_table *ht, const void *key)
248 int location = find_mapping (ht, key);
252 return ht->mappings[location].value;
255 /* Like hash_table_get, but writes out the pointers to both key and
256 value. Returns non-zero on success. */
259 hash_table_get_pair (struct hash_table *ht, const void *lookup_key,
260 void *orig_key, void *value)
262 int location = find_mapping (ht, lookup_key);
267 struct mapping *mp = ht->mappings + location;
269 *(void **)orig_key = mp->key;
271 *(void **)value = mp->value;
276 /* Return 1 if KEY exists in HT, 0 otherwise. */
279 hash_table_exists (struct hash_table *ht, const void *key)
281 return find_mapping (ht, key) >= 0;
284 #define MAX(i, j) (((i) >= (j)) ? (i) : (j))
286 /* Grow hash table HT as necessary, and rehash all the key-value
290 grow_hash_table (struct hash_table *ht)
293 struct mapping *old_mappings = ht->mappings;
294 int old_count = ht->count; /* for assert() below */
295 int old_size = ht->size;
297 /* To minimize the number of regrowth, we'd like to resize the hash
298 table exponentially. Normally, this would be done by doubling
299 ht->size (and round it to next prime) on each regrow:
301 ht->size = prime_size (ht->size * 2);
303 But it is possible that the table has large fullness because of
304 the many deleted entries. If that is the case, we don't want to
305 blindly grow the table; we just want to rehash it. For that
306 reason, we use ht->count as the relevant parameter. MAX is used
307 only because we don't want to actually shrink the table. (But
308 maybe that's wrong.) */
310 int needed_size = prime_size (ht->count * 3);
311 ht->size = MAX (old_size, needed_size);
314 printf ("growing from %d to %d\n", old_size, ht->size);
317 ht->mappings = xmalloc (ht->size * sizeof (struct mapping));
318 memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
320 /* Need to reset these two; hash_table_put will reinitialize them. */
323 for (i = 0; i < old_size; i++)
325 struct mapping *mp = old_mappings + i;
326 void *mp_key = mp->key;
328 if (!EMPTY_ENTRY_P (mp_key)
329 && !DELETED_ENTRY_P (mp_key))
330 hash_table_put (ht, mp_key, mp->value);
332 assert (ht->count == old_count);
333 xfree (old_mappings);
336 /* Put VALUE in the hash table HT under the key KEY. This regrows the
337 table if necessary. */
340 hash_table_put (struct hash_table *ht, const void *key, void *value)
342 /* Cannot use find_mapping here because we're actually looking for
345 struct mapping *mappings = ht->mappings;
347 int location = ht->hash_function (key) % size;
350 struct mapping *mp = mappings + location;
351 void *mp_key = mp->key;
353 if (EMPTY_ENTRY_P (mp_key))
358 mp->key = (void *)key; /* const? */
362 else if (DELETED_ENTRY_P (mp_key)
363 || !ht->test_function (key, mp_key))
365 if (++location == size)
368 else /* equal to key and not deleted */
370 /* We're replacing an existing entry, so ht->count and
371 ht->fullness remain unchanged. */
375 if (ht->fullness * 4 > ht->size * 3)
376 /* When fullness exceeds 75% of size, regrow the table. */
377 grow_hash_table (ht);
380 /* Remove KEY from HT. */
383 hash_table_remove (struct hash_table *ht, const void *key)
385 int location = find_mapping (ht, key);
390 struct mapping *mappings = ht->mappings;
391 struct mapping *mp = mappings + location;
392 /* We don't really remove an entry from the hash table: we just
393 mark it as deleted. This is because there may be other
394 entries located after this entry whose hash points to a
395 location before this entry. (Example: keys A, B and C have
396 the same hash. If you were to really *delete* B from the
397 table, C could no longer be found.) */
399 /* Optimization addendum: if the mapping that follows LOCATION
400 is already empty, that is a sure sign that nobody depends on
401 LOCATION being non-empty. (This is because we're using
402 linear probing. This would not be the case with double
403 hashing.) In that case, we may safely delete the mapping. */
405 /* This could be generalized so that the all the non-empty
406 locations following LOCATION are simply shifted leftward. It
407 would make deletion a bit slower, but it would remove the
408 ugly DELETED_ENTRY_P checks from all the rest of the code,
409 making the whole thing faster. */
410 int location_after = (location + 1) == ht->size ? 0 : location + 1;
411 struct mapping *mp_after = mappings + location_after;
413 if (EMPTY_ENTRY_P (mp_after->key))
415 mp->key = ENTRY_EMPTY;
419 mp->key = ENTRY_DELETED;
426 /* Clear HT of all entries. After calling this function, the count
427 and the fullness of the hash table will be zero. The size will
431 hash_table_clear (struct hash_table *ht)
433 memset (ht->mappings, '\0', ht->size * sizeof (struct mapping));
438 /* Map MAPFUN over all the mappings in hash table HT. MAPFUN is
439 called with three arguments: the key, the value, and the CLOSURE.
440 Don't add or remove entries from HT while hash_table_map is being
441 called, or strange things may happen. */
444 hash_table_map (struct hash_table *ht,
445 int (*mapfun) (void *, void *, void *),
448 struct mapping *mappings = ht->mappings;
450 for (i = 0; i < ht->size; i++)
452 struct mapping *mp = mappings + i;
453 void *mp_key = mp->key;
455 if (!EMPTY_ENTRY_P (mp_key)
456 && !DELETED_ENTRY_P (mp_key))
457 if (mapfun (mp_key, mp->value, closure))
462 /* Return the number of elements in the hash table. This is not the
463 same as the physical size of the hash table, which is always
464 greater than the number of elements. */
466 hash_table_count (struct hash_table *ht)
471 /* Support for hash tables whose keys are strings. */
473 /* supposedly from the Dragon Book P436. */
475 string_hash (const void *sv)
478 unsigned const char *x = (unsigned const char *) sv;
484 if ((g = h & 0xf0000000) != 0)
485 h = (h ^ (g >> 24)) ^ g;
492 /* If I ever need it: hashing of integers. */
495 inthash (unsigned int key)
510 string_cmp (const void *s1, const void *s2)
512 return !strcmp ((const char *)s1, (const char *)s2);
515 /* Return a hash table of initial size INITIAL_SIZE suitable to use
519 make_string_hash_table (int initial_size)
521 return hash_table_new (initial_size, string_hash, string_cmp);
531 print_hash_table_mapper (void *key, void *value, void *count)
534 printf ("%s: %s\n", (const char *)key, (char *)value);
539 print_hash (struct hash_table *sht)
542 hash_table_map (sht, print_hash_table_mapper, &debug_count);
543 assert (debug_count == sht->count);
549 struct hash_table *ht = make_string_hash_table (0);
551 while ((fgets (line, sizeof (line), stdin)))
553 int len = strlen (line);
557 if (!hash_table_exists (ht, line))
558 hash_table_put (ht, strdup (line), "here I am!");
563 if (hash_table_get_pair (ht, line, &line_copy, NULL))
565 hash_table_remove (ht, line);
575 printf ("%d %d %d\n", ht->count, ht->fullness, ht->size);