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. */
55 # define xmalloc malloc
56 # define xrealloc realloc
60 # define TOLOWER(x) ('A' <= (x) && (x) <= 'Z' ? (x) - 32 : (x))
65 Hash tables are a technique used to implement mapping between
66 objects with near-constant-time access and storage. The table
67 associates keys to values, and a value can be very quickly
68 retrieved by providing the key. Fast lookup tables are typically
69 implemented as hash tables.
72 hash_table_new -- creates the table.
73 hash_table_destroy -- destroys the table.
74 hash_table_put -- establishes or updates key->value mapping.
75 hash_table_get -- retrieves value of key.
76 hash_table_get_pair -- get key/value pair for key.
77 hash_table_contains -- test whether the table contains key.
78 hash_table_remove -- remove the key->value mapping for key.
79 hash_table_map -- iterate through table mappings.
80 hash_table_clear -- clear hash table contents.
81 hash_table_count -- return the number of entries in the table.
83 The hash table grows internally as new entries are added and is not
84 limited in size, except by available memory. The table doubles
85 with each resize, which ensures that the amortized time per
86 operation remains constant.
88 By default, tables created by hash_table_new consider the keys to
89 be equal if their pointer values are the same. You can use
90 make_string_hash_table to create tables whose keys are considered
91 equal if their string contents are the same. In the general case,
92 the criterion of equality used to compare keys is specified at
93 table creation time with two callback functions, "hash" and "test".
94 The hash function transforms the key into an arbitrary number that
95 must be the same for two equal keys. The test function accepts two
96 keys and returns non-zero if they are to be considered equal.
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 The hash table is implemented as an open-addressed table with
107 linear probing collision resolution.
109 In regular language, it means that all the hash entries (pairs of
110 pointers key and value) are stored in a contiguous array. The
111 position of each mapping is determined by the hash value of its key
112 and the size of the table: location := hash(key) % size. If two
113 different keys end up on the same position (collide), the one that
114 came second is placed at the next empty position following the
115 occupied place. This collision resolution technique is called
118 There are more advanced collision resolution methods (quadratic
119 probing, double hashing), but we don't use them because they incur
120 more non-sequential access to the array, which results in worse CPU
121 cache behavior. Linear probing works well as long as the
122 count/size ratio (fullness) is kept below 75%. We make sure to
123 grow and rehash the table whenever this threshold is exceeded.
125 Collisions make deletion tricky because clearing a position
126 followed by a colliding entry would make the position seem empty
127 and the colliding entry not found. One solution is to leave a
128 "tombstone" instead of clearing the entry, and another is to
129 carefully rehash the entries immediately following the deleted one.
130 We use the latter method because it results in less bookkeeping and
131 faster retrieval at the (slight) expense of deletion. */
133 /* Maximum allowed fullness: when hash table's fullness exceeds this
134 value, the table is resized. */
135 #define HASH_MAX_FULLNESS 0.75
137 /* The hash table size is multiplied by this factor (and then rounded
138 to the next prime) with each resize. This guarantees infrequent
140 #define HASH_RESIZE_FACTOR 2
147 typedef unsigned long (*hashfun_t) PARAMS ((const void *));
148 typedef int (*testfun_t) PARAMS ((const void *, const void *));
151 hashfun_t hash_function;
152 testfun_t test_function;
154 struct mapping *mappings; /* pointer to the table entries. */
155 int size; /* size of the array. */
157 int count; /* number of non-empty entries. */
158 int resize_threshold; /* after size exceeds this number of
159 entries, resize the table. */
160 int prime_offset; /* the offset of the current prime in
164 /* We use the all-bits-set constant (INVALID_PTR) marker to mean that
165 a mapping is empty. It is unaligned and therefore illegal as a
166 pointer. INVALID_PTR_BYTE (0xff) is the one-byte value used to
167 initialize the mappings array as empty.
169 The all-bits-set value is a better choice than NULL because it
170 allows the use of NULL/0 keys. Since the keys are either integers
171 or pointers, the only key that cannot be used is the integer value
172 -1. This is acceptable because it still allows the use of
173 nonnegative integer keys. */
175 #define INVALID_PTR ((void *) ~(unsigned long)0)
177 # define UCHAR_MAX 0xff
179 #define INVALID_PTR_BYTE UCHAR_MAX
181 #define NON_EMPTY(mp) ((mp)->key != INVALID_PTR)
182 #define MARK_AS_EMPTY(mp) ((mp)->key = INVALID_PTR)
184 /* "Next" mapping is the mapping after MP, but wrapping back to
185 MAPPINGS when MP would reach MAPPINGS+SIZE. */
186 #define NEXT_MAPPING(mp, mappings, size) (mp != mappings + (size - 1) \
189 /* Loop over non-empty mappings starting at MP. */
190 #define LOOP_NON_EMPTY(mp, mappings, size) \
191 for (; NON_EMPTY (mp); mp = NEXT_MAPPING (mp, mappings, size))
193 /* Return the position of KEY in hash table SIZE large, hash function
195 #define HASH_POSITION(key, hashfun, size) ((hashfun) (key) % size)
197 /* Find a prime near, but greather than or equal to SIZE. The primes
198 are looked up from a table with a selection of primes convenient
201 PRIME_OFFSET is a minor optimization: it specifies start position
202 for the search for the large enough prime. The final offset is
203 stored in the same variable. That way the list of primes does not
204 have to be scanned from the beginning each time around. */
207 prime_size (int size, int *prime_offset)
209 static const unsigned long primes [] = {
210 13, 19, 29, 41, 59, 79, 107, 149, 197, 263, 347, 457, 599, 787, 1031,
211 1361, 1777, 2333, 3037, 3967, 5167, 6719, 8737, 11369, 14783,
212 19219, 24989, 32491, 42257, 54941, 71429, 92861, 120721, 156941,
213 204047, 265271, 344857, 448321, 582821, 757693, 985003, 1280519,
214 1664681, 2164111, 2813353, 3657361, 4754591, 6180989, 8035301,
215 10445899, 13579681, 17653589, 22949669, 29834603, 38784989,
216 50420551, 65546729, 85210757, 110774011, 144006217, 187208107,
217 243370577, 316381771, 411296309, 534685237, 695090819, 903618083,
218 1174703521, 1527114613, 1985248999,
219 (unsigned long)0x99d43ea5, (unsigned long)0xc7fa5177
223 for (i = *prime_offset; i < countof (primes); i++)
224 if (primes[i] >= size)
226 /* Set the offset to the next prime. That is safe because,
227 next time we are called, it will be with a larger SIZE,
228 which means we could never return the same prime anyway.
229 (If that is not the case, the caller can simply reset
231 *prime_offset = i + 1;
239 static unsigned long ptrhash PARAMS ((const void *));
240 static int ptrcmp PARAMS ((const void *, const void *));
242 /* Create a hash table with hash function HASH_FUNCTION and test
243 function TEST_FUNCTION. The table is empty (its count is 0), but
244 pre-allocated to store at least ITEMS items.
246 ITEMS is the number of items that the table can accept without
247 needing to resize. It is useful when creating a table that is to
248 be immediately filled with a known number of items. In that case,
249 the regrows are a waste of time, and specifying ITEMS correctly
250 will avoid them altogether.
252 Note that hash tables grow dynamically regardless of ITEMS. The
253 only use of ITEMS is to preallocate the table and avoid unnecessary
254 dynamic regrows. Don't bother making ITEMS prime because it's not
255 used as size unchanged. To start with a small table that grows as
256 needed, simply specify zero ITEMS.
258 If hash and test callbacks are not specified, identity mapping is
259 assumed, i.e. pointer values are used for key comparison. (Common
260 Lisp calls such tables EQ hash tables, and Java calls them
261 IdentityHashMaps.) If your keys require different comparison,
262 specify hash and test functions. For easy use of C strings as hash
263 keys, you can use the convenience functions make_string_hash_table
264 and make_nocase_string_hash_table. */
267 hash_table_new (int items,
268 unsigned long (*hash_function) (const void *),
269 int (*test_function) (const void *, const void *))
272 struct hash_table *ht = xnew (struct hash_table);
274 ht->hash_function = hash_function ? hash_function : ptrhash;
275 ht->test_function = test_function ? test_function : ptrcmp;
277 /* If the size of struct hash_table ever becomes a concern, this
278 field can go. (Wget doesn't create many hashes.) */
279 ht->prime_offset = 0;
281 /* Calculate the size that ensures that the table will store at
282 least ITEMS keys without the need to resize. */
283 size = 1 + items / HASH_MAX_FULLNESS;
284 size = prime_size (size, &ht->prime_offset);
286 ht->resize_threshold = size * HASH_MAX_FULLNESS;
287 /*assert (ht->resize_threshold >= items);*/
289 ht->mappings = xnew_array (struct mapping, ht->size);
290 /* Mark mappings as empty. We use 0xff rather than 0 to mark empty
291 keys because it allows us to use NULL/0 as keys. */
292 memset (ht->mappings, INVALID_PTR_BYTE, size * sizeof (struct mapping));
299 /* Free the data associated with hash table HT. */
302 hash_table_destroy (struct hash_table *ht)
304 xfree (ht->mappings);
308 /* The heart of most functions in this file -- find the mapping whose
309 KEY is equal to key, using linear probing. Returns the mapping
310 that matches KEY, or the first empty mapping if none matches. */
312 static inline struct mapping *
313 find_mapping (const struct hash_table *ht, const void *key)
315 struct mapping *mappings = ht->mappings;
317 struct mapping *mp = mappings + HASH_POSITION (key, ht->hash_function, size);
318 testfun_t equals = ht->test_function;
320 LOOP_NON_EMPTY (mp, mappings, size)
321 if (equals (key, mp->key))
326 /* Get the value that corresponds to the key KEY in the hash table HT.
327 If no value is found, return NULL. Note that NULL is a legal value
328 for value; if you are storing NULLs in your hash table, you can use
329 hash_table_contains to be sure that a (possibly NULL) value exists
330 in the table. Or, you can use hash_table_get_pair instead of this
334 hash_table_get (const struct hash_table *ht, const void *key)
336 struct mapping *mp = find_mapping (ht, key);
343 /* Like hash_table_get, but writes out the pointers to both key and
344 value. Returns non-zero on success. */
347 hash_table_get_pair (const struct hash_table *ht, const void *lookup_key,
348 void *orig_key, void *value)
350 struct mapping *mp = find_mapping (ht, lookup_key);
354 *(void **)orig_key = mp->key;
356 *(void **)value = mp->value;
363 /* Return 1 if HT contains KEY, 0 otherwise. */
366 hash_table_contains (const struct hash_table *ht, const void *key)
368 struct mapping *mp = find_mapping (ht, key);
369 return NON_EMPTY (mp);
372 /* Grow hash table HT as necessary, and rehash all the key-value
376 grow_hash_table (struct hash_table *ht)
378 hashfun_t hasher = ht->hash_function;
379 struct mapping *old_mappings = ht->mappings;
380 struct mapping *old_end = ht->mappings + ht->size;
381 struct mapping *mp, *mappings;
384 newsize = prime_size (ht->size * HASH_RESIZE_FACTOR, &ht->prime_offset);
386 printf ("growing from %d to %d; fullness %.2f%% to %.2f%%\n",
388 100.0 * ht->count / ht->size,
389 100.0 * ht->count / newsize);
393 ht->resize_threshold = newsize * HASH_MAX_FULLNESS;
395 mappings = xnew_array (struct mapping, newsize);
396 memset (mappings, INVALID_PTR_BYTE, newsize * sizeof (struct mapping));
397 ht->mappings = mappings;
399 for (mp = old_mappings; mp < old_end; mp++)
402 struct mapping *new_mp;
403 /* We don't need to test for uniqueness of keys because they
404 come from the hash table and are therefore known to be
406 new_mp = mappings + HASH_POSITION (mp->key, hasher, newsize);
407 LOOP_NON_EMPTY (new_mp, mappings, newsize)
412 xfree (old_mappings);
415 /* Put VALUE in the hash table HT under the key KEY. This regrows the
416 table if necessary. */
419 hash_table_put (struct hash_table *ht, const void *key, void *value)
421 struct mapping *mp = find_mapping (ht, key);
424 /* update existing item */
425 mp->key = (void *)key; /* const? */
430 /* If adding the item would make the table exceed max. fullness,
431 grow the table first. */
432 if (ht->count >= ht->resize_threshold)
434 grow_hash_table (ht);
435 mp = find_mapping (ht, key);
440 mp->key = (void *)key; /* const? */
444 /* Remove a mapping that matches KEY from HT. Return 0 if there was
445 no such entry; return 1 if an entry was removed. */
448 hash_table_remove (struct hash_table *ht, const void *key)
450 struct mapping *mp = find_mapping (ht, key);
456 struct mapping *mappings = ht->mappings;
457 hashfun_t hasher = ht->hash_function;
462 /* Rehash all the entries following MP. The alternative
463 approach is to mark the entry as deleted, i.e. create a
464 "tombstone". That speeds up removal, but leaves a lot of
465 garbage and slows down hash_table_get and hash_table_put. */
467 mp = NEXT_MAPPING (mp, mappings, size);
468 LOOP_NON_EMPTY (mp, mappings, size)
470 const void *key2 = mp->key;
471 struct mapping *mp_new;
473 /* Find the new location for the key. */
474 mp_new = mappings + HASH_POSITION (key2, hasher, size);
475 LOOP_NON_EMPTY (mp_new, mappings, size)
476 if (key2 == mp_new->key)
477 /* The mapping MP (key2) is already where we want it (in
478 MP_NEW's "chain" of keys.) */
491 /* Clear HT of all entries. After calling this function, the count
492 and the fullness of the hash table will be zero. The size will
496 hash_table_clear (struct hash_table *ht)
498 memset (ht->mappings, INVALID_PTR_BYTE, ht->size * sizeof (struct mapping));
502 /* Map MAPFUN over all the mappings in hash table HT. MAPFUN is
503 called with three arguments: the key, the value, and MAPARG.
505 It is undefined what happens if you add or remove entries in the
506 hash table while hash_table_map is running. The exception is the
507 entry you're currently mapping over; you may remove or change that
511 hash_table_map (struct hash_table *ht,
512 int (*mapfun) (void *, void *, void *),
515 struct mapping *mp = ht->mappings;
516 struct mapping *end = ht->mappings + ht->size;
518 for (; mp < end; mp++)
524 if (mapfun (key, mp->value, maparg))
526 /* hash_table_remove might have moved the adjacent
528 if (mp->key != key && NON_EMPTY (mp))
533 /* Return the number of elements in the hash table. This is not the
534 same as the physical size of the hash table, which is always
535 greater than the number of elements. */
538 hash_table_count (const struct hash_table *ht)
543 /* Functions from this point onward are meant for convenience and
544 don't strictly belong to this file. However, this is as good a
545 place for them as any. */
547 /* Rules for creating custom hash and test functions:
549 - The test function returns non-zero for keys that are considered
550 "equal", zero otherwise.
552 - The hash function returns a number that represents the
553 "distinctness" of the object. In more precise terms, it means
554 that for any two objects that test "equal" under the test
555 function, the hash function MUST produce the same result.
557 This does not mean that all different objects must produce
558 different values (that would be "perfect" hashing), only that
559 non-distinct objects must produce the same values! For instance,
560 a hash function that returns 0 for any given object is a
561 perfectly valid (albeit extremely bad) hash function. A hash
562 function that hashes a string by adding up all its characters is
563 another example of a valid (but quite bad) hash function.
565 It is not hard to make hash and test functions agree about
566 equality. For example, if the test function compares strings
567 case-insensitively, the hash function can lower-case the
568 characters when calculating the hash value. That ensures that
569 two strings differing only in case will hash the same.
571 - If you care about performance, choose a hash function with as
572 good "spreading" as possible. A good hash function will use all
573 the bits of the input when calculating the hash, and will react
574 to even small changes in input with a completely different
575 output. Finally, don't make the hash function itself overly
576 slow, because you'll be incurring a non-negligible overhead to
577 all hash table operations. */
580 * Support for hash tables whose keys are strings.
584 /* 31 bit hash function. Taken from Gnome's glib, modified to use
587 We used to use the popular hash function from the Dragon Book, but
588 this one seems to perform much better. */
591 string_hash (const void *key)
597 for (p += 1; *p != '\0'; p++)
598 h = (h << 5) - h + *p;
603 /* Frontend for strcmp usable for hash tables. */
606 string_cmp (const void *s1, const void *s2)
608 return !strcmp ((const char *)s1, (const char *)s2);
611 /* Return a hash table of preallocated to store at least ITEMS items
612 suitable to use strings as keys. */
615 make_string_hash_table (int items)
617 return hash_table_new (items, string_hash, string_cmp);
621 * Support for hash tables whose keys are strings, but which are
622 * compared case-insensitively.
626 /* Like string_hash, but produce the same hash regardless of the case. */
629 string_hash_nocase (const void *key)
632 unsigned int h = TOLOWER (*p);
635 for (p += 1; *p != '\0'; p++)
636 h = (h << 5) - h + TOLOWER (*p);
641 /* Like string_cmp, but doing case-insensitive compareison. */
644 string_cmp_nocase (const void *s1, const void *s2)
646 return !strcasecmp ((const char *)s1, (const char *)s2);
649 /* Like make_string_hash_table, but uses string_hash_nocase and
650 string_cmp_nocase. */
653 make_nocase_string_hash_table (int items)
655 return hash_table_new (items, string_hash_nocase, string_cmp_nocase);
658 /* Hashing of numeric values, such as pointers and integers.
660 This implementation is the Robert Jenkins' 32 bit Mix Function,
661 with a simple adaptation for 64-bit values. It offers excellent
662 spreading of values and doesn't need to know the hash table size to
663 work (unlike the very popular Knuth's multiplication hash). */
666 ptrhash (const void *ptr)
668 unsigned long key = (unsigned long)ptr;
691 ptrcmp (const void *ptr1, const void *ptr2)
702 print_hash_table_mapper (void *key, void *value, void *count)
705 printf ("%s: %s\n", (const char *)key, (char *)value);
710 print_hash (struct hash_table *sht)
713 hash_table_map (sht, print_hash_table_mapper, &debug_count);
714 assert (debug_count == sht->count);
720 struct hash_table *ht = make_string_hash_table (0);
722 while ((fgets (line, sizeof (line), stdin)))
724 int len = strlen (line);
728 if (!hash_table_contains (ht, line))
729 hash_table_put (ht, strdup (line), "here I am!");
734 if (hash_table_get_pair (ht, line, &line_copy, NULL))
736 hash_table_remove (ht, line);
746 printf ("%d %d\n", ht->count, ht->size);