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 The above means that all the hash entries (pairs of pointers, key
110 and value) are stored in a contiguous array. The position of each
111 mapping is determined by the hash value of its key and the size of
112 the table: location := hash(key) % size. If two different keys end
113 up on the same position (collide), the one that came second is
114 placed at the next empty position following the occupied place.
115 This collision resolution technique is called "linear probing".
117 There are more advanced collision resolution methods (quadratic
118 probing, double hashing), but we don't use them because they incur
119 more non-sequential access to the array, which results in worse CPU
120 cache behavior. Linear probing works well as long as the
121 count/size ratio (fullness) is kept below 75%. We make sure to
122 grow and rehash the table whenever this threshold is exceeded.
124 Collisions make deletion tricky because clearing a position
125 followed by a colliding entry would make the position seem empty
126 and the colliding entry not found. One solution is to leave a
127 "tombstone" instead of clearing the entry, and another is to
128 carefully rehash the entries immediately following the deleted one.
129 We use the latter method because it results in less bookkeeping and
130 faster retrieval at the (slight) expense of deletion. */
132 /* Maximum allowed fullness: when hash table's fullness exceeds this
133 value, the table is resized. */
134 #define HASH_MAX_FULLNESS 0.75
136 /* The hash table size is multiplied by this factor (and then rounded
137 to the next prime) with each resize. This guarantees infrequent
139 #define HASH_RESIZE_FACTOR 2
146 typedef unsigned long (*hashfun_t) PARAMS ((const void *));
147 typedef int (*testfun_t) PARAMS ((const void *, const void *));
150 hashfun_t hash_function;
151 testfun_t test_function;
153 struct mapping *mappings; /* pointer to the table entries. */
154 int size; /* size of the array. */
156 int count; /* number of non-empty entries. */
157 int resize_threshold; /* after size exceeds this number of
158 entries, resize the table. */
159 int prime_offset; /* the offset of the current prime in
163 /* We use the all-bits-set constant (INVALID_PTR) marker to mean that
164 a mapping is empty. It is unaligned and therefore illegal as a
165 pointer. INVALID_PTR_BYTE (0xff) is the one-byte value used to
166 initialize the mappings array as empty.
168 The all-bits-set value is a better choice than NULL because it
169 allows the use of NULL/0 keys. Since the keys are either integers
170 or pointers, the only key that cannot be used is the integer value
171 -1. This is acceptable because it still allows the use of
172 nonnegative integer keys. */
174 #define INVALID_PTR ((void *) ~(unsigned long)0)
176 # define UCHAR_MAX 0xff
178 #define INVALID_PTR_BYTE UCHAR_MAX
180 #define NON_EMPTY(mp) ((mp)->key != INVALID_PTR)
181 #define MARK_AS_EMPTY(mp) ((mp)->key = INVALID_PTR)
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 /* Return the position of KEY in hash table SIZE large, hash function
194 #define HASH_POSITION(key, hashfun, size) ((hashfun) (key) % size)
196 /* Find a prime near, but greather than or equal to SIZE. The primes
197 are looked up from a table with a selection of primes convenient
200 PRIME_OFFSET is a minor optimization: it specifies start position
201 for the search for the large enough prime. The final offset is
202 stored in the same variable. That way the list of primes does not
203 have to be scanned from the beginning each time around. */
206 prime_size (int size, int *prime_offset)
208 static const unsigned long primes [] = {
209 13, 19, 29, 41, 59, 79, 107, 149, 197, 263, 347, 457, 599, 787, 1031,
210 1361, 1777, 2333, 3037, 3967, 5167, 6719, 8737, 11369, 14783,
211 19219, 24989, 32491, 42257, 54941, 71429, 92861, 120721, 156941,
212 204047, 265271, 344857, 448321, 582821, 757693, 985003, 1280519,
213 1664681, 2164111, 2813353, 3657361, 4754591, 6180989, 8035301,
214 10445899, 13579681, 17653589, 22949669, 29834603, 38784989,
215 50420551, 65546729, 85210757, 110774011, 144006217, 187208107,
216 243370577, 316381771, 411296309, 534685237, 695090819, 903618083,
217 1174703521, 1527114613, 1985248999,
218 (unsigned long)0x99d43ea5, (unsigned long)0xc7fa5177
222 for (i = *prime_offset; i < countof (primes); i++)
223 if (primes[i] >= size)
225 /* Set the offset to the next prime. That is safe because,
226 next time we are called, it will be with a larger SIZE,
227 which means we could never return the same prime anyway.
228 (If that is not the case, the caller can simply reset
230 *prime_offset = i + 1;
238 static unsigned long ptrhash PARAMS ((const void *));
239 static int ptrcmp PARAMS ((const void *, const void *));
241 /* Create a hash table with hash function HASH_FUNCTION and test
242 function TEST_FUNCTION. The table is empty (its count is 0), but
243 pre-allocated to store at least ITEMS items.
245 ITEMS is the number of items that the table can accept without
246 needing to resize. It is useful when creating a table that is to
247 be immediately filled with a known number of items. In that case,
248 the regrows are a waste of time, and specifying ITEMS correctly
249 will avoid them altogether.
251 Note that hash tables grow dynamically regardless of ITEMS. The
252 only use of ITEMS is to preallocate the table and avoid unnecessary
253 dynamic regrows. Don't bother making ITEMS prime because it's not
254 used as size unchanged. To start with a small table that grows as
255 needed, simply specify zero ITEMS.
257 If hash and test callbacks are not specified, identity mapping is
258 assumed, i.e. pointer values are used for key comparison. (Common
259 Lisp calls such tables EQ hash tables, and Java calls them
260 IdentityHashMaps.) If your keys require different comparison,
261 specify hash and test functions. For easy use of C strings as hash
262 keys, you can use the convenience functions make_string_hash_table
263 and make_nocase_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 use NULL/0 as keys. */
291 memset (ht->mappings, INVALID_PTR_BYTE, 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 (key, ht->hash_function, size);
317 testfun_t equals = 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 hashfun_t hasher = ht->hash_function;
378 struct mapping *old_mappings = ht->mappings;
379 struct mapping *old_end = ht->mappings + ht->size;
380 struct mapping *mp, *mappings;
383 newsize = prime_size (ht->size * HASH_RESIZE_FACTOR, &ht->prime_offset);
385 printf ("growing from %d to %d; fullness %.2f%% to %.2f%%\n",
387 100.0 * ht->count / ht->size,
388 100.0 * ht->count / newsize);
392 ht->resize_threshold = newsize * HASH_MAX_FULLNESS;
394 mappings = xnew_array (struct mapping, newsize);
395 memset (mappings, INVALID_PTR_BYTE, newsize * sizeof (struct mapping));
396 ht->mappings = mappings;
398 for (mp = old_mappings; mp < old_end; mp++)
401 struct mapping *new_mp;
402 /* We don't need to test for uniqueness of keys because they
403 come from the hash table and are therefore known to be
405 new_mp = mappings + HASH_POSITION (mp->key, hasher, newsize);
406 LOOP_NON_EMPTY (new_mp, mappings, newsize)
411 xfree (old_mappings);
414 /* Put VALUE in the hash table HT under the key KEY. This regrows the
415 table if necessary. */
418 hash_table_put (struct hash_table *ht, const void *key, void *value)
420 struct mapping *mp = find_mapping (ht, key);
423 /* update existing item */
424 mp->key = (void *)key; /* const? */
429 /* If adding the item would make the table exceed max. fullness,
430 grow the table first. */
431 if (ht->count >= ht->resize_threshold)
433 grow_hash_table (ht);
434 mp = find_mapping (ht, key);
439 mp->key = (void *)key; /* const? */
443 /* Remove a mapping that matches KEY from HT. Return 0 if there was
444 no such entry; return 1 if an entry was removed. */
447 hash_table_remove (struct hash_table *ht, const void *key)
449 struct mapping *mp = find_mapping (ht, key);
455 struct mapping *mappings = ht->mappings;
456 hashfun_t hasher = ht->hash_function;
461 /* Rehash all the entries following MP. The alternative
462 approach is to mark the entry as deleted, i.e. create a
463 "tombstone". That speeds up removal, but leaves a lot of
464 garbage and slows down hash_table_get and hash_table_put. */
466 mp = NEXT_MAPPING (mp, mappings, size);
467 LOOP_NON_EMPTY (mp, mappings, size)
469 const void *key2 = mp->key;
470 struct mapping *mp_new;
472 /* Find the new location for the key. */
473 mp_new = mappings + HASH_POSITION (key2, hasher, size);
474 LOOP_NON_EMPTY (mp_new, mappings, size)
475 if (key2 == mp_new->key)
476 /* The mapping MP (key2) is already where we want it (in
477 MP_NEW's "chain" of keys.) */
490 /* Clear HT of all entries. After calling this function, the count
491 and the fullness of the hash table will be zero. The size will
495 hash_table_clear (struct hash_table *ht)
497 memset (ht->mappings, INVALID_PTR_BYTE, ht->size * sizeof (struct mapping));
501 /* Map MAPFUN over all the mappings in hash table HT. MAPFUN is
502 called with three arguments: the key, the value, and MAPARG.
504 It is undefined what happens if you add or remove entries in the
505 hash table while hash_table_map is running. The exception is the
506 entry you're currently mapping over; you may remove or change that
510 hash_table_map (struct hash_table *ht,
511 int (*mapfun) (void *, void *, void *),
514 struct mapping *mp = ht->mappings;
515 struct mapping *end = ht->mappings + ht->size;
517 for (; mp < end; mp++)
523 if (mapfun (key, mp->value, maparg))
525 /* hash_table_remove might have moved the adjacent
527 if (mp->key != key && NON_EMPTY (mp))
532 /* Return the number of elements in the hash table. This is not the
533 same as the physical size of the hash table, which is always
534 greater than the number of elements. */
537 hash_table_count (const struct hash_table *ht)
542 /* Functions from this point onward are meant for convenience and
543 don't strictly belong to this file. However, this is as good a
544 place for them as any. */
546 /* Rules for creating custom hash and test functions:
548 - The test function returns non-zero for keys that are considered
549 "equal", zero otherwise.
551 - The hash function returns a number that represents the
552 "distinctness" of the object. In more precise terms, it means
553 that for any two objects that test "equal" under the test
554 function, the hash function MUST produce the same result.
556 This does not mean that all different objects must produce
557 different values (that would be "perfect" hashing), only that
558 non-distinct objects must produce the same values! For instance,
559 a hash function that returns 0 for any given object is a
560 perfectly valid (albeit extremely bad) hash function. A hash
561 function that hashes a string by adding up all its characters is
562 another example of a valid (but quite bad) hash function.
564 It is not hard to make hash and test functions agree about
565 equality. For example, if the test function compares strings
566 case-insensitively, the hash function can lower-case the
567 characters when calculating the hash value. That ensures that
568 two strings differing only in case will hash the same.
570 - If you care about performance, choose a hash function with as
571 good "spreading" as possible. A good hash function will use all
572 the bits of the input when calculating the hash, and will react
573 to even small changes in input with a completely different
574 output. Finally, don't make the hash function itself overly
575 slow, because you'll be incurring a non-negligible overhead to
576 all hash table operations. */
579 * Support for hash tables whose keys are strings.
583 /* 31 bit hash function. Taken from Gnome's glib, modified to use
586 We used to use the popular hash function from the Dragon Book, but
587 this one seems to perform much better. */
590 string_hash (const void *key)
596 for (p += 1; *p != '\0'; p++)
597 h = (h << 5) - h + *p;
602 /* Frontend for strcmp usable for hash tables. */
605 string_cmp (const void *s1, const void *s2)
607 return !strcmp ((const char *)s1, (const char *)s2);
610 /* Return a hash table of preallocated to store at least ITEMS items
611 suitable to use strings as keys. */
614 make_string_hash_table (int items)
616 return hash_table_new (items, string_hash, string_cmp);
620 * Support for hash tables whose keys are strings, but which are
621 * compared case-insensitively.
625 /* Like string_hash, but produce the same hash regardless of the case. */
628 string_hash_nocase (const void *key)
631 unsigned int h = TOLOWER (*p);
634 for (p += 1; *p != '\0'; p++)
635 h = (h << 5) - h + TOLOWER (*p);
640 /* Like string_cmp, but doing case-insensitive compareison. */
643 string_cmp_nocase (const void *s1, const void *s2)
645 return !strcasecmp ((const char *)s1, (const char *)s2);
648 /* Like make_string_hash_table, but uses string_hash_nocase and
649 string_cmp_nocase. */
652 make_nocase_string_hash_table (int items)
654 return hash_table_new (items, string_hash_nocase, string_cmp_nocase);
657 /* Hashing of numeric values, such as pointers and integers.
659 This implementation is the Robert Jenkins' 32 bit Mix Function,
660 with a simple adaptation for 64-bit values. It offers excellent
661 spreading of values and doesn't need to know the hash table size to
662 work (unlike the very popular Knuth's multiplication hash). */
665 ptrhash (const void *ptr)
667 unsigned long key = (unsigned long)ptr;
690 ptrcmp (const void *ptr1, const void *ptr2)
701 print_hash_table_mapper (void *key, void *value, void *count)
704 printf ("%s: %s\n", (const char *)key, (char *)value);
709 print_hash (struct hash_table *sht)
712 hash_table_map (sht, print_hash_table_mapper, &debug_count);
713 assert (debug_count == sht->count);
719 struct hash_table *ht = make_string_hash_table (0);
721 while ((fgets (line, sizeof (line), stdin)))
723 int len = strlen (line);
727 if (!hash_table_contains (ht, line))
728 hash_table_put (ht, strdup (line), "here I am!");
733 if (hash_table_get_pair (ht, line, &line_copy, NULL))
735 hash_table_remove (ht, line);
745 printf ("%d %d\n", ht->count, ht->size);