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. */
30 /* With -DSTANDALONE, this file can be compiled outside Wget source
31 tree. To test, also use -DTEST. */
44 /* If running without Autoconf, go ahead and assume presence of
45 standard C89 headers. */
55 /* Get Wget's utility headers. */
59 /* Make do without them. */
60 # define xnew(x) xmalloc (sizeof (x))
61 # define xnew_array(type, x) xmalloc (sizeof (type) * (x))
62 # define xmalloc malloc /* or something that exits
63 if not enough memory */
65 # define countof(x) (sizeof (x) / sizeof ((x)[0]))
66 # define TOLOWER(x) ('A' <= (x) && (x) <= 'Z' ? (x) - 32 : (x))
74 Hash tables are a technique used to implement mapping between
75 objects with near-constant-time access and storage. The table
76 associates keys to values, and a value can be very quickly
77 retrieved by providing the key. Fast lookup tables are typically
78 implemented as hash tables.
81 hash_table_new -- creates the table.
82 hash_table_destroy -- destroys the table.
83 hash_table_put -- establishes or updates key->value mapping.
84 hash_table_get -- retrieves value of key.
85 hash_table_get_pair -- get key/value pair for key.
86 hash_table_contains -- test whether the table contains key.
87 hash_table_remove -- remove the key->value mapping for key.
88 hash_table_map -- iterate through table mappings.
89 hash_table_clear -- clear hash table contents.
90 hash_table_count -- return the number of entries in the table.
92 The hash table grows internally as new entries are added and is not
93 limited in size, except by available memory. The table doubles
94 with each resize, which ensures that the amortized time per
95 operation remains constant.
97 By default, tables created by hash_table_new consider the keys to
98 be equal if their pointer values are the same. You can use
99 make_string_hash_table to create tables whose keys are considered
100 equal if their string contents are the same. In the general case,
101 the criterion of equality used to compare keys is specified at
102 table creation time with two callback functions, "hash" and "test".
103 The hash function transforms the key into an arbitrary number that
104 must be the same for two equal keys. The test function accepts two
105 keys and returns non-zero if they are to be considered equal.
107 Note that neither keys nor values are copied when inserted into the
108 hash table, so they must exist for the lifetime of the table. This
109 means that e.g. the use of static strings is OK, but objects with a
110 shorter life-time need to be copied (with strdup() or the like in
111 the case of strings) before being inserted. */
115 The hash table is implemented as an open-addressed table with
116 linear probing collision resolution.
118 The above means that all the hash entries (pairs of pointers, key
119 and value) are stored in a contiguous array. The position of each
120 mapping is determined by the hash value of its key and the size of
121 the table: location := hash(key) % size. If two different keys end
122 up on the same position (collide), the one that came second is
123 placed at the next empty position following the occupied place.
124 This collision resolution technique is called "linear probing".
126 There are more advanced collision resolution methods (quadratic
127 probing, double hashing), but we don't use them because they incur
128 more non-sequential access to the array, which results in worse CPU
129 cache behavior. Linear probing works well as long as the
130 count/size ratio (fullness) is kept below 75%. We make sure to
131 grow and rehash the table whenever this threshold is exceeded.
133 Collisions make deletion tricky because clearing a position
134 followed by a colliding entry would make the position seem empty
135 and the colliding entry not found. One solution is to leave a
136 "tombstone" instead of clearing the entry, and another is to
137 carefully rehash the entries immediately following the deleted one.
138 We use the latter method because it results in less bookkeeping and
139 faster retrieval at the (slight) expense of deletion. */
141 /* Maximum allowed fullness: when hash table's fullness exceeds this
142 value, the table is resized. */
143 #define HASH_MAX_FULLNESS 0.75
145 /* The hash table size is multiplied by this factor (and then rounded
146 to the next prime) with each resize. This guarantees infrequent
148 #define HASH_RESIZE_FACTOR 2
155 typedef unsigned long (*hashfun_t) PARAMS ((const void *));
156 typedef int (*testfun_t) PARAMS ((const void *, const void *));
159 hashfun_t hash_function;
160 testfun_t test_function;
162 struct mapping *mappings; /* pointer to the table entries. */
163 int size; /* size of the array. */
165 int count; /* number of non-empty entries. */
166 int resize_threshold; /* after size exceeds this number of
167 entries, resize the table. */
168 int prime_offset; /* the offset of the current prime in
172 /* We use the all-bits-set constant (INVALID_PTR) marker to mean that
173 a mapping is empty. It is unaligned and therefore illegal as a
174 pointer. INVALID_PTR_BYTE (0xff) is the one-byte value used to
175 initialize the mappings array as empty.
177 The all-bits-set value is a better choice than NULL because it
178 allows the use of NULL/0 keys. Since the keys are either integers
179 or pointers, the only key that cannot be used is the integer value
180 -1. This is acceptable because it still allows the use of
181 nonnegative integer keys. */
183 #define INVALID_PTR ((void *) ~(unsigned long)0)
185 # define UCHAR_MAX 0xff
187 #define INVALID_PTR_BYTE UCHAR_MAX
189 #define NON_EMPTY(mp) ((mp)->key != INVALID_PTR)
190 #define MARK_AS_EMPTY(mp) ((mp)->key = INVALID_PTR)
192 /* "Next" mapping is the mapping after MP, but wrapping back to
193 MAPPINGS when MP would reach MAPPINGS+SIZE. */
194 #define NEXT_MAPPING(mp, mappings, size) (mp != mappings + (size - 1) \
197 /* Loop over non-empty mappings starting at MP. */
198 #define LOOP_NON_EMPTY(mp, mappings, size) \
199 for (; NON_EMPTY (mp); mp = NEXT_MAPPING (mp, mappings, size))
201 /* Return the position of KEY in hash table SIZE large, hash function
203 #define HASH_POSITION(key, hashfun, size) ((hashfun) (key) % size)
205 /* Find a prime near, but greather than or equal to SIZE. The primes
206 are looked up from a table with a selection of primes convenient
209 PRIME_OFFSET is a minor optimization: it specifies start position
210 for the search for the large enough prime. The final offset is
211 stored in the same variable. That way the list of primes does not
212 have to be scanned from the beginning each time around. */
215 prime_size (int size, int *prime_offset)
217 static const int primes[] = {
218 13, 19, 29, 41, 59, 79, 107, 149, 197, 263, 347, 457, 599, 787, 1031,
219 1361, 1777, 2333, 3037, 3967, 5167, 6719, 8737, 11369, 14783,
220 19219, 24989, 32491, 42257, 54941, 71429, 92861, 120721, 156941,
221 204047, 265271, 344857, 448321, 582821, 757693, 985003, 1280519,
222 1664681, 2164111, 2813353, 3657361, 4754591, 6180989, 8035301,
223 10445899, 13579681, 17653589, 22949669, 29834603, 38784989,
224 50420551, 65546729, 85210757, 110774011, 144006217, 187208107,
225 243370577, 316381771, 411296309, 534685237, 695090819, 903618083,
226 1174703521, 1527114613, 1837299131, 2147483647
230 for (i = *prime_offset; i < countof (primes); i++)
231 if (primes[i] >= size)
233 /* Set the offset to the next prime. That is safe because,
234 next time we are called, it will be with a larger SIZE,
235 which means we could never return the same prime anyway.
236 (If that is not the case, the caller can simply reset
238 *prime_offset = i + 1;
246 static int cmp_pointer PARAMS ((const void *, const void *));
248 /* Create a hash table with hash function HASH_FUNCTION and test
249 function TEST_FUNCTION. The table is empty (its count is 0), but
250 pre-allocated to store at least ITEMS items.
252 ITEMS is the number of items that the table can accept without
253 needing to resize. It is useful when creating a table that is to
254 be immediately filled with a known number of items. In that case,
255 the regrows are a waste of time, and specifying ITEMS correctly
256 will avoid them altogether.
258 Note that hash tables grow dynamically regardless of ITEMS. The
259 only use of ITEMS is to preallocate the table and avoid unnecessary
260 dynamic regrows. Don't bother making ITEMS prime because it's not
261 used as size unchanged. To start with a small table that grows as
262 needed, simply specify zero ITEMS.
264 If hash and test callbacks are not specified, identity mapping is
265 assumed, i.e. pointer values are used for key comparison. (Common
266 Lisp calls such tables EQ hash tables, and Java calls them
267 IdentityHashMaps.) If your keys require different comparison,
268 specify hash and test functions. For easy use of C strings as hash
269 keys, you can use the convenience functions make_string_hash_table
270 and make_nocase_string_hash_table. */
273 hash_table_new (int items,
274 unsigned long (*hash_function) (const void *),
275 int (*test_function) (const void *, const void *))
278 struct hash_table *ht = xnew (struct hash_table);
280 ht->hash_function = hash_function ? hash_function : hash_pointer;
281 ht->test_function = test_function ? test_function : cmp_pointer;
283 /* If the size of struct hash_table ever becomes a concern, this
284 field can go. (Wget doesn't create many hashes.) */
285 ht->prime_offset = 0;
287 /* Calculate the size that ensures that the table will store at
288 least ITEMS keys without the need to resize. */
289 size = 1 + items / HASH_MAX_FULLNESS;
290 size = prime_size (size, &ht->prime_offset);
292 ht->resize_threshold = size * HASH_MAX_FULLNESS;
293 /*assert (ht->resize_threshold >= items);*/
295 ht->mappings = xnew_array (struct mapping, ht->size);
297 /* Mark mappings as empty. We use 0xff rather than 0 to mark empty
298 keys because it allows us to use NULL/0 as keys. */
299 memset (ht->mappings, INVALID_PTR_BYTE, size * sizeof (struct mapping));
306 /* Free the data associated with hash table HT. */
309 hash_table_destroy (struct hash_table *ht)
311 xfree (ht->mappings);
315 /* The heart of most functions in this file -- find the mapping whose
316 KEY is equal to key, using linear probing. Returns the mapping
317 that matches KEY, or the first empty mapping if none matches. */
319 static inline struct mapping *
320 find_mapping (const struct hash_table *ht, const void *key)
322 struct mapping *mappings = ht->mappings;
324 struct mapping *mp = mappings + HASH_POSITION (key, ht->hash_function, size);
325 testfun_t equals = ht->test_function;
327 LOOP_NON_EMPTY (mp, mappings, size)
328 if (equals (key, mp->key))
333 /* Get the value that corresponds to the key KEY in the hash table HT.
334 If no value is found, return NULL. Note that NULL is a legal value
335 for value; if you are storing NULLs in your hash table, you can use
336 hash_table_contains to be sure that a (possibly NULL) value exists
337 in the table. Or, you can use hash_table_get_pair instead of this
341 hash_table_get (const struct hash_table *ht, const void *key)
343 struct mapping *mp = find_mapping (ht, key);
350 /* Like hash_table_get, but writes out the pointers to both key and
351 value. Returns non-zero on success. */
354 hash_table_get_pair (const struct hash_table *ht, const void *lookup_key,
355 void *orig_key, void *value)
357 struct mapping *mp = find_mapping (ht, lookup_key);
361 *(void **)orig_key = mp->key;
363 *(void **)value = mp->value;
370 /* Return 1 if HT contains KEY, 0 otherwise. */
373 hash_table_contains (const struct hash_table *ht, const void *key)
375 struct mapping *mp = find_mapping (ht, key);
376 return NON_EMPTY (mp);
379 /* Grow hash table HT as necessary, and rehash all the key-value
383 grow_hash_table (struct hash_table *ht)
385 hashfun_t hasher = ht->hash_function;
386 struct mapping *old_mappings = ht->mappings;
387 struct mapping *old_end = ht->mappings + ht->size;
388 struct mapping *mp, *mappings;
391 newsize = prime_size (ht->size * HASH_RESIZE_FACTOR, &ht->prime_offset);
393 printf ("growing from %d to %d; fullness %.2f%% to %.2f%%\n",
395 100.0 * ht->count / ht->size,
396 100.0 * ht->count / newsize);
400 ht->resize_threshold = newsize * HASH_MAX_FULLNESS;
402 mappings = xnew_array (struct mapping, newsize);
403 memset (mappings, INVALID_PTR_BYTE, newsize * sizeof (struct mapping));
404 ht->mappings = mappings;
406 for (mp = old_mappings; mp < old_end; mp++)
409 struct mapping *new_mp;
410 /* We don't need to test for uniqueness of keys because they
411 come from the hash table and are therefore known to be
413 new_mp = mappings + HASH_POSITION (mp->key, hasher, newsize);
414 LOOP_NON_EMPTY (new_mp, mappings, newsize)
419 xfree (old_mappings);
422 /* Put VALUE in the hash table HT under the key KEY. This regrows the
423 table if necessary. */
426 hash_table_put (struct hash_table *ht, const void *key, void *value)
428 struct mapping *mp = find_mapping (ht, key);
431 /* update existing item */
432 mp->key = (void *)key; /* const? */
437 /* If adding the item would make the table exceed max. fullness,
438 grow the table first. */
439 if (ht->count >= ht->resize_threshold)
441 grow_hash_table (ht);
442 mp = find_mapping (ht, key);
447 mp->key = (void *)key; /* const? */
451 /* Remove a mapping that matches KEY from HT. Return 0 if there was
452 no such entry; return 1 if an entry was removed. */
455 hash_table_remove (struct hash_table *ht, const void *key)
457 struct mapping *mp = find_mapping (ht, key);
463 struct mapping *mappings = ht->mappings;
464 hashfun_t hasher = ht->hash_function;
469 /* Rehash all the entries following MP. The alternative
470 approach is to mark the entry as deleted, i.e. create a
471 "tombstone". That speeds up removal, but leaves a lot of
472 garbage and slows down hash_table_get and hash_table_put. */
474 mp = NEXT_MAPPING (mp, mappings, size);
475 LOOP_NON_EMPTY (mp, mappings, size)
477 const void *key2 = mp->key;
478 struct mapping *mp_new;
480 /* Find the new location for the key. */
481 mp_new = mappings + HASH_POSITION (key2, hasher, size);
482 LOOP_NON_EMPTY (mp_new, mappings, size)
483 if (key2 == mp_new->key)
484 /* The mapping MP (key2) is already where we want it (in
485 MP_NEW's "chain" of keys.) */
498 /* Clear HT of all entries. After calling this function, the count
499 and the fullness of the hash table will be zero. The size will
503 hash_table_clear (struct hash_table *ht)
505 memset (ht->mappings, INVALID_PTR_BYTE, ht->size * sizeof (struct mapping));
509 /* Map MAPFUN over all the mappings in hash table HT. MAPFUN is
510 called with three arguments: the key, the value, and MAPARG.
512 It is undefined what happens if you add or remove entries in the
513 hash table while hash_table_map is running. The exception is the
514 entry you're currently mapping over; you may remove or change that
518 hash_table_map (struct hash_table *ht,
519 int (*mapfun) (void *, void *, void *),
522 struct mapping *mp = ht->mappings;
523 struct mapping *end = ht->mappings + ht->size;
525 for (; mp < end; mp++)
531 if (mapfun (key, mp->value, maparg))
533 /* hash_table_remove might have moved the adjacent
535 if (mp->key != key && NON_EMPTY (mp))
540 /* Return the number of elements in the hash table. This is not the
541 same as the physical size of the hash table, which is always
542 greater than the number of elements. */
545 hash_table_count (const struct hash_table *ht)
550 /* Functions from this point onward are meant for convenience and
551 don't strictly belong to this file. However, this is as good a
552 place for them as any. */
554 /* Guidelines for creating custom hash and test functions:
556 - The test function returns non-zero for keys that are considered
557 "equal", zero otherwise.
559 - The hash function returns a number that represents the
560 "distinctness" of the object. In more precise terms, it means
561 that for any two objects that test "equal" under the test
562 function, the hash function MUST produce the same result.
564 This does not mean that all different objects must produce
565 different values (that would be "perfect" hashing), only that
566 non-distinct objects must produce the same values! For instance,
567 a hash function that returns 0 for any given object is a
568 perfectly valid (albeit extremely bad) hash function. A hash
569 function that hashes a string by adding up all its characters is
570 another example of a valid (but quite bad) hash function.
572 It is not hard to make hash and test functions agree about
573 equality. For example, if the test function compares strings
574 case-insensitively, the hash function can lower-case the
575 characters when calculating the hash value. That ensures that
576 two strings differing only in case will hash the same.
578 - If you care about performance, choose a hash function with as
579 good "spreading" as possible. A good hash function will use all
580 the bits of the input when calculating the hash, and will react
581 to even small changes in input with a completely different
582 output. Finally, don't make the hash function itself overly
583 slow, because you'll be incurring a non-negligible overhead to
584 all hash table operations. */
587 * Support for hash tables whose keys are strings.
591 /* 31 bit hash function. Taken from Gnome's glib, modified to use
594 We used to use the popular hash function from the Dragon Book, but
595 this one seems to perform much better. */
598 hash_string (const void *key)
604 for (p += 1; *p != '\0'; p++)
605 h = (h << 5) - h + *p;
610 /* Frontend for strcmp usable for hash tables. */
613 cmp_string (const void *s1, const void *s2)
615 return !strcmp ((const char *)s1, (const char *)s2);
618 /* Return a hash table of preallocated to store at least ITEMS items
619 suitable to use strings as keys. */
622 make_string_hash_table (int items)
624 return hash_table_new (items, hash_string, cmp_string);
628 * Support for hash tables whose keys are strings, but which are
629 * compared case-insensitively.
633 /* Like hash_string, but produce the same hash regardless of the case. */
636 hash_string_nocase (const void *key)
639 unsigned int h = TOLOWER (*p);
642 for (p += 1; *p != '\0'; p++)
643 h = (h << 5) - h + TOLOWER (*p);
648 /* Like string_cmp, but doing case-insensitive compareison. */
651 string_cmp_nocase (const void *s1, const void *s2)
653 return !strcasecmp ((const char *)s1, (const char *)s2);
656 /* Like make_string_hash_table, but uses string_hash_nocase and
657 string_cmp_nocase. */
660 make_nocase_string_hash_table (int items)
662 return hash_table_new (items, hash_string_nocase, string_cmp_nocase);
665 /* Hashing of numeric values, such as pointers and integers.
667 This implementation is the Robert Jenkins' 32 bit Mix Function,
668 with a simple adaptation for 64-bit values. It offers excellent
669 spreading of values and doesn't need to know the hash table size to
670 work (unlike the very popular Knuth's multiplication hash). */
673 hash_pointer (const void *ptr)
675 unsigned long key = (unsigned long)ptr;
698 cmp_pointer (const void *ptr1, const void *ptr2)
709 print_hash_table_mapper (void *key, void *value, void *count)
712 printf ("%s: %s\n", (const char *)key, (char *)value);
717 print_hash (struct hash_table *sht)
720 hash_table_map (sht, print_hash_table_mapper, &debug_count);
721 assert (debug_count == sht->count);
727 struct hash_table *ht = make_string_hash_table (0);
729 while ((fgets (line, sizeof (line), stdin)))
731 int len = strlen (line);
735 if (!hash_table_contains (ht, line))
736 hash_table_put (ht, strdup (line), "here I am!");
741 if (hash_table_get_pair (ht, line, &line_copy, NULL))
743 hash_table_remove (ht, line);
753 printf ("%d %d\n", ht->count, ht->size);