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. */
54 /* Get Wget's utility headers. */
58 /* Make do without them. */
59 # define xnew(x) xmalloc (sizeof (x))
60 # define xnew_array(type, x) xmalloc (sizeof (type) * (x))
61 # define xmalloc malloc /* or something that exits
62 if not enough memory */
64 # define countof(x) (sizeof (x) / sizeof ((x)[0]))
65 # define TOLOWER(x) ('A' <= (x) && (x) <= 'Z' ? (x) - 32 : (x))
73 Hash tables are a technique used to implement mapping between
74 objects with near-constant-time access and storage. The table
75 associates keys to values, and a value can be very quickly
76 retrieved by providing the key. Fast lookup tables are typically
77 implemented as hash tables.
80 hash_table_new -- creates the table.
81 hash_table_destroy -- destroys the table.
82 hash_table_put -- establishes or updates key->value mapping.
83 hash_table_get -- retrieves value of key.
84 hash_table_get_pair -- get key/value pair for key.
85 hash_table_contains -- test whether the table contains key.
86 hash_table_remove -- remove the key->value mapping for key.
87 hash_table_map -- iterate through table mappings.
88 hash_table_clear -- clear hash table contents.
89 hash_table_count -- return the number of entries in the table.
91 The hash table grows internally as new entries are added and is not
92 limited in size, except by available memory. The table doubles
93 with each resize, which ensures that the amortized time per
94 operation remains constant.
96 By default, tables created by hash_table_new consider the keys to
97 be equal if their pointer values are the same. You can use
98 make_string_hash_table to create tables whose keys are considered
99 equal if their string contents are the same. In the general case,
100 the criterion of equality used to compare keys is specified at
101 table creation time with two callback functions, "hash" and "test".
102 The hash function transforms the key into an arbitrary number that
103 must be the same for two equal keys. The test function accepts two
104 keys and returns non-zero if they are to be considered equal.
106 Note that neither keys nor values are copied when inserted into the
107 hash table, so they must exist for the lifetime of the table. This
108 means that e.g. the use of static strings is OK, but objects with a
109 shorter life-time need to be copied (with strdup() or the like in
110 the case of strings) before being inserted. */
114 The hash table is implemented as an open-addressed table with
115 linear probing collision resolution.
117 The above means that all the hash entries (pairs of pointers, key
118 and value) are stored in a contiguous array. The position of each
119 mapping is determined by the hash value of its key and the size of
120 the table: location := hash(key) % size. If two different keys end
121 up on the same position (collide), the one that came second is
122 placed at the next empty position following the occupied place.
123 This collision resolution technique is called "linear probing".
125 There are more advanced collision resolution methods (quadratic
126 probing, double hashing), but we don't use them because they incur
127 more non-sequential access to the array, which results in worse CPU
128 cache behavior. Linear probing works well as long as the
129 count/size ratio (fullness) is kept below 75%. We make sure to
130 grow and rehash the table whenever this threshold is exceeded.
132 Collisions make deletion tricky because clearing a position
133 followed by a colliding entry would make the position seem empty
134 and the colliding entry not found. One solution is to leave a
135 "tombstone" instead of clearing the entry, and another is to
136 carefully rehash the entries immediately following the deleted one.
137 We use the latter method because it results in less bookkeeping and
138 faster retrieval at the (slight) expense of deletion. */
140 /* Maximum allowed fullness: when hash table's fullness exceeds this
141 value, the table is resized. */
142 #define HASH_MAX_FULLNESS 0.75
144 /* The hash table size is multiplied by this factor (and then rounded
145 to the next prime) with each resize. This guarantees infrequent
147 #define HASH_RESIZE_FACTOR 2
154 typedef unsigned long (*hashfun_t) PARAMS ((const void *));
155 typedef int (*testfun_t) PARAMS ((const void *, const void *));
158 hashfun_t hash_function;
159 testfun_t test_function;
161 struct mapping *mappings; /* pointer to the table entries. */
162 int size; /* size of the array. */
164 int count; /* number of non-empty entries. */
165 int resize_threshold; /* after size exceeds this number of
166 entries, resize the table. */
167 int prime_offset; /* the offset of the current prime in
171 /* We use the all-bits-set constant (INVALID_PTR) marker to mean that
172 a mapping is empty. It is unaligned and therefore illegal as a
173 pointer. INVALID_PTR_BYTE (0xff) is the one-byte value used to
174 initialize the mappings array as empty.
176 The all-bits-set value is a better choice than NULL because it
177 allows the use of NULL/0 keys. Since the keys are either integers
178 or pointers, the only key that cannot be used is the integer value
179 -1. This is acceptable because it still allows the use of
180 nonnegative integer keys. */
182 #define INVALID_PTR ((void *) ~(unsigned long)0)
184 # define UCHAR_MAX 0xff
186 #define INVALID_PTR_BYTE UCHAR_MAX
188 #define NON_EMPTY(mp) ((mp)->key != INVALID_PTR)
189 #define MARK_AS_EMPTY(mp) ((mp)->key = INVALID_PTR)
191 /* "Next" mapping is the mapping after MP, but wrapping back to
192 MAPPINGS when MP would reach MAPPINGS+SIZE. */
193 #define NEXT_MAPPING(mp, mappings, size) (mp != mappings + (size - 1) \
196 /* Loop over non-empty mappings starting at MP. */
197 #define LOOP_NON_EMPTY(mp, mappings, size) \
198 for (; NON_EMPTY (mp); mp = NEXT_MAPPING (mp, mappings, size))
200 /* Return the position of KEY in hash table SIZE large, hash function
202 #define HASH_POSITION(key, hashfun, size) ((hashfun) (key) % size)
204 /* Find a prime near, but greather than or equal to SIZE. The primes
205 are looked up from a table with a selection of primes convenient
208 PRIME_OFFSET is a minor optimization: it specifies start position
209 for the search for the large enough prime. The final offset is
210 stored in the same variable. That way the list of primes does not
211 have to be scanned from the beginning each time around. */
214 prime_size (int size, int *prime_offset)
216 static const unsigned long primes [] = {
217 13, 19, 29, 41, 59, 79, 107, 149, 197, 263, 347, 457, 599, 787, 1031,
218 1361, 1777, 2333, 3037, 3967, 5167, 6719, 8737, 11369, 14783,
219 19219, 24989, 32491, 42257, 54941, 71429, 92861, 120721, 156941,
220 204047, 265271, 344857, 448321, 582821, 757693, 985003, 1280519,
221 1664681, 2164111, 2813353, 3657361, 4754591, 6180989, 8035301,
222 10445899, 13579681, 17653589, 22949669, 29834603, 38784989,
223 50420551, 65546729, 85210757, 110774011, 144006217, 187208107,
224 243370577, 316381771, 411296309, 534685237, 695090819, 903618083,
225 1174703521, 1527114613, 1985248999,
226 (unsigned long)0x99d43ea5, (unsigned long)0xc7fa5177
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 unsigned long ptrhash PARAMS ((const void *));
247 static int ptrcmp PARAMS ((const void *, const void *));
249 /* Create a hash table with hash function HASH_FUNCTION and test
250 function TEST_FUNCTION. The table is empty (its count is 0), but
251 pre-allocated to store at least ITEMS items.
253 ITEMS is the number of items that the table can accept without
254 needing to resize. It is useful when creating a table that is to
255 be immediately filled with a known number of items. In that case,
256 the regrows are a waste of time, and specifying ITEMS correctly
257 will avoid them altogether.
259 Note that hash tables grow dynamically regardless of ITEMS. The
260 only use of ITEMS is to preallocate the table and avoid unnecessary
261 dynamic regrows. Don't bother making ITEMS prime because it's not
262 used as size unchanged. To start with a small table that grows as
263 needed, simply specify zero ITEMS.
265 If hash and test callbacks are not specified, identity mapping is
266 assumed, i.e. pointer values are used for key comparison. (Common
267 Lisp calls such tables EQ hash tables, and Java calls them
268 IdentityHashMaps.) If your keys require different comparison,
269 specify hash and test functions. For easy use of C strings as hash
270 keys, you can use the convenience functions make_string_hash_table
271 and make_nocase_string_hash_table. */
274 hash_table_new (int items,
275 unsigned long (*hash_function) (const void *),
276 int (*test_function) (const void *, const void *))
279 struct hash_table *ht = xnew (struct hash_table);
281 ht->hash_function = hash_function ? hash_function : ptrhash;
282 ht->test_function = test_function ? test_function : ptrcmp;
284 /* If the size of struct hash_table ever becomes a concern, this
285 field can go. (Wget doesn't create many hashes.) */
286 ht->prime_offset = 0;
288 /* Calculate the size that ensures that the table will store at
289 least ITEMS keys without the need to resize. */
290 size = 1 + items / HASH_MAX_FULLNESS;
291 size = prime_size (size, &ht->prime_offset);
293 ht->resize_threshold = size * HASH_MAX_FULLNESS;
294 /*assert (ht->resize_threshold >= items);*/
296 ht->mappings = xnew_array (struct mapping, ht->size);
298 /* Mark mappings as empty. We use 0xff rather than 0 to mark empty
299 keys because it allows us to use NULL/0 as keys. */
300 memset (ht->mappings, INVALID_PTR_BYTE, size * sizeof (struct mapping));
307 /* Free the data associated with hash table HT. */
310 hash_table_destroy (struct hash_table *ht)
312 xfree (ht->mappings);
316 /* The heart of most functions in this file -- find the mapping whose
317 KEY is equal to key, using linear probing. Returns the mapping
318 that matches KEY, or the first empty mapping if none matches. */
320 static inline struct mapping *
321 find_mapping (const struct hash_table *ht, const void *key)
323 struct mapping *mappings = ht->mappings;
325 struct mapping *mp = mappings + HASH_POSITION (key, ht->hash_function, size);
326 testfun_t equals = ht->test_function;
328 LOOP_NON_EMPTY (mp, mappings, size)
329 if (equals (key, mp->key))
334 /* Get the value that corresponds to the key KEY in the hash table HT.
335 If no value is found, return NULL. Note that NULL is a legal value
336 for value; if you are storing NULLs in your hash table, you can use
337 hash_table_contains to be sure that a (possibly NULL) value exists
338 in the table. Or, you can use hash_table_get_pair instead of this
342 hash_table_get (const struct hash_table *ht, const void *key)
344 struct mapping *mp = find_mapping (ht, key);
351 /* Like hash_table_get, but writes out the pointers to both key and
352 value. Returns non-zero on success. */
355 hash_table_get_pair (const struct hash_table *ht, const void *lookup_key,
356 void *orig_key, void *value)
358 struct mapping *mp = find_mapping (ht, lookup_key);
362 *(void **)orig_key = mp->key;
364 *(void **)value = mp->value;
371 /* Return 1 if HT contains KEY, 0 otherwise. */
374 hash_table_contains (const struct hash_table *ht, const void *key)
376 struct mapping *mp = find_mapping (ht, key);
377 return NON_EMPTY (mp);
380 /* Grow hash table HT as necessary, and rehash all the key-value
384 grow_hash_table (struct hash_table *ht)
386 hashfun_t hasher = ht->hash_function;
387 struct mapping *old_mappings = ht->mappings;
388 struct mapping *old_end = ht->mappings + ht->size;
389 struct mapping *mp, *mappings;
392 newsize = prime_size (ht->size * HASH_RESIZE_FACTOR, &ht->prime_offset);
394 printf ("growing from %d to %d; fullness %.2f%% to %.2f%%\n",
396 100.0 * ht->count / ht->size,
397 100.0 * ht->count / newsize);
401 ht->resize_threshold = newsize * HASH_MAX_FULLNESS;
403 mappings = xnew_array (struct mapping, newsize);
404 memset (mappings, INVALID_PTR_BYTE, newsize * sizeof (struct mapping));
405 ht->mappings = mappings;
407 for (mp = old_mappings; mp < old_end; mp++)
410 struct mapping *new_mp;
411 /* We don't need to test for uniqueness of keys because they
412 come from the hash table and are therefore known to be
414 new_mp = mappings + HASH_POSITION (mp->key, hasher, newsize);
415 LOOP_NON_EMPTY (new_mp, mappings, newsize)
420 xfree (old_mappings);
423 /* Put VALUE in the hash table HT under the key KEY. This regrows the
424 table if necessary. */
427 hash_table_put (struct hash_table *ht, const void *key, void *value)
429 struct mapping *mp = find_mapping (ht, key);
432 /* update existing item */
433 mp->key = (void *)key; /* const? */
438 /* If adding the item would make the table exceed max. fullness,
439 grow the table first. */
440 if (ht->count >= ht->resize_threshold)
442 grow_hash_table (ht);
443 mp = find_mapping (ht, key);
448 mp->key = (void *)key; /* const? */
452 /* Remove a mapping that matches KEY from HT. Return 0 if there was
453 no such entry; return 1 if an entry was removed. */
456 hash_table_remove (struct hash_table *ht, const void *key)
458 struct mapping *mp = find_mapping (ht, key);
464 struct mapping *mappings = ht->mappings;
465 hashfun_t hasher = ht->hash_function;
470 /* Rehash all the entries following MP. The alternative
471 approach is to mark the entry as deleted, i.e. create a
472 "tombstone". That speeds up removal, but leaves a lot of
473 garbage and slows down hash_table_get and hash_table_put. */
475 mp = NEXT_MAPPING (mp, mappings, size);
476 LOOP_NON_EMPTY (mp, mappings, size)
478 const void *key2 = mp->key;
479 struct mapping *mp_new;
481 /* Find the new location for the key. */
482 mp_new = mappings + HASH_POSITION (key2, hasher, size);
483 LOOP_NON_EMPTY (mp_new, mappings, size)
484 if (key2 == mp_new->key)
485 /* The mapping MP (key2) is already where we want it (in
486 MP_NEW's "chain" of keys.) */
499 /* Clear HT of all entries. After calling this function, the count
500 and the fullness of the hash table will be zero. The size will
504 hash_table_clear (struct hash_table *ht)
506 memset (ht->mappings, INVALID_PTR_BYTE, ht->size * sizeof (struct mapping));
510 /* Map MAPFUN over all the mappings in hash table HT. MAPFUN is
511 called with three arguments: the key, the value, and MAPARG.
513 It is undefined what happens if you add or remove entries in the
514 hash table while hash_table_map is running. The exception is the
515 entry you're currently mapping over; you may remove or change that
519 hash_table_map (struct hash_table *ht,
520 int (*mapfun) (void *, void *, void *),
523 struct mapping *mp = ht->mappings;
524 struct mapping *end = ht->mappings + ht->size;
526 for (; mp < end; mp++)
532 if (mapfun (key, mp->value, maparg))
534 /* hash_table_remove might have moved the adjacent
536 if (mp->key != key && NON_EMPTY (mp))
541 /* Return the number of elements in the hash table. This is not the
542 same as the physical size of the hash table, which is always
543 greater than the number of elements. */
546 hash_table_count (const struct hash_table *ht)
551 /* Functions from this point onward are meant for convenience and
552 don't strictly belong to this file. However, this is as good a
553 place for them as any. */
555 /* Rules for creating custom hash and test functions:
557 - The test function returns non-zero for keys that are considered
558 "equal", zero otherwise.
560 - The hash function returns a number that represents the
561 "distinctness" of the object. In more precise terms, it means
562 that for any two objects that test "equal" under the test
563 function, the hash function MUST produce the same result.
565 This does not mean that all different objects must produce
566 different values (that would be "perfect" hashing), only that
567 non-distinct objects must produce the same values! For instance,
568 a hash function that returns 0 for any given object is a
569 perfectly valid (albeit extremely bad) hash function. A hash
570 function that hashes a string by adding up all its characters is
571 another example of a valid (but quite bad) hash function.
573 It is not hard to make hash and test functions agree about
574 equality. For example, if the test function compares strings
575 case-insensitively, the hash function can lower-case the
576 characters when calculating the hash value. That ensures that
577 two strings differing only in case will hash the same.
579 - If you care about performance, choose a hash function with as
580 good "spreading" as possible. A good hash function will use all
581 the bits of the input when calculating the hash, and will react
582 to even small changes in input with a completely different
583 output. Finally, don't make the hash function itself overly
584 slow, because you'll be incurring a non-negligible overhead to
585 all hash table operations. */
588 * Support for hash tables whose keys are strings.
592 /* 31 bit hash function. Taken from Gnome's glib, modified to use
595 We used to use the popular hash function from the Dragon Book, but
596 this one seems to perform much better. */
599 string_hash (const void *key)
605 for (p += 1; *p != '\0'; p++)
606 h = (h << 5) - h + *p;
611 /* Frontend for strcmp usable for hash tables. */
614 string_cmp (const void *s1, const void *s2)
616 return !strcmp ((const char *)s1, (const char *)s2);
619 /* Return a hash table of preallocated to store at least ITEMS items
620 suitable to use strings as keys. */
623 make_string_hash_table (int items)
625 return hash_table_new (items, string_hash, string_cmp);
629 * Support for hash tables whose keys are strings, but which are
630 * compared case-insensitively.
634 /* Like string_hash, but produce the same hash regardless of the case. */
637 string_hash_nocase (const void *key)
640 unsigned int h = TOLOWER (*p);
643 for (p += 1; *p != '\0'; p++)
644 h = (h << 5) - h + TOLOWER (*p);
649 /* Like string_cmp, but doing case-insensitive compareison. */
652 string_cmp_nocase (const void *s1, const void *s2)
654 return !strcasecmp ((const char *)s1, (const char *)s2);
657 /* Like make_string_hash_table, but uses string_hash_nocase and
658 string_cmp_nocase. */
661 make_nocase_string_hash_table (int items)
663 return hash_table_new (items, string_hash_nocase, string_cmp_nocase);
666 /* Hashing of numeric values, such as pointers and integers.
668 This implementation is the Robert Jenkins' 32 bit Mix Function,
669 with a simple adaptation for 64-bit values. It offers excellent
670 spreading of values and doesn't need to know the hash table size to
671 work (unlike the very popular Knuth's multiplication hash). */
674 ptrhash (const void *ptr)
676 unsigned long key = (unsigned long)ptr;
699 ptrcmp (const void *ptr1, const void *ptr2)
710 print_hash_table_mapper (void *key, void *value, void *count)
713 printf ("%s: %s\n", (const char *)key, (char *)value);
718 print_hash (struct hash_table *sht)
721 hash_table_map (sht, print_hash_table_mapper, &debug_count);
722 assert (debug_count == sht->count);
728 struct hash_table *ht = make_string_hash_table (0);
730 while ((fgets (line, sizeof (line), stdin)))
732 int len = strlen (line);
736 if (!hash_table_contains (ht, line))
737 hash_table_put (ht, strdup (line), "here I am!");
742 if (hash_table_get_pair (ht, line, &line_copy, NULL))
744 hash_table_remove (ht, line);
754 printf ("%d %d\n", ht->count, ht->size);