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5da6c26f | 1 | /* An expandable hash tables datatype. |
8c8eb750 | 2 | Copyright (C) 1999, 2000, 2001, 2002 Free Software Foundation, Inc. |
5da6c26f | 3 | Contributed by Vladimir Makarov (vmakarov@cygnus.com). |
4 | ||
5 | This file is part of the libiberty library. | |
6 | Libiberty is free software; you can redistribute it and/or | |
7 | modify it under the terms of the GNU Library General Public | |
8 | License as published by the Free Software Foundation; either | |
9 | version 2 of the License, or (at your option) any later version. | |
10 | ||
11 | Libiberty 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 GNU | |
14 | Library General Public License for more details. | |
15 | ||
16 | You should have received a copy of the GNU Library General Public | |
17 | License along with libiberty; see the file COPYING.LIB. If | |
18 | not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, | |
19 | Boston, MA 02111-1307, USA. */ | |
20 | ||
21 | /* This package implements basic hash table functionality. It is possible | |
22 | to search for an entry, create an entry and destroy an entry. | |
23 | ||
24 | Elements in the table are generic pointers. | |
25 | ||
26 | The size of the table is not fixed; if the occupancy of the table | |
27 | grows too high the hash table will be expanded. | |
28 | ||
29 | The abstract data implementation is based on generalized Algorithm D | |
30 | from Knuth's book "The art of computer programming". Hash table is | |
31 | expanded by creation of new hash table and transferring elements from | |
32 | the old table to the new table. */ | |
33 | ||
34 | #ifdef HAVE_CONFIG_H | |
35 | #include "config.h" | |
36 | #endif | |
37 | ||
d7f8de75 | 38 | #include <sys/types.h> |
39 | ||
5da6c26f | 40 | #ifdef HAVE_STDLIB_H |
41 | #include <stdlib.h> | |
42 | #endif | |
43 | ||
317ab997 | 44 | #ifdef HAVE_STRING_H |
45 | #include <string.h> | |
46 | #endif | |
47 | ||
bd41a79e | 48 | #include <stdio.h> |
49 | ||
5da6c26f | 50 | #include "libiberty.h" |
51 | #include "hashtab.h" | |
52 | ||
5da6c26f | 53 | /* This macro defines reserved value for empty table entry. */ |
54 | ||
696d6593 | 55 | #define EMPTY_ENTRY ((PTR) 0) |
5da6c26f | 56 | |
57 | /* This macro defines reserved value for table entry which contained | |
58 | a deleted element. */ | |
59 | ||
696d6593 | 60 | #define DELETED_ENTRY ((PTR) 1) |
5da6c26f | 61 | |
07c797e3 | 62 | static unsigned long higher_prime_number PARAMS ((unsigned long)); |
c9dfb8ae | 63 | static hashval_t hash_pointer PARAMS ((const void *)); |
64 | static int eq_pointer PARAMS ((const void *, const void *)); | |
e4c2dc6e | 65 | static int htab_expand PARAMS ((htab_t)); |
696d6593 | 66 | static PTR *find_empty_slot_for_expand PARAMS ((htab_t, hashval_t)); |
c9dfb8ae | 67 | |
68 | /* At some point, we could make these be NULL, and modify the | |
69 | hash-table routines to handle NULL specially; that would avoid | |
70 | function-call overhead for the common case of hashing pointers. */ | |
71 | htab_hash htab_hash_pointer = hash_pointer; | |
72 | htab_eq htab_eq_pointer = eq_pointer; | |
07c797e3 | 73 | |
54b3a5af | 74 | /* The following function returns a nearest prime number which is |
75 | greater than N, and near a power of two. */ | |
5da6c26f | 76 | |
77 | static unsigned long | |
07c967f9 | 78 | higher_prime_number (n) |
79 | unsigned long n; | |
5da6c26f | 80 | { |
54b3a5af | 81 | /* These are primes that are near, but slightly smaller than, a |
82 | power of two. */ | |
542e9271 | 83 | static const unsigned long primes[] = { |
1de479ad | 84 | (unsigned long) 7, |
85 | (unsigned long) 13, | |
86 | (unsigned long) 31, | |
87 | (unsigned long) 61, | |
88 | (unsigned long) 127, | |
89 | (unsigned long) 251, | |
90 | (unsigned long) 509, | |
91 | (unsigned long) 1021, | |
92 | (unsigned long) 2039, | |
93 | (unsigned long) 4093, | |
94 | (unsigned long) 8191, | |
95 | (unsigned long) 16381, | |
96 | (unsigned long) 32749, | |
97 | (unsigned long) 65521, | |
98 | (unsigned long) 131071, | |
99 | (unsigned long) 262139, | |
100 | (unsigned long) 524287, | |
101 | (unsigned long) 1048573, | |
102 | (unsigned long) 2097143, | |
103 | (unsigned long) 4194301, | |
104 | (unsigned long) 8388593, | |
105 | (unsigned long) 16777213, | |
106 | (unsigned long) 33554393, | |
107 | (unsigned long) 67108859, | |
108 | (unsigned long) 134217689, | |
109 | (unsigned long) 268435399, | |
110 | (unsigned long) 536870909, | |
111 | (unsigned long) 1073741789, | |
112 | (unsigned long) 2147483647, | |
113 | /* 4294967291L */ | |
2938048c | 114 | ((unsigned long) 2147483647) + ((unsigned long) 2147483644), |
54b3a5af | 115 | }; |
116 | ||
542e9271 | 117 | const unsigned long *low = &primes[0]; |
118 | const unsigned long *high = &primes[sizeof(primes) / sizeof(primes[0])]; | |
54b3a5af | 119 | |
120 | while (low != high) | |
121 | { | |
542e9271 | 122 | const unsigned long *mid = low + (high - low) / 2; |
54b3a5af | 123 | if (n > *mid) |
124 | low = mid + 1; | |
125 | else | |
126 | high = mid; | |
127 | } | |
128 | ||
129 | /* If we've run out of primes, abort. */ | |
130 | if (n > *low) | |
131 | { | |
132 | fprintf (stderr, "Cannot find prime bigger than %lu\n", n); | |
133 | abort (); | |
134 | } | |
135 | ||
136 | return *low; | |
5da6c26f | 137 | } |
138 | ||
c9dfb8ae | 139 | /* Returns a hash code for P. */ |
140 | ||
8afd4145 | 141 | static hashval_t |
c9dfb8ae | 142 | hash_pointer (p) |
696d6593 | 143 | const PTR p; |
c9dfb8ae | 144 | { |
e51b357b | 145 | return (hashval_t) ((long)p >> 3); |
c9dfb8ae | 146 | } |
147 | ||
148 | /* Returns non-zero if P1 and P2 are equal. */ | |
149 | ||
8afd4145 | 150 | static int |
c9dfb8ae | 151 | eq_pointer (p1, p2) |
696d6593 | 152 | const PTR p1; |
153 | const PTR p2; | |
c9dfb8ae | 154 | { |
155 | return p1 == p2; | |
156 | } | |
157 | ||
5da6c26f | 158 | /* This function creates table with length slightly longer than given |
159 | source length. Created hash table is initiated as empty (all the | |
160 | hash table entries are EMPTY_ENTRY). The function returns the | |
1f3233d1 | 161 | created hash table, or NULL if memory allocation fails. */ |
5da6c26f | 162 | |
07c967f9 | 163 | htab_t |
1f3233d1 | 164 | htab_create_alloc (size, hash_f, eq_f, del_f, alloc_f, free_f) |
5da6c26f | 165 | size_t size; |
07c967f9 | 166 | htab_hash hash_f; |
167 | htab_eq eq_f; | |
3fdd387a | 168 | htab_del del_f; |
1f3233d1 | 169 | htab_alloc alloc_f; |
170 | htab_free free_f; | |
5da6c26f | 171 | { |
07c967f9 | 172 | htab_t result; |
5da6c26f | 173 | |
174 | size = higher_prime_number (size); | |
1f3233d1 | 175 | result = (htab_t) (*alloc_f) (1, sizeof (struct htab)); |
e4c2dc6e | 176 | if (result == NULL) |
177 | return NULL; | |
1f3233d1 | 178 | result->entries = (PTR *) (*alloc_f) (size, sizeof (PTR)); |
e4c2dc6e | 179 | if (result->entries == NULL) |
180 | { | |
1f3233d1 | 181 | if (free_f != NULL) |
182 | (*free_f) (result); | |
e4c2dc6e | 183 | return NULL; |
184 | } | |
e4c2dc6e | 185 | result->size = size; |
186 | result->hash_f = hash_f; | |
187 | result->eq_f = eq_f; | |
188 | result->del_f = del_f; | |
1f3233d1 | 189 | result->alloc_f = alloc_f; |
190 | result->free_f = free_f; | |
5da6c26f | 191 | return result; |
192 | } | |
193 | ||
194 | /* This function frees all memory allocated for given hash table. | |
195 | Naturally the hash table must already exist. */ | |
196 | ||
197 | void | |
07c967f9 | 198 | htab_delete (htab) |
199 | htab_t htab; | |
5da6c26f | 200 | { |
3fdd387a | 201 | int i; |
2b3dbc20 | 202 | |
3fdd387a | 203 | if (htab->del_f) |
204 | for (i = htab->size - 1; i >= 0; i--) | |
2b3dbc20 | 205 | if (htab->entries[i] != EMPTY_ENTRY |
206 | && htab->entries[i] != DELETED_ENTRY) | |
207 | (*htab->del_f) (htab->entries[i]); | |
3fdd387a | 208 | |
1f3233d1 | 209 | if (htab->free_f != NULL) |
210 | { | |
211 | (*htab->free_f) (htab->entries); | |
212 | (*htab->free_f) (htab); | |
213 | } | |
5da6c26f | 214 | } |
215 | ||
216 | /* This function clears all entries in the given hash table. */ | |
217 | ||
218 | void | |
07c967f9 | 219 | htab_empty (htab) |
220 | htab_t htab; | |
5da6c26f | 221 | { |
3fdd387a | 222 | int i; |
2b3dbc20 | 223 | |
3fdd387a | 224 | if (htab->del_f) |
225 | for (i = htab->size - 1; i >= 0; i--) | |
2b3dbc20 | 226 | if (htab->entries[i] != EMPTY_ENTRY |
227 | && htab->entries[i] != DELETED_ENTRY) | |
228 | (*htab->del_f) (htab->entries[i]); | |
3fdd387a | 229 | |
696d6593 | 230 | memset (htab->entries, 0, htab->size * sizeof (PTR)); |
5da6c26f | 231 | } |
232 | ||
ed26da85 | 233 | /* Similar to htab_find_slot, but without several unwanted side effects: |
234 | - Does not call htab->eq_f when it finds an existing entry. | |
235 | - Does not change the count of elements/searches/collisions in the | |
236 | hash table. | |
237 | This function also assumes there are no deleted entries in the table. | |
238 | HASH is the hash value for the element to be inserted. */ | |
2b3dbc20 | 239 | |
696d6593 | 240 | static PTR * |
ed26da85 | 241 | find_empty_slot_for_expand (htab, hash) |
242 | htab_t htab; | |
7669680f | 243 | hashval_t hash; |
ed26da85 | 244 | { |
245 | size_t size = htab->size; | |
ed26da85 | 246 | unsigned int index = hash % size; |
8c8eb750 | 247 | PTR *slot = htab->entries + index; |
248 | hashval_t hash2; | |
249 | ||
250 | if (*slot == EMPTY_ENTRY) | |
251 | return slot; | |
252 | else if (*slot == DELETED_ENTRY) | |
253 | abort (); | |
ed26da85 | 254 | |
8c8eb750 | 255 | hash2 = 1 + hash % (size - 2); |
ed26da85 | 256 | for (;;) |
257 | { | |
8c8eb750 | 258 | index += hash2; |
259 | if (index >= size) | |
260 | index -= size; | |
2b3dbc20 | 261 | |
8c8eb750 | 262 | slot = htab->entries + index; |
ed26da85 | 263 | if (*slot == EMPTY_ENTRY) |
264 | return slot; | |
2b3dbc20 | 265 | else if (*slot == DELETED_ENTRY) |
ed26da85 | 266 | abort (); |
ed26da85 | 267 | } |
268 | } | |
269 | ||
5da6c26f | 270 | /* The following function changes size of memory allocated for the |
271 | entries and repeatedly inserts the table elements. The occupancy | |
272 | of the table after the call will be about 50%. Naturally the hash | |
273 | table must already exist. Remember also that the place of the | |
e4c2dc6e | 274 | table entries is changed. If memory allocation failures are allowed, |
275 | this function will return zero, indicating that the table could not be | |
276 | expanded. If all goes well, it will return a non-zero value. */ | |
5da6c26f | 277 | |
e4c2dc6e | 278 | static int |
07c967f9 | 279 | htab_expand (htab) |
280 | htab_t htab; | |
5da6c26f | 281 | { |
696d6593 | 282 | PTR *oentries; |
283 | PTR *olimit; | |
284 | PTR *p; | |
1f3233d1 | 285 | PTR *nentries; |
07c967f9 | 286 | |
287 | oentries = htab->entries; | |
288 | olimit = oentries + htab->size; | |
289 | ||
290 | htab->size = higher_prime_number (htab->size * 2); | |
e4c2dc6e | 291 | |
1f3233d1 | 292 | nentries = (PTR *) (*htab->alloc_f) (htab->size, sizeof (PTR *)); |
293 | if (nentries == NULL) | |
294 | return 0; | |
295 | htab->entries = nentries; | |
07c967f9 | 296 | |
297 | htab->n_elements -= htab->n_deleted; | |
298 | htab->n_deleted = 0; | |
299 | ||
300 | p = oentries; | |
301 | do | |
302 | { | |
696d6593 | 303 | PTR x = *p; |
2b3dbc20 | 304 | |
07c967f9 | 305 | if (x != EMPTY_ENTRY && x != DELETED_ENTRY) |
306 | { | |
696d6593 | 307 | PTR *q = find_empty_slot_for_expand (htab, (*htab->hash_f) (x)); |
2b3dbc20 | 308 | |
07c967f9 | 309 | *q = x; |
310 | } | |
2b3dbc20 | 311 | |
07c967f9 | 312 | p++; |
313 | } | |
314 | while (p < olimit); | |
2b3dbc20 | 315 | |
1f3233d1 | 316 | if (htab->free_f != NULL) |
317 | (*htab->free_f) (oentries); | |
e4c2dc6e | 318 | return 1; |
5da6c26f | 319 | } |
320 | ||
07c967f9 | 321 | /* This function searches for a hash table entry equal to the given |
322 | element. It cannot be used to insert or delete an element. */ | |
323 | ||
696d6593 | 324 | PTR |
ed26da85 | 325 | htab_find_with_hash (htab, element, hash) |
07c967f9 | 326 | htab_t htab; |
696d6593 | 327 | const PTR element; |
7669680f | 328 | hashval_t hash; |
5da6c26f | 329 | { |
7669680f | 330 | unsigned int index; |
331 | hashval_t hash2; | |
07c967f9 | 332 | size_t size; |
696d6593 | 333 | PTR entry; |
07c967f9 | 334 | |
335 | htab->searches++; | |
336 | size = htab->size; | |
07c967f9 | 337 | index = hash % size; |
5da6c26f | 338 | |
07c797e3 | 339 | entry = htab->entries[index]; |
340 | if (entry == EMPTY_ENTRY | |
341 | || (entry != DELETED_ENTRY && (*htab->eq_f) (entry, element))) | |
342 | return entry; | |
343 | ||
344 | hash2 = 1 + hash % (size - 2); | |
345 | ||
07c967f9 | 346 | for (;;) |
5da6c26f | 347 | { |
07c967f9 | 348 | htab->collisions++; |
349 | index += hash2; | |
350 | if (index >= size) | |
351 | index -= size; | |
07c797e3 | 352 | |
353 | entry = htab->entries[index]; | |
354 | if (entry == EMPTY_ENTRY | |
355 | || (entry != DELETED_ENTRY && (*htab->eq_f) (entry, element))) | |
356 | return entry; | |
5da6c26f | 357 | } |
07c967f9 | 358 | } |
359 | ||
ed26da85 | 360 | /* Like htab_find_slot_with_hash, but compute the hash value from the |
361 | element. */ | |
2b3dbc20 | 362 | |
696d6593 | 363 | PTR |
ed26da85 | 364 | htab_find (htab, element) |
365 | htab_t htab; | |
696d6593 | 366 | const PTR element; |
ed26da85 | 367 | { |
368 | return htab_find_with_hash (htab, element, (*htab->hash_f) (element)); | |
369 | } | |
370 | ||
07c967f9 | 371 | /* This function searches for a hash table slot containing an entry |
372 | equal to the given element. To delete an entry, call this with | |
373 | INSERT = 0, then call htab_clear_slot on the slot returned (possibly | |
374 | after doing some checks). To insert an entry, call this with | |
e4c2dc6e | 375 | INSERT = 1, then write the value you want into the returned slot. |
376 | When inserting an entry, NULL may be returned if memory allocation | |
377 | fails. */ | |
07c967f9 | 378 | |
696d6593 | 379 | PTR * |
ed26da85 | 380 | htab_find_slot_with_hash (htab, element, hash, insert) |
07c967f9 | 381 | htab_t htab; |
696d6593 | 382 | const PTR element; |
7669680f | 383 | hashval_t hash; |
2b3dbc20 | 384 | enum insert_option insert; |
07c967f9 | 385 | { |
696d6593 | 386 | PTR *first_deleted_slot; |
7669680f | 387 | unsigned int index; |
388 | hashval_t hash2; | |
07c967f9 | 389 | size_t size; |
8c8eb750 | 390 | PTR entry; |
07c967f9 | 391 | |
e4c2dc6e | 392 | if (insert == INSERT && htab->size * 3 <= htab->n_elements * 4 |
393 | && htab_expand (htab) == 0) | |
394 | return NULL; | |
07c967f9 | 395 | |
396 | size = htab->size; | |
07c967f9 | 397 | index = hash % size; |
398 | ||
5da6c26f | 399 | htab->searches++; |
07c967f9 | 400 | first_deleted_slot = NULL; |
401 | ||
8c8eb750 | 402 | entry = htab->entries[index]; |
403 | if (entry == EMPTY_ENTRY) | |
404 | goto empty_entry; | |
405 | else if (entry == DELETED_ENTRY) | |
406 | first_deleted_slot = &htab->entries[index]; | |
407 | else if ((*htab->eq_f) (entry, element)) | |
408 | return &htab->entries[index]; | |
409 | ||
410 | hash2 = 1 + hash % (size - 2); | |
07c967f9 | 411 | for (;;) |
5da6c26f | 412 | { |
8c8eb750 | 413 | htab->collisions++; |
414 | index += hash2; | |
415 | if (index >= size) | |
416 | index -= size; | |
417 | ||
418 | entry = htab->entries[index]; | |
07c967f9 | 419 | if (entry == EMPTY_ENTRY) |
8c8eb750 | 420 | goto empty_entry; |
421 | else if (entry == DELETED_ENTRY) | |
07c967f9 | 422 | { |
423 | if (!first_deleted_slot) | |
424 | first_deleted_slot = &htab->entries[index]; | |
425 | } | |
8c8eb750 | 426 | else if ((*htab->eq_f) (entry, element)) |
2b3dbc20 | 427 | return &htab->entries[index]; |
5da6c26f | 428 | } |
8c8eb750 | 429 | |
430 | empty_entry: | |
431 | if (insert == NO_INSERT) | |
432 | return NULL; | |
433 | ||
434 | htab->n_elements++; | |
435 | ||
436 | if (first_deleted_slot) | |
437 | { | |
438 | *first_deleted_slot = EMPTY_ENTRY; | |
439 | return first_deleted_slot; | |
440 | } | |
441 | ||
442 | return &htab->entries[index]; | |
5da6c26f | 443 | } |
444 | ||
ed26da85 | 445 | /* Like htab_find_slot_with_hash, but compute the hash value from the |
446 | element. */ | |
2b3dbc20 | 447 | |
696d6593 | 448 | PTR * |
ed26da85 | 449 | htab_find_slot (htab, element, insert) |
450 | htab_t htab; | |
696d6593 | 451 | const PTR element; |
2b3dbc20 | 452 | enum insert_option insert; |
ed26da85 | 453 | { |
454 | return htab_find_slot_with_hash (htab, element, (*htab->hash_f) (element), | |
455 | insert); | |
456 | } | |
457 | ||
07c967f9 | 458 | /* This function deletes an element with the given value from hash |
459 | table. If there is no matching element in the hash table, this | |
460 | function does nothing. */ | |
5da6c26f | 461 | |
462 | void | |
07c967f9 | 463 | htab_remove_elt (htab, element) |
464 | htab_t htab; | |
696d6593 | 465 | PTR element; |
5da6c26f | 466 | { |
696d6593 | 467 | PTR *slot; |
5da6c26f | 468 | |
2b3dbc20 | 469 | slot = htab_find_slot (htab, element, NO_INSERT); |
07c967f9 | 470 | if (*slot == EMPTY_ENTRY) |
471 | return; | |
472 | ||
3fdd387a | 473 | if (htab->del_f) |
474 | (*htab->del_f) (*slot); | |
475 | ||
07c967f9 | 476 | *slot = DELETED_ENTRY; |
477 | htab->n_deleted++; | |
5da6c26f | 478 | } |
479 | ||
07c967f9 | 480 | /* This function clears a specified slot in a hash table. It is |
481 | useful when you've already done the lookup and don't want to do it | |
482 | again. */ | |
21a7d507 | 483 | |
484 | void | |
07c967f9 | 485 | htab_clear_slot (htab, slot) |
486 | htab_t htab; | |
696d6593 | 487 | PTR *slot; |
21a7d507 | 488 | { |
489 | if (slot < htab->entries || slot >= htab->entries + htab->size | |
490 | || *slot == EMPTY_ENTRY || *slot == DELETED_ENTRY) | |
491 | abort (); | |
2b3dbc20 | 492 | |
3fdd387a | 493 | if (htab->del_f) |
494 | (*htab->del_f) (*slot); | |
2b3dbc20 | 495 | |
21a7d507 | 496 | *slot = DELETED_ENTRY; |
07c967f9 | 497 | htab->n_deleted++; |
21a7d507 | 498 | } |
499 | ||
500 | /* This function scans over the entire hash table calling | |
501 | CALLBACK for each live entry. If CALLBACK returns false, | |
502 | the iteration stops. INFO is passed as CALLBACK's second | |
503 | argument. */ | |
504 | ||
505 | void | |
07c967f9 | 506 | htab_traverse (htab, callback, info) |
507 | htab_t htab; | |
508 | htab_trav callback; | |
696d6593 | 509 | PTR info; |
21a7d507 | 510 | { |
696d6593 | 511 | PTR *slot = htab->entries; |
512 | PTR *limit = slot + htab->size; | |
2b3dbc20 | 513 | |
07c967f9 | 514 | do |
515 | { | |
696d6593 | 516 | PTR x = *slot; |
2b3dbc20 | 517 | |
07c967f9 | 518 | if (x != EMPTY_ENTRY && x != DELETED_ENTRY) |
ed26da85 | 519 | if (!(*callback) (slot, info)) |
07c967f9 | 520 | break; |
521 | } | |
522 | while (++slot < limit); | |
21a7d507 | 523 | } |
524 | ||
2b3dbc20 | 525 | /* Return the current size of given hash table. */ |
5da6c26f | 526 | |
527 | size_t | |
07c967f9 | 528 | htab_size (htab) |
529 | htab_t htab; | |
5da6c26f | 530 | { |
531 | return htab->size; | |
532 | } | |
533 | ||
2b3dbc20 | 534 | /* Return the current number of elements in given hash table. */ |
5da6c26f | 535 | |
536 | size_t | |
07c967f9 | 537 | htab_elements (htab) |
538 | htab_t htab; | |
5da6c26f | 539 | { |
07c967f9 | 540 | return htab->n_elements - htab->n_deleted; |
5da6c26f | 541 | } |
542 | ||
2b3dbc20 | 543 | /* Return the fraction of fixed collisions during all work with given |
544 | hash table. */ | |
5da6c26f | 545 | |
07c967f9 | 546 | double |
547 | htab_collisions (htab) | |
548 | htab_t htab; | |
5da6c26f | 549 | { |
2b3dbc20 | 550 | if (htab->searches == 0) |
07c967f9 | 551 | return 0.0; |
2b3dbc20 | 552 | |
553 | return (double) htab->collisions / (double) htab->searches; | |
5da6c26f | 554 | } |
80f07f6c | 555 | |
3f3e622a | 556 | /* Hash P as a null-terminated string. |
557 | ||
558 | Copied from gcc/hashtable.c. Zack had the following to say with respect | |
559 | to applicability, though note that unlike hashtable.c, this hash table | |
560 | implementation re-hashes rather than chain buckets. | |
561 | ||
562 | http://gcc.gnu.org/ml/gcc-patches/2001-08/msg01021.html | |
563 | From: Zack Weinberg <zackw@panix.com> | |
564 | Date: Fri, 17 Aug 2001 02:15:56 -0400 | |
565 | ||
566 | I got it by extracting all the identifiers from all the source code | |
567 | I had lying around in mid-1999, and testing many recurrences of | |
568 | the form "H_n = H_{n-1} * K + c_n * L + M" where K, L, M were either | |
569 | prime numbers or the appropriate identity. This was the best one. | |
570 | I don't remember exactly what constituted "best", except I was | |
571 | looking at bucket-length distributions mostly. | |
572 | ||
573 | So it should be very good at hashing identifiers, but might not be | |
574 | as good at arbitrary strings. | |
575 | ||
576 | I'll add that it thoroughly trounces the hash functions recommended | |
577 | for this use at http://burtleburtle.net/bob/hash/index.html, both | |
578 | on speed and bucket distribution. I haven't tried it against the | |
579 | function they just started using for Perl's hashes. */ | |
80f07f6c | 580 | |
581 | hashval_t | |
582 | htab_hash_string (p) | |
583 | const PTR p; | |
584 | { | |
585 | const unsigned char *str = (const unsigned char *) p; | |
586 | hashval_t r = 0; | |
587 | unsigned char c; | |
588 | ||
589 | while ((c = *str++) != 0) | |
590 | r = r * 67 + c - 113; | |
591 | ||
592 | return r; | |
593 | } |