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[binutils.git] / libiberty / hashtab.c
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1 /* An expandable hash tables datatype.
2 Copyright (C) 1999, 2000, 2001, 2002, 2003, 2004
3 Free Software Foundation, Inc.
4 Contributed by Vladimir Makarov (vmakarov@cygnus.com).
6 This file is part of the libiberty library.
7 Libiberty is free software; you can redistribute it and/or
8 modify it under the terms of the GNU Library General Public
9 License as published by the Free Software Foundation; either
10 version 2 of the License, or (at your option) any later version.
12 Libiberty is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15 Library General Public License for more details.
17 You should have received a copy of the GNU Library General Public
18 License along with libiberty; see the file COPYING.LIB. If
19 not, write to the Free Software Foundation, Inc., 51 Franklin Street - Fifth Floor,
20 Boston, MA 02110-1301, USA. */
22 /* This package implements basic hash table functionality. It is possible
23 to search for an entry, create an entry and destroy an entry.
25 Elements in the table are generic pointers.
27 The size of the table is not fixed; if the occupancy of the table
28 grows too high the hash table will be expanded.
30 The abstract data implementation is based on generalized Algorithm D
31 from Knuth's book "The art of computer programming". Hash table is
32 expanded by creation of new hash table and transferring elements from
33 the old table to the new table. */
35 #ifdef HAVE_CONFIG_H
36 #include "config.h"
37 #endif
39 #include <sys/types.h>
41 #ifdef HAVE_STDLIB_H
42 #include <stdlib.h>
43 #endif
44 #ifdef HAVE_STRING_H
45 #include <string.h>
46 #endif
47 #ifdef HAVE_MALLOC_H
48 #include <malloc.h>
49 #endif
50 #ifdef HAVE_LIMITS_H
51 #include <limits.h>
52 #endif
53 #ifdef HAVE_STDINT_H
54 #include <stdint.h>
55 #endif
57 #include <stdio.h>
59 #include "libiberty.h"
60 #include "ansidecl.h"
61 #include "hashtab.h"
63 #ifndef CHAR_BIT
64 #define CHAR_BIT 8
65 #endif
67 static unsigned int higher_prime_index (unsigned long);
68 static hashval_t htab_mod_1 (hashval_t, hashval_t, hashval_t, int);
69 static hashval_t htab_mod (hashval_t, htab_t);
70 static hashval_t htab_mod_m2 (hashval_t, htab_t);
71 static hashval_t hash_pointer (const void *);
72 static int eq_pointer (const void *, const void *);
73 static int htab_expand (htab_t);
74 static PTR *find_empty_slot_for_expand (htab_t, hashval_t);
76 /* At some point, we could make these be NULL, and modify the
77 hash-table routines to handle NULL specially; that would avoid
78 function-call overhead for the common case of hashing pointers. */
79 htab_hash htab_hash_pointer = hash_pointer;
80 htab_eq htab_eq_pointer = eq_pointer;
82 /* Table of primes and multiplicative inverses.
84 Note that these are not minimally reduced inverses. Unlike when generating
85 code to divide by a constant, we want to be able to use the same algorithm
86 all the time. All of these inverses (are implied to) have bit 32 set.
88 For the record, here's the function that computed the table; it's a
89 vastly simplified version of the function of the same name from gcc. */
91 #if 0
92 unsigned int
93 ceil_log2 (unsigned int x)
95 int i;
96 for (i = 31; i >= 0 ; --i)
97 if (x > (1u << i))
98 return i+1;
99 abort ();
102 unsigned int
103 choose_multiplier (unsigned int d, unsigned int *mlp, unsigned char *shiftp)
105 unsigned long long mhigh;
106 double nx;
107 int lgup, post_shift;
108 int pow, pow2;
109 int n = 32, precision = 32;
111 lgup = ceil_log2 (d);
112 pow = n + lgup;
113 pow2 = n + lgup - precision;
115 nx = ldexp (1.0, pow) + ldexp (1.0, pow2);
116 mhigh = nx / d;
118 *shiftp = lgup - 1;
119 *mlp = mhigh;
120 return mhigh >> 32;
122 #endif
124 struct prime_ent
126 hashval_t prime;
127 hashval_t inv;
128 hashval_t inv_m2; /* inverse of prime-2 */
129 hashval_t shift;
132 static struct prime_ent const prime_tab[] = {
133 { 7, 0x24924925, 0x9999999b, 2 },
134 { 13, 0x3b13b13c, 0x745d1747, 3 },
135 { 31, 0x08421085, 0x1a7b9612, 4 },
136 { 61, 0x0c9714fc, 0x15b1e5f8, 5 },
137 { 127, 0x02040811, 0x0624dd30, 6 },
138 { 251, 0x05197f7e, 0x073260a5, 7 },
139 { 509, 0x01824366, 0x02864fc8, 8 },
140 { 1021, 0x00c0906d, 0x014191f7, 9 },
141 { 2039, 0x0121456f, 0x0161e69e, 10 },
142 { 4093, 0x00300902, 0x00501908, 11 },
143 { 8191, 0x00080041, 0x00180241, 12 },
144 { 16381, 0x000c0091, 0x00140191, 13 },
145 { 32749, 0x002605a5, 0x002a06e6, 14 },
146 { 65521, 0x000f00e2, 0x00110122, 15 },
147 { 131071, 0x00008001, 0x00018003, 16 },
148 { 262139, 0x00014002, 0x0001c004, 17 },
149 { 524287, 0x00002001, 0x00006001, 18 },
150 { 1048573, 0x00003001, 0x00005001, 19 },
151 { 2097143, 0x00004801, 0x00005801, 20 },
152 { 4194301, 0x00000c01, 0x00001401, 21 },
153 { 8388593, 0x00001e01, 0x00002201, 22 },
154 { 16777213, 0x00000301, 0x00000501, 23 },
155 { 33554393, 0x00001381, 0x00001481, 24 },
156 { 67108859, 0x00000141, 0x000001c1, 25 },
157 { 134217689, 0x000004e1, 0x00000521, 26 },
158 { 268435399, 0x00000391, 0x000003b1, 27 },
159 { 536870909, 0x00000019, 0x00000029, 28 },
160 { 1073741789, 0x0000008d, 0x00000095, 29 },
161 { 2147483647, 0x00000003, 0x00000007, 30 },
162 /* Avoid "decimal constant so large it is unsigned" for 4294967291. */
163 { 0xfffffffb, 0x00000006, 0x00000008, 31 }
166 /* The following function returns an index into the above table of the
167 nearest prime number which is greater than N, and near a power of two. */
169 static unsigned int
170 higher_prime_index (unsigned long n)
172 unsigned int low = 0;
173 unsigned int high = sizeof(prime_tab) / sizeof(prime_tab[0]);
175 while (low != high)
177 unsigned int mid = low + (high - low) / 2;
178 if (n > prime_tab[mid].prime)
179 low = mid + 1;
180 else
181 high = mid;
184 /* If we've run out of primes, abort. */
185 if (n > prime_tab[low].prime)
187 fprintf (stderr, "Cannot find prime bigger than %lu\n", n);
188 abort ();
191 return low;
194 /* Returns a hash code for P. */
196 static hashval_t
197 hash_pointer (const PTR p)
199 return (hashval_t) ((long)p >> 3);
202 /* Returns non-zero if P1 and P2 are equal. */
204 static int
205 eq_pointer (const PTR p1, const PTR p2)
207 return p1 == p2;
211 /* The parens around the function names in the next two definitions
212 are essential in order to prevent macro expansions of the name.
213 The bodies, however, are expanded as expected, so they are not
214 recursive definitions. */
216 /* Return the current size of given hash table. */
218 #define htab_size(htab) ((htab)->size)
220 size_t
221 (htab_size) (htab_t htab)
223 return htab_size (htab);
226 /* Return the current number of elements in given hash table. */
228 #define htab_elements(htab) ((htab)->n_elements - (htab)->n_deleted)
230 size_t
231 (htab_elements) (htab_t htab)
233 return htab_elements (htab);
236 /* Return X % Y. */
238 static inline hashval_t
239 htab_mod_1 (hashval_t x, hashval_t y, hashval_t inv, int shift)
241 /* The multiplicative inverses computed above are for 32-bit types, and
242 requires that we be able to compute a highpart multiply. */
243 #ifdef UNSIGNED_64BIT_TYPE
244 __extension__ typedef UNSIGNED_64BIT_TYPE ull;
245 if (sizeof (hashval_t) * CHAR_BIT <= 32)
247 hashval_t t1, t2, t3, t4, q, r;
249 t1 = ((ull)x * inv) >> 32;
250 t2 = x - t1;
251 t3 = t2 >> 1;
252 t4 = t1 + t3;
253 q = t4 >> shift;
254 r = x - (q * y);
256 return r;
258 #endif
260 /* Otherwise just use the native division routines. */
261 return x % y;
264 /* Compute the primary hash for HASH given HTAB's current size. */
266 static inline hashval_t
267 htab_mod (hashval_t hash, htab_t htab)
269 const struct prime_ent *p = &prime_tab[htab->size_prime_index];
270 return htab_mod_1 (hash, p->prime, p->inv, p->shift);
273 /* Compute the secondary hash for HASH given HTAB's current size. */
275 static inline hashval_t
276 htab_mod_m2 (hashval_t hash, htab_t htab)
278 const struct prime_ent *p = &prime_tab[htab->size_prime_index];
279 return 1 + htab_mod_1 (hash, p->prime - 2, p->inv_m2, p->shift);
282 /* This function creates table with length slightly longer than given
283 source length. Created hash table is initiated as empty (all the
284 hash table entries are HTAB_EMPTY_ENTRY). The function returns the
285 created hash table, or NULL if memory allocation fails. */
287 htab_t
288 htab_create_alloc (size_t size, htab_hash hash_f, htab_eq eq_f,
289 htab_del del_f, htab_alloc alloc_f, htab_free free_f)
291 htab_t result;
292 unsigned int size_prime_index;
294 size_prime_index = higher_prime_index (size);
295 size = prime_tab[size_prime_index].prime;
297 result = (htab_t) (*alloc_f) (1, sizeof (struct htab));
298 if (result == NULL)
299 return NULL;
300 result->entries = (PTR *) (*alloc_f) (size, sizeof (PTR));
301 if (result->entries == NULL)
303 if (free_f != NULL)
304 (*free_f) (result);
305 return NULL;
307 result->size = size;
308 result->size_prime_index = size_prime_index;
309 result->hash_f = hash_f;
310 result->eq_f = eq_f;
311 result->del_f = del_f;
312 result->alloc_f = alloc_f;
313 result->free_f = free_f;
314 return result;
317 /* As above, but use the variants of alloc_f and free_f which accept
318 an extra argument. */
320 htab_t
321 htab_create_alloc_ex (size_t size, htab_hash hash_f, htab_eq eq_f,
322 htab_del del_f, void *alloc_arg,
323 htab_alloc_with_arg alloc_f,
324 htab_free_with_arg free_f)
326 htab_t result;
327 unsigned int size_prime_index;
329 size_prime_index = higher_prime_index (size);
330 size = prime_tab[size_prime_index].prime;
332 result = (htab_t) (*alloc_f) (alloc_arg, 1, sizeof (struct htab));
333 if (result == NULL)
334 return NULL;
335 result->entries = (PTR *) (*alloc_f) (alloc_arg, size, sizeof (PTR));
336 if (result->entries == NULL)
338 if (free_f != NULL)
339 (*free_f) (alloc_arg, result);
340 return NULL;
342 result->size = size;
343 result->size_prime_index = size_prime_index;
344 result->hash_f = hash_f;
345 result->eq_f = eq_f;
346 result->del_f = del_f;
347 result->alloc_arg = alloc_arg;
348 result->alloc_with_arg_f = alloc_f;
349 result->free_with_arg_f = free_f;
350 return result;
353 /* Update the function pointers and allocation parameter in the htab_t. */
355 void
356 htab_set_functions_ex (htab_t htab, htab_hash hash_f, htab_eq eq_f,
357 htab_del del_f, PTR alloc_arg,
358 htab_alloc_with_arg alloc_f, htab_free_with_arg free_f)
360 htab->hash_f = hash_f;
361 htab->eq_f = eq_f;
362 htab->del_f = del_f;
363 htab->alloc_arg = alloc_arg;
364 htab->alloc_with_arg_f = alloc_f;
365 htab->free_with_arg_f = free_f;
368 /* These functions exist solely for backward compatibility. */
370 #undef htab_create
371 htab_t
372 htab_create (size_t size, htab_hash hash_f, htab_eq eq_f, htab_del del_f)
374 return htab_create_alloc (size, hash_f, eq_f, del_f, xcalloc, free);
377 htab_t
378 htab_try_create (size_t size, htab_hash hash_f, htab_eq eq_f, htab_del del_f)
380 return htab_create_alloc (size, hash_f, eq_f, del_f, calloc, free);
383 /* This function frees all memory allocated for given hash table.
384 Naturally the hash table must already exist. */
386 void
387 htab_delete (htab_t htab)
389 size_t size = htab_size (htab);
390 PTR *entries = htab->entries;
391 int i;
393 if (htab->del_f)
394 for (i = size - 1; i >= 0; i--)
395 if (entries[i] != HTAB_EMPTY_ENTRY && entries[i] != HTAB_DELETED_ENTRY)
396 (*htab->del_f) (entries[i]);
398 if (htab->free_f != NULL)
400 (*htab->free_f) (entries);
401 (*htab->free_f) (htab);
403 else if (htab->free_with_arg_f != NULL)
405 (*htab->free_with_arg_f) (htab->alloc_arg, entries);
406 (*htab->free_with_arg_f) (htab->alloc_arg, htab);
410 /* This function clears all entries in the given hash table. */
412 void
413 htab_empty (htab_t htab)
415 size_t size = htab_size (htab);
416 PTR *entries = htab->entries;
417 int i;
419 if (htab->del_f)
420 for (i = size - 1; i >= 0; i--)
421 if (entries[i] != HTAB_EMPTY_ENTRY && entries[i] != HTAB_DELETED_ENTRY)
422 (*htab->del_f) (entries[i]);
424 /* Instead of clearing megabyte, downsize the table. */
425 if (size > 1024*1024 / sizeof (PTR))
427 int nindex = higher_prime_index (1024 / sizeof (PTR));
428 int nsize = prime_tab[nindex].prime;
430 if (htab->free_f != NULL)
431 (*htab->free_f) (htab->entries);
432 else if (htab->free_with_arg_f != NULL)
433 (*htab->free_with_arg_f) (htab->alloc_arg, htab->entries);
434 if (htab->alloc_with_arg_f != NULL)
435 htab->entries = (PTR *) (*htab->alloc_with_arg_f) (htab->alloc_arg, nsize,
436 sizeof (PTR *));
437 else
438 htab->entries = (PTR *) (*htab->alloc_f) (nsize, sizeof (PTR *));
439 htab->size = nsize;
440 htab->size_prime_index = nindex;
442 else
443 memset (entries, 0, size * sizeof (PTR));
444 htab->n_deleted = 0;
445 htab->n_elements = 0;
448 /* Similar to htab_find_slot, but without several unwanted side effects:
449 - Does not call htab->eq_f when it finds an existing entry.
450 - Does not change the count of elements/searches/collisions in the
451 hash table.
452 This function also assumes there are no deleted entries in the table.
453 HASH is the hash value for the element to be inserted. */
455 static PTR *
456 find_empty_slot_for_expand (htab_t htab, hashval_t hash)
458 hashval_t index = htab_mod (hash, htab);
459 size_t size = htab_size (htab);
460 PTR *slot = htab->entries + index;
461 hashval_t hash2;
463 if (*slot == HTAB_EMPTY_ENTRY)
464 return slot;
465 else if (*slot == HTAB_DELETED_ENTRY)
466 abort ();
468 hash2 = htab_mod_m2 (hash, htab);
469 for (;;)
471 index += hash2;
472 if (index >= size)
473 index -= size;
475 slot = htab->entries + index;
476 if (*slot == HTAB_EMPTY_ENTRY)
477 return slot;
478 else if (*slot == HTAB_DELETED_ENTRY)
479 abort ();
483 /* The following function changes size of memory allocated for the
484 entries and repeatedly inserts the table elements. The occupancy
485 of the table after the call will be about 50%. Naturally the hash
486 table must already exist. Remember also that the place of the
487 table entries is changed. If memory allocation failures are allowed,
488 this function will return zero, indicating that the table could not be
489 expanded. If all goes well, it will return a non-zero value. */
491 static int
492 htab_expand (htab_t htab)
494 PTR *oentries;
495 PTR *olimit;
496 PTR *p;
497 PTR *nentries;
498 size_t nsize, osize, elts;
499 unsigned int oindex, nindex;
501 oentries = htab->entries;
502 oindex = htab->size_prime_index;
503 osize = htab->size;
504 olimit = oentries + osize;
505 elts = htab_elements (htab);
507 /* Resize only when table after removal of unused elements is either
508 too full or too empty. */
509 if (elts * 2 > osize || (elts * 8 < osize && osize > 32))
511 nindex = higher_prime_index (elts * 2);
512 nsize = prime_tab[nindex].prime;
514 else
516 nindex = oindex;
517 nsize = osize;
520 if (htab->alloc_with_arg_f != NULL)
521 nentries = (PTR *) (*htab->alloc_with_arg_f) (htab->alloc_arg, nsize,
522 sizeof (PTR *));
523 else
524 nentries = (PTR *) (*htab->alloc_f) (nsize, sizeof (PTR *));
525 if (nentries == NULL)
526 return 0;
527 htab->entries = nentries;
528 htab->size = nsize;
529 htab->size_prime_index = nindex;
530 htab->n_elements -= htab->n_deleted;
531 htab->n_deleted = 0;
533 p = oentries;
536 PTR x = *p;
538 if (x != HTAB_EMPTY_ENTRY && x != HTAB_DELETED_ENTRY)
540 PTR *q = find_empty_slot_for_expand (htab, (*htab->hash_f) (x));
542 *q = x;
545 p++;
547 while (p < olimit);
549 if (htab->free_f != NULL)
550 (*htab->free_f) (oentries);
551 else if (htab->free_with_arg_f != NULL)
552 (*htab->free_with_arg_f) (htab->alloc_arg, oentries);
553 return 1;
556 /* This function searches for a hash table entry equal to the given
557 element. It cannot be used to insert or delete an element. */
560 htab_find_with_hash (htab_t htab, const PTR element, hashval_t hash)
562 hashval_t index, hash2;
563 size_t size;
564 PTR entry;
566 htab->searches++;
567 size = htab_size (htab);
568 index = htab_mod (hash, htab);
570 entry = htab->entries[index];
571 if (entry == HTAB_EMPTY_ENTRY
572 || (entry != HTAB_DELETED_ENTRY && (*htab->eq_f) (entry, element)))
573 return entry;
575 hash2 = htab_mod_m2 (hash, htab);
576 for (;;)
578 htab->collisions++;
579 index += hash2;
580 if (index >= size)
581 index -= size;
583 entry = htab->entries[index];
584 if (entry == HTAB_EMPTY_ENTRY
585 || (entry != HTAB_DELETED_ENTRY && (*htab->eq_f) (entry, element)))
586 return entry;
590 /* Like htab_find_slot_with_hash, but compute the hash value from the
591 element. */
594 htab_find (htab_t htab, const PTR element)
596 return htab_find_with_hash (htab, element, (*htab->hash_f) (element));
599 /* This function searches for a hash table slot containing an entry
600 equal to the given element. To delete an entry, call this with
601 insert=NO_INSERT, then call htab_clear_slot on the slot returned
602 (possibly after doing some checks). To insert an entry, call this
603 with insert=INSERT, then write the value you want into the returned
604 slot. When inserting an entry, NULL may be returned if memory
605 allocation fails. */
607 PTR *
608 htab_find_slot_with_hash (htab_t htab, const PTR element,
609 hashval_t hash, enum insert_option insert)
611 PTR *first_deleted_slot;
612 hashval_t index, hash2;
613 size_t size;
614 PTR entry;
616 size = htab_size (htab);
617 if (insert == INSERT && size * 3 <= htab->n_elements * 4)
619 if (htab_expand (htab) == 0)
620 return NULL;
621 size = htab_size (htab);
624 index = htab_mod (hash, htab);
626 htab->searches++;
627 first_deleted_slot = NULL;
629 entry = htab->entries[index];
630 if (entry == HTAB_EMPTY_ENTRY)
631 goto empty_entry;
632 else if (entry == HTAB_DELETED_ENTRY)
633 first_deleted_slot = &htab->entries[index];
634 else if ((*htab->eq_f) (entry, element))
635 return &htab->entries[index];
637 hash2 = htab_mod_m2 (hash, htab);
638 for (;;)
640 htab->collisions++;
641 index += hash2;
642 if (index >= size)
643 index -= size;
645 entry = htab->entries[index];
646 if (entry == HTAB_EMPTY_ENTRY)
647 goto empty_entry;
648 else if (entry == HTAB_DELETED_ENTRY)
650 if (!first_deleted_slot)
651 first_deleted_slot = &htab->entries[index];
653 else if ((*htab->eq_f) (entry, element))
654 return &htab->entries[index];
657 empty_entry:
658 if (insert == NO_INSERT)
659 return NULL;
661 if (first_deleted_slot)
663 htab->n_deleted--;
664 *first_deleted_slot = HTAB_EMPTY_ENTRY;
665 return first_deleted_slot;
668 htab->n_elements++;
669 return &htab->entries[index];
672 /* Like htab_find_slot_with_hash, but compute the hash value from the
673 element. */
675 PTR *
676 htab_find_slot (htab_t htab, const PTR element, enum insert_option insert)
678 return htab_find_slot_with_hash (htab, element, (*htab->hash_f) (element),
679 insert);
682 /* This function deletes an element with the given value from hash
683 table (the hash is computed from the element). If there is no matching
684 element in the hash table, this function does nothing. */
686 void
687 htab_remove_elt (htab_t htab, PTR element)
689 htab_remove_elt_with_hash (htab, element, (*htab->hash_f) (element));
693 /* This function deletes an element with the given value from hash
694 table. If there is no matching element in the hash table, this
695 function does nothing. */
697 void
698 htab_remove_elt_with_hash (htab_t htab, PTR element, hashval_t hash)
700 PTR *slot;
702 slot = htab_find_slot_with_hash (htab, element, hash, NO_INSERT);
703 if (*slot == HTAB_EMPTY_ENTRY)
704 return;
706 if (htab->del_f)
707 (*htab->del_f) (*slot);
709 *slot = HTAB_DELETED_ENTRY;
710 htab->n_deleted++;
713 /* This function clears a specified slot in a hash table. It is
714 useful when you've already done the lookup and don't want to do it
715 again. */
717 void
718 htab_clear_slot (htab_t htab, PTR *slot)
720 if (slot < htab->entries || slot >= htab->entries + htab_size (htab)
721 || *slot == HTAB_EMPTY_ENTRY || *slot == HTAB_DELETED_ENTRY)
722 abort ();
724 if (htab->del_f)
725 (*htab->del_f) (*slot);
727 *slot = HTAB_DELETED_ENTRY;
728 htab->n_deleted++;
731 /* This function scans over the entire hash table calling
732 CALLBACK for each live entry. If CALLBACK returns false,
733 the iteration stops. INFO is passed as CALLBACK's second
734 argument. */
736 void
737 htab_traverse_noresize (htab_t htab, htab_trav callback, PTR info)
739 PTR *slot;
740 PTR *limit;
742 slot = htab->entries;
743 limit = slot + htab_size (htab);
747 PTR x = *slot;
749 if (x != HTAB_EMPTY_ENTRY && x != HTAB_DELETED_ENTRY)
750 if (!(*callback) (slot, info))
751 break;
753 while (++slot < limit);
756 /* Like htab_traverse_noresize, but does resize the table when it is
757 too empty to improve effectivity of subsequent calls. */
759 void
760 htab_traverse (htab_t htab, htab_trav callback, PTR info)
762 if (htab_elements (htab) * 8 < htab_size (htab))
763 htab_expand (htab);
765 htab_traverse_noresize (htab, callback, info);
768 /* Return the fraction of fixed collisions during all work with given
769 hash table. */
771 double
772 htab_collisions (htab_t htab)
774 if (htab->searches == 0)
775 return 0.0;
777 return (double) htab->collisions / (double) htab->searches;
780 /* Hash P as a null-terminated string.
782 Copied from gcc/hashtable.c. Zack had the following to say with respect
783 to applicability, though note that unlike hashtable.c, this hash table
784 implementation re-hashes rather than chain buckets.
786 http://gcc.gnu.org/ml/gcc-patches/2001-08/msg01021.html
787 From: Zack Weinberg <zackw@panix.com>
788 Date: Fri, 17 Aug 2001 02:15:56 -0400
790 I got it by extracting all the identifiers from all the source code
791 I had lying around in mid-1999, and testing many recurrences of
792 the form "H_n = H_{n-1} * K + c_n * L + M" where K, L, M were either
793 prime numbers or the appropriate identity. This was the best one.
794 I don't remember exactly what constituted "best", except I was
795 looking at bucket-length distributions mostly.
797 So it should be very good at hashing identifiers, but might not be
798 as good at arbitrary strings.
800 I'll add that it thoroughly trounces the hash functions recommended
801 for this use at http://burtleburtle.net/bob/hash/index.html, both
802 on speed and bucket distribution. I haven't tried it against the
803 function they just started using for Perl's hashes. */
805 hashval_t
806 htab_hash_string (const PTR p)
808 const unsigned char *str = (const unsigned char *) p;
809 hashval_t r = 0;
810 unsigned char c;
812 while ((c = *str++) != 0)
813 r = r * 67 + c - 113;
815 return r;
818 /* DERIVED FROM:
819 --------------------------------------------------------------------
820 lookup2.c, by Bob Jenkins, December 1996, Public Domain.
821 hash(), hash2(), hash3, and mix() are externally useful functions.
822 Routines to test the hash are included if SELF_TEST is defined.
823 You can use this free for any purpose. It has no warranty.
824 --------------------------------------------------------------------
828 --------------------------------------------------------------------
829 mix -- mix 3 32-bit values reversibly.
830 For every delta with one or two bit set, and the deltas of all three
831 high bits or all three low bits, whether the original value of a,b,c
832 is almost all zero or is uniformly distributed,
833 * If mix() is run forward or backward, at least 32 bits in a,b,c
834 have at least 1/4 probability of changing.
835 * If mix() is run forward, every bit of c will change between 1/3 and
836 2/3 of the time. (Well, 22/100 and 78/100 for some 2-bit deltas.)
837 mix() was built out of 36 single-cycle latency instructions in a
838 structure that could supported 2x parallelism, like so:
839 a -= b;
840 a -= c; x = (c>>13);
841 b -= c; a ^= x;
842 b -= a; x = (a<<8);
843 c -= a; b ^= x;
844 c -= b; x = (b>>13);
846 Unfortunately, superscalar Pentiums and Sparcs can't take advantage
847 of that parallelism. They've also turned some of those single-cycle
848 latency instructions into multi-cycle latency instructions. Still,
849 this is the fastest good hash I could find. There were about 2^^68
850 to choose from. I only looked at a billion or so.
851 --------------------------------------------------------------------
853 /* same, but slower, works on systems that might have 8 byte hashval_t's */
854 #define mix(a,b,c) \
856 a -= b; a -= c; a ^= (c>>13); \
857 b -= c; b -= a; b ^= (a<< 8); \
858 c -= a; c -= b; c ^= ((b&0xffffffff)>>13); \
859 a -= b; a -= c; a ^= ((c&0xffffffff)>>12); \
860 b -= c; b -= a; b = (b ^ (a<<16)) & 0xffffffff; \
861 c -= a; c -= b; c = (c ^ (b>> 5)) & 0xffffffff; \
862 a -= b; a -= c; a = (a ^ (c>> 3)) & 0xffffffff; \
863 b -= c; b -= a; b = (b ^ (a<<10)) & 0xffffffff; \
864 c -= a; c -= b; c = (c ^ (b>>15)) & 0xffffffff; \
868 --------------------------------------------------------------------
869 hash() -- hash a variable-length key into a 32-bit value
870 k : the key (the unaligned variable-length array of bytes)
871 len : the length of the key, counting by bytes
872 level : can be any 4-byte value
873 Returns a 32-bit value. Every bit of the key affects every bit of
874 the return value. Every 1-bit and 2-bit delta achieves avalanche.
875 About 36+6len instructions.
877 The best hash table sizes are powers of 2. There is no need to do
878 mod a prime (mod is sooo slow!). If you need less than 32 bits,
879 use a bitmask. For example, if you need only 10 bits, do
880 h = (h & hashmask(10));
881 In which case, the hash table should have hashsize(10) elements.
883 If you are hashing n strings (ub1 **)k, do it like this:
884 for (i=0, h=0; i<n; ++i) h = hash( k[i], len[i], h);
886 By Bob Jenkins, 1996. bob_jenkins@burtleburtle.net. You may use this
887 code any way you wish, private, educational, or commercial. It's free.
889 See http://burtleburtle.net/bob/hash/evahash.html
890 Use for hash table lookup, or anything where one collision in 2^32 is
891 acceptable. Do NOT use for cryptographic purposes.
892 --------------------------------------------------------------------
895 hashval_t
896 iterative_hash (const PTR k_in /* the key */,
897 register size_t length /* the length of the key */,
898 register hashval_t initval /* the previous hash, or
899 an arbitrary value */)
901 register const unsigned char *k = (const unsigned char *)k_in;
902 register hashval_t a,b,c,len;
904 /* Set up the internal state */
905 len = length;
906 a = b = 0x9e3779b9; /* the golden ratio; an arbitrary value */
907 c = initval; /* the previous hash value */
909 /*---------------------------------------- handle most of the key */
910 #ifndef WORDS_BIGENDIAN
911 /* On a little-endian machine, if the data is 4-byte aligned we can hash
912 by word for better speed. This gives nondeterministic results on
913 big-endian machines. */
914 if (sizeof (hashval_t) == 4 && (((size_t)k)&3) == 0)
915 while (len >= 12) /* aligned */
917 a += *(hashval_t *)(k+0);
918 b += *(hashval_t *)(k+4);
919 c += *(hashval_t *)(k+8);
920 mix(a,b,c);
921 k += 12; len -= 12;
923 else /* unaligned */
924 #endif
925 while (len >= 12)
927 a += (k[0] +((hashval_t)k[1]<<8) +((hashval_t)k[2]<<16) +((hashval_t)k[3]<<24));
928 b += (k[4] +((hashval_t)k[5]<<8) +((hashval_t)k[6]<<16) +((hashval_t)k[7]<<24));
929 c += (k[8] +((hashval_t)k[9]<<8) +((hashval_t)k[10]<<16)+((hashval_t)k[11]<<24));
930 mix(a,b,c);
931 k += 12; len -= 12;
934 /*------------------------------------- handle the last 11 bytes */
935 c += length;
936 switch(len) /* all the case statements fall through */
938 case 11: c+=((hashval_t)k[10]<<24);
939 case 10: c+=((hashval_t)k[9]<<16);
940 case 9 : c+=((hashval_t)k[8]<<8);
941 /* the first byte of c is reserved for the length */
942 case 8 : b+=((hashval_t)k[7]<<24);
943 case 7 : b+=((hashval_t)k[6]<<16);
944 case 6 : b+=((hashval_t)k[5]<<8);
945 case 5 : b+=k[4];
946 case 4 : a+=((hashval_t)k[3]<<24);
947 case 3 : a+=((hashval_t)k[2]<<16);
948 case 2 : a+=((hashval_t)k[1]<<8);
949 case 1 : a+=k[0];
950 /* case 0: nothing left to add */
952 mix(a,b,c);
953 /*-------------------------------------------- report the result */
954 return c;