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Diffstat (limited to 'common/hash.c')
| -rw-r--r-- | common/hash.c | 380 |
1 files changed, 380 insertions, 0 deletions
diff --git a/common/hash.c b/common/hash.c new file mode 100644 index 0000000..789e85c --- /dev/null +++ b/common/hash.c @@ -0,0 +1,380 @@ +/* + * Copyright (c) 2004, Nate Nielsen + * All rights reserved. + * + * Redistribution and use in source and binary forms, with or without + * modification, are permitted provided that the following conditions + * are met: + * + * * Redistributions of source code must retain the above + * copyright notice, this list of conditions and the + * following disclaimer. + * * Redistributions in binary form must reproduce the + * above copyright notice, this list of conditions and + * the following disclaimer in the documentation and/or + * other materials provided with the distribution. + * * The names of contributors to this software may not be + * used to endorse or promote products derived from this + * software without specific prior written permission. + * + * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS + * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT + * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS + * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE + * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, + * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, + * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS + * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED + * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, + * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF + * THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH + * DAMAGE. + */ + +/* + * Originally from apache 2.0 + * Modifications for general use by <nielsen@memberwebs.com> + */ + +/* Copyright 2000-2004 The Apache Software Foundation + * + * Licensed under the Apache License, Version 2.0 (the "License"); + * you may not use this file except in compliance with the License. + * You may obtain a copy of the License at + * + * http://www.apache.org/licenses/LICENSE-2.0 + * + * Unless required by applicable law or agreed to in writing, software + * distributed under the License is distributed on an "AS IS" BASIS, + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. + * See the License for the specific language governing permissions and + * limitations under the License. + */ + +#include <sys/types.h> +#include <stdlib.h> +#include "hash.h" + +#define KEY_DATA(he) ((he)->key) + +/* + * The internal form of a hash table. + * + * The table is an array indexed by the hash of the key; collisions + * are resolved by hanging a linked list of hash entries off each + * element of the array. Although this is a really simple design it + * isn't too bad given that pools have a low allocation overhead. + */ + +typedef struct hsh_entry_t hsh_entry_t; + +struct hsh_entry_t +{ + hsh_entry_t* next; + unsigned int hash; + const void* key; + size_t klen; + const void* val; +}; + +/* + * Data structure for iterating through a hash table. + * + * We keep a pointer to the next hash entry here to allow the current + * hash entry to be freed or otherwise mangled between calls to + * hsh_next(). + */ +struct hsh_index_t +{ + hsh_t* ht; + hsh_entry_t* ths; + hsh_entry_t* next; + unsigned int index; +}; + +/* + * The size of the array is always a power of two. We use the maximum + * index rather than the size so that we can use bitwise-AND for + * modular arithmetic. + * The count of hash entries may be greater depending on the chosen + * collision rate. + */ +struct hsh_t +{ + hsh_entry_t** array; + hsh_index_t iterator; /* For hsh_first(...) */ + unsigned int count; + unsigned int max; +}; + + +#define INITIAL_MAX 15 /* tunable == 2^n - 1 */ +#define int_malloc malloc +#define int_calloc calloc +#define int_free free + +/* + * Hash creation functions. + */ + +static hsh_entry_t** alloc_array(hsh_t* ht, unsigned int max) +{ + return (hsh_entry_t**)int_calloc(sizeof(*(ht->array)), (max + 1)); +} + +hsh_t* hsh_create() +{ + hsh_t* ht = int_malloc(sizeof(hsh_t)); + if(ht) + { + ht->count = 0; + ht->max = INITIAL_MAX; + ht->array = alloc_array(ht, ht->max); + if(!ht->array) + { + int_free(ht); + return NULL; + } + } + return ht; +} + +void hsh_free(hsh_t* ht) +{ + hsh_index_t* hi; + + for(hi = hsh_first(ht); hi; hi = hsh_next(hi)) + int_free(hi->ths); + + if(ht->array) + int_free(ht->array); + + int_free(ht); +} + +/* + * Hash iteration functions. + */ + +hsh_index_t* hsh_next(hsh_index_t* hi) +{ + hi->ths = hi->next; + while(!hi->ths) + { + if(hi->index > hi->ht->max) + return NULL; + + hi->ths = hi->ht->array[hi->index++]; + } + hi->next = hi->ths->next; + return hi; +} + +hsh_index_t* hsh_first(hsh_t* ht) +{ + hsh_index_t* hi = &ht->iterator; + + hi->ht = ht; + hi->index = 0; + hi->ths = NULL; + hi->next = NULL; + return hsh_next(hi); +} + +void* hsh_this(hsh_index_t* hi, const void** key, size_t* klen) +{ + if(key) + *key = KEY_DATA(hi->ths); + if(klen) + *klen = hi->ths->klen; + return (void*)hi->ths->val; +} + + +/* + * Expanding a hash table + */ + +static int expand_array(hsh_t* ht) +{ + hsh_index_t* hi; + hsh_entry_t** new_array; + unsigned int new_max; + + new_max = ht->max * 2 + 1; + new_array = alloc_array(ht, new_max); + + if(!new_array) + return 0; + + for(hi = hsh_first(ht); hi; hi = hsh_next(hi)) + { + unsigned int i = hi->ths->hash & new_max; + hi->ths->next = new_array[i]; + new_array[i] = hi->ths; + } + + if(ht->array) + free(ht->array); + + ht->array = new_array; + ht->max = new_max; + return 1; +} + +/* + * This is where we keep the details of the hash function and control + * the maximum collision rate. + * + * If val is non-NULL it creates and initializes a new hash entry if + * there isn't already one there; it returns an updatable pointer so + * that hash entries can be removed. + */ + +static hsh_entry_t** find_entry(hsh_t* ht, const void* key, size_t klen, const void* val) +{ + hsh_entry_t** hep; + hsh_entry_t* he; + const unsigned char* p; + unsigned int hash; + size_t i; + + /* + * This is the popular `times 33' hash algorithm which is used by + * perl and also appears in Berkeley DB. This is one of the best + * known hash functions for strings because it is both computed + * very fast and distributes very well. + * + * The originator may be Dan Bernstein but the code in Berkeley DB + * cites Chris Torek as the source. The best citation I have found + * is "Chris Torek, Hash function for text in C, Usenet message + * <27038@mimsy.umd.edu> in comp.lang.c , October, 1990." in Rich + * Salz's USENIX 1992 paper about INN which can be found at + * <http://citeseer.nj.nec.com/salz92internetnews.html>. + * + * The magic of number 33, i.e. why it works better than many other + * constants, prime or not, has never been adequately explained by + * anyone. So I try an explanation: if one experimentally tests all + * multipliers between 1 and 256 (as I did while writing a low-level + * data structure library some time ago) one detects that even + * numbers are not useable at all. The remaining 128 odd numbers + * (except for the number 1) work more or less all equally well. + * They all distribute in an acceptable way and this way fill a hash + * table with an average percent of approx. 86%. + * + * If one compares the chi^2 values of the variants (see + * Bob Jenkins ``Hashing Frequently Asked Questions'' at + * http://burtleburtle.net/bob/hash/hashfaq.html for a description + * of chi^2), the number 33 not even has the best value. But the + * number 33 and a few other equally good numbers like 17, 31, 63, + * 127 and 129 have nevertheless a great advantage to the remaining + * numbers in the large set of possible multipliers: their multiply + * operation can be replaced by a faster operation based on just one + * shift plus either a single addition or subtraction operation. And + * because a hash function has to both distribute good _and_ has to + * be very fast to compute, those few numbers should be preferred. + * + * -- Ralf S. Engelschall <rse@engelschall.com> + */ + hash = 0; + + if(klen == HSH_KEY_STRING) + { + for(p = key; *p; p++) + hash = hash * 33 + *p; + + klen = p - (const unsigned char *)key; + } + else + { + for(p = key, i = klen; i; i--, p++) + hash = hash * 33 + *p; + } + + /* scan linked list */ + for(hep = &ht->array[hash & ht->max], he = *hep; + he; hep = &he->next, he = *hep) + { + if(he->hash == hash && + he->klen == klen && + memcmp(KEY_DATA(he), key, klen) == 0) + break; + } + + if(he || !val) + return hep; + + /* add a new entry for non-NULL val */ + he = int_malloc(sizeof(*he)); + + if(he) + { + /* Key points to external data */ + he->key = key; + he->klen = klen; + + he->next = NULL; + he->hash = hash; + he->val = val; + + *hep = he; + ht->count++; + } + + return hep; +} + +void* hsh_get(hsh_t* ht, const void *key, size_t klen) +{ + hsh_entry_t** he = find_entry(ht, key, klen, NULL); + + if(he && *he) + return (void*)((*he)->val); + else + return NULL; +} + +int hsh_set(hsh_t* ht, const void* key, size_t klen, void* val) +{ + hsh_entry_t** hep = find_entry(ht, key, klen, val); + + if(hep && *hep) + { + /* replace entry */ + (*hep)->val = val; + + /* check that the collision rate isn't too high */ + if(ht->count > ht->max) + { + if(!expand_array(ht)) + return 0; + } + + return 1; + } + + return 0; +} + +void* hsh_rem(hsh_t* ht, const void* key, size_t klen) +{ + hsh_entry_t** hep = find_entry(ht, key, klen, NULL); + void* val = NULL; + + if(hep && *hep) + { + hsh_entry_t* old = *hep; + *hep = (*hep)->next; + --ht->count; + val = (void*)old->val; + free(old); + } + + return val; +} + +unsigned int hsh_count(hsh_t* ht) +{ + return ht->count; +} + |