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echttp_hash.c
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echttp_hash.c
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/* echttp - Embedded HTTP server.
*
* Copyright 2019, Pascal Martin
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version 2
* of the License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor,
* Boston, MA 02110-1301, USA.
*
* ------------------------------------------------------------------------
*
* A small, low memory footprint toolkit to create hash tables.
*
* (The current implementation is a transition step, to maintain compatibility
* with a few applications and until these applications have migrated to the
* new API. The description below describes the final design.)
*
* This module provides a way to associate a string key with an array index.
* It is up to the application to declare the array with a data structure
* appropriate to their needs.
*
* This module supports two types of index: unique keys and non-unique keys.
*
* On a unique key index, each key appears only once. Trying to create an
* already existing key causes the index for the existing key to be returned.
* This type of index can be used to create a key/value store (see file
* echttp_catalog.c).
*
* On a non-unique key index, each key may appear more than once. It is
* possible to walk the list of records matching a specific key. This
* type of index is very useful to optimize searches like "retrieve all
* the children of a specific parent".
*
* unsigned int echttp_hash_signature (const char *name);
*
* Calculate a signature for the provided key. A signature is the hash
* code _before_ applying the modulo operation. In this respect it is
* a more unique and discriminant value that the hash code. It is also
* independent from the hash modulo (if ever that one changes when the
* table expands).
*
* void echttp_hash_reset (echttp_hash *d, echttp_hash_action *action);
*
* Erase all data in the given hash. After this call the hash is empty.
* The action may be used to free name and value if required.
* A null action is ignored.
*
* int echttp_hash_find (echttp_hash *d, const char *name);
*
* Return 0 if the item was not found, the item's index if found.
*
* int echttp_hash_next (echttp_hash *d, int from, const char *name);
*
* Return 0 if no other matching item was found after from, the item's
* index if found. (This is used for non-unique indexes only.)
*
* int echttp_hash_add (echttp_hash *d, const char *name);
*
* Add a new item, even if the key already exists.
* Return 0 if the hash table is full, the new item's index otherwise.
* (This is used for non-unique indexes only.)
*
* int echttp_hash_iterate (echttp_hash *d,
* const char *name, echttp_hash_action *action);
*
* Scan the items in the hash index and call action for each matching
* items. If name is null, action is called for all items in the hash
* index. The action must not be null (what would be the point?).
* If the action returns 0, the iteration continue until no more item
* is found; otherwise the iteration stops and returns the last
* processed index.
*
* void echttp_hash_insert (echttp_hash *d, const char *name);
*
* Insert a new item if it did not exist already. Return 0 if the hash
* table is full and cannot accomodate any new item, or the item's index
* otherwise (This is used for unique indexes only.)
*
* void echttp_hash_set (echttp_hash *d,
* const char *name, const char *value);
*
* Insert a new item, or change the value of an item that already exists.
* (Compatibility API, to be phased out.)
*
* const char *echttp_hash_refresh (echttp_hash *d,
* const char *name, void *value,
* time_t timestamp);
*
* Same as echttp_hash_set, but also update the item's timestamp.
* (The timestamp can be used by the application to detect old items
* that should be ignored.)
* TBD: the timestamp should really be managed by the caller..
* (Compatibility API, to be phased out.)
*
* void *echttp_hash_get (echttp_hash *d, const char *name);
*
* Retrieve the value associated with the provided key. Returns 0 when
* the key is not found. If there are duplicates items, the value of
* the first one found is returned: do not use with non-unique indexes.
* (Compatibility API, to be phased out.)
*
* LIMITATIONS
*
* The current implementation is limited to a maximum of 256 entries. This
* module was never meant to be used for datasets with millions of items.
*/
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <ctype.h>
#include "echttp_hash.h"
/* This hash function is derived from Daniel J. Bernstein's djb2 hash function.
*
* It was made case independent because several HTTP entities are not case
* sensitive. We do not apply the modulo here: the full hash value will be
* used later to accelerate string search when there is collision.
*/
unsigned int echttp_hash_signature (const char *name) {
unsigned int hash = 5381;
int c;
while (c = *name++)
hash = ((hash << 5) + hash) + tolower(c); /* hash * 33 + c */
return hash;
}
void echttp_hash_reset (echttp_hash *d, echttp_hash_action *action) {
int i;
for (i = 1; i < ECHTTP_MAX_SYMBOL; ++i) {
if (d->item[i].name && action)
action (i, d->item[i].name);
d->item[i].name = d->item[i].value = 0;
d->item[i].next = 0;
d->item[i].timestamp = 0;
}
for (i = 0; i < ECHTTP_HASH; ++i) {
d->index[i] = 0;
}
d->count = 0;
}
static int echttp_hash_forage (echttp_hash *d, int start,
const char *name, int signature) {
int i;
for (i = start; i > 0; i = d->item[i].next) {
if (d->item[i].signature != signature) continue;
if (strcasecmp(name, d->item[i].name) == 0) {
return i;
}
}
return 0;
}
int echttp_hash_find (echttp_hash *d, const char *name) {
if (!d) return 0;
unsigned int signature = echttp_hash_signature(name);
int hash = signature % ECHTTP_HASH;
return echttp_hash_forage (d, d->index[hash], name, signature);
}
int echttp_hash_next (echttp_hash *d, int from, const char *name) {
if (from <= 0 || from >= ECHTTP_MAX_SYMBOL) return 0;
if (!d) return 0;
return echttp_hash_forage (d, d->item[from].next,
name, d->item[from].signature);
}
int echttp_hash_add (echttp_hash *d, const char *name) {
if (!d) return 0;
unsigned int signature = echttp_hash_signature(name);
int hash = signature % ECHTTP_HASH;
int index = d->count + 1;
if (index >= ECHTTP_MAX_SYMBOL) {
fprintf (stderr, "Too many symbols in hash.\n");
return 0;
}
d->item[index].name = name;
d->item[index].value = 0;
d->item[index].signature = signature;
d->item[index].next = d->index[hash];
d->index[hash] = index;
d->count = index;
return index;
}
int echttp_hash_iterate (echttp_hash *d,
const char *name, echttp_hash_action *action) {
if (!d) return 0;
int i;
if (!name) {
for (i = 1; i < d->count; ++i) {
if (action (i, d->item[i].name)) return i;
}
} else {
for (i = echttp_hash_find (d, name);
i > 0; i = echttp_hash_next(d, i, name)) {
if (action (i, name)) return i;
}
}
return 0;
}
int echttp_hash_insert (echttp_hash *d, const char *name) {
if (!d) return 0;
int index = echttp_hash_find (d, name);
if (index <= 0) {
index = echttp_hash_add (d, name);
}
return index;
}
void *echttp_hash_refresh (echttp_hash *d,
const char *name, void *value, time_t timestamp) {
int index = echttp_hash_insert (d, name);
void *old;
if (index > 0) {
old = d->item[index].value;
d->item[index].value = value;
d->item[index].timestamp = timestamp;
} else {
old = 0;
}
return old;
}
void echttp_hash_set (echttp_hash *d, const char *name, void *value) {
echttp_hash_refresh (d, name, value, 0);
}
void *echttp_hash_get (echttp_hash *d, const char *name) {
int i = echttp_hash_find (d, name);
if (i > 0) return d->item[i].value;
return 0;
}