libgd 2.0 / gdcache.c

#ifdef HAVE_LIBTTF

/* 
 * gdcache.c
 *
 * Caches of pointers to user structs in which the least-recently-used 
 * element is replaced in the event of a cache miss after the cache has 
 * reached a given size.
 *
 * John Ellson  (ellson@lucent.com)  Oct 31, 1997
 *
 * Test this with:
 *		 gcc -o gdcache -g -Wall -DTEST gdcache.c
 *
 * The cache is implemented by a singly-linked list of elements
 * each containing a pointer to a user struct that is being managed by
 * the cache.
 *
 * The head structure has a pointer to the most-recently-used
 * element, and elements are moved to this position in the list each
 * time they are used.  The head also contains pointers to three
 * user defined functions: 
 *		- a function to test if a cached userdata matches some keydata 
 *		- a function to provide a new userdata struct to the cache 
 *          if there has been a cache miss.
 *		- a function to release a userdata struct when it is
 *          no longer being managed by the cache
 *
 * In the event of a cache miss the cache is allowed to grow up to
 * a specified maximum size.  After the maximum size is reached then
 * the least-recently-used element is discarded to make room for the 
 * new.  The most-recently-returned value is always left at the 
 * beginning of the list after retrieval.
 *
 * In the current implementation the cache is traversed by a linear
 * search from most-recent to least-recent.  This linear search
 * probably limits the usefulness of this implementation to cache
 * sizes of a few tens of elements.
 */

#include "gdcache.h"

/*********************************************************/
/* implementation                                        */
/*********************************************************/


/* create a new cache */
gdCache_head_t *
gdCacheCreate(
	int					size,
	gdCacheTestFn_t		gdCacheTest,
	gdCacheFetchFn_t	gdCacheFetch,
	gdCacheReleaseFn_t	gdCacheRelease ) 
{
	gdCache_head_t *head; 

	head = (gdCache_head_t *)malloc(sizeof(gdCache_head_t));
	head->mru = NULL;
	head->size = size;
	head->gdCacheTest = gdCacheTest;
	head->gdCacheFetch = gdCacheFetch;
	head->gdCacheRelease = gdCacheRelease;
	return head;
}

void
gdCacheDelete( gdCache_head_t *head )
{
	gdCache_element_t *elem, *prev;

	elem = head->mru;
	while(elem) {
		(*(head->gdCacheRelease))(elem->userdata);
		prev = elem;
		elem = elem->next;
		free((char *)prev);
	}
	free((char *)head);
}

void *
gdCacheGet( gdCache_head_t *head, void *keydata )
{
	int				i=0;
	gdCache_element_t *elem, *prev = NULL, *prevprev = NULL;
	void			*userdata;

	elem = head->mru;
	while(elem) {
		if ((*(head->gdCacheTest))(elem->userdata, keydata)) {
			if (i) {  /* if not already most-recently-used */
				/* relink to top of list */
				prev->next = elem->next;
				elem->next = head->mru;
				head->mru = elem;
			}
			return elem->userdata;
		}
		prevprev = prev;
		prev = elem;
		elem = elem->next;
		i++;
	}
	userdata = (*(head->gdCacheFetch))(&(head->error), keydata);
	if (! userdata) {
		/* if there was an error in the fetch then don't cache */
		return NULL;
	}
	if (i < head->size) {  /* cache still growing - add new elem */
		elem = (gdCache_element_t *)malloc(sizeof(gdCache_element_t));
	}
	else { /* cache full - replace least-recently-used */
		/* preveprev becomes new end of list */
		prevprev->next = NULL;
		elem = prev;
		(*(head->gdCacheRelease))(elem->userdata);
	}
	/* relink to top of list */
	elem->next = head->mru;
	head->mru = elem;
	elem->userdata = userdata;
	return userdata;
}



/*********************************************************/
/* test stub                                             */
/*********************************************************/


#ifdef TEST

#include <stdio.h>

typedef struct {
	int key;
	int value;
} key_value_t;

static int
cacheTest( void *map, void *key )
{
	return (((key_value_t *)map)->key == *(int *)key);
}

static void *
cacheFetch( char **error, void *key )
{
	key_value_t *map;

	map = (key_value_t *)malloc(sizeof(key_value_t));
	map->key = *(int *)key;
	map->value = 3;

	*error = NULL;
	return (void *)map;
}

static void
cacheRelease( void *map)
{
	free( (char *)map );
}

int
main(char *argv[], int argc)
{
	gdCache_head_t	*cacheTable;
	int 			elem, key;

	cacheTable = gdCacheCreate(3, cacheTest, cacheFetch, cacheRelease);

	key = 20;
	elem = *(int *)gdCacheGet(cacheTable, &key);
	key = 30;
	elem = *(int *)gdCacheGet(cacheTable, &key);
	key = 40;
	elem = *(int *)gdCacheGet(cacheTable, &key);
	key = 50;
	elem = *(int *)gdCacheGet(cacheTable, &key);
	key = 30;
	elem = *(int *)gdCacheGet(cacheTable, &key);
	key = 30;
	elem = *(int *)gdCacheGet(cacheTable, &key);

	gdCacheDelete(cacheTable);

	return 0;
}

#endif /* TEST */
#endif /* HAVE_LIBTTF */
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