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/*****************************************************************************
* "Gif-Lib" - Yet another gif library. *
* *
* Written by: Gershon Elber IBM PC Ver 0.1, Jun. 1989 *
******************************************************************************
* Module to support the following operations: *
* *
* 1. InitHashTable - initialize hash table. *
* 2. ClearHashTable - clear the hash table to an empty state. *
* 2. InsertHashTable - insert one item into data structure. *
* 3. ExistsHashTable - test if item exists in data structure. *
* *
* This module is used to hash the GIF codes during encoding. *
******************************************************************************
* History: *
* 14 Jun 89 - Version 1.0 by Gershon Elber. *
*****************************************************************************/
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "gif_lib.h"
#include "gif_hash.h"
#include "gif_lib_private.h"
/* #define DEBUG_HIT_RATE Debug number of misses per hash Insert/Exists. */
#ifdef DEBUG_HIT_RATE
static long NumberOfTests = 0,
NumberOfMisses = 0;
#endif /* DEBUG_HIT_RATE */
static int KeyItem(UINT32 Item);
/******************************************************************************
* Initialize HashTable - allocate the memory needed and clear it. *
******************************************************************************/
GifHashTableType *_InitHashTable(void)
{
GifHashTableType *HashTable;
if ((HashTable = (GifHashTableType *) malloc(sizeof(GifHashTableType)))
== NULL)
return NULL;
_ClearHashTable(HashTable);
return HashTable;
}
/******************************************************************************
* Routine to clear the HashTable to an empty state. *
* This part is a little machine depended. Use the commented part otherwise. *
******************************************************************************/
void _ClearHashTable(GifHashTableType *HashTable)
{
memset(HashTable -> HTable, 0xFF, HT_SIZE * sizeof(UINT32));
}
/******************************************************************************
* Routine to insert a new Item into the HashTable. The data is assumed to be *
* new one. *
******************************************************************************/
void _InsertHashTable(GifHashTableType *HashTable, UINT32 Key, int Code)
{
int HKey = KeyItem(Key);
UINT32 *HTable = HashTable -> HTable;
#ifdef DEBUG_HIT_RATE
NumberOfTests++;
NumberOfMisses++;
#endif /* DEBUG_HIT_RATE */
while (HT_GET_KEY(HTable[HKey]) != 0xFFFFFL) {
#ifdef DEBUG_HIT_RATE
NumberOfMisses++;
#endif /* DEBUG_HIT_RATE */
HKey = (HKey + 1) & HT_KEY_MASK;
}
HTable[HKey] = HT_PUT_KEY(Key) | HT_PUT_CODE(Code);
}
/******************************************************************************
* Routine to test if given Key exists in HashTable and if so returns its code *
* Returns the Code if key was found, -1 if not. *
******************************************************************************/
int _ExistsHashTable(GifHashTableType *HashTable, UINT32 Key)
{
int HKey = KeyItem(Key);
UINT32 *HTable = HashTable -> HTable, HTKey;
#ifdef DEBUG_HIT_RATE
NumberOfTests++;
NumberOfMisses++;
#endif /* DEBUG_HIT_RATE */
while ((HTKey = HT_GET_KEY(HTable[HKey])) != 0xFFFFFL) {
#ifdef DEBUG_HIT_RATE
NumberOfMisses++;
#endif /* DEBUG_HIT_RATE */
if (Key == HTKey) return HT_GET_CODE(HTable[HKey]);
HKey = (HKey + 1) & HT_KEY_MASK;
}
return -1;
}
/******************************************************************************
* Routine to generate an HKey for the hashtable out of the given unique key. *
* The given Key is assumed to be 20 bits as follows: lower 8 bits are the *
* new postfix character, while the upper 12 bits are the prefix code. *
* Because the average hit ratio is only 2 (2 hash references per entry), *
* evaluating more complex keys (such as twin prime keys) does not worth it! *
******************************************************************************/
static int KeyItem(UINT32 Item)
{
return ((Item >> 12) ^ Item) & HT_KEY_MASK;
}
#ifdef DEBUG_HIT_RATE
/******************************************************************************
* Debugging routine to print the hit ratio - number of times the hash table *
* was tested per operation. This routine was used to test the KeyItem routine *
******************************************************************************/
void HashTablePrintHitRatio(void)
{
printf("Hash Table Hit Ratio is %ld/%ld = %ld%%.\n",
NumberOfMisses, NumberOfTests,
NumberOfMisses * 100 / NumberOfTests);
}
#endif /* DEBUG_HIT_RATE */
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