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132 lines
4.4 KiB
C
132 lines
4.4 KiB
C
/*****************************************************************************
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gif_hash.c -- module to support the following operations:
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1. InitHashTable - initialize hash table.
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2. ClearHashTable - clear the hash table to an empty state.
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2. InsertHashTable - insert one item into data structure.
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3. ExistsHashTable - test if item exists in data structure.
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This module is used to hash the GIF codes during encoding.
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*****************************************************************************/
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//#include <unistd.h>
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#include <stdint.h>
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#include <stdlib.h>
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#include <fcntl.h>
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#include <stdio.h>
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#include <string.h>
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#include "gif_lib.h"
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#include "gif_hash.h"
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#include "gif_lib_private.h"
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/* #define DEBUG_HIT_RATE Debug number of misses per hash Insert/Exists. */
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#ifdef DEBUG_HIT_RATE
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static long NumberOfTests = 0,
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NumberOfMisses = 0;
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#endif /* DEBUG_HIT_RATE */
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static int KeyItem(uint32_t Item);
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/******************************************************************************
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Initialize HashTable - allocate the memory needed and clear it. *
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******************************************************************************/
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GifHashTableType *_InitHashTable(void)
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{
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GifHashTableType *HashTable;
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if ((HashTable = (GifHashTableType *) malloc(sizeof(GifHashTableType)))
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== NULL)
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return NULL;
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_ClearHashTable(HashTable);
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return HashTable;
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}
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/******************************************************************************
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Routine to clear the HashTable to an empty state. *
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This part is a little machine depended. Use the commented part otherwise. *
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******************************************************************************/
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void _ClearHashTable(GifHashTableType *HashTable)
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{
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memset(HashTable -> HTable, 0xFF, HT_SIZE * sizeof(uint32_t));
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}
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/******************************************************************************
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Routine to insert a new Item into the HashTable. The data is assumed to be *
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new one. *
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******************************************************************************/
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void _InsertHashTable(GifHashTableType *HashTable, uint32_t Key, int Code)
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{
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int HKey = KeyItem(Key);
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uint32_t *HTable = HashTable -> HTable;
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#ifdef DEBUG_HIT_RATE
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NumberOfTests++;
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NumberOfMisses++;
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#endif /* DEBUG_HIT_RATE */
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while (HT_GET_KEY(HTable[HKey]) != 0xFFFFFL) {
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#ifdef DEBUG_HIT_RATE
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NumberOfMisses++;
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#endif /* DEBUG_HIT_RATE */
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HKey = (HKey + 1) & HT_KEY_MASK;
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}
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HTable[HKey] = HT_PUT_KEY(Key) | HT_PUT_CODE(Code);
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}
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/******************************************************************************
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Routine to test if given Key exists in HashTable and if so returns its code *
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Returns the Code if key was found, -1 if not. *
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******************************************************************************/
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int _ExistsHashTable(GifHashTableType *HashTable, uint32_t Key)
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{
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int HKey = KeyItem(Key);
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uint32_t *HTable = HashTable -> HTable, HTKey;
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#ifdef DEBUG_HIT_RATE
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NumberOfTests++;
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NumberOfMisses++;
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#endif /* DEBUG_HIT_RATE */
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while ((HTKey = HT_GET_KEY(HTable[HKey])) != 0xFFFFFL) {
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#ifdef DEBUG_HIT_RATE
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NumberOfMisses++;
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#endif /* DEBUG_HIT_RATE */
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if (Key == HTKey) return HT_GET_CODE(HTable[HKey]);
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HKey = (HKey + 1) & HT_KEY_MASK;
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}
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return -1;
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}
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/******************************************************************************
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Routine to generate an HKey for the hashtable out of the given unique key. *
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The given Key is assumed to be 20 bits as follows: lower 8 bits are the *
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new postfix character, while the upper 12 bits are the prefix code. *
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Because the average hit ratio is only 2 (2 hash references per entry), *
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evaluating more complex keys (such as twin prime keys) does not worth it! *
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******************************************************************************/
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static int KeyItem(uint32_t Item)
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{
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return ((Item >> 12) ^ Item) & HT_KEY_MASK;
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}
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#ifdef DEBUG_HIT_RATE
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/******************************************************************************
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Debugging routine to print the hit ratio - number of times the hash table *
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was tested per operation. This routine was used to test the KeyItem routine *
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******************************************************************************/
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void HashTablePrintHitRatio(void)
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{
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printf("Hash Table Hit Ratio is %ld/%ld = %ld%%.\n",
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NumberOfMisses, NumberOfTests,
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NumberOfMisses * 100 / NumberOfTests);
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}
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#endif /* DEBUG_HIT_RATE */
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/* end */
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