156 lines
5.3 KiB
C
156 lines
5.3 KiB
C
/*****************************************************************************
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* "Gif-Lib" - Yet another gif library. *
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* *
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* Written by: Gershon Elber IBM PC Ver 0.1, Jun. 1989 *
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******************************************************************************
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* Module to support the following operations: *
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* *
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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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* *
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* This module is used to hash the GIF codes during encoding. *
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******************************************************************************
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* History: *
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* 14 Jun 89 - Version 1.0 by Gershon Elber. *
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*****************************************************************************/
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#ifdef __MSDOS__
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#include <io.h>
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#include <alloc.h>
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#include <sys\stat.h>
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#else
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#include <sys/types.h>
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#include <sys/stat.h>
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#endif /* __MSDOS__ */
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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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#define PROGRAM_NAME "GIF_LIBRARY"
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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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#ifdef SYSV
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static char *VersionStr =
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"Gif library module,\t\tGershon Elber\n\
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(C) Copyright 1989 Gershon Elber, Non commercial use only.\n";
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#else
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static char *VersionStr =
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PROGRAM_NAME
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" IBMPC "
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GIF_LIB_VERSION
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" Gershon Elber, "
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__DATE__ ", " __TIME__ "\n"
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"(C) Copyright 1989 Gershon Elber, Non commercial use only.\n";
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#endif /* SYSV */
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static int KeyItem(unsigned long 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(long));
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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, unsigned long Key, int Code)
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{
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int HKey = KeyItem(Key);
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unsigned long *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, unsigned long Key)
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{
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int HKey = KeyItem(Key);
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unsigned long *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(unsigned long 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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