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Update LZ encoding algorithm to iterate positively instead of negatively

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Rachel 2024-10-16 22:47:11 -07:00
parent 0a893d4d3e
commit 791ab44576
1 changed files with 24 additions and 16 deletions
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40
lz.c
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@ -1,7 +1,9 @@
// Copyright (c) 2015 YamaArashi
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include "global.h"
#include "lz.h"
@ -92,6 +94,7 @@ unsigned char *LZCompress(unsigned char *src, int srcSize, int *compressedSize,
int srcPos = 0;
int destPos = 4;
int lookbackStart = 0;
for (;;) {
unsigned char *flags = &dest[destPos++];
@ -100,38 +103,43 @@ unsigned char *LZCompress(unsigned char *src, int srcSize, int *compressedSize,
for (int i = 0; i < 8; i++) {
int bestBlockDistance = 0;
int bestBlockSize = 0;
int blockDistance = minDistance;
while (blockDistance <= srcPos && blockDistance <= 0x1000) {
int blockStart = srcPos - blockDistance;
int blockSize = 0;
int blockStart = lookbackStart;
while (blockStart != srcPos) {
int blockSize = 0;
while (blockSize < 18
&& srcPos + blockSize < srcSize
&& src[blockStart + blockSize] == src[srcPos + blockSize])
blockSize++;
while (blockSize < 18
&& srcPos + blockSize < srcSize
&& src[blockStart + blockSize] == src[srcPos + blockSize])
blockSize++;
if (blockSize > bestBlockSize) {
bestBlockDistance = blockDistance;
bestBlockSize = blockSize;
if (blockSize > bestBlockSize
&& srcPos - blockStart >= minDistance) {
bestBlockDistance = srcPos - blockStart;
bestBlockSize = blockSize;
if (blockSize == 18)
break;
}
if (blockSize == 18)
break;
}
blockDistance++;
}
blockStart++;
}
int lookbackJump = 0;
if (bestBlockSize >= 3) {
*flags |= (0x80 >> i);
srcPos += bestBlockSize;
lookbackJump = srcPos - lookbackStart > 0x1000 ? bestBlockSize : 0;
bestBlockSize -= 3;
bestBlockDistance--;
dest[destPos++] = (bestBlockSize << 4) | ((unsigned int)bestBlockDistance >> 8);
dest[destPos++] = (unsigned char)bestBlockDistance;
} else {
lookbackJump = (int)(srcPos - lookbackStart > 0x1000);
dest[destPos++] = src[srcPos++];
}
lookbackStart += lookbackJump;
if (srcPos == srcSize) {
// Pad to multiple of 4 bytes.