mirror of
https://github.com/CTCaer/hekate
synced 2024-09-06 16:18:59 +00:00
180 lines
6.5 KiB
C
180 lines
6.5 KiB
C
![]() |
/*************************************************************************
|
||
|
* Name: lz.c
|
||
|
* Author: Marcus Geelnard
|
||
|
* Description: LZ77 coder/decoder implementation.
|
||
|
* Reentrant: Yes
|
||
|
*
|
||
|
* The LZ77 compression scheme is a substitutional compression scheme
|
||
|
* proposed by Abraham Lempel and Jakob Ziv in 1977. It is very simple in
|
||
|
* its design, and uses no fancy bit level compression.
|
||
|
*
|
||
|
* This is my first attempt at an implementation of a LZ77 code/decoder.
|
||
|
*
|
||
|
* The principle of the LZ77 compression algorithm is to store repeated
|
||
|
* occurrences of strings as references to previous occurrences of the same
|
||
|
* string. The point is that the reference consumes less space than the
|
||
|
* string itself, provided that the string is long enough (in this
|
||
|
* implementation, the string has to be at least 4 bytes long, since the
|
||
|
* minimum coded reference is 3 bytes long). Also note that the term
|
||
|
* "string" refers to any kind of byte sequence (it does not have to be
|
||
|
* an ASCII string, for instance).
|
||
|
*
|
||
|
* The coder uses a brute force approach to finding string matches in the
|
||
|
* history buffer (or "sliding window", if you wish), which is very, very
|
||
|
* slow. I recon the complexity is somewhere between O(n^2) and O(n^3),
|
||
|
* depending on the input data.
|
||
|
*
|
||
|
* There is also a faster implementation that uses a large working buffer
|
||
|
* in which a "jump table" is stored, which is used to quickly find
|
||
|
* possible string matches (see the source code for LZ_CompressFast() for
|
||
|
* more information). The faster method is an order of magnitude faster,
|
||
|
* but still quite slow compared to other compression methods.
|
||
|
*
|
||
|
* The upside is that decompression is very fast, and the compression ratio
|
||
|
* is often very good.
|
||
|
*
|
||
|
* The reference to a string is coded as a (length,offset) pair, where the
|
||
|
* length indicates the length of the string, and the offset gives the
|
||
|
* offset from the current data position. To distinguish between string
|
||
|
* references and literal strings (uncompressed bytes), a string reference
|
||
|
* is preceded by a marker byte, which is chosen as the least common byte
|
||
|
* symbol in the input data stream (this marker byte is stored in the
|
||
|
* output stream as the first byte).
|
||
|
*
|
||
|
* Occurrences of the marker byte in the stream are encoded as the marker
|
||
|
* byte followed by a zero byte, which means that occurrences of the marker
|
||
|
* byte have to be coded with two bytes.
|
||
|
*
|
||
|
* The lengths and offsets are coded in a variable length fashion, allowing
|
||
|
* values of any magnitude (up to 4294967295 in this implementation).
|
||
|
*
|
||
|
* With this compression scheme, the worst case compression result is
|
||
|
* (257/256)*insize + 1.
|
||
|
*
|
||
|
*-------------------------------------------------------------------------
|
||
|
* Copyright (c) 2003-2006 Marcus Geelnard
|
||
|
*
|
||
|
* This software is provided 'as-is', without any express or implied
|
||
|
* warranty. In no event will the authors be held liable for any damages
|
||
|
* arising from the use of this software.
|
||
|
*
|
||
|
* Permission is granted to anyone to use this software for any purpose,
|
||
|
* including commercial applications, and to alter it and redistribute it
|
||
|
* freely, subject to the following restrictions:
|
||
|
*
|
||
|
* 1. The origin of this software must not be misrepresented; you must not
|
||
|
* claim that you wrote the original software. If you use this software
|
||
|
* in a product, an acknowledgment in the product documentation would
|
||
|
* be appreciated but is not required.
|
||
|
*
|
||
|
* 2. Altered source versions must be plainly marked as such, and must not
|
||
|
* be misrepresented as being the original software.
|
||
|
*
|
||
|
* 3. This notice may not be removed or altered from any source
|
||
|
* distribution.
|
||
|
*
|
||
|
* Marcus Geelnard
|
||
|
* marcus.geelnard at home.se
|
||
|
*************************************************************************/
|
||
|
|
||
|
|
||
|
/*************************************************************************
|
||
|
* INTERNAL FUNCTIONS *
|
||
|
*************************************************************************/
|
||
|
|
||
|
|
||
|
/*************************************************************************
|
||
|
* _LZ_ReadVarSize() - Read unsigned integer with variable number of
|
||
|
* bytes depending on value.
|
||
|
*************************************************************************/
|
||
|
|
||
|
static int _LZ_ReadVarSize( unsigned int * x, const unsigned char * buf )
|
||
|
{
|
||
|
unsigned int y, b, num_bytes;
|
||
|
|
||
|
/* Read complete value (stop when byte contains zero in 8:th bit) */
|
||
|
y = 0;
|
||
|
num_bytes = 0;
|
||
|
do
|
||
|
{
|
||
|
b = (unsigned int) (*buf ++);
|
||
|
y = (y << 7) | (b & 0x0000007f);
|
||
|
++ num_bytes;
|
||
|
}
|
||
|
while( b & 0x00000080 );
|
||
|
|
||
|
/* Store value in x */
|
||
|
*x = y;
|
||
|
|
||
|
/* Return number of bytes read */
|
||
|
return num_bytes;
|
||
|
}
|
||
|
|
||
|
|
||
|
|
||
|
/*************************************************************************
|
||
|
* PUBLIC FUNCTIONS *
|
||
|
*************************************************************************/
|
||
|
|
||
|
|
||
|
/*************************************************************************
|
||
|
* LZ_Uncompress() - Uncompress a block of data using an LZ77 decoder.
|
||
|
* in - Input (compressed) buffer.
|
||
|
* out - Output (uncompressed) buffer. This buffer must be large
|
||
|
* enough to hold the uncompressed data.
|
||
|
* insize - Number of input bytes.
|
||
|
*************************************************************************/
|
||
|
|
||
|
void LZ_Uncompress( const unsigned char *in, unsigned char *out,
|
||
|
unsigned int insize )
|
||
|
{
|
||
|
unsigned char marker, symbol;
|
||
|
unsigned int i, inpos, outpos, length, offset;
|
||
|
|
||
|
/* Do we have anything to uncompress? */
|
||
|
if( insize < 1 )
|
||
|
{
|
||
|
return;
|
||
|
}
|
||
|
|
||
|
/* Get marker symbol from input stream */
|
||
|
marker = in[ 0 ];
|
||
|
inpos = 1;
|
||
|
|
||
|
/* Main decompression loop */
|
||
|
outpos = 0;
|
||
|
do
|
||
|
{
|
||
|
symbol = in[ inpos ++ ];
|
||
|
if( symbol == marker )
|
||
|
{
|
||
|
/* We had a marker byte */
|
||
|
if( in[ inpos ] == 0 )
|
||
|
{
|
||
|
/* It was a single occurrence of the marker byte */
|
||
|
out[ outpos ++ ] = marker;
|
||
|
++ inpos;
|
||
|
}
|
||
|
else
|
||
|
{
|
||
|
/* Extract true length and offset */
|
||
|
inpos += _LZ_ReadVarSize( &length, &in[ inpos ] );
|
||
|
inpos += _LZ_ReadVarSize( &offset, &in[ inpos ] );
|
||
|
|
||
|
/* Copy corresponding data from history window */
|
||
|
for( i = 0; i < length; ++ i )
|
||
|
{
|
||
|
out[ outpos ] = out[ outpos - offset ];
|
||
|
++ outpos;
|
||
|
}
|
||
|
}
|
||
|
}
|
||
|
else
|
||
|
{
|
||
|
/* No marker, plain copy */
|
||
|
out[ outpos ++ ] = symbol;
|
||
|
}
|
||
|
}
|
||
|
while( inpos < insize );
|
||
|
}
|