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547 lines
17 KiB
C
547 lines
17 KiB
C
//
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// Name: lz.c
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// Author: Marcus Geelnard
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// Description: LZ77 coder/decoder implementation.
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// Reentrant: Yes
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// $ATH_LICENSE_NULL$
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//
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// The LZ77 compression scheme is a substitutional compression scheme
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// proposed by Abraham Lempel and Jakob Ziv in 1977. It is very simple in
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// its design, and uses no fancy bit level compression.
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//
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// This is my first attempt at an implementation of a LZ77 code/decoder.
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//
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// The principle of the LZ77 compression algorithm is to store repeated
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// occurrences of strings as references to previous occurrences of the same
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// string. The point is that the reference consumes less space than the
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// string itself, provided that the string is long enough (in this
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// implementation, the string has to be at least 4 bytes long, since the
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// minimum coded reference is 3 bytes long). Also note that the term
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// "string" refers to any kind of byte sequence (it does not have to be
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// an ASCII string, for instance).
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//
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// The coder uses a brute force approach to finding string matches in the
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// history buffer (or "sliding window", if you wish), which is very, very
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// slow. I recon the complexity is somewhere between O(n^2) and O(n^3),
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// depending on the input data.
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//
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// There is also a faster implementation that uses a large working buffer
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// in which a "jump table" is stored, which is used to quickly find
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// possible string matches (see the source code for LZ_CompressFast() for
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// more information). The faster method is an order of magnitude faster,
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// but still quite slow compared to other compression methods.
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//
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// The upside is that decompression is very fast, and the compression ratio
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// is often very good.
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//
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// The reference to a string is coded as a (length,offset) pair, where the
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// length indicates the length of the string, and the offset gives the
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// offset from the current data position. To distinguish between string
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// references and literal strings (uncompressed bytes), a string reference
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// is preceded by a marker byte, which is chosen as the least common byte
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// symbol in the input data stream (this marker byte is stored in the
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// output stream as the first byte).
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//
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// Occurrences of the marker byte in the stream are encoded as the marker
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// byte followed by a zero byte, which means that occurrences of the marker
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// byte have to be coded with two bytes.
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//
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// The lengths and offsets are coded in a variable length fashion, allowing
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// values of any magnitude (up to 4294967295 in this implementation).
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//
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// With this compression scheme, the worst case compression result is
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// (257/256)*insize + 1.
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//
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//------------------------------------------------------------------------
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// Copyright (c) 2003-2006 Marcus Geelnard
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//
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// This software is provided 'as-is', without any express or implied
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// warranty. In no event will the authors be held liable for any damages
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// arising from the use of this software.
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//
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// Permission is granted to anyone to use this software for any purpose,
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// including commercial applications, and to alter it and redistribute it
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// freely, subject to the following restrictions:
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//
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// 1. The origin of this software must not be misrepresented; you must not
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// claim that you wrote the original software. If you use this software
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// in a product, an acknowledgment in the product documentation would
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// be appreciated but is not required.
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//
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// 2. Altered source versions must be plainly marked as such, and must not
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// be misrepresented as being the original software.
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//
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// 3. This notice may not be removed or altered from any source
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// distribution.
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//
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// Marcus Geelnard
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// marcus.geelnard at home.se
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//
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//
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// This file has been altered from the original version.
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//
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/*************************************************************************
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* Constants used for LZ77 coding
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*************************************************************************/
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/* Maximum offset (can be any size < 2^31). Lower values give faster
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compression, while higher values gives better compression. The default
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value of 100000 is quite high. Experiment to see what works best for
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you. */
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#define LZ_MAX_OFFSET 100000
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/*************************************************************************
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* INTERNAL FUNCTIONS *
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*************************************************************************/
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/*************************************************************************
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* _LZ_StringCompare() - Return maximum length string match.
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*************************************************************************/
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static unsigned int _LZ_StringCompare( unsigned char * str1,
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unsigned char * str2, unsigned int minlen, unsigned int maxlen )
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{
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unsigned int len;
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for( len = minlen; (len < maxlen) && (str1[len] == str2[len]); ++ len );
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return len;
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}
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/*************************************************************************
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* _LZ_WriteVarSize() - Write unsigned integer with variable number of
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* bytes depending on value.
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*************************************************************************/
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static int _LZ_WriteVarSize( unsigned int x, unsigned char * buf )
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{
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unsigned int y;
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int num_bytes, i, b;
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/* Determine number of bytes needed to store the number x */
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y = x >> 3;
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for( num_bytes = 5; num_bytes >= 2; -- num_bytes )
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{
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if( y & 0xfe000000 ) break;
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y <<= 7;
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}
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/* Write all bytes, seven bits in each, with 8:th bit set for all */
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/* but the last byte. */
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for( i = num_bytes-1; i >= 0; -- i )
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{
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b = (x >> (i*7)) & 0x0000007f;
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if( i > 0 )
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{
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b |= 0x00000080;
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}
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*buf ++ = (unsigned char) b;
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}
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/* Return number of bytes written */
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return num_bytes;
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}
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/*************************************************************************
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* _LZ_ReadVarSize() - Read unsigned integer with variable number of
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* bytes depending on value.
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*************************************************************************/
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static int _LZ_ReadVarSize( unsigned int * x, unsigned char * buf )
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{
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unsigned int y, b, num_bytes;
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/* Read complete value (stop when byte contains zero in 8:th bit) */
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y = 0;
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num_bytes = 0;
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do
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{
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b = (unsigned int) (*buf ++);
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y = (y << 7) | (b & 0x0000007f);
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++ num_bytes;
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}
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while( b & 0x00000080 );
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/* Store value in x */
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*x = y;
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/* Return number of bytes read */
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return num_bytes;
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}
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/*************************************************************************
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* PUBLIC FUNCTIONS *
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*************************************************************************/
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/*************************************************************************
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* LZ_Compress() - Compress a block of data using an LZ77 coder.
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* in - Input (uncompressed) buffer.
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* out - Output (compressed) buffer. This buffer must be 0.4% larger
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* than the input buffer, plus one byte.
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* insize - Number of input bytes.
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* The function returns the size of the compressed data.
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*************************************************************************/
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int LZ_Compress( unsigned char *in, unsigned char *out,
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unsigned int insize )
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{
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unsigned char marker, symbol;
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unsigned int inpos, outpos, bytesleft, i;
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unsigned int maxoffset, offset, bestoffset;
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unsigned int maxlength, length, bestlength;
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unsigned int histogram[ 256 ];
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unsigned char *ptr1, *ptr2;
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/* Do we have anything to compress? */
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if( insize < 1 )
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{
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return 0;
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}
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/* Create histogram */
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for( i = 0; i < 256; ++ i )
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{
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histogram[ i ] = 0;
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}
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for( i = 0; i < insize; ++ i )
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{
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++ histogram[ in[ i ] ];
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}
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/* Find the least common byte, and use it as the marker symbol */
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marker = 0;
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for( i = 1; i < 256; ++ i )
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{
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if( histogram[ i ] < histogram[ marker ] )
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{
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marker = i;
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}
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}
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/* Remember the marker symbol for the decoder */
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out[ 0 ] = marker;
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/* Start of compression */
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inpos = 0;
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outpos = 1;
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/* Main compression loop */
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bytesleft = insize;
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do
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{
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/* Determine most distant position */
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if( inpos > LZ_MAX_OFFSET ) maxoffset = LZ_MAX_OFFSET;
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else maxoffset = inpos;
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/* Get pointer to current position */
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ptr1 = &in[ inpos ];
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/* Search history window for maximum length string match */
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bestlength = 3;
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bestoffset = 0;
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for( offset = 3; offset <= maxoffset; ++ offset )
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{
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/* Get pointer to candidate string */
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ptr2 = &ptr1[ -(int)offset ];
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/* Quickly determine if this is a candidate (for speed) */
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if( (ptr1[ 0 ] == ptr2[ 0 ]) &&
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(ptr1[ bestlength ] == ptr2[ bestlength ]) )
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{
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/* Determine maximum length for this offset */
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maxlength = (bytesleft < offset ? bytesleft : offset);
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/* Count maximum length match at this offset */
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length = _LZ_StringCompare( ptr1, ptr2, 0, maxlength );
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/* Better match than any previous match? */
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if( length > bestlength )
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{
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bestlength = length;
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bestoffset = offset;
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}
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}
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}
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/* Was there a good enough match? */
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if( (bestlength >= 8) ||
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((bestlength == 4) && (bestoffset <= 0x0000007f)) ||
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((bestlength == 5) && (bestoffset <= 0x00003fff)) ||
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((bestlength == 6) && (bestoffset <= 0x001fffff)) ||
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((bestlength == 7) && (bestoffset <= 0x0fffffff)) )
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{
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out[ outpos ++ ] = (unsigned char) marker;
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outpos += _LZ_WriteVarSize( bestlength, &out[ outpos ] );
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outpos += _LZ_WriteVarSize( bestoffset, &out[ outpos ] );
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inpos += bestlength;
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bytesleft -= bestlength;
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}
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else
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{
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/* Output single byte (or two bytes if marker byte) */
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symbol = in[ inpos ++ ];
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out[ outpos ++ ] = symbol;
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if( symbol == marker )
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{
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out[ outpos ++ ] = 0;
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}
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-- bytesleft;
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}
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}
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while( bytesleft > 3 );
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/* Dump remaining bytes, if any */
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while( inpos < insize )
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{
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if( in[ inpos ] == marker )
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{
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out[ outpos ++ ] = marker;
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out[ outpos ++ ] = 0;
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}
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else
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{
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out[ outpos ++ ] = in[ inpos ];
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}
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++ inpos;
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}
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return outpos;
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}
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/*************************************************************************
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* LZ_CompressFast() - Compress a block of data using an LZ77 coder.
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* in - Input (uncompressed) buffer.
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* out - Output (compressed) buffer. This buffer must be 0.4% larger
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* than the input buffer, plus one byte.
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* insize - Number of input bytes.
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* work - Pointer to a temporary buffer (internal working buffer), which
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* must be able to hold (insize+65536) unsigned integers.
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* The function returns the size of the compressed data.
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*************************************************************************/
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int LZ_CompressFast( unsigned char *in, unsigned char *out,
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unsigned int insize, unsigned int *work )
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{
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unsigned char marker, symbol;
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unsigned int inpos, outpos, bytesleft, i, index, symbols;
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unsigned int offset, bestoffset;
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unsigned int maxlength, length, bestlength;
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unsigned int histogram[ 256 ], *lastindex, *jumptable;
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unsigned char *ptr1, *ptr2;
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/* Do we have anything to compress? */
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if( insize < 1 )
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{
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return 0;
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}
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/* Assign arrays to the working area */
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lastindex = work;
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jumptable = &work[ 65536 ];
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/* Build a "jump table". Here is how the jump table works:
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jumptable[i] points to the nearest previous occurrence of the same
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symbol pair as in[i]:in[i+1], so in[i] == in[jumptable[i]] and
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in[i+1] == in[jumptable[i]+1], and so on... Following the jump table
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gives a dramatic boost for the string search'n'match loop compared
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to doing a brute force search. The jump table is built in O(n) time,
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so it is a cheap operation in terms of time, but it is expensice in
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terms of memory consumption. */
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for( i = 0; i < 65536; ++ i )
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{
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lastindex[ i ] = 0xffffffff;
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}
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for( i = 0; i < insize-1; ++ i )
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{
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symbols = (((unsigned int)in[i]) << 8) | ((unsigned int)in[i+1]);
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index = lastindex[ symbols ];
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lastindex[ symbols ] = i;
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jumptable[ i ] = index;
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}
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jumptable[ insize-1 ] = 0xffffffff;
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/* Create histogram */
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for( i = 0; i < 256; ++ i )
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{
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histogram[ i ] = 0;
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}
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for( i = 0; i < insize; ++ i )
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{
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++ histogram[ in[ i ] ];
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}
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/* Find the least common byte, and use it as the marker symbol */
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marker = 0;
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for( i = 1; i < 256; ++ i )
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{
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if( histogram[ i ] < histogram[ marker ] )
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{
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marker = i;
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}
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}
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/* Remember the marker symbol for the decoder */
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out[ 0 ] = marker;
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/* Start of compression */
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inpos = 0;
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outpos = 1;
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/* Main compression loop */
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bytesleft = insize;
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do
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{
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/* Get pointer to current position */
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ptr1 = &in[ inpos ];
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/* Search history window for maximum length string match */
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bestlength = 3;
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bestoffset = 0;
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index = jumptable[ inpos ];
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while( (index != 0xffffffff) && ((inpos - index) < LZ_MAX_OFFSET) )
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{
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/* Get pointer to candidate string */
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ptr2 = &in[ index ];
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/* Quickly determine if this is a candidate (for speed) */
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if( ptr2[ bestlength ] == ptr1[ bestlength ] )
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{
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/* Determine maximum length for this offset */
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offset = inpos - index;
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maxlength = (bytesleft < offset ? bytesleft : offset);
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/* Count maximum length match at this offset */
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length = _LZ_StringCompare( ptr1, ptr2, 2, maxlength );
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/* Better match than any previous match? */
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if( length > bestlength )
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{
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bestlength = length;
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bestoffset = offset;
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}
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}
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/* Get next possible index from jump table */
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index = jumptable[ index ];
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}
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/* Was there a good enough match? */
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if( (bestlength >= 8) ||
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((bestlength == 4) && (bestoffset <= 0x0000007f)) ||
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((bestlength == 5) && (bestoffset <= 0x00003fff)) ||
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((bestlength == 6) && (bestoffset <= 0x001fffff)) ||
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((bestlength == 7) && (bestoffset <= 0x0fffffff)) )
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{
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out[ outpos ++ ] = (unsigned char) marker;
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outpos += _LZ_WriteVarSize( bestlength, &out[ outpos ] );
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outpos += _LZ_WriteVarSize( bestoffset, &out[ outpos ] );
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inpos += bestlength;
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bytesleft -= bestlength;
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}
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else
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{
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/* Output single byte (or two bytes if marker byte) */
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symbol = in[ inpos ++ ];
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out[ outpos ++ ] = symbol;
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if( symbol == marker )
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{
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out[ outpos ++ ] = 0;
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}
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-- bytesleft;
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}
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}
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while( bytesleft > 3 );
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/* Dump remaining bytes, if any */
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while( inpos < insize )
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{
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if( in[ inpos ] == marker )
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{
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out[ outpos ++ ] = marker;
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out[ outpos ++ ] = 0;
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}
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else
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{
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out[ outpos ++ ] = in[ inpos ];
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}
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++ inpos;
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}
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return outpos;
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}
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/*************************************************************************
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* LZ_Uncompress() - Uncompress a block of data using an LZ77 decoder.
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* in - Input (compressed) buffer.
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* out - Output (uncompressed) buffer. This buffer must be large
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* enough to hold the uncompressed data.
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* insize - Number of input bytes.
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*************************************************************************/
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int LZ_Uncompress( unsigned char *in, unsigned char *out,
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unsigned int insize )
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{
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unsigned char marker, symbol;
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unsigned int i, inpos, outpos, length, offset;
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/* Do we have anything to uncompress? */
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if( insize < 1 )
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{
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return 0;
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}
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/* Get marker symbol from input stream */
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marker = in[ 0 ];
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inpos = 1;
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/* Main decompression loop */
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outpos = 0;
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do
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{
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symbol = in[ inpos ++ ];
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if( symbol == marker )
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{
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/* We had a marker byte */
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if( in[ inpos ] == 0 )
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{
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/* It was a single occurrence of the marker byte */
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out[ outpos ++ ] = marker;
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++ inpos;
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}
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else
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{
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/* Extract true length and offset */
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inpos += _LZ_ReadVarSize( &length, &in[ inpos ] );
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inpos += _LZ_ReadVarSize( &offset, &in[ inpos ] );
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/* Copy corresponding data from history window */
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for( i = 0; i < length; ++ i )
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{
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out[ outpos ] = out[ outpos - offset ];
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++ outpos;
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}
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}
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}
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else
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{
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/* No marker, plain copy */
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out[ outpos ++ ] = symbol;
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}
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}
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while( inpos < insize );
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return outpos;
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}
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