/*
* xpress-decompress.c
*
* XPRESS decompression routines.
*/
/*
*
* Copyright (C) 2012, 2013 Eric Biggers
*
* This file is part of wimlib, a library for working with WIM files.
*
* wimlib is free software; you can redistribute it and/or modify it under the
* terms of the GNU General Public License as published by the Free
* Software Foundation; either version 3 of the License, or (at your option)
* any later version.
*
* wimlib is distributed in the hope that it will be useful, but WITHOUT ANY
* WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
* A PARTICULAR PURPOSE. See the GNU General Public License for more
* details.
*
* You should have received a copy of the GNU General Public License
* along with wimlib; if not, see http://www.gnu.org/licenses/.
*/
/*
* The XPRESS compression format is a LZ77 and Huffman-code based algorithm.
* That means it is quite similar to LZX compression, but XPRESS is slightly
* simpler, so it is a little faster to compress and decompress.
*
* The XPRESS compression format is mostly documented in a file called "[MS-XCA]
* Xpress Compression Algorithm". In the MSDN library, it can currently be
* found under Open Specifications => Protocols => Windows Protocols => Windows
* Server Protocols => [MS-XCA] Xpress Compression Algorithm". The format in
* WIMs is specifically the algorithm labeled as the "LZ77+Huffman Algorithm"
* (there apparently are some other versions of XPRESS as well).
*
* If you are already familiar with the LZ77 algorithm and Huffman coding, the
* XPRESS format is fairly simple. The compressed data begins with 256 bytes
* that contain 512 4-bit integers that are the lengths of the symbols in the
* Huffman tree used for decoding compressed literals. This is the only Huffman
* tree that is used for the entirety of the compressed data, and the codeword
* lengths are not encoded with a pretree.
*
* The rest of the compressed data is Huffman-encoded symbols. Values 0 through
* 255 are literal bytes. Values 256 through 511 are matches and may require
* extra bits or bytes to be read to get the match offset and match length.
*
* There is no notion of a "compressed block" in the XPRESS format, so in the
* XPRESS format, each WIM chunk (32768 bytes) will always use only one Huffman
* tree.
*
* The trickiest part is probably the fact that literal bytes for match lengths
* are encoded "separately" from the bitstream.
*
* Also, a caveat--- according to Microsoft's documentation for XPRESS,
*
* "Some implementation of the decompression algorithm expect an extra
* symbol to mark the end of the data. Specifically, some implementations
* fail during decompression if the Huffman symbol 256 is not found after
* the actual data."
*
* This is the case for WIM files--- in we must write this extra symbol "256" at
* the end. Otherwise Microsoft's software will fail to decompress the
* XPRESS-compressed data.
*
* However, wimlib's decompressor in this file currently does not care if this
* extra symbol is there or not.
*/
#ifdef HAVE_CONFIG_H
# include "config.h"
#endif
#include "wimlib.h"
#include "wimlib/assert.h"
#define XPRESS_DECOMP
#include "wimlib/decompress.h"
#include "wimlib/util.h"
#include "wimlib/xpress.h"
/*
* Decodes a symbol @huffsym that begins an XPRESS match.
*
* The low 8 bits of the symbol are divided into:
*
* bits 0-3: length header
* bits 4-7: index of high-order bit of match offset
*
* Note: taking the low 8 bits of the symbol is the same as subtracting 256, the
* number of symbols reserved for literals.
*
* Returns the match length, or -1 on error.
*/
static int
xpress_decode_match(unsigned huffsym, unsigned window_pos,
unsigned window_len, u8 window[restrict],
struct input_bitstream * restrict istream)
{
unsigned match_len;
unsigned match_offset;
u8 match_sym = (u8)huffsym;
u8 len_hdr = match_sym & 0xf;
u8 offset_bsr = match_sym >> 4;
int ret;
u8 *match_dest;
u8 *match_src;
unsigned i;
ret = bitstream_read_bits(istream, offset_bsr, &match_offset);
if (ret)
return ret;
match_offset |= (1 << offset_bsr);
if (len_hdr == 0xf) {
ret = bitstream_read_byte(istream);
if (ret < 0)
return ret;
match_len = ret;
if (match_len == 0xff) {
ret = bitstream_read_byte(istream);
if (ret < 0)
return ret;
match_len = ret;
ret = bitstream_read_byte(istream);
if (ret < 0)
return ret;
match_len |= (ret << 8);
} else {
match_len += 0xf;
}
} else {
match_len = len_hdr;
}
match_len += XPRESS_MIN_MATCH;
/* Verify that the match is in the bounds of the part of the window
* currently in use, then copy the source of the match to the current
* position. */
match_dest = window + window_pos;
match_src = match_dest - match_offset;
if (window_pos + match_len > window_len) {
DEBUG("XPRESS decompression error: match of length %u "
"bytes overflows window", match_len);
return -1;
}
if (match_src < window) {
DEBUG("XPRESS decompression error: match of length %u bytes "
"references data before window (match_offset = %u, "
"window_pos = %u)", match_len, match_offset, window_pos);
return -1;
}
for (i = 0; i < match_len; i++)
match_dest[i] = match_src[i];
return match_len;
}
/* Decodes the Huffman-encoded matches and literal bytes in a block of
* XPRESS-encoded data. */
static int
xpress_decompress_block(struct input_bitstream * restrict istream,
u8 uncompressed_data[restrict],
unsigned uncompressed_len,
const u8 lens[restrict],
const u16 decode_table[restrict])
{
unsigned curpos;
unsigned huffsym;
int ret;
int match_len;
curpos = 0;
while (curpos < uncompressed_len) {
ret = read_huffsym(istream, decode_table, lens,
XPRESS_NUM_SYMBOLS, XPRESS_TABLEBITS,
&huffsym, XPRESS_MAX_CODEWORD_LEN);
if (ret)
return ret;
if (huffsym < XPRESS_NUM_CHARS) {
uncompressed_data[curpos++] = huffsym;
} else {
match_len = xpress_decode_match(huffsym,
curpos,
uncompressed_len,
uncompressed_data,
istream);
if (match_len < 0)
return match_len;
curpos += match_len;
}
}
return 0;
}
/* API function documented in wimlib.h */
WIMLIBAPI int
wimlib_xpress_decompress(const void * restrict _compressed_data, unsigned compressed_len,
void * restrict uncompressed_data, unsigned uncompressed_len)
{
u8 lens[XPRESS_NUM_SYMBOLS];
u16 decode_table[(1 << XPRESS_TABLEBITS) + 2 * XPRESS_NUM_SYMBOLS]
_aligned_attribute(DECODE_TABLE_ALIGNMENT);
struct input_bitstream istream;
u8 *lens_p;
const u8 *compressed_data;
unsigned i;
int ret;
compressed_data = _compressed_data;
lens_p = lens;
DEBUG2("compressed_len = %d, uncompressed_len = %d",
compressed_len, uncompressed_len);
/* XPRESS uses only one Huffman tree. It contains 512 symbols, and the
* code lengths of these symbols are given literally as 4-bit integers
* in the first 256 bytes of the compressed data.
*/
if (compressed_len < XPRESS_NUM_SYMBOLS / 2) {
DEBUG("xpress_decompress(): Compressed length too short!");
return -1;
}
for (i = 0; i < XPRESS_NUM_SYMBOLS / 2; i++) {
*lens_p++ = compressed_data[i] & 0xf;
*lens_p++ = compressed_data[i] >> 4;
}
ret = make_huffman_decode_table(decode_table, XPRESS_NUM_SYMBOLS,
XPRESS_TABLEBITS, lens,
XPRESS_MAX_CODEWORD_LEN);
if (ret)
return ret;
init_input_bitstream(&istream, compressed_data + XPRESS_NUM_SYMBOLS / 2,
compressed_len - XPRESS_NUM_SYMBOLS / 2);
return xpress_decompress_block(&istream, uncompressed_data,
uncompressed_len, lens,
decode_table);
}