Appendix A Reference source code
/* Lzd - Educational decompressor for lzip files
Copyright (C) 2013 Antonio Diaz Diaz.
This program is free software: you have unlimited permission
to copy, distribute and modify it.
This program 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.
*/
/*
Exit status: 0 for a normal exit, 1 for environmental problems
(file not found, invalid flags, I/O errors, etc), 2 to indicate a
corrupt or invalid input file.
*/
#include <algorithm>
#include <cerrno>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <stdint.h>
#include <unistd.h>
class State
{
int st;
public:
enum { states = 12 };
State() : st( 0 ) {}
int operator()() const { return st; }
bool is_char() const { return st < 7; }
void set_char()
{
static const int next[states] = { 0, 0, 0, 0, 1, 2, 3, 4, 5, 6, 4, 5 };
st = next[st];
}
void set_match() { st = ( st < 7 ) ? 7 : 10; }
void set_rep() { st = ( st < 7 ) ? 8 : 11; }
void set_short_rep() { st = ( st < 7 ) ? 9 : 11; }
};
enum {
min_dictionary_size = 1 << 12,
max_dictionary_size = 1 << 29,
literal_context_bits = 3,
pos_state_bits = 2,
pos_states = 1 << pos_state_bits,
pos_state_mask = pos_states - 1,
len_states = 4,
dis_slot_bits = 6,
start_dis_model = 4,
end_dis_model = 14,
modeled_distances = 1 << (end_dis_model / 2), // 128
dis_align_bits = 4,
dis_align_size = 1 << dis_align_bits,
len_low_bits = 3,
len_mid_bits = 3,
len_high_bits = 8,
len_low_symbols = 1 << len_low_bits,
len_mid_symbols = 1 << len_mid_bits,
len_high_symbols = 1 << len_high_bits,
max_len_symbols = len_low_symbols + len_mid_symbols + len_high_symbols,
min_match_len = 2, // must be 2
bit_model_move_bits = 5,
bit_model_total_bits = 11,
bit_model_total = 1 << bit_model_total_bits };
struct Bit_model
{
int probability;
Bit_model() : probability( bit_model_total / 2 ) {}
};
struct Len_model
{
Bit_model choice1;
Bit_model choice2;
Bit_model bm_low[pos_states][len_low_symbols];
Bit_model bm_mid[pos_states][len_mid_symbols];
Bit_model bm_high[len_high_symbols];
};
class CRC32
{
uint32_t data[256]; // Table of CRCs of all 8-bit messages.
public:
CRC32()
{
for( unsigned n = 0; n < 256; ++n )
{
unsigned c = n;
for( int k = 0; k < 8; ++k )
{ if( c & 1 ) c = 0xEDB88320U ^ ( c >> 1 ); else c >>= 1; }
data[n] = c;
}
}
void update_buf( uint32_t & crc, const uint8_t * const buffer,
const int size ) const
{
for( int i = 0; i < size; ++i )
crc = data[(crc^buffer[i])&0xFF] ^ ( crc >> 8 );
}
};
const CRC32 crc32;
typedef uint8_t File_header[6]; // 0-3 magic, 4 version, 5 coded_dict_size
typedef uint8_t File_trailer[20];
// 0-3 CRC32 of the uncompressed data
// 4-11 size of the uncompressed data
// 12-19 member size including header and trailer
class Range_decoder
{
uint32_t code;
uint32_t range;
public:
Range_decoder() : code( 0 ), range( 0xFFFFFFFFU )
{
for( int i = 0; i < 5; ++i ) code = (code << 8) | get_byte();
}
uint8_t get_byte() { return std::getc( stdin ); }
int decode( const int num_bits )
{
int symbol = 0;
for( int i = 0; i < num_bits; ++i )
{
range >>= 1;
symbol <<= 1;
if( code >= range ) { code -= range; symbol |= 1; }
if( range <= 0x00FFFFFFU ) // normalize
{ range <<= 8; code = (code << 8) | get_byte(); }
}
return symbol;
}
int decode_bit( Bit_model & bm )
{
int symbol;
const uint32_t bound = ( range >> bit_model_total_bits ) * bm.probability;
if( code < bound )
{
range = bound;
bm.probability += (bit_model_total - bm.probability) >> bit_model_move_bits;
symbol = 0;
}
else
{
range -= bound;
code -= bound;
bm.probability -= bm.probability >> bit_model_move_bits;
symbol = 1;
}
if( range <= 0x00FFFFFFU ) // normalize
{ range <<= 8; code = (code << 8) | get_byte(); }
return symbol;
}
int decode_tree( Bit_model bm[], const int num_bits )
{
int symbol = 1;
for( int i = 0; i < num_bits; ++i )
symbol = ( symbol << 1 ) | decode_bit( bm[symbol] );
return symbol - (1 << num_bits);
}
int decode_tree_reversed( Bit_model bm[], const int num_bits )
{
int symbol = decode_tree( bm, num_bits );
int reversed_symbol = 0;
for( int i = 0; i < num_bits; ++i )
{
reversed_symbol = ( reversed_symbol << 1 ) | ( symbol & 1 );
symbol >>= 1;
}
return reversed_symbol;
}
int decode_matched( Bit_model bm[], const int match_byte )
{
Bit_model * const bm1 = bm + 0x100;
int symbol = 1;
for( int i = 7; i >= 0; --i )
{
const int match_bit = ( match_byte >> i ) & 1;
const int bit = decode_bit( bm1[(match_bit<<8)+symbol] );
symbol = ( symbol << 1 ) | bit;
if( match_bit != bit )
{
while( symbol < 0x100 )
symbol = ( symbol << 1 ) | decode_bit( bm[symbol] );
break;
}
}
return symbol - 0x100;
}
int decode_len( Len_model & lm, const int pos_state )
{
if( decode_bit( lm.choice1 ) == 0 )
return decode_tree( lm.bm_low[pos_state], len_low_bits );
if( decode_bit( lm.choice2 ) == 0 )
return len_low_symbols +
decode_tree( lm.bm_mid[pos_state], len_mid_bits );
return len_low_symbols + len_mid_symbols +
decode_tree( lm.bm_high, len_high_bits );
}
};
class LZ_decoder
{
unsigned long long partial_data_pos;
Range_decoder rdec;
const unsigned dictionary_size;
uint8_t * const buffer; // output buffer
unsigned pos; // current pos in buffer
unsigned stream_pos; // first byte not yet written to stdout
uint32_t crc_;
void flush_data();
uint8_t get_byte( const unsigned distance ) const
{
unsigned i = pos - distance - 1;
if( pos <= distance ) i += dictionary_size;
return buffer[i];
}
void put_byte( const uint8_t b )
{
buffer[pos] = b;
if( ++pos >= dictionary_size ) flush_data();
}
public:
LZ_decoder( const unsigned dict_size )
:
partial_data_pos( 0 ),
dictionary_size( dict_size ),
buffer( new uint8_t[dictionary_size] ),
pos( 0 ),
stream_pos( 0 ),
crc_( 0xFFFFFFFFU )
{ buffer[dictionary_size-1] = 0; } // prev_byte of first_byte
~LZ_decoder() { delete[] buffer; }
unsigned crc() const { return crc_ ^ 0xFFFFFFFFU; }
unsigned long long data_position() const { return partial_data_pos + pos; }
bool decode_member();
};
void LZ_decoder::flush_data()
{
if( pos > stream_pos )
{
const unsigned size = pos - stream_pos;
crc32.update_buf( crc_, buffer + stream_pos, size );
errno = 0;
if( std::fwrite( buffer + stream_pos, 1, size, stdout ) != size )
{ std::fprintf( stderr, "Write error: %s\n", std::strerror( errno ) );
std::exit( 1 ); }
if( pos >= dictionary_size ) { partial_data_pos += pos; pos = 0; }
stream_pos = pos;
}
}
bool LZ_decoder::decode_member() // Returns false if error
{
Bit_model bm_literal[1<<literal_context_bits][0x300];
Bit_model bm_match[State::states][pos_states];
Bit_model bm_rep[State::states];
Bit_model bm_rep0[State::states];
Bit_model bm_rep1[State::states];
Bit_model bm_rep2[State::states];
Bit_model bm_len[State::states][pos_states];
Bit_model bm_dis_slot[len_states][1<<dis_slot_bits];
Bit_model bm_dis[modeled_distances-end_dis_model];
Bit_model bm_align[dis_align_size];
Len_model match_len_model;
Len_model rep_len_model;
unsigned rep0 = 0; // rep[0-3] latest four distances
unsigned rep1 = 0; // used for efficient coding of
unsigned rep2 = 0; // repeated distances
unsigned rep3 = 0;
State state;
while( !std::feof( stdin ) && !std::ferror( stdin ) )
{
const int pos_state = data_position() & pos_state_mask;
if( rdec.decode_bit( bm_match[state()][pos_state] ) == 0 ) // 1st bit
{
const uint8_t prev_byte = get_byte( 0 );
const int literal_state = prev_byte >> ( 8 - literal_context_bits );
Bit_model * const bm = bm_literal[literal_state];
if( state.is_char() )
put_byte( rdec.decode_tree( bm, 8 ) );
else
put_byte( rdec.decode_matched( bm, get_byte( rep0 ) ) );
state.set_char();
}
else
{
int len;
if( rdec.decode_bit( bm_rep[state()] ) != 0 ) // 2nd bit
{
if( rdec.decode_bit( bm_rep0[state()] ) != 0 ) // 3rd bit
{
unsigned distance;
if( rdec.decode_bit( bm_rep1[state()] ) == 0 ) // 4th bit
distance = rep1;
else
{
if( rdec.decode_bit( bm_rep2[state()] ) == 0 ) // 5th bit
distance = rep2;
else
{ distance = rep3; rep3 = rep2; }
rep2 = rep1;
}
rep1 = rep0;
rep0 = distance;
}
else
{
if( rdec.decode_bit( bm_len[state()][pos_state] ) == 0 ) // 4th bit
{ state.set_short_rep(); put_byte( get_byte( rep0 ) ); continue; }
}
state.set_rep();
len = min_match_len + rdec.decode_len( rep_len_model, pos_state );
}
else
{
rep3 = rep2; rep2 = rep1; rep1 = rep0;
len = min_match_len + rdec.decode_len( match_len_model, pos_state );
const int len_state = std::min( len - min_match_len, len_states - 1 );
const int dis_slot =
rdec.decode_tree( bm_dis_slot[len_state], dis_slot_bits );
if( dis_slot < start_dis_model ) rep0 = dis_slot;
else
{
const int direct_bits = ( dis_slot >> 1 ) - 1;
rep0 = ( 2 | ( dis_slot & 1 ) ) << direct_bits;
if( dis_slot < end_dis_model )
rep0 += rdec.decode_tree_reversed( bm_dis + rep0 - dis_slot - 1,
direct_bits );
else
{
rep0 += rdec.decode( direct_bits - dis_align_bits ) << dis_align_bits;
rep0 += rdec.decode_tree_reversed( bm_align, dis_align_bits );
if( rep0 == 0xFFFFFFFFU ) // Marker found
{
flush_data();
return ( len == min_match_len ); // End Of Stream marker
}
}
}
state.set_match();
if( rep0 >= dictionary_size || ( rep0 >= pos && !partial_data_pos ) )
return false;
}
for( int i = 0; i < len; ++i )
put_byte( get_byte( rep0 ) );
}
}
return false;
}
int main( const int argc, const char * const argv[] )
{
if( argc > 1 )
{
std::printf( "Lzd %s - Educational decompressor for lzip files.\n",
PROGVERSION );
std::printf( "Study the source to learn how a lzip decompressor works.\n"
"See the lzip manual for an explanation of the code.\n"
"It is not safe to use lzd for any real work.\n"
"\nUsage: %s < file.lz > file\n", argv[0] );
std::printf( "Lzd decompresses from standard input to standard output.\n"
"\nCopyright (C) 2013 Antonio Diaz Diaz.\n"
"This is free software: you are free to change and redistribute it.\n"
"There is NO WARRANTY, to the extent permitted by law.\n"
"Report bugs to lzip-bug@nongnu.org\n"
"Lzd home page: http://www.nongnu.org/lzip/lzd.html\n" );
return 0;
}
for( bool first_member = true; ; first_member = false )
{
File_header header;
for( int i = 0; i < 6; ++i )
header[i] = std::getc( stdin );
if( std::feof( stdin ) || std::memcmp( header, "LZIP", 4 ) != 0 )
{
if( first_member )
{ std::fprintf( stderr, "Bad magic number (file not in lzip format)\n" );
return 2; }
break;
}
if( header[4] != 1 )
{
std::fprintf( stderr, "Version %d member format not supported.\n",
header[4] );
return 2;
}
unsigned dict_size = 1 << ( header[5] & 0x1F );
dict_size -= ( dict_size / 16 ) * ( ( header[5] >> 5 ) & 7 );
if( dict_size < min_dictionary_size || dict_size > max_dictionary_size )
{ std::fprintf( stderr, "Invalid dictionary size in member header\n" );
return 2; }
LZ_decoder decoder( dict_size );
if( !decoder.decode_member() )
{ std::fprintf( stderr, "Data error\n" ); return 2; }
File_trailer trailer;
for( int i = 0; i < 20; ++i ) trailer[i] = std::getc( stdin );
unsigned crc = 0;
for( int i = 3; i >= 0; --i ) { crc <<= 8; crc += trailer[i]; }
unsigned long long data_size = 0;
for( int i = 11; i >= 4; --i ) { data_size <<= 8; data_size += trailer[i]; }
if( crc != decoder.crc() || data_size != decoder.data_position() )
{ std::fprintf( stderr, "CRC error\n" ); return 2; }
}
if( std::fclose( stdout ) != 0 )
{ std::fprintf( stderr, "Can't close stdout: %s\n", std::strerror( errno ) );
return 1; }
return 0;
}
This document was generated on October 10, 2013 using texi2html 5.0.
