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Appendix A Reference source code
********************************

/* Lzd - Educational decompressor for the lzip format
   Copyright (C) 2013-2024 Antonio Diaz Diaz.

   This program is free software. Redistribution and use in source and
   binary forms, with or without modification, are permitted provided
   that the following conditions are met:

   1. Redistributions of source code must retain the above copyright
   notice, this list of conditions, and the following disclaimer.

   2. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions, and the following disclaimer in the
   documentation and/or other materials provided with the distribution.

   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 command-line options, I/O errors, etc), 2 to
   indicate a corrupt or invalid input file.
*/

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#if defined __MSVCRT__ || defined __OS2__ || defined __DJGPP__
#include 
#include 
#endif


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()
    {
    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,
  literal_pos_state_bits = 0,				// not used
  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;


enum { header_size = 6, trailer_size = 20 };
typedef uint8_t Lzip_header[header_size]; // 0-3 magic bytes
					  //   4 version
					  //   5 coded dictionary size
typedef uint8_t Lzip_trailer[trailer_size];
			//  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
  {
  unsigned long long member_pos;
  uint32_t code;
  uint32_t range;

public:
  Range_decoder()
    : member_pos( header_size ), code( 0 ), range( 0xFFFFFFFFU )
    {
    get_byte();			// discard first byte of the LZMA stream
    for( int i = 0; i < 4; ++i ) code = ( code << 8 ) | get_byte();
    }

  uint8_t get_byte() { ++member_pos; return std::getc( stdin ); }
  unsigned long long member_position() const { return member_pos; }

  unsigned decode( const int num_bits )
    {
    unsigned symbol = 0;
    for( int i = num_bits; i > 0; --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;
    }

  bool decode_bit( Bit_model & bm )
    {
    bool 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
      {
      code -= bound;
      range -= bound;
      bm.probability -= bm.probability >> bit_model_move_bits;
      symbol = 1;
      }
    if( range <= 0x00FFFFFFU )				// normalize
      { range <<= 8; code = ( code << 8 ) | get_byte(); }
    return symbol;
    }

  unsigned decode_tree( Bit_model bm[], const int num_bits )
    {
    unsigned symbol = 1;
    for( int i = 0; i < num_bits; ++i )
      symbol = ( symbol << 1 ) | decode_bit( bm[symbol] );
    return symbol - ( 1 << num_bits );
    }

  unsigned decode_tree_reversed( Bit_model bm[], const int num_bits )
    {
    unsigned symbol = decode_tree( bm, num_bits );
    unsigned reversed_symbol = 0;
    for( int i = 0; i < num_bits; ++i )
      {
      reversed_symbol = ( reversed_symbol << 1 ) | ( symbol & 1 );
      symbol >>= 1;
      }
    return reversed_symbol;
    }

  unsigned decode_matched( Bit_model bm[], const unsigned match_byte )
    {
    unsigned symbol = 1;
    for( int i = 7; i >= 0; --i )
      {
      const bool match_bit = ( match_byte >> i ) & 1;
      const bool bit = decode_bit( bm[symbol+(match_bit<<8)+0x100] );
      symbol = ( symbol << 1 ) | bit;
      if( match_bit != bit )
        {
        while( symbol < 0x100 )
          symbol = ( symbol << 1 ) | decode_bit( bm[symbol] );
        break;
        }
      }
    return symbol & 0xFF;
    }

  unsigned decode_len( Len_model & lm, const int pos_state )
    {
    if( decode_bit( lm.choice1 ) == 0 )
      return min_match_len +
             decode_tree( lm.bm_low[pos_state], len_low_bits );
    if( decode_bit( lm.choice2 ) == 0 )
      return min_match_len + len_low_symbols +
             decode_tree( lm.bm_mid[pos_state], len_mid_bits );
    return min_match_len + 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_;
  bool pos_wrapped;

  void flush_data();

  uint8_t peek( const unsigned distance ) const
    {
    if( pos > distance ) return buffer[pos - distance - 1];
    if( pos_wrapped ) return buffer[dictionary_size + pos - distance - 1];
    return 0;			// prev_byte of first byte
    }

  void put_byte( const uint8_t b )
    {
    buffer[pos] = b;
    if( ++pos >= dictionary_size ) flush_data();
    }

public:
  explicit 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 ),
    pos_wrapped( false )
    {}

  ~LZ_decoder() { delete[] buffer; }

  unsigned crc() const { return crc_ ^ 0xFFFFFFFFU; }
  unsigned long long data_position() const
    { return partial_data_pos + pos; }
  uint8_t get_byte() { return rdec.get_byte(); }
  unsigned long long member_position() const
    { return rdec.member_position(); }

  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 );
    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; pos_wrapped = true; }
    stream_pos = pos;
    }
  }


bool LZ_decoder::decode_member()	// Return false if error
  {
  Bit_model bm_literal[1<> ( 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, peek( rep0 ) ) );
      state.set_char();
      continue;
      }
    // match or repeated match
    int len;
    if( rdec.decode_bit( bm_rep[state()] ) != 0 )		// 2nd bit
      {
      if( rdec.decode_bit( bm_rep0[state()] ) == 0 )		// 3rd bit
        {
        if( rdec.decode_bit( bm_len[state()][pos_state] ) == 0 ) // 4th bit
          { state.set_short_rep(); put_byte( peek( rep0 ) ); continue; }
        }
      else
        {
        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;
        }
      state.set_rep();
      len = rdec.decode_len( rep_len_model, pos_state );
      }
    else					// match
      {
      rep3 = rep2; rep2 = rep1; rep1 = rep0;
      len = rdec.decode_len( match_len_model, pos_state );
      const int len_state = std::min( len - min_match_len, len_states - 1 );
      rep0 = rdec.decode_tree( bm_dis_slot[len_state], dis_slot_bits );
      if( rep0 >= start_dis_model )
        {
        const unsigned dis_slot = rep0;
        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 ),
                                             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 && !pos_wrapped ) )
        { flush_data(); return false; }
      }
    for( int i = 0; i < len; ++i ) put_byte( peek( rep0 ) );
    }
  flush_data();
  return false;
  }


int main( const int argc, const char * const argv[] )
  {
  if( argc > 2 || ( argc == 2 && std::strcmp( argv[1], "-d" ) != 0 ) )
    {
    std::printf(
      "Lzd %s - Educational decompressor for the lzip format.\n"
      "Study the source code to learn how a lzip decompressor works.\n"
      "See the lzip manual for an explanation of the code.\n"
      "\nUsage: %s [-d] < file.lz > file\n"
      "Lzd decompresses from standard input to standard output.\n"
      "\nCopyright (C) 2024 Antonio Diaz Diaz.\n"
      "License 2-clause BSD.\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",
      PROGVERSION, argv[0] );
    return 0;
    }

#if defined __MSVCRT__ || defined __OS2__ || defined __DJGPP__
  setmode( STDIN_FILENO, O_BINARY );
  setmode( STDOUT_FILENO, O_BINARY );
#endif

  for( bool first_member = true; ; first_member = false )
    {
    Lzip_header header;				// check header
    for( int i = 0; i < header_size; ++i ) header[i] = std::getc( stdin );
    if( std::feof( stdin ) || std::memcmp( header, "LZIP\x01", 5 ) != 0 )
      {
      if( first_member )
        { std::fputs( "Bad magic number (file not in lzip format).\n",
                      stderr ); return 2; }
      break;					// ignore trailing data
      }
    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::fputs( "Invalid dictionary size in member header.\n", stderr );
        return 2; }

    LZ_decoder decoder( dict_size );		// decode LZMA stream
    if( !decoder.decode_member() )
      { std::fputs( "Data error\n", stderr ); return 2; }

    Lzip_trailer trailer;			// check trailer
    for( int i = 0; i < trailer_size; ++i ) trailer[i] = decoder.get_byte();
    int retval = 0;
    unsigned crc = 0;
    for( int i = 3; i >= 0; --i ) crc = ( crc << 8 ) + trailer[i];
    if( crc != decoder.crc() )
      { std::fputs( "CRC mismatch\n", stderr ); retval = 2; }

    unsigned long long data_size = 0;
    for( int i = 11; i >= 4; --i )
      data_size = ( data_size << 8 ) + trailer[i];
    if( data_size != decoder.data_position() )
      { std::fputs( "Data size mismatch\n", stderr ); retval = 2; }

    unsigned long long member_size = 0;
    for( int i = 19; i >= 12; --i )
      member_size = ( member_size << 8 ) + trailer[i];
    if( member_size != decoder.member_position() )
      { std::fputs( "Member size mismatch\n", stderr ); retval = 2; }
    if( retval ) return retval;
    }

  if( std::fclose( stdout ) != 0 )
    { std::fprintf( stderr, "Error closing stdout: %s\n",
                    std::strerror( errno ) ); return 1; }
  return 0;
  }