Class: Cabriolet::Decompressors::LZSS

Inherits:
Base
  • Object
show all
Defined in:
lib/cabriolet/decompressors/lzss.rb

Overview

LZSS decompressor for LZSS-compressed CAB data

LZSS (Lempel-Ziv-Storer-Szymanski) is a derivative of LZ77 compression. It uses a 4096-byte sliding window with a control byte mechanism to indicate whether the next operation is a literal byte copy or a match from the window history.

Constant Summary collapse

WINDOW_SIZE =

LZSS algorithm constants

4096
WINDOW_FILL = 
0x20
MODE_EXPAND =

LZSS modes

0
MODE_MSHELP = 
1
MODE_QBASIC = 
2

Instance Attribute Summary collapse

Attributes inherited from Base

#buffer_size, #input, #io_system, #output

Instance Method Summary collapse

Methods inherited from Base

#free

Constructor Details

#initialize(io_system, input, output, buffer_size, mode = MODE_EXPAND) ⇒ LZSS

Initialize LZSS decompressor

Parameters:



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# File 'lib/cabriolet/decompressors/lzss.rb', line 30

def initialize(io_system, input, output, buffer_size,
               mode = MODE_EXPAND)
  super(io_system, input, output, buffer_size)
  @mode = mode
  @window = (WINDOW_FILL.chr * WINDOW_SIZE).b
  @window_pos = initialize_window_position
  @input_buffer = ""
  @input_pos = 0
  @invert = mode == MODE_MSHELP ? 0xFF : 0x00
  @output_buffer = String.new(encoding: Encoding::BINARY, capacity: 4096)
end

Instance Attribute Details

#mode ⇒ Object (readonly)

Returns the value of attribute mode.



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# File 'lib/cabriolet/decompressors/lzss.rb', line 21

def mode
  @mode
end

#window ⇒ Object (readonly)

Returns the value of attribute window.



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# File 'lib/cabriolet/decompressors/lzss.rb', line 21

def window
  @window
end

#window_pos ⇒ Object (readonly)

Returns the value of attribute window_pos.



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# File 'lib/cabriolet/decompressors/lzss.rb', line 21

def window_pos
  @window_pos
end

Instance Method Details

#decompress(bytes = nil) ⇒ Integer

Decompress LZSS data

Parameters:

  • bytes (Integer, nil) (defaults to: nil) —

    Maximum number of output bytes to write (nil or 0 = until EOF)

Returns:

  • (Integer) —

    Number of bytes decompressed



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# File 'lib/cabriolet/decompressors/lzss.rb', line 46

def decompress(bytes = nil)
  bytes_written = 0
  # Only enforce limit if bytes is a positive integer
  enforce_limit = bytes&.positive?

  loop do
    # Check if we've reached the output byte limit (only when limit is enforced)
    break if enforce_limit && bytes_written >= bytes

    # Read control byte
    control_byte = read_input_byte
    break if control_byte.nil?

    control_byte ^= @invert

    # Process each bit in the control byte
    8.times do |bit_index|
      # Check output limit before each operation (only when limit is enforced)
      break if enforce_limit && bytes_written >= bytes

      mask = 1 << bit_index

      if control_byte.anybits?(mask)
        # Bit is 1: literal byte
        literal = read_input_byte
        break if literal.nil?

        @window.setbyte(@window_pos, literal)
        buffer_output_byte(literal)
        bytes_written += 1

        @window_pos = (@window_pos + 1) & (WINDOW_SIZE - 1)
      else
        # Bit is 0: match from window
        offset_low = read_input_byte
        break if offset_low.nil?

        offset_high_and_length = read_input_byte
        break if offset_high_and_length.nil?

        # Decode match position and length
        match_pos = offset_low | ((offset_high_and_length & 0xF0) << 4)
        length = (offset_high_and_length & 0x0F) + 3

        # Copy from window
        length.times do
          # Check if we've reached the limit mid-match
          break if enforce_limit && bytes_written >= bytes

          byte = @window.getbyte(match_pos)
          @window.setbyte(@window_pos, byte)
          buffer_output_byte(byte)
          bytes_written += 1

          @window_pos = (@window_pos + 1) & (WINDOW_SIZE - 1)
          match_pos = (match_pos + 1) & (WINDOW_SIZE - 1)
        end
      end
    end
  end

  flush_output_buffer
  bytes_written
end