Class: Origami::Filter::LZW

Inherits:
Object
  • Object
show all
Includes:
Origami::Filter, Predictor
Defined in:
lib/origami/filters/lzw.rb

Overview

Class representing a filter used to encode and decode data with LZW compression algorithm.

Constant Summary collapse

EOD =

:nodoc:

257
CLEARTABLE =

:nodoc:

256

Constants included from Predictor

Predictor::NONE, Predictor::PNG_AVERAGE, Predictor::PNG_NONE, Predictor::PNG_OPTIMUM, Predictor::PNG_PAETH, Predictor::PNG_SUB, Predictor::PNG_UP, Predictor::TIFF

Constants included from Origami::Filter

A85, AHx, CCF, Fl, RL

Instance Method Summary collapse

Methods included from Predictor

included, #initialize

Methods included from Origami::Filter

included, #initialize

Instance Method Details

#decode(string) ⇒ Object

Decodes given data using LZW compression method.

stream

The data to decode.



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# File 'lib/origami/filters/lzw.rb', line 80

def decode(string)
  result = "".b
  bstring = Utils::BitReader.new(string)
  table, codesize = reset_state
  prevbyte = nil

  until bstring.eod?
    byte = bstring.read(codesize)
    break if byte == EOD

    if byte == CLEARTABLE
      table, codesize = reset_state
      prevbyte = nil
      redo
    end

    begin
      codesize = decode_codeword_size(table)
      result << decode_byte(table, prevbyte, byte, codesize)
    rescue InvalidLZWDataError => error
      error.message.concat " (bit pos #{bstring.pos - codesize})"
      error.input_data = string
      error.decoded_data = result
      raise(error)
    end

    prevbyte = byte
  end

  post_prediction(result)
end

#encode(string) ⇒ Object

Encodes given data using LZW compression method.

stream

The data to encode.



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# File 'lib/origami/filters/lzw.rb', line 42

def encode(string)
  input = pre_prediction(string)

  table, codesize = reset_state
  result = Utils::BitWriter.new
  result.write(CLEARTABLE, codesize)

  s = ''
  input.each_byte do |byte|
    char = byte.chr

    if table.size == 4096
      result.write(CLEARTABLE, codesize)
      table, _ = reset_state
    end

    codesize = table.size.bit_length

    it = s + char
    if table.has_key?(it)
      s = it
    else
      result.write(table[s], codesize)
      table[it] = table.size
      s = char
    end
  end

  result.write(table[s], codesize) unless s.empty?
  result.write(EOD, codesize)

  result.final.to_s
end