Class: PDF::Reader::LZW

Inherits:
Object
  • Object
show all
Defined in:
lib/pdf/reader/lzw.rb

Overview

A general class for decoding LZW compressed data. LZW can be used in PDF files to compresses streams, usually for image data sourced from a TIFF file.

See the following links for more information:

ref http://www.fileformat.info/format/tiff/corion-lzw.htm
ref http://marknelson.us/1989/10/01/lzw-data-compression/

The PDF spec also has some data on the algorithm.

Defined Under Namespace

Classes: BitStream, StringTable

Constant Summary collapse

CODE_EOD =

end of data

257
CODE_CLEAR_TABLE =

clear table

256

Class Method Summary collapse

Class Method Details

.decode(data) ⇒ Object

Decompresses a LZW compressed string.



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# File 'lib/pdf/reader/lzw.rb', line 91

def self.decode(data)
  stream = BitStream.new(data.to_s, 9) # size of codes between 9 and 12 bits
  string_table = StringTable.new
  result = "".dup
  until (code = stream.read) == CODE_EOD
    if code == CODE_CLEAR_TABLE
      stream.set_bits_in_chunk(9)
      string_table = StringTable.new
      code = stream.read
      break if code == CODE_EOD
      result << string_table[code]
      old_code = code
    else
      string = string_table[code]
      if string
        result << string
        string_table.add create_new_string(string_table, old_code, code)
        old_code = code
      else
        new_string = create_new_string(string_table, old_code, old_code)
        result << new_string
        string_table.add new_string
        old_code = code
      end
      #increase de size of the codes when limit reached
      if string_table.string_table_pos == 511
        stream.set_bits_in_chunk(10)
      elsif string_table.string_table_pos == 1023
        stream.set_bits_in_chunk(11)
      elsif string_table.string_table_pos == 2047
        stream.set_bits_in_chunk(12)
      end
    end
  end
  result
end