Class: Origami::Filter::RunLength

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

Overview

Class representing a Filter used to encode and decode data using RLE compression algorithm.

Constant Summary collapse

EOD =

:nodoc:

128

Constants included from Origami::Filter

A85, AHx, CCF, Fl, RL

Instance Method Summary collapse

Methods included from Origami::Filter

included, #initialize

Instance Method Details

#decode(stream) ⇒ Object

Decodes data using RLE decompression method.

stream

The data to decode.



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# File 'lib/origami/filters/runlength.rb', line 69

def decode(stream)
  result = "".b

  i = 0
  until (i >= stream.length) || (stream[i].ord == EOD)

    # At least two bytes are required.
    if i > stream.length - 2
      raise InvalidRunLengthDataError.new("Truncated run-length data", input_data: stream, decoded_data: result)
    end

    length = stream[i].ord
    if length < EOD
      result << stream[i + 1, length + 1]
      i = i + length + 2
    else
      result << stream[i + 1] * (257 - length)
      i += 2
    end
  end

  # Check if offset is beyond the end of data.
  if i > stream.length
    raise InvalidRunLengthDataError.new("Truncated run-length data", input_data: stream, decoded_data: result)
  end

  result
end

#encode(stream) ⇒ Object

Encodes data using RLE compression method.

stream

The data to encode.



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# File 'lib/origami/filters/runlength.rb', line 38

def encode(stream)
  result = "".b
  i = 0

  while i < stream.size

    # How many identical bytes coming?
    length = compute_run_length(stream, i)

    # If more than 1, then compress them.
    if length > 1
      result << (257 - length).chr << stream[i]

    # Otherwise how many different bytes to copy?
    else
      next_pos = find_next_run(stream, i)
      length = next_pos - i

      result << (length - 1).chr << stream[i, length]

    end
    i += length
  end

  result << EOD.chr
end