Module: Flipbook::GIF::LZW

Defined in:
lib/flipbook/gif/lzw.rb,
lib/flipbook/gif/reader.rb

Class Method Summary collapse

Class Method Details

.decode(bytes, minimum_code_size, expected_size) ⇒ Object

Raises:



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# File 'lib/flipbook/gif/reader.rb', line 186

def decode(bytes, minimum_code_size, expected_size)
  clear = 1 << minimum_code_size
  ending = clear + 1
  prefix = Array.new(4096)
  suffix = Array.new(4096)
  clear.times { |index| suffix[index] = index }
  next_code = ending + 1
  width = minimum_code_size + 1
  bit_offset = 0
  previous = nil
  first = nil
  output = String.new(capacity: expected_size, encoding: Encoding::BINARY)
  ended = false

  loop do
    code = read_code(bytes, bit_offset, width)
    break unless code

    bit_offset += width
    if code == clear
      clear.times { |index| suffix[index] = index }
      next_code = ending + 1
      width = minimum_code_size + 1
      previous = nil
      next
    end
    if code == ending
      ended = true
      break
    end

    if code < next_code && suffix[code]
      entry, first = expand(code, clear, prefix, suffix)
    elsif code == next_code && previous
      entry, first = expand(previous, clear, prefix, suffix)
      entry << first
    else
      raise Error, "invalid GIF LZW code"
    end
    output << entry.pack("C*")
    raise Error, "GIF frame data exceeds its dimensions" if output.bytesize > expected_size

    if previous && next_code < 4096
      prefix[next_code] = previous
      suffix[next_code] = first
      next_code += 1
      width += 1 if next_code == (1 << width) && width < 12
    end
    previous = code
  end
  raise Error, "truncated GIF LZW data" unless ended && output.bytesize == expected_size

  output.bytes
end

.encode(indices, minimum_code_size) ⇒ Object

Raises:

  • (ArgumentError) 


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# File 'lib/flipbook/gif/lzw.rb', line 8

def encode(indices, minimum_code_size)
  minimum_code_size = Integer(minimum_code_size)
  raise ArgumentError, "minimum code size must be between 2 and 8" unless minimum_code_size.between?(2, 8)

  clear_code = 1 << minimum_code_size
  end_code = clear_code + 1
  next_code = end_code + 1
  code_width = minimum_code_size + 1
  dictionary = {}
  writer = BitWriter.new
  writer.write(clear_code, code_width)
  bytes = String(indices).b
  return writer.tap { |bits| bits.write(end_code, code_width) }.sub_blocks if bytes.empty?

  prefix = bytes.getbyte(0)
  bytes.byteslice(1..).to_s.each_byte do |byte|
    key = (prefix << 8) | byte
    if dictionary.key?(key)
      prefix = dictionary[key]
      next
    end

    writer.write(prefix, code_width)
    if next_code < 4096
      dictionary[key] = next_code
      next_code += 1
      code_width += 1 if next_code > (1 << code_width) && code_width < 12
    else
      writer.write(clear_code, code_width)
      dictionary.clear
      next_code = end_code + 1
      code_width = minimum_code_size + 1
    end
    prefix = byte
  end
  writer.write(prefix, code_width)
  code_width += 1 if next_code == (1 << code_width) && code_width < 12
  writer.write(end_code, code_width)
  writer.sub_blocks
end