Parse PNG file

pngcheck.rb

# http://www.w3.org/TR/PNG/#5DataRep
class PNGCheck
  def initialize(path)
    @path = path
  end

  def check_all!
    expected_signature = [137, 80, 78, 71, 13, 10, 26, 10]
    file.seek 0
    signature_data = file.read(8)
    raise "unable to read signature, not enough bytes. found #{signature_data}" unless signature_data.length == 8
    signature = signature_data.unpack("C*")
    raise "invalid signature: #{signature.inspect} expected #{expected_signature.inspect}" unless signature == expected_signature

    chunks = 0
    loop do
      break unless process_chunk
      chunks += 1
    end
    puts "File is valid!"
  end

  def process_chunk
    length_data = file.read(4)
    return false if length_data.nil? || length_data == ""
    raise "expected 4 bytes for LENGTH, got '#{length_data}'" unless length_data.length == 4
    length = length_data.unpack("N").first
    type = file.read(4)
    raise "expected 4 bytes for TYPE, got '#{type}'" unless type.length == 4
    data = file.read(length)
    raise "expected #{length} bytes for DATA, got #{data.length}" unless data.length == length
    crc_data = file.read(4)
    raise "expected 4 bytes for CRC, got '#{crc_data}'" unless crc_data.length == 4
    crc = crc_data.unpack("N").first
    crc_check = Zlib.crc32(type + data)
    raise "CRC did not check" unless crc == crc_check
    puts "#{type} #{length} bytes CRC: #{crc} POS: #{file.pos}"
    true
  end

  def file
    @file ||= File.open(@path, "rb")
  end
end