Module: Tessel::PNG::Decoder

Defined in:

lib/tessel.rb

Constant Summary

ADAM7 =

[[0, 0, 8, 8], [4, 0, 8, 8], [0, 4, 4, 8], [2, 0, 4, 4], [0, 2, 2, 4], [1, 0, 2, 2], [0, 1, 1, 2]].freeze

MAX_METADATA_BYTES =

1_048_576

Class Method Summary

• .decode(bytes, max_pixels: 16384 * 16384, max_metadata_bytes: MAX_METADATA_BYTES, verify_crc: true) ⇒ Object

Class Method Details

.decode(bytes, max_pixels: 16384 * 16384, max_metadata_bytes: MAX_METADATA_BYTES, verify_crc: true) ⇒ Object

# File 'lib/tessel.rb', line 427

def decode(bytes, max_pixels: 16384 * 16384, max_metadata_bytes: MAX_METADATA_BYTES, verify_crc: true)
  bytes = String(bytes).b
  raise DecodeError, "invalid PNG signature" unless bytes.start_with?(SIGNATURE)
  raise ArgumentError, "max_metadata_bytes must be non-negative" unless max_metadata_bytes.is_a?(Integer) && !max_metadata_bytes.negative?

  chunks = []
  offset = SIGNATURE.bytesize
  while offset < bytes.bytesize
    raise DecodeError.new("truncated PNG chunk", offset: offset) if offset + 12 > bytes.bytesize
    length = bytes.byteslice(offset, 4).unpack1("N")
    type = bytes.byteslice(offset + 4, 4)
    raise DecodeError.new("invalid PNG chunk length", offset: offset, chunk: type) if length > 0x7fff_ffff
    raise DecodeError.new("invalid PNG chunk type", offset: offset, chunk: type) unless type.match?(/\A[A-Za-z]{4}\z/)
    data_start = offset + 8
    data_end = data_start + length
    raise DecodeError.new("truncated PNG chunk", offset: offset, chunk: type) if data_end + 4 > bytes.bytesize
    data = bytes.byteslice(data_start, length)
    actual_crc = bytes.byteslice(data_end, 4).unpack1("N")
    if verify_crc && Zlib.crc32(type + data) != actual_crc
      raise DecodeError.new("PNG CRC mismatch", offset: offset, chunk: type)
    end
    chunks << [type, data]
    offset = data_end + 4
    break if type == "IEND"
  end
  raise DecodeError.new("trailing data after IEND", offset: offset) unless offset == bytes.bytesize
  raise DecodeError, "PNG is missing IEND" unless chunks.last&.first == "IEND"
  raise DecodeError, "IEND must be empty" unless chunks.last[1].empty?
  raise DecodeError, "PNG is missing IDAT" unless chunks.any? { |type, _| type == "IDAT" }
  raise DecodeError, "PNG must have one IHDR" unless chunks.count { |type, _| type == "IHDR" } == 1
  idat_index = chunks.index { |type, _| type == "IDAT" }
  %w[PLTE tRNS].each do |type|
    positions = chunks.each_index.select { |index| chunks[index][0] == type }
    raise DecodeError, "duplicate PNG #{type} chunk" if positions.length > 1
    raise DecodeError, "invalid PNG chunk order" if positions.first && positions.first > idat_index
  end
  plte_index = chunks.index { |type, _| type == "PLTE" }
  trns_index = chunks.index { |type, _| type == "tRNS" }
  raise DecodeError, "invalid PNG chunk order" if plte_index && trns_index && trns_index < plte_index
  raise UnsupportedError, "unknown critical PNG chunk" if chunks.any? { |type, _| type.getbyte(0) < 97 && !%w[IHDR PLTE IDAT IEND].include?(type) }
  idat_started = false
  idat_ended = false
  chunks.each do |type, _|
    if type == "IDAT"
      raise DecodeError, "PNG IDAT chunks must be consecutive" if idat_ended
      idat_started = true
    elsif idat_started && type != "IEND"
      idat_ended = true
    end
  end
  parse(chunks, max_pixels: max_pixels, max_metadata_bytes: max_metadata_bytes)
rescue Zlib::Error => e
  raise DecodeError, "invalid PNG data: #{e.message}"
end