joshuaflanagan · August 29, 2015 14:13 · Jan 16, 2015 · Jan 16, 2015
diff --git a/pngcheck.rb b/pngcheck.rb
@@ -1,3 +1,4 @@
+# http://www.w3.org/TR/PNG/#5DataRep
 class PNGCheck
   def initialize(path)
     @path = path

diff --git a/pngcheck.rb b/pngcheck.rb
@@ -0,0 +1,43 @@
+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