import struct

class PNGFilter:
    """Skips non-critical chunks in a PNG file."""
    magic = b"\x89PNG\r\n\x1a\n"
    def __init__(self):
        self.inbuffer = b""
        self.critchunk = False
        self.chunkleft = None

    def filter(self, data):
        self.inbuffer += data
        if self.chunkleft is None:
            if len(self.inbuffer) < 8:
                return b""
            if not self.inbuffer.startswith(self.magic):
                raise ValueError("PNG file magic not found")
            self.inbuffer = self.inbuffer[8:]
            self.chunkleft = 0
        ret = b""
        while True:
            if self.chunkleft == 0:
                if len(self.inbuffer) < 8:
                    break
                self.chunkleft, chunktype = struct.unpack(">I4s",
                        self.inbuffer[:8])
                self.chunkleft += 12 # len, type, crc
                self.critchunk = chunktype[0].isupper()
                if self.critchunk:
                    print("critical chunk %s %r" % (chunktype, self.inbuffer[8:16]))
            n = min(self.chunkleft, len(self.inbuffer))
            if self.critchunk:
                ret += self.inbuffer[:n]
            self.inbuffer = self.inbuffer[n:]
            self.chunkleft -= n
            if self.chunkleft:
                break
        return ret

    def flush(self):
        ret = self.inbuffer
        self.inbuffer = b""
        return ret

    def copy(self):
        new = PNGFilter()
        new.inbuffer = self.inbuffer
        new.critchunk = self.critchunk
        new.chunkleft = self.chunkleft
        return new