def METHOD_NAME(self):
    self.offset = 0
    header = self.fileobj.read(16)
    if header == b"":
        self.chunk = ""
        return
    if len(header) != 16:
        raise Exception("Corrupted PBZX payload?")
    decompressed_size, compressed_size = struct.unpack(">QQ", header)
    chunk = self.fileobj.read(compressed_size)
    if compressed_size == decompressed_size:
        self.chunk = chunk
    else:
        self.chunk = lzma.decompress(chunk)
        if len(self.chunk) != decompressed_size:
            raise Exception("Corrupted PBZX payload?")