readHeader

    fn readHeader(self: *Iterator) !?Header {
        if (self.padding > 0) {
            try self.reader.discardAll(self.padding);
        }
        const n = try self.reader.readSliceShort(&self.header_buffer);
        if (n == 0) return null;
        if (n < Header.SIZE) return error.UnexpectedEndOfStream;
        const header = Header{ .bytes = self.header_buffer[0..Header.SIZE] };
        if (try header.checkChksum() == 0) return null;
        return header;
    }