import struct


def checksum(data: bytes) -> bytes:
	ck_a = 0
	ck_b = 0
	for i in range(len(data)):
		ck_a = (ck_a + data[i]) & 0xFF
		ck_b = (ck_b + ck_a) & 0xFF

	return struct.pack("BB", ck_a, ck_b)

def frame_create(_class: int, _id: int, _data: bytes) -> bytes:
	hdr = b'\xb5\x62'
	frame = struct.pack('B', _class)
	frame += struct.pack('B', _id)
	frame += struct.pack('<H', len(_data))
	frame += _data
	check = checksum(frame)

	return hdr + frame + check

def frame_parse(frame: bytes):
	hdr = frame[0:2]
	if hdr != b'\xb5\x62':
		return None

	_class = struct.unpack('B', frame[2:3])[0]
	_id = struct.unpack('B', frame[3:4])[0]
	_length = struct.unpack('<H', frame[4:6])[0]
	if (6+_length+2) != len(frame):
		return None

	_data = frame[6:-2]
	_check_soll = frame[-2:]
	_check_ist = checksum(frame[2:-2])
	if _check_soll != _check_ist:
		return None

	return _data, _class, _id

if __name__ == "__main__":
	result = frame_create(1, 2, b'Hallo')
	print(result)
	payload, msg_class, msg_id = frame_parse(result)
	print(payload)