import struct

UBX_SYNC_WORD = b'\xb5\x62'

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)

class Hdr:
	def __init__(self, _class: int, _id: int, _length: int):
		self.mclass = _class
		self.mid = _id
		self.length = _length


def frame_parse_hdr(frame: bytes) -> Hdr|None:
	hdr = Hdr(0,0,0)

	if len(frame) < 4:
		return None

	hdr.mclass = struct.unpack('B', frame[0:1])[0]
	hdr.mid = struct.unpack('B', frame[1:2])[0]
	hdr.length = struct.unpack('<H', frame[2:4])[0]

	return hdr

def frame_parse(frame: bytes):
	_data = None
	_hdr = Hdr(0,0,0)

	while True:
		_hdr = frame_parse_hdr(frame[0:4])
		if _hdr is None:
			break

		if (4+_hdr.length+2) != len(frame):
			break

		_check_soll = frame[-2:]
		_check_ist = checksum(frame[0:-2])
		if _check_soll != _check_ist:
			break

		_data = frame[4:-2]
		break

	return _data, _hdr

class UbxPacketizer:
	def __init__(self, sync_bytes: bytes = UBX_SYNC_WORD):
		self.sync_bytes = sync_bytes
		self.packet = b''
		self.wait_sync = True
		self.sync_count = 0
		self.d_save = b''

	def reset(self):
		self.packet = b''
		self.wait_sync = True
		self.sync_count = 0

	def on_recv(self, msg: bytes):
		frames = []
		for d in msg:
			if d == self.sync_bytes[self.sync_count]:
				self.d_save += struct.pack('B', d)
				self.sync_count += 1
				if self.sync_count == len(self.sync_bytes):
					self.sync_count = 0
					self.d_save = b''
					self.packet = b''
					self.wait_sync = False
			else:
				if self.sync_count > 0:
					self.packet += self.d_save
					self.d_save = b''
					self.sync_count = 0
				if not self.wait_sync:
					self.packet += struct.pack('B', d)
					data, hdr = frame_parse(self.packet)
					if data is not None:
						if hdr.length == len(data):
							frames.append({"cls": hdr.mclass, "id": hdr.mid, "payload": data})
							self.packet = b''
							self.wait_sync = True
		return frames