CRC Computation

crc.py

ATM = 0b100000111
AAL = 0b11000110011
CCITT41 = 0b10001000000100001
IEEE802 = 0b100000100110000010001010110110101

def compute_crc(frame: bytes, crc: int = 0, poly: int = ATM) -> int:
	"""Compute the CRC of the given frame

	>>> crc = compute_crc(b"Hello world !")
	>>> compute_crc(b"Hello world !", crc) == 0
	True
	>>> compute_crc(b"Good bye !", crc) != 0
	True
	"""

	window = 0
	byteno = 0
	end = False

	while True:
		while window.bit_length() < poly.bit_length():
			if byteno < len(frame):
				window = (window << 8) | frame[byteno]
				byteno += 1

			elif end:
				return window

			else:
				end = True
				window = (window << (poly.bit_length() - 1)) | crc

		rest_window_length = window.bit_length() - poly.bit_length()
		crc_window = window >> rest_window_length
		rest_window = window & ~(~0 << rest_window_length)
		window = ((crc_window ^ poly) << rest_window_length) | rest_window


if __name__ == "__main__":
	from doctest import testmod
	testmod()