def _encode_message(cmd):
"""Add message preamble and calculate checksum, add padding."""
length = len(cmd) + _STRUCT_PREAMBLE.size + _STRUCT_CODA.size
preamble = _STRUCT_PREAMBLE.pack(_STX, length)
message = preamble + cmd + bytes((_ETX, ))
checksum = _STRUCT_CHECKSUM.pack(lifescan.crc_ccitt(message))
# Pad the message to match the size of the register.
return message + checksum + bytes(_REGISTER_SIZE - 2 - len(message))
def _extract_message(register):
"""Parse the message preamble and verify checksums."""
stx, length = _STRUCT_PREAMBLE.unpack_from(register)
if stx != _STX:
raise lifescan.MalformedCommand('invalid STX byte: %02x' % stx)
if length > _REGISTER_SIZE:
raise lifescan.MalformedCommand('invalid length: %d > REGISTER_SIZE' %
length)
# 2 is the length of the checksum, so it should be ignored.
calculated_checksum = lifescan.crc_ccitt(register[:(length - 2)])
coda_offset = length - _STRUCT_CODA.size
etx, encoded_checksum = _STRUCT_CODA.unpack_from(register[coda_offset:])
if etx != _ETX:
raise lifescan.MalformedCommand('invalid ETX byte: %02x' % etx)
if encoded_checksum != calculated_checksum:
raise exceptions.InvalidChecksum(encoded_checksum, calculated_checksum)
response = register[_STRUCT_PREAMBLE.size:coda_offset]
return response
def checksum(self):
return lifescan.crc_ccitt(self.cmd[:_IDX_CHECKSUM].tobytes())
def test_crc_array(self):
cmd_array = array.array('B', b'\x02\x06\x08\x03')
self.assertEqual(
0x62C2,
lifescan.crc_ccitt(cmd_array))
def test_crc(self):
self.assertEqual(
0x41cd,
lifescan.crc_ccitt(b'\x02\x06\x06\x03'))
def test_crc_array(self):
cmd_array = array.array('B', b'\x02\x06\x08\x03')
self.assertEqual(0x62C2, lifescan.crc_ccitt(cmd_array))
def test_crc(self):
self.assertEqual(0x41cd, lifescan.crc_ccitt(b'\x02\x06\x06\x03'))
def test_crc_array(self):
cmd_array = array.array("B", b"\x02\x06\x08\x03")
self.assertEqual(0x62C2, lifescan.crc_ccitt(cmd_array))
def test_crc(self):
self.assertEqual(0x41CD, lifescan.crc_ccitt(b"\x02\x06\x06\x03"))
def testCrc(self):
self.assertEqual(0x41cd, lifescan.crc_ccitt(b'\x02\x06\x06\x03'))
cmd_array = array.array('B', b'\x02\x06\x08\x03')
self.assertEqual(0x62C2, lifescan.crc_ccitt(cmd_array))
