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# -*- coding: utf-8 -*-
"""Common utility functions for LifeScan meters."""
__author__ = 'Diego Elio Pettenò'
__email__ = 'flameeyes@flameeyes.eu'
__copyright__ = 'Copyright © 2013, Diego Elio Pettenò'
__license__ = 'MIT'
from glucometerutils import exceptions
class MissingChecksum(exceptions.InvalidResponse):
"""The response misses the expected 4-digits checksum."""
def __init__(self, response):
self.message = 'Response is missing checksum: %s' % response
class InvalidSerialNumber(exceptions.Error):
"""The serial number is not as expected."""
def __init__(self, serial_number):
self.message = 'Serial number %s is invalid.' % serial_number
class MalformedCommand(exceptions.InvalidResponse):
def __init__(self, message):
exceptions.InvalidResponse.__init__(
self, 'Malformed command: %s' % message)
def crc_ccitt(data):
"""Calculate the CRC-16-CCITT with LifeScan's common seed.
Args:
data: (bytes) the data to calculate the checksum of
Returns:
(int) The 16-bit integer value of the CRC-CCITT calculated.
This function uses the non-default 0xFFFF seed as used by multiple
LifeScan meters.
"""
crc = 0xffff
for byte in data:
crc = (crc >> 8) & 0xffff | (crc << 8) & 0xffff
crc ^= byte
crc ^= (crc & 0xff) >> 4
crc ^= (((crc << 8) & 0xffff) << 4) & 0xffff
crc ^= (crc & 0xff) << 5
return (crc & 0xffff)
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