checksum-xmodem.py

#!/usr/bin/env python
# -*- coding: utf8 -*-

# CRC CCITT
#
# comes in 3 flavous
# (XModem)  starting value: 0x0000
#           starting value: 0xffff
#           starting value: 0x1d0f
# 
# Cristian NAVALICI cristian.navalici at gmail dot com


from ctypes import c_ushort


def calc_polynomial(list_):
    r = 0
    for p in list_:
        r += 2 ** p
    return r


class CRCCCITT32(object):
    crc_ccitt_tab = []
    #polynomials = (32, 26, 23, 22, 16, 12, 11, 10, 8, 7, 5, 4, 2, 1, 0)
    #crc_ccitt_constant = calc_polynomial(polynomials[1:]) # 0x04c11db7
    crc_ccitt_constant = 0x1021

    def __init__(self):
        self.starting_value = 0x0000
        if not self.crc_ccitt_tab:
            self.init_crc_ccitt() # initialize the precalculated tables

    def calculate(self, string = ''):
        if not isinstance(string, str):
            raise Exception("Please provide a string as argument for calculation.")
        if not string:
            return 0
        crcValue = self.starting_value
        for c in string:
            tmp = (c_ushort(crcValue >> 8).value) ^ ord(c)
            crcValue = (c_ushort(crcValue << 8).value) ^ int(self.crc_ccitt_tab[tmp], 0)
        return crcValue

    def init_crc_ccitt(self):
        '''The algorithm use tables with precalculated values'''
        for i in range(0, 256):
            crc = 0
            c = i << 8
            for j in range(0, 8):
                if ((crc ^ c) & 0x8000):
                    crc = c_ushort(crc << 1).value ^ self.crc_ccitt_constant
                else:
                    crc = c_ushort(crc << 1).value
                c = c_ushort(c << 1).value # equiv c = c << 1
            self.crc_ccitt_tab.append(hex(crc))


crc = CRCCCITT32()
x = '\x04\x03\x02\x01\x08\x07\x06\x05'
crc.calculate(x)
r = crc.calculate(x)
print(hex(r))