crc

#!/usr/bin/env python

from math import log, ceil


def print_help():
    print("""
Script to generate Cyclic Redundancy Check codeword for
Networks and Internet Systems.

# Usage: `$ ./crc`
    """)


def getCRC(dataword, generator):
    dword = int(dataword, 2)
    l_gen = len(generator)

    # append 0s to dividend
    dividend = dword << (l_gen - 1)
    dividend = divMod2(dividend, generator)

    # finally, AND the initial dividend with the remainder (=dividend)
    codeword = dword if not dividend else (dword << (l_gen-1) | dividend)
    print("CRC:\t\t\t", bin(dividend).lstrip("-0b").zfill(l_gen-1))
    print("Codeword:\t\t", bin(codeword).lstrip("-0b"))


def divMod2(dividend, generator):
    l_gen = len(generator)

    # shift specifies the no. of least significant bits not being XORed
    shift = ceil(log(dividend + 1, 2)) - l_gen

    # ceil(log(dividend+1 , 2)) is the no. of binary
    # digits in dividend
    generator = int(generator, 2)

    while dividend >= generator or shift >= 0:
        # bitwise XOR the MSBs of dividend with generator
        # replace the operated MSBs from the dividend with
        # remainder generated
        rem = (dividend >> shift) ^ generator
        dividend = (dividend & ((1 << shift) - 1)) | (rem << shift)

        # change shift variable
        shift = ceil(log(dividend+1, 2)) - l_gen

    return dividend


def hasError(message, generator):
    msg = int(message, 2)
    l_gen = len(generator)
    r = divMod2(msg, generator)
    remainder = bin(r).lstrip("-0b").zfill(l_gen-1)
    print("Remainder:\t\t", remainder if remainder else None)
    return r != 0


if __name__ == '__main__':
    try:
        op = int(input("1. Find k-1 bit CRC for k bit generator\n" +
                       "2. Check if data received has error\n" +
                       "Choose 1 or 2:\t\t "))
        if op == 1:
            dataword = input("\nMessage data:\t\t ")
            generator = input("Generator:\t\t ")
            getCRC(dataword, generator)
        elif op == 2:
            received = input("\nMessage received:\t ")
            generator = input("Generator:\t\t ")
            print("Error:\t\t\t", hasError(received, generator))
        else:
            print("Wrong input!")
    except Exception as e:
        print(e)
        print_help()