checksum_calc

#!/usr/bin/env python


def bit_not(n, numbits=16):
    return (1 << numbits) - 1 - n


class ChecksumCalculator:

    def __init__(self, tgt):
        self.tgt = tgt

    def _conv_16bits(self):
        res_list = []
        temp = "".join(self.tgt.split(" "))
        i = 0
        j = 0
        for x in range(len(temp) // 4):
            unit = ""
            while i < 4:
                unit += temp[j]
                i += 1
                j += 1
            res_list.append(unit)
            i = 0
        return res_list[:-1]

    def _calc_sum(self, bits=None):
        s = 0
        if not bits:
            bits=self._conv_16bits()
        for item in bits:
            s += int(item, 16)

        res = hex(s).split('x')[-1]
        res = "000" +res[0] + " " + res[1:]
        return res.upper()

    def combine_inverse(self):
        s = self._calc_sum().split(" ")
        t = "".join(self._calc_sum(s)[3:].split(" "))

        combined = hex(bit_not(int(bin(int(t, 16)), 2))).split('x')[-1].upper()

        return " ".join(self._conv_16bits()) + " " + combined


if __name__ == "__main__":
    pkt_header = input("Please enter a packet header string to process: ").upper()
    job = ChecksumCalculator(pkt_header)
    print("Done! Result is: %s." % job.combine_inverse())