rfc1982_serial_number/rfc1982_serial_number.py

#!/usr/bin/env python3

class Serial:

    modulus = 2 ** 32

    def __init__(self, val):
        self._val = int(val)
        if self._val < 0 or self._val >= self.modulus:
            raise ValueError

    def __add__(self, val):
        val = int(val)
        if val < 0 or val > (self.modulus - 1) >> 1:
            raise ValueError
        return type(self)((self._val + val) & (self.modulus - 1))

    def __le__(self, other):
        return (self.modulus - int(self) + int(other)) & (self.modulus >> 1) == 0

    def __ge__(self, other):
        return (self.modulus + int(self) - int(other)) & (self.modulus >> 1) == 0

    def __eq__(self, other):
        return int(self) == int(other)

    def __ne__(self, other):
        return int(self) != int(other)

    def __lt__(self, other):
        return self != other and self <= other

    def __gt__(self, other):
        return self != other and self >= other

    def __int__(self):
        return self._val

    def __str__(self):
        return f"{int(self):{len(str(self.modulus))}d}"

def find_intermediate(start, finish):
    if start < finish:
        return None
    else:
        wrap = start + ((Serial.modulus >> 1) - 2)
        assert start < wrap and wrap < finish
        return wrap

def main():
    from argparse import ArgumentParser

    ap = ArgumentParser()
    ap.add_argument("start", type = Serial)
    ap.add_argument("finish", type = Serial)
    args = ap.parse_args()

    wrap = find_intermediate(args.start, args.finish)

    print(f"Start at {args.start!s}")

    if wrap is None:
        print("No step needed")
    else:
        print(f"Step via {wrap!s}")

    print(f"End at   {args.finish!s}")

if __name__ == "__main__":
    main()