#!/usr/bin/env python

import base64
import hmac
import re
import struct
import sys
import time
from dataclasses import dataclass
import defl
from defl import cl
from defl import log
from dataclasses import dataclass, field

@dataclass(slots=True, kw_only=True, frozen=False)
class OTP:
    cert: str = field(kw_only=False, repr=True)

    def __post_init__(_):
        _.cert = re.compile(r'\s', flags=re.I).sub(r'', _.cert)
        assert re.compile(r'^[0-9A-Za-z]+$', flags=re.I).search(_.cert)

    @staticmethod
    def _hotp(key, counter, digits=6, digest='sha1'):
        key = base64.b32decode(key.upper() + '=' * ((8 - len(key)) % 8))
        counter = struct.pack('>Q', counter)
        mac = hmac.new(key, counter, digest).digest()
        offset = mac[-1] & 0x0f
        binary = struct.unpack('>L', mac[offset:offset + 4])[0] & 0x7fffffff
        return str(binary)[-digits:].rjust(digits, '0')

    @staticmethod
    def _totp(key, time_step=30, digits=6, digest='sha1'):
        return OTP._hotp(key, int(time.time() / time_step), digits, digest)

    def generate(_):
        return OTP._totp(_.cert)

if __name__ == '__main__':
    res = OTP(cert=sys.argv[1])
    log.info(res.generate())