Method __construct_byset

src/dateutil/rrule.py:1032–1077

If a `BYXXX` sequence is passed to the constructor at the same level as `FREQ` (e.g. `FREQ=HOURLY,BYHOUR={2,4,7},INTERVAL=3`), there are some specifications which cannot be reached given some starting conditions. This occurs whenever the interval is not coprime with

(self, start, byxxx, base)

                    ii.rebuild(year, month)

    def __construct_byset(self, start, byxxx, base):
        """
        If a `BYXXX` sequence is passed to the constructor at the same level as
        `FREQ` (e.g. `FREQ=HOURLY,BYHOUR={2,4,7},INTERVAL=3`), there are some
        specifications which cannot be reached given some starting conditions.

        This occurs whenever the interval is not coprime with the base of a
        given unit and the difference between the starting position and the
        ending position is not coprime with the greatest common denominator
        between the interval and the base. For example, with a FREQ of hourly
        starting at 17:00 and an interval of 4, the only valid values for
        BYHOUR would be {21, 1, 5, 9, 13, 17}, because 4 and 24 are not
        coprime.

        :param start:
            Specifies the starting position.
        :param byxxx:
            An iterable containing the list of allowed values.
        :param base:
            The largest allowable value for the specified frequency (e.g.
            24 hours, 60 minutes).

        This does not preserve the type of the iterable, returning a set, since
        the values should be unique and the order is irrelevant, this will
        speed up later lookups.

        In the event of an empty set, raises a :exception:`ValueError`, as this
        results in an empty rrule.
        """

        cset = set()

        # Support a single byxxx value.
        if isinstance(byxxx, integer_types):
            byxxx = (byxxx, )

        for num in byxxx:
            i_gcd = gcd(self._interval, base)
            # Use divmod rather than % because we need to wrap negative nums.
            if i_gcd == 1 or divmod(num - start, i_gcd)[1] == 0:
                cset.add(num)

        if len(cset) == 0:
            raise ValueError("Invalid rrule byxxx generates an empty set.")

        return cset

    def __mod_distance(self, value, byxxx, base):
        """