import datetime

import arrow


def diff_month(d1, d2):
    return (d1.year - d2.year) * 12 + d1.month - d2.month


def calc_current_period(start_date, period, now=None, unit='months'):
    now = now or datetime.datetime.now()

    now = arrow.get(now)

    if unit == 'months':
        if now.day < start_date.day:
            now = now.shift(months=-1)

        start_date = arrow.get(start_date)

        remainder = diff_month(now, start_date) % period

        current_start_date = now.shift(months=-remainder).replace(day=start_date.day)
        current_end_date = now.shift(months=period - remainder).replace(day=start_date.day)
        return current_start_date.date(), current_end_date.date()
    elif unit == 'days':
        start_date = arrow.get(start_date)

        remainder = (now - start_date).days % period

        current_start_date = now.shift(days=-remainder)
        current_end_date = now.shift(days=period - remainder)
        return current_start_date.date(), current_end_date.date()
    else:
        raise ValueError


# now 2.6
class NewDateTime(datetime.datetime):
    @classmethod
    def now(cls, tz=None):
        return cls(2018, 2, 6)


datetime.datetime = NewDateTime

assert calc_current_period(datetime.datetime(2018, 2, 7), 1) == (datetime.date(2018, 1, 7), datetime.date(2018, 2, 7))
assert calc_current_period(datetime.datetime(2018, 2, 6), 1) == (datetime.date(2018, 2, 6), datetime.date(2018, 3, 6))
assert calc_current_period(datetime.datetime(2018, 2, 5), 1) == (datetime.date(2018, 2, 5), datetime.date(2018, 3, 5))
assert calc_current_period(datetime.datetime(2018, 2, 7), 2) == (datetime.date(2017, 12, 7), datetime.date(2018, 2, 7))
assert calc_current_period(datetime.datetime(2018, 2, 6), 2) == (datetime.date(2018, 2, 6), datetime.date(2018, 4, 6))
assert calc_current_period(datetime.datetime(2018, 2, 5), 2) == (datetime.date(2018, 2, 5), datetime.date(2018, 4, 5))
assert calc_current_period(datetime.datetime(2018, 1, 7), 1) == (datetime.date(2018, 1, 7), datetime.date(2018, 2, 7))
assert calc_current_period(datetime.datetime(2018, 1, 6), 1) == (datetime.date(2018, 2, 6), datetime.date(2018, 3, 6))
assert calc_current_period(datetime.datetime(2018, 1, 5), 1) == (datetime.date(2018, 2, 5), datetime.date(2018, 3, 5))
assert calc_current_period(datetime.datetime(2018, 1, 7), 2) == (datetime.date(2018, 1, 7), datetime.date(2018, 3, 7))
assert calc_current_period(datetime.datetime(2018, 1, 6), 2) == (datetime.date(2018, 1, 6), datetime.date(2018, 3, 6))
assert calc_current_period(datetime.datetime(2018, 1, 5), 2) == (datetime.date(2018, 1, 5), datetime.date(2018, 3, 5))
assert calc_current_period(datetime.datetime(2018, 3, 7), 2) == (datetime.date(2018, 1, 7), datetime.date(2018, 3, 7))
assert calc_current_period(datetime.datetime(2018, 3, 6), 2) == (datetime.date(2018, 1, 6), datetime.date(2018, 3, 6))
assert calc_current_period(datetime.datetime(2018, 3, 5), 2) == (datetime.date(2018, 1, 5), datetime.date(2018, 3, 5))

assert calc_current_period(datetime.datetime(2018, 2, 6), 1, unit='days') == (
    datetime.date(2018, 2, 6), datetime.date(2018, 2, 7))
assert calc_current_period(datetime.datetime(2018, 2, 5), 1, unit='days') == (
    datetime.date(2018, 2, 6), datetime.date(2018, 2, 7))
assert calc_current_period(datetime.datetime(2018, 2, 7), 1, unit='days') == (
    datetime.date(2018, 2, 6), datetime.date(2018, 2, 7))

assert calc_current_period(datetime.datetime(2018, 1, 6), 1, unit='days') == (
    datetime.date(2018, 2, 6), datetime.date(2018, 2, 7))
assert calc_current_period(datetime.datetime(2018, 1, 5), 1, unit='days') == (
    datetime.date(2018, 2, 6), datetime.date(2018, 2, 7))
assert calc_current_period(datetime.datetime(2018, 1, 7), 1, unit='days') == (
    datetime.date(2018, 2, 6), datetime.date(2018, 2, 7))

assert calc_current_period(datetime.datetime(2018, 3, 6), 1, unit='days') == (
    datetime.date(2018, 2, 6), datetime.date(2018, 2, 7))
assert calc_current_period(datetime.datetime(2018, 3, 5), 1, unit='days') == (
    datetime.date(2018, 2, 6), datetime.date(2018, 2, 7))
assert calc_current_period(datetime.datetime(2018, 3, 7), 1, unit='days') == (
    datetime.date(2018, 2, 6), datetime.date(2018, 2, 7))

assert calc_current_period(datetime.datetime(2018, 2, 6), 2, unit='days') == (
    datetime.date(2018, 2, 6), datetime.date(2018, 2, 8))
assert calc_current_period(datetime.datetime(2018, 2, 5), 2, unit='days') == (
    datetime.date(2018, 2, 5), datetime.date(2018, 2, 7))
assert calc_current_period(datetime.datetime(2018, 2, 7), 2, unit='days') == (
    datetime.date(2018, 2, 5), datetime.date(2018, 2, 7))

assert calc_current_period(datetime.datetime(2018, 1, 6), 2, unit='days') == (
    datetime.date(2018, 2, 5), datetime.date(2018, 2, 7))
assert calc_current_period(datetime.datetime(2018, 1, 5), 2, unit='days') == (
    datetime.date(2018, 2, 6), datetime.date(2018, 2, 8))
assert calc_current_period(datetime.datetime(2018, 1, 7), 2, unit='days') == (
    datetime.date(2018, 2, 6), datetime.date(2018, 2, 8))

assert calc_current_period(datetime.datetime(2018, 3, 6), 2, unit='days') == (
    datetime.date(2018, 2, 6), datetime.date(2018, 2, 8))
assert calc_current_period(datetime.datetime(2018, 3, 5), 2, unit='days') == (
    datetime.date(2018, 2, 5), datetime.date(2018, 2, 7))
assert calc_current_period(datetime.datetime(2018, 3, 7), 2, unit='days') == (
    datetime.date(2018, 2, 5), datetime.date(2018, 2, 7))