from datetime import datetime, timedelta
from typing import List, Optional

from analytics.lib.counts import CountStat
from zerver.lib.timestamp import floor_to_day, floor_to_hour, verify_UTC

# If min_length is None, returns end_times from ceiling(start) to floor(end), inclusive.
# If min_length is greater than 0, pads the list to the left.
# So informally, time_range(Sep 20, Sep 22, day, None) returns [Sep 20, Sep 21, Sep 22],
# and time_range(Sep 20, Sep 22, day, 5) returns [Sep 18, Sep 19, Sep 20, Sep 21, Sep 22]
def time_range(start: datetime, end: datetime, frequency: str,
               min_length: Optional[int]) -> List[datetime]:
    verify_UTC(start)
    verify_UTC(end)
    if frequency == CountStat.HOUR:
        end = floor_to_hour(end)
        step = timedelta(hours=1)
    elif frequency == CountStat.DAY:
        end = floor_to_day(end)
        step = timedelta(days=1)
    else:
        raise AssertionError("Unknown frequency: %s" % (frequency,))

    times = []
    if min_length is not None:
        start = min(start, end - (min_length-1)*step)
    current = end
    while current >= start:
        times.append(current)
        current -= step
    return list(reversed(times))