from datetime import datetime
from dateutil.relativedelta import relativedelta


def keep_date(now, snapshots, date_convert, delta, count):
    keep = set()
    if count < 1:
        return keep
    start_date = date_convert(now) - delta*(count-1)
    for snapshot in snapshots:
        if snapshot.date() >= start_date:
            keep.add(snapshot)
            start_date = date_convert(snapshot) + delta
        if start_date > now.date():
            break
    return keep


def keep_year(now, snapshots, count):
    return keep_date(now, snapshots, lambda d: d.date().replace(day=1, month=1), relativedelta(years=1), count)


def keep_month(now, snapshots, count):
    return keep_date(now, snapshots, lambda d: d.date().replace(day=1), relativedelta(months=1), count)


def keep_day(now, snapshots, count):
    return keep_date(now, snapshots, lambda d: d.date(), relativedelta(days=1), count)


def keep_time(now, snapshots, convert, delta, count):
    keep = set()
    if count < 1:
        return keep
    start = convert(now) - delta*(count-1)
    for snapshot in snapshots:
        if snapshot >= start:
            keep.add(snapshot)
            start = convert(snapshot) + delta
        if start > now:
            break
    return keep


def keep_hour(now, snapshots, count):
    return keep_time(now, snapshots, lambda d: d.replace(minute=0, second=0, microsecond=0), relativedelta(hours=1),
                     count)


def keep_frequent(now, snapshots, count):
    return keep_time(now, snapshots, lambda d: d.replace(minute=d.minute//15*15, second=0, microsecond=0),
                     relativedelta(minutes=15), count)


def snapshots_to_keep(now, snapshots, yearly=0, monthly=0, weekly=0, daily=0, hourly=0, frequently=0):
    snapshots.sort()
    keep = set()
    keep.union(keep_year(now, snapshots, yearly))
    keep.union(keep_month(now, snapshots, monthly))
    keep.union(keep_day(now, snapshots, daily))
    return keep


def main():
    print("Hello, world!")