# steward/core/weekly.py
"""A weekly slot: a weekday and a time in a time zone, with a window after it. A
slot fires once, and only while its window is open, so a Steward that was down
over the slot starts late rather than a day late. A schedule is anything with
weekday, at_time, time_zone, window_minutes, enabled, last_slot_at and created_at."""
from __future__ import annotations

from datetime import datetime, time, timedelta, timezone

from steward.core.formatting import zone

WEEKDAYS = ("Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday")


def parse_at(at_time: str) -> time:
    h, m = (int(x) for x in at_time.split(":", 1))
    if not (0 <= h < 24 and 0 <= m < 60):
        raise ValueError(at_time)
    return time(h, m)


def last_slot(plan, now: datetime) -> datetime:
    """The most recent scheduled time at or before `now`, in UTC."""
    tz = zone(plan.time_zone)
    local = now.astimezone(tz)
    day = local.date() - timedelta(days=(local.weekday() - plan.weekday) % 7)
    slot = datetime.combine(day, parse_at(plan.at_time), tzinfo=tz)
    if slot > local:
        slot -= timedelta(days=7)
    return slot.astimezone(timezone.utc)


def fired_slot(plan) -> datetime | None:
    """The last slot that fired, or None while none has. A new schedule's
    last_slot_at holds the moment it was made, so its first slot is the
    next one rather than one already past; that moment is no slot."""
    at = plan.last_slot_at
    if at is None:
        return None
    return at if last_slot(plan, at) == at else None


def next_slot(plan, now: datetime) -> datetime:
    return last_slot(plan, now) + timedelta(days=7)


def is_due(plan, now: datetime) -> datetime | None:
    """The slot to fire now, or None."""
    if not plan.enabled:
        return None
    slot = last_slot(plan, now)
    since = plan.last_slot_at or plan.created_at
    if since is not None and slot <= since:
        return None
    if now - slot >= timedelta(minutes=plan.window_minutes):
        return None
    return slot


def describe(plan) -> str:
    """"Sundays at 03:00 (Europe/London), up to 4 h" for a page."""
    hours = plan.window_minutes / 60
    span = f"{hours:g} h" if plan.window_minutes % 60 == 0 else f"{plan.window_minutes} min"
    return f"{WEEKDAYS[plan.weekday]}s at {plan.at_time} ({plan.time_zone}), up to {span}"