"""Slot generation for the booking module (#16).

Pure functions only -- no I/O. Callers (booking_api.py) fetch a resource's
hours/existing bookings via booking_db.py and pass them in here, which keeps
this module trivially unit-testable against hand-computed expectations,
including across a DST transition.
"""
from datetime import date, datetime, timedelta, timezone
from zoneinfo import ZoneInfo


def generate_slots(hours_by_weekday, duration_minutes, date_from, date_to,
                    tz_name, now, min_notice_minutes=60, max_advance_days=30,
                    buffer_minutes=0, busy=()):
    """Return a sorted list of tz-aware UTC datetimes, one per bookable slot
    start, for [date_from, date_to] inclusive.

    hours_by_weekday: {0..6: (opens_at time, closes_at time)}, 0=Monday,
        matching date.weekday(); a missing weekday means closed that day.
    tz_name: IANA zone the hours are local to (e.g. "Europe/Berlin").
    now: tz-aware datetime "now" is measured from, for min-notice/max-advance.
    busy: iterable of (start, end) tz-aware datetimes already booked on this
        resource -- a candidate slot within buffer_minutes of one is dropped.
    """
    tz = ZoneInfo(tz_name)
    duration = timedelta(minutes=duration_minutes)
    buffer_td = timedelta(minutes=buffer_minutes)
    earliest = now + timedelta(minutes=min_notice_minutes)
    latest = now + timedelta(days=max_advance_days)
    busy_utc = [(s.astimezone(timezone.utc), e.astimezone(timezone.utc)) for s, e in busy]

    slots = []
    day = date_from
    while day <= date_to:
        hours = hours_by_weekday.get(day.weekday())
        if hours:
            opens_at, closes_at = hours
            cursor = datetime.combine(day, opens_at, tzinfo=tz)
            close = datetime.combine(day, closes_at, tzinfo=tz)
            while cursor + duration <= close:
                start_utc = cursor.astimezone(timezone.utc)
                end_utc = (cursor + duration).astimezone(timezone.utc)
                if (earliest <= start_utc <= latest
                        and not _conflicts(start_utc, end_utc, busy_utc, buffer_td)):
                    slots.append(start_utc)
                cursor += duration
        day += timedelta(days=1)
    return slots


def _conflicts(start, end, busy_utc, buffer_td):
    for b_start, b_end in busy_utc:
        if start < b_end + buffer_td and end + buffer_td > b_start:
            return True
    return False