"""Calendar recurrence expansion across a DST boundary (SPEC §A.3, acceptance A.7.2)."""

from __future__ import annotations

from datetime import UTC, datetime, timedelta
from zoneinfo import ZoneInfo

from webmail_plugin_calendar.recurrence import expand_occurrences  # type: ignore[import-not-found]

WARSAW = ZoneInfo("Europe/Warsaw")


def _local(occ: list[tuple[datetime, datetime]]) -> dict[str, datetime]:
    # Map "YYYY-MM-DD" (Warsaw date) -> the UTC start of that occurrence.
    return {s.astimezone(WARSAW).date().isoformat(): s for s, _ in occ}


def test_weekly_recurrence_keeps_local_time_across_dst() -> None:
    # Monday 2026-03-23 10:00 Europe/Warsaw == 09:00 UTC (UTC+1, before DST).
    # DST starts 2026-03-29, so from 03-30 the offset is UTC+2.
    dtstart = datetime(2026, 3, 23, 9, 0, tzinfo=UTC)
    occ = expand_occurrences(
        dtstart=dtstart,
        duration=timedelta(hours=1),
        rrule="FREQ=WEEKLY;BYDAY=MO",
        exdates=[],
        range_start=datetime(2026, 3, 1, tzinfo=UTC),
        range_end=datetime(2026, 4, 30, tzinfo=UTC),
        tz="Europe/Warsaw",
    )
    by_date = _local(occ)

    # Every occurrence is at 10:00 local wall-clock, on both sides of the DST change.
    for start, _ in occ:
        assert start.astimezone(WARSAW).strftime("%H:%M") == "10:00"

    # ...but the UTC offset moves across the boundary.
    assert by_date["2026-03-23"].hour == 9  # UTC+1
    assert by_date["2026-03-30"].hour == 8  # UTC+2 (DST)
    assert by_date["2026-04-06"].hour == 8


def test_exdates_remove_instances() -> None:
    dtstart = datetime(2026, 3, 23, 9, 0, tzinfo=UTC)
    occ = expand_occurrences(
        dtstart=dtstart,
        duration=timedelta(hours=1),
        rrule="FREQ=WEEKLY;BYDAY=MO",
        # Exclude the 2026-03-30 occurrence (10:00 Warsaw == 08:00 UTC under DST).
        exdates=[datetime(2026, 3, 30, 8, 0, tzinfo=UTC)],
        range_start=datetime(2026, 3, 1, tzinfo=UTC),
        range_end=datetime(2026, 4, 30, tzinfo=UTC),
        tz="Europe/Warsaw",
    )
    dates = set(_local(occ))
    assert "2026-03-23" in dates
    assert "2026-03-30" not in dates  # excluded
    assert "2026-04-06" in dates


def test_single_event_no_rrule() -> None:
    dtstart = datetime(2026, 6, 1, 12, 0, tzinfo=UTC)
    occ = expand_occurrences(
        dtstart=dtstart,
        duration=timedelta(minutes=30),
        rrule=None,
        exdates=[],
        range_start=datetime(2026, 6, 1, tzinfo=UTC),
        range_end=datetime(2026, 6, 2, tzinfo=UTC),
        tz="UTC",
    )
    assert occ == [(dtstart, dtstart + timedelta(minutes=30))]

    # Outside the window -> nothing.
    empty = expand_occurrences(
        dtstart=dtstart,
        duration=timedelta(minutes=30),
        rrule=None,
        exdates=[],
        range_start=datetime(2026, 7, 1, tzinfo=UTC),
        range_end=datetime(2026, 7, 2, tzinfo=UTC),
        tz="UTC",
    )
    assert empty == []