"""Devices that announced they went offline.

The firmware's LWT (unclean disconnect) and its graceful-disconnect message both
publish {"state": "offline", "ok": false} to status/heartbeat, retained. Online
status is otherwise "a heartbeat newer than 90s", which would keep showing a
device that just dropped as online for up to 90s. This remembers the marker so
/mqtt/status and the ping loop treat the device as offline right away.

In-memory only, which is enough: on backend start the broker replays each
device's retained heartbeat, so a device whose last word was "offline" gets
re-marked, and any live heartbeat clears the mark.
"""

_offline: set[str] = set()


def mark_offline(serial: str) -> None:
    _offline.add(serial)


def mark_alive(serial: str) -> None:
    _offline.discard(serial)


def is_marked_offline(serial: str) -> bool:
    return serial in _offline