"""Server-Sent Event bus for pushing updates to displays.

Events are scoped by user_id so that each user's display only receives
updates about their own data (alarms, calendar, pomodoro, ...).
"""
import json
import queue
import threading


class EventBus:
    """Simple pub/sub event system with SSE support.

    publish(event_type, data, user_id=None): when user_id is given, only
    subscribers registered for that user receive the event.
    """

    def __init__(self):
        self._subscribers: list[tuple[int | None, queue.Queue]] = []
        self._lock = threading.Lock()

    def subscribe(self, user_id: int | None = None) -> queue.Queue:
        """Subscribe to events. user_id=None receives ALL events (broadcast)."""
        q: queue.Queue = queue.Queue()
        with self._lock:
            self._subscribers.append((user_id, q))
        return q

    def unsubscribe(self, q: queue.Queue):
        with self._lock:
            self._subscribers = [(uid, sq) for uid, sq in self._subscribers if sq is not q]

    def publish(self, event_type: str, data: dict, user_id: int | None = None):
        msg = json.dumps({"type": event_type, "data": data})
        with self._lock:
            dead = []
            for sub_uid, q in self._subscribers:
                if sub_uid is not None and user_id is not None and sub_uid != user_id:
                    continue
                try:
                    q.put_nowait(msg)
                except queue.Full:
                    dead.append((sub_uid, q))
            for item in dead:
                self._subscribers.remove(item)