// Live updates: one multiplexed WebSocket with auto-reconnect and a 15 s
// polling fallback signal (§2.2). Subscribers register per channel.
const subs = new Map(); // channel -> Set<fn>
let socket = null;
let attempts = 0;
let fallbackPolling = false;
const pollListeners = new Set();

export function subscribe(channel, fn) {
  if (!subs.has(channel)) subs.set(channel, new Set());
  subs.get(channel).add(fn);
  ensureSocket();
  return () => subs.get(channel).delete(fn);
}

// onPollFallback registers a callback invoked every 15 s while the socket is
// down, so pages can refresh their data without WS.
export function onPollFallback(fn) {
  pollListeners.add(fn);
}

export function send(channel, event, data) {
  if (socket && socket.readyState === WebSocket.OPEN) {
    socket.send(JSON.stringify({ channel, event, data }));
  }
}

let pollTimer = null;
function startPolling() {
  if (fallbackPolling) return;
  fallbackPolling = true;
  pollTimer = setInterval(() => pollListeners.forEach((fn) => fn()), 15000);
}
function stopPolling() {
  fallbackPolling = false;
  if (pollTimer) clearInterval(pollTimer);
}

function ensureSocket() {
  if (socket && socket.readyState <= WebSocket.OPEN) return;
  const proto = window.location.protocol === 'https:' ? 'wss:' : 'ws:';
  socket = new WebSocket(`${proto}//${window.location.host}/ws`);
  socket.addEventListener('open', () => { attempts = 0; stopPolling(); });
  socket.addEventListener('message', (e) => {
    try {
      const msg = JSON.parse(e.data);
      (subs.get(msg.channel) || []).forEach((fn) => fn(msg.event, msg.data));
    } catch (err) { /* ignore malformed frames */ }
  });
  socket.addEventListener('close', () => {
    startPolling();
    attempts += 1;
    const delay = Math.min(30000, 1000 * 2 ** attempts);
    setTimeout(ensureSocket, delay);
  });
  socket.addEventListener('error', () => socket.close());
}