export const CHANNEL = 'mai-call-sync';

export type SyncMessage =
  | { type: 'queue-update'; pendingCount: number }
  | { type: 'synced'; clientRequestId: string }
  | { type: 'dead-letter'; clientRequestId: string };

let _ch: BroadcastChannel | null = null;

function ch(): BroadcastChannel | null {
  if (typeof window === 'undefined') return null;
  if (!_ch) _ch = new BroadcastChannel(CHANNEL);
  return _ch;
}

export function broadcast(msg: SyncMessage): void {
  ch()?.postMessage(msg);
}

export function subscribeBroadcast(handler: (msg: SyncMessage) => void): () => void {
  const c = ch();
  if (!c) return () => {};
  const listener = (e: MessageEvent<SyncMessage>) => handler(e.data);
  c.addEventListener('message', listener);
  return () => c.removeEventListener('message', listener);
}