export function connectWs(onMessage) {
  let ws, retry = 0, closed = false;
  function open() {
    const proto = location.protocol === 'https:' ? 'wss' : 'ws';
    ws = new WebSocket(`${proto}://${location.host}/ws`);
    ws.addEventListener('open', () => { retry = 0; });
    ws.addEventListener('message', (ev) => {
      try { onMessage(JSON.parse(ev.data)); } catch {}
    });
    ws.addEventListener('close', () => {
      if (closed) return;
      retry = Math.min(retry + 1, 6);
      setTimeout(open, 500 * 2 ** retry);
    });
    ws.addEventListener('error', () => ws.close());
  }
  open();
  return {
    send(msg) { if (ws?.readyState === WebSocket.OPEN) ws.send(JSON.stringify(msg)); },
    close()   { closed = true; ws?.close(); }
  };
}