// Subscribes to /ws/events/:id and emits per-message callbacks.
//
// Auto-reconnects with exponential backoff up to 30s. Returns a cleanup
// fn the caller invokes (e.g. inside onUnmounted).

interface WSMessage {
  type: string
  event_id: string
  payload: any
  timestamp: string
}

export function useEventWS(eventId: string, onMessage: (msg: WSMessage) => void) {
  if (import.meta.server) return () => {}

  const config = useRuntimeConfig()
  const base = (config.public.wsBase as string) || ''
  const url = `${base}/ws/events/${eventId}`

  let ws: WebSocket | null = null
  let attempt = 0
  let stopped = false
  let reconnectTimer: ReturnType<typeof setTimeout> | null = null

  function connect() {
    if (stopped) return
    ws = new WebSocket(url)

    ws.onopen = () => {
      attempt = 0
    }
    ws.onmessage = (e) => {
      try {
        const msg = JSON.parse(e.data) as WSMessage
        onMessage(msg)
      } catch {
        /* ignore */
      }
    }
    ws.onclose = () => {
      if (stopped) return
      const backoff = Math.min(30_000, 500 * Math.pow(2, attempt++))
      reconnectTimer = setTimeout(connect, backoff)
    }
    ws.onerror = () => {
      ws?.close()
    }
  }

  connect()

  return function stop() {
    stopped = true
    if (reconnectTimer) clearTimeout(reconnectTimer)
    ws?.close()
  }
}