/**
 * AG-UI-style SSE event helpers.
 *
 * We don't implement the full AG-UI protocol — we use a simplified event set
 * that maps cleanly to our chat UX:
 *
 *   text_delta       — append text to the current assistant message
 *   tool_start       — model is calling a tool (renders collapsible block)
 *   tool_result      — local handler finished (fills the block)
 *   navigate         — render a clickable navigation button inline
 *   done             — terminal event; carries usage + final assistant message id
 *   error            — terminal event; carries error detail
 *
 * Each is emitted as one SSE message:
 *
 *   event: <name>
 *   data:  <json>
 *
 *   <blank line>
 */
export type AGUIEvent =
  | { type: "text_delta"; delta: string }
  | { type: "tool_start"; id: string; name: string; args: Record<string, unknown> }
  | { type: "tool_result"; id: string; result: unknown; durationMs?: number }
  | { type: "navigate"; target: string; label: string }
  | { type: "done"; provider: string; model: string; usage?: Record<string, unknown>; messageId?: string }
  | { type: "error"; message: string };

/**
 * Encode an event into the byte-stream chunks expected by the SSE protocol.
 */
export function encodeEvent(ev: AGUIEvent): Uint8Array {
  const enc = new TextEncoder();
  const json = JSON.stringify(ev);
  return enc.encode(`event: ${ev.type}\ndata: ${json}\n\n`);
}

/**
 * Helper that creates a ReadableStream + a typed `emit()` callback to push
 * events into it. Caller closes the stream by calling `emit({type:"done"})` or
 * `close()`.
 */
export function createEventStream(): {
  stream: ReadableStream<Uint8Array>;
  emit: (ev: AGUIEvent) => void;
  close: () => void;
} {
  let controller!: ReadableStreamDefaultController<Uint8Array>;
  const stream = new ReadableStream<Uint8Array>({
    start(c) {
      controller = c;
    },
  });
  return {
    stream,
    emit(ev) {
      try {
        controller.enqueue(encodeEvent(ev));
      } catch { /* stream closed */ }
    },
    close() {
      try { controller.close(); } catch { /* already closed */ }
    },
  };
}