/**
 * Minimal SSE writer for Route Handlers. Each event is one JSON object per `data:` line.
 * Heartbeats keep ngrok / proxies from closing idle connections during long reasoning.
 */
export function sseResponse(run: (emit: (event: unknown) => void, signal: AbortSignal) => Promise<void>, opts: { requestId?: string; heartbeatMs?: number } = {}): Response {
  const encoder = new TextEncoder();
  const abort = new AbortController();
  let closed = false;
  const stream = new ReadableStream<Uint8Array>({
    start(controller) {
      const emit = (event: unknown) => {
        if (closed) return;
        try {
          controller.enqueue(encoder.encode(`data: ${JSON.stringify(event)}\n\n`));
        } catch {
          closed = true;
        }
      };
      const hb = setInterval(() => {
        if (closed) return;
        try {
          controller.enqueue(encoder.encode(`: ping\n\n`));
        } catch {
          closed = true;
        }
      }, opts.heartbeatMs ?? 15_000);
      run(emit, abort.signal)
        .catch((e) => {
          emit({ type: "error", error: { code: "INTERNAL_ERROR", message: (e as Error)?.message ?? "Internal error", retryable: false } });
        })
        .finally(() => {
          clearInterval(hb);
          if (!closed) {
            closed = true;
            try {
              controller.close();
            } catch {
              /* already closed */
            }
          }
        });
    },
    cancel() {
      closed = true;
      abort.abort();
    },
  });
  return new Response(stream, {
    headers: {
      "Content-Type": "text/event-stream; charset=utf-8",
      "Cache-Control": "no-cache, no-transform",
      Connection: "keep-alive",
      "X-Accel-Buffering": "no",
      ...(opts.requestId ? { "X-Request-Id": opts.requestId } : {}),
    },
  });
}