/**
 * A1 阶段的占位引擎：把用户输入回显成一条助手消息，用来打通 HTTP / SSE 链路。
 *
 * 它刻意不做任何「理解」，也不调用 Provider——避免把假能力写进真实链路。
 * A3.1 定义正式 AgentRuntime、A3.2 写 FakeRuntime 之后，本文件整体替换。
 */
import type { EventBus } from './events.js';

export interface EchoRunInput {
  sessionId: string;
  runId: string;
  messageId: string;
  text: string;
  bus: EventBus;
  signal: AbortSignal;
}

export interface EchoRunResult {
  text: string;
  finishReason: 'stop' | 'cancelled';
}

const CHUNK_SIZE = 6;
const CHUNK_DELAY_MS = 25;

const sleep = (ms: number): Promise<void> => new Promise((resolve) => setTimeout(resolve, ms));

export async function runEcho(input: EchoRunInput): Promise<EchoRunResult> {
  const { bus, signal, sessionId, runId, messageId } = input;
  const replyMessageId = `${messageId}-reply`;
  const reply = `（A1 占位引擎）收到：${input.text}`;
  let emitted = '';

  bus.publish({ sessionId, runId, type: 'run.state', data: { state: 'running' } });

  for (let index = 0; index < reply.length; index += CHUNK_SIZE) {
    if (signal.aborted) return finish(true);
    const piece = reply.slice(index, index + CHUNK_SIZE);
    emitted += piece;
    bus.publish({ sessionId, runId, type: 'message.delta', data: { messageId: replyMessageId, text: piece } });
    await sleep(CHUNK_DELAY_MS);
  }

  if (signal.aborted) return finish(true);
  bus.publish({ sessionId, runId, type: 'message.completed', data: { messageId: replyMessageId, finishReason: 'stop' } });
  bus.publish({ sessionId, runId, type: 'run.state', data: { state: 'idle' } });
  return { text: emitted, finishReason: 'stop' };

  function finish(cancelled: boolean): EchoRunResult {
    // 取消路径：cancelling → 一条带 cancelled 结束原因的消息 → 回到 idle。
    // 'cancelled' 这个运行状态当前不使用（A3.1 定稿再确认语义）。
    bus.publish({ sessionId, runId, type: 'run.state', data: { state: 'cancelling' } });
    bus.publish({
      sessionId,
      runId,
      type: 'message.completed',
      data: { messageId: replyMessageId, finishReason: cancelled ? 'cancelled' : 'stop' },
    });
    bus.publish({ sessionId, runId, type: 'run.state', data: { state: 'idle' } });
    return { text: emitted, finishReason: 'cancelled' };
  }
}