import type { SendMessageResponse } from "@duoc-thu/shared-types";
import { buildMockResponse } from "./mockFixtures";

/**
 * Sends a question to the real RAG backend through the app's own route
 * handler, which holds the service URL and maps the response.
 *
 * The mock is retained but is now opt-in via `NEXT_PUBLIC_USE_MOCK_CHAT`, and
 * every mocked answer is labelled as such in its own text. Silently falling
 * back to a plausible-looking fake answer is the exact failure this project
 * cannot afford: a fabricated dose that looks like a real one.
 */
const USE_MOCK = process.env.NEXT_PUBLIC_USE_MOCK_CHAT === "true";

export class ChatUnavailableError extends Error {
  constructor(readonly status: number) {
    super(`Chat backend unavailable (${status})`);
    this.name = "ChatUnavailableError";
  }
}

export async function sendChatMessage(
  content: string,
  conversationId?: string
): Promise<SendMessageResponse> {
  if (USE_MOCK) {
    await new Promise((resolve) => setTimeout(resolve, 400));
    return buildMockResponse(content);
  }

  const response = await fetch("/api/chat", {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify({ content, conversationId }),
  });

  if (!response.ok) {
    // Surfaced to the user as an error state, never as an answer.
    throw new ChatUnavailableError(response.status);
  }

  return (await response.json()) as SendMessageResponse;
}