/**
 * Fetches link previews for the create form, and proxies the remote images
 * that the frontend's thumbnails can't load directly.
 *
 * - `GET /api/preview?url=` — signed in. Answers the preview of a link, and
 *   whether its page could be reached.
 * - `GET /api/proxy-image?url=` — public. Serves a remote raster image through
 *   this site; 400 when it isn't one, 502 when its host can't be reached.
 *
 * @module
 */

import { Router } from "@oak/oak";
import { isValidHttpUrl } from "../remote/fetch.ts";
import { fetchProxiedImage } from "../remote/image-proxy.ts";
import { tryFetchRichContent } from "../services/rich-content-service.ts";
import {
  APIErrorCode,
  type APIFailure,
  type APIResponse,
} from "../../shared/api.ts";
import type { LinkPreview } from "../../shared/dump.ts";
import { authMiddleware } from "../middleware/auth.ts";

/**
 * Mounted by `api/main.ts` behind the error, Open Graph and CORS middleware.
 * An `APIException` thrown by a handler becomes the JSON error response there.
 */
const previewRouter: Router = new Router();

// Signed-in only: an open preview would let anyone make the server fetch pages.
previewRouter.get("/api/preview", authMiddleware, async (ctx) => {
  const url = ctx.request.url.searchParams.get("url") ?? "";
  if (!isValidHttpUrl(url)) {
    const responseBody: APIFailure = {
      success: false,
      error: { code: APIErrorCode.BAD_REQUEST, message: "Invalid URL" },
    };
    ctx.response.status = 400;
    ctx.response.body = responseBody;
    return;
  }
  const { ok, content } = await tryFetchRichContent(url);
  const responseBody: APIResponse<LinkPreview> = {
    success: true,
    data: { reached: ok, richContent: content ?? null },
  };
  ctx.response.body = responseBody;
});

/**
 * Answers with a proxied image. It is sent from this site's origin, so it is
 * sandboxed in case a browser opens it as a document.
 */
function sendImage(
  ctx: { response: { headers: Headers; body?: unknown } },
  bytes: Uint8Array,
  contentType: string,
): void {
  ctx.response.headers.set("Content-Type", contentType);
  ctx.response.headers.set("X-Content-Type-Options", "nosniff");
  ctx.response.headers.set(
    "Content-Security-Policy",
    "sandbox; default-src 'none'",
  );
  ctx.response.headers.set("Cache-Control", "public, max-age=86400");
  ctx.response.body = bytes;
}

// The frontend sends a thumbnail here when it is plain HTTP, which an HTTPS
// page won't load, or when its host's hotlink protection refused the browser.
previewRouter.get("/api/proxy-image", async (ctx) => {
  const url = ctx.request.url.searchParams.get("url") ?? "";
  if (!isValidHttpUrl(url)) {
    ctx.response.status = 400;
    return;
  }

  try {
    const image = await fetchProxiedImage(url);
    if (!image) {
      ctx.response.status = 400;
      return;
    }
    sendImage(ctx, image.bytes, image.contentType);
  } catch {
    ctx.response.status = 502;
  }
});

export default previewRouter;