/**
 * Previews any page no other provider claims, for `rich-content-service.ts`,
 * from whatever metadata its HTML carries.
 *
 * @module
 */

import type { RichContent } from "../../../shared/dump.ts";
import type { RichContentProvider } from "./provider.ts";
import { siteHost } from "../../../shared/url.ts";
import {
  extractBestIcon,
  extractFirstContentImage,
  extractJsonLd,
  extractLargeImage,
  extractMaskIconColor,
  extractMetaName,
  extractOgTag,
  extractPageTitle,
  extractThemeColor,
  normalizeCssColor,
} from "../html.ts";
import { fetchWithTimeout, readTextCapped } from "../fetch.ts";

/**
 * Matches every URL, so it comes last in the service's list. Each field falls
 * back from OG tags to Twitter tags, JSON-LD and the markup itself. The icon
 * and brand color feed the placeholder the frontend draws when there is no
 * thumbnail.
 */
export const genericProvider: RichContentProvider = {
  name: "generic",

  matches(_url: string): boolean {
    return true;
  },

  async fetch(url: string): Promise<RichContent> {
    const res = await fetchWithTimeout(url);
    const contentType = res.headers.get("content-type") ?? "";

    if (!contentType.startsWith("text/html")) {
      await res.body?.cancel();
      return { type: "generic", url };
    }

    const html = await readTextCapped(res);
    const ld = extractJsonLd(html);

    // An og:url naming another page (often the home page) means the OG tags
    // are the site's defaults, not this page's: its title and image are
    // skipped.
    const ogUrl = extractOgTag(html, "url");
    const useOg = !ogUrl || (() => {
      try {
        const ogPath = new URL(ogUrl).pathname.replace(/\/+$/, "") || "/";
        const pagePath = new URL(url).pathname.replace(/\/+$/, "") || "/";
        return ogPath === pagePath;
      } catch {
        return true;
      }
    })();

    const title = (useOg ? extractOgTag(html, "title") : undefined) ??
      extractMetaName(html, "twitter:title") ??
      ld.title ??
      extractPageTitle(html);

    const siteName = extractOgTag(html, "site_name") ??
      siteHost(url);

    const description = extractOgTag(html, "description") ??
      extractMetaName(html, "twitter:description") ??
      ld.description ??
      extractMetaName(html, "description");

    // Never the favicon: a 16×16 icon cropped into a thumbnail looks broken,
    // and the placeholder drawn instead shows the icon properly.
    const thumbnailUrl = (useOg ? extractOgTag(html, "image") : undefined) ??
      extractMetaName(html, "twitter:image") ??
      ld.thumbnailUrl ??
      extractLargeImage(html, url) ??
      extractFirstContentImage(html, url);

    // `/favicon.ico` is a guess. When it doesn't load, the placeholder shows
    // the site's initial.
    const faviconUrl = extractBestIcon(html, url) ??
      `${new URL(url).origin}/favicon.ico`;

    const accentColor = normalizeCssColor(
      extractThemeColor(html) ??
        extractMetaName(html, "msapplication-TileColor") ??
        extractMaskIconColor(html),
    );

    return {
      type: "generic",
      url,
      title,
      description,
      thumbnailUrl,
      faviconUrl,
      accentColor,
      siteName,
    };
  },
};