/**
 * Fills the placeholders of the frontend's `index.html` with settings read at
 * runtime, so they change with a restart and no rebuild. The static and OG
 * middlewares serve what it returns.
 *
 * @module
 */

import {
  BANDCAMP_PLAYER,
  DEFAULT_STYLE,
  OG_SITE_NAME,
  SITE_EMOJI,
  SMTPS_URL,
} from "../config.ts";
import { emojiToCodepoint } from "./site-icons.ts";

/**
 * Versions the icon URLs, so browsers fetch new icons when the emoji changes.
 */
const ICON_VERSION = emojiToCodepoint(SITE_EMOJI);

/**
 * Caches each root's filled file until its mtime changes, as a rebuild under a
 * running server does, so a navigation costs a stat instead of a read.
 */
const indexHtml = new Map<string, { mtime: number; html: string }>();

/**
 * Returns `${root}/index.html` with the site name, emoji, icon version,
 * Bandcamp player mode, default style and whether email is on filled in.
 *
 * @throws {Error} When the file can't be read.
 */
export async function loadIndexHtml(root: string): Promise<string> {
  const path = `${root}/index.html`;
  const mtime = (await Deno.stat(path)).mtime?.getTime() ?? 0;
  const cached = indexHtml.get(root);
  if (cached && cached.mtime === mtime) return cached.html;
  const html = (await Deno.readTextFile(path))
    .replaceAll("__SITE_NAME__", OG_SITE_NAME)
    .replaceAll("__SITE_EMOJI__", SITE_EMOJI)
    .replaceAll("__ICON_VERSION__", ICON_VERSION)
    .replaceAll("__BANDCAMP_PLAYER__", BANDCAMP_PLAYER)
    .replaceAll("__DEFAULT_STYLE__", DEFAULT_STYLE)
    .replaceAll("__EMAIL_ENABLED__", SMTPS_URL ? "on" : "off");
  indexHtml.set(root, { mtime, html });
  return html;
}