/**
 * The built frontend's `index.html`, with its runtime placeholders filled: the
 * site name, emoji, icon version and Bandcamp player mode.
 *
 * @module
 */

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

/** The cache-busting version of the icon URLs. */
const ICON_VERSION = emojiToCodepoint(SITE_EMOJI);

/**
 * `index.html` with its placeholders filled, by root. Kept until the file
 * changes (a rebuild under a running server): a stat per navigation instead of
 * a read and four replaceAll.
 */
const indexHtml = new Map<string, { mtime: number; html: string }>();

/**
 * Returns `${root}/index.html` with its placeholders filled.
 *
 * @throws {Error} When the file can't be read.
 */
export async function loadIndexHtml(root: string): Promise<string> {
  const path = `${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);
  indexHtml.set(root, { mtime, html });
  return html;
}