import type { i18n } from "i18next";
import type { locales } from "@shared/utils/date";
import { unicodeCLDRtoBCP47 } from "@shared/utils/date";
import { isRTLLanguage } from "@shared/utils/rtl";
import Desktop from "./Desktop";

/**
 * Detects the user's language based on the browser's language settings. A
 * language without a region, such as "ja", resolves to its most likely region.
 *
 * @returns The user's language in CLDR format (en_US)
 */
export function detectLanguage() {
  try {
    const { language, region } = new Intl.Locale(navigator.language).maximize();
    if (region) {
      return `${language}_${region}` as keyof typeof locales;
    }
  } catch (_err) {
    // invalid language tag, fall back to splitting the string
  }

  const [ln, r] = navigator.language.split("-");
  const region = (r || ln).toUpperCase();
  return `${ln}_${region}` as keyof typeof locales;
}

/**
 * Changes the language of the app, and updates the spellchecker language
 * if running in the desktop shell.
 *
 * @param locale The locale to change to, in CLDR format (en_US)
 * @param instance The i18n instance to use
 */
export async function changeLanguage(
  locale: string | null | undefined,
  instance: i18n
) {
  // Languages are stored in en_US format in the database, however the
  // frontend translation framework (i18next) expects en-US
  const localeBCP = locale ? unicodeCLDRtoBCP47(locale) : undefined;

  if (localeBCP && instance.languages?.[0] !== localeBCP) {
    await instance.changeLanguage(localeBCP);
    await Desktop.bridge?.setSpellCheckerLanguages(["en-US", localeBCP]);
  }

  if (typeof document !== "undefined") {
    document.documentElement.dir = isRTLLanguage(locale) ? "rtl" : "ltr";
  }
}