import { importIntl, importLocaleData, importMessages } from './import';
import cookie from './cookie';
import { normalizeLocale, getLanguageCodeFromLocale, getNavigatorLocale } from './utils';

/**
 * Get a locale for the user that is supported
 *
 * It considers normalized locales from
 * - the cookie (first full locale, then extracted language)
 * - the navigator locale (first full, then extracted language)
 * - default: 'en'
 *
 * and returns the first match in the normalized supported locales
 *
 * @param {string[]} supportedLocales
 * @returns {string} normalized locale which is supported (or 'en')
 */
export const getLocale = supportedLocales => {
  const cookieLocale = normalizeLocale(cookie.get('locale'));
  const navigatorLocale = normalizeLocale(getNavigatorLocale());
  const normalizedSuppportedLocales = supportedLocales.map(locale => normalizeLocale(locale));

  const desiredLocales = [
    cookieLocale,
    getLanguageCodeFromLocale(cookieLocale),
    navigatorLocale,
    getLanguageCodeFromLocale(navigatorLocale),
    'en',
  ];

  const matchedLocale = desiredLocales.reduce((result, desiredLocale) => {
    if (!result && normalizedSuppportedLocales.includes(desiredLocale)) {
      return desiredLocale;
    }

    return result;
  }, false);

  return matchedLocale || 'en';
};

/**
 * Initialise I18n asynchronously
 * - determine locale
 * - load Intl polyfill (only executed when window.Intl is not present)
 * - load `react-intl` locale data if not english locale
 * - load translation messages if not english
 *
 * It returns a promise that will always resolve: errors are caught internally, and a default language is returned
 *
 * @async
 * @returns {Promise<object>} Resolves to an object with locale, locale data (optional) and translation messages (optional)
 */
export default async function initI18n({ supportedLocales = ['en'] } = {}) {
  try {
    const locale = getLocale(supportedLocales);

    const imports = [importIntl()];
    if (locale !== 'en') {
      imports.push(importLocaleData(locale), importMessages(locale));
    }

    const [, localeData, messages] = await Promise.all(imports);
    return {
      locale,
      localeData,
      messages,
    };
  } catch (error) {
    return {
      locale: 'en',
    };
  }
}