import i18n from 'i18next';
import { initReactI18next } from 'react-i18next';
import commonEn from './languages/en/common.json';
import commonId from './languages/id/common.json';
import validationEn from './languages/en/validation.json';
import validationId from './languages/id/validation.json';

const DEFAULT_LANGUAGE = 'en';
const SUPPORTED_LANGUAGES = ['en', 'id'] as const;

export type SupportedLanguage = (typeof SUPPORTED_LANGUAGES)[number];

export const resources = {
  en: { common: commonEn.common, validation: validationEn.validation },
  id: { common: commonId.common, validation: validationId.validation },
} as const;

export interface I18nStorageAdapter {
  getLanguage(): Promise<string | null>;
  setLanguage(lng: string): Promise<void>;
}

export interface I18nConfig {
  storageAdapter?: I18nStorageAdapter;
}

// Store the adapter module-wide so manager.ts can access it
export let globalStorageAdapter: I18nStorageAdapter | undefined;

/**
 * Bootstraps the central i18n engine.
 *
 * It accepts an optional storage adapter to read the initial language.
 */
export async function setupI18n(config: I18nConfig = {}, defaultLanguage?: string): Promise<void> {
  globalStorageAdapter = config.storageAdapter;

  let initialLng = defaultLanguage ? defaultLanguage : DEFAULT_LANGUAGE;
  try {
    if (globalStorageAdapter) {
      const storedLng = await globalStorageAdapter.getLanguage();
      if (storedLng && SUPPORTED_LANGUAGES.includes(storedLng as SupportedLanguage)) {
        initialLng = storedLng;
      }
    }
  } catch (err) {
    console.warn('[i18n] Failed to read locale from storage', err);
  }

  await i18n.use(initReactI18next).init({
    resources,
    lng: initialLng,
    fallbackLng: DEFAULT_LANGUAGE,
    defaultNS: 'common',
    interpolation: {
      escapeValue: false, // React already escapes values
    },
  });

  // Apply initial language to the DOM for SEO/Accessibility
  if (typeof document !== 'undefined') {
    document.documentElement.lang = i18n.language;
  }

  // Ensure DOM updates whenever the language changes later
  i18n.on('languageChanged', (lng) => {
    if (typeof document !== 'undefined') {
      document.documentElement.lang = lng;
    }
  });
}