import * as Localization from 'expo-localization';
import i18n from 'i18next';
import { initReactI18next } from 'react-i18next';

import { getItemSync, setItem } from '@/utils/kvStore';
import en from './en';
import zh from './zh';

export const LANGUAGE_PREFERENCE_KEY = 'app_language_preference';
export const SUPPORTED_LANGUAGES = ['zh', 'en'] as const;
export type AppLanguage = typeof SUPPORTED_LANGUAGES[number];

const fallbackLanguage: AppLanguage = 'zh';

const resources = {
  zh: {
    translation: zh,
  },
  en: {
    translation: en,
  },
};

export const isSupportedLanguage = (language?: string | null): language is AppLanguage => {
  if (!language) return false;
  return SUPPORTED_LANGUAGES.some((code) => language === code || language.startsWith(`${code}-`));
};

export const getNormalizedLanguage = (language?: string | null): AppLanguage => {
  if (!language) return fallbackLanguage;
  const normalized = SUPPORTED_LANGUAGES.find((code) => language === code || language.startsWith(`${code}-`));
  return normalized ?? fallbackLanguage;
};

const getStoredLanguage = (): AppLanguage | null => {
  try {
    const stored = getItemSync?.(LANGUAGE_PREFERENCE_KEY) as AppLanguage | null | undefined;
    if (stored && isSupportedLanguage(stored)) {
      return stored;
    }
  } catch (error) {
    // ignore storage errors and fall back to device preference
  }
  return null;
};

const getDeviceLanguage = (): AppLanguage | null => {
  try {
    const locales = Localization.getLocales();
    const preferred = locales.find((locale) => locale.languageCode && isSupportedLanguage(locale.languageCode));
    return preferred?.languageCode as AppLanguage | undefined || null;
  } catch (error) {
    return null;
  }
};

const initialLanguage = getStoredLanguage() ?? getDeviceLanguage() ?? fallbackLanguage;

void i18n.use(initReactI18next).init({
  compatibilityJSON: 'v4',
  resources,
  lng: initialLanguage,
  fallbackLng: fallbackLanguage,
  interpolation: {
    escapeValue: false,
  },
  returnNull: false,
});

export const changeAppLanguage = async (language: AppLanguage) => {
  const nextLanguage = isSupportedLanguage(language) ? language : fallbackLanguage;
  await i18n.changeLanguage(nextLanguage);
  await setItem(LANGUAGE_PREFERENCE_KEY, nextLanguage);
};

export default i18n;