export const FALLBACK_LANG = "zh";
export const SUPPORTED_LANGUAGES = ["zh", "en"] as const;
export type AppLanguage = (typeof SUPPORTED_LANGUAGES)[number];

/** Cookie name read by `@unly/universal-language-detector`. */
export const I18N_COOKIE_NAME = "i18next";

export function isAppLanguage(value: unknown): value is AppLanguage {
  return value === "zh" || value === "en";
}

export function normalizeAppLanguage(value: unknown): AppLanguage | null {
  if (typeof value !== "string") return null;
  const lower = value.trim().toLowerCase();
  if (lower === "zh" || lower.startsWith("zh-")) return "zh";
  if (lower === "en" || lower.startsWith("en-")) return "en";
  return null;
}

export function setLanguageCookie(lang: AppLanguage) {
  if (typeof document === "undefined") return;
  document.cookie = `${I18N_COOKIE_NAME}=${lang}; Path=/; SameSite=Lax; Max-Age=31536000`;
}