import { cookies, headers } from "next/headers";
import { DEFAULT_LOCALE, LOCALES, LOCALE_COOKIE, type Locale } from "./messages";

function isLocale(v: string | undefined | null): v is Locale {
  return !!v && (LOCALES as string[]).includes(v);
}

/**
 * Resolve the current locale on the server.
 *  1. Explicit cookie
 *  2. Accept-Language header (en* → en, otherwise zh)
 *  3. Default
 */
export async function getServerLocale(): Promise<Locale> {
  const cookieStore = await cookies();
  const cookieLocale = cookieStore.get(LOCALE_COOKIE)?.value;
  if (isLocale(cookieLocale)) return cookieLocale;

  try {
    const h = await headers();
    const accept = h.get("accept-language") || "";
    const first = accept.split(",")[0]?.trim().toLowerCase() || "";
    if (first.startsWith("en")) return "en";
    if (first.startsWith("zh")) return "zh";
  } catch {
    // headers() can throw outside request scope
  }

  return DEFAULT_LOCALE;
}