import { defaultLocale, isLocale, type Locale } from "./config";

export function negotiateAcceptLanguage(header: string | null): Locale {
  if (!header) return defaultLocale;
  const ranked = header
    .split(",")
    .map((part) => {
      const [tag, ...params] = part.trim().split(";");
      const q = params.map((p) => p.trim()).find((p) => p.startsWith("q="));
      const quality = q ? Number.parseFloat(q.slice(2)) : 1;
      return { base: tag.trim().toLowerCase().split("-")[0], quality };
    })
    .filter((r) => r.base && !Number.isNaN(r.quality))
    .sort((a, b) => b.quality - a.quality);
  for (const { base } of ranked) {
    if (isLocale(base)) return base;
  }
  return defaultLocale;
}