/**
 * Backend runtime language detection.
 *
 * Fetches /api/v1/language once at module load (app start) and caches the
 * result. Components subscribe via React's useSyncExternalStore; there is no
 * per-component fetch or useEffect.
 *
 * Pattern: module-level fetch + listener set, same subscription approach as
 * enterprise billingStatus.ts.
 */

type Listener = () => void;
const listeners = new Set<Listener>();

let backendLanguage: string | null = null;

// Kick off the fetch at module load — app start, not component mount.
const promise: Promise<string> = fetch('/api/v1/language')
  .then((r) => r.json())
  .then((body: { data?: { language?: string } }) => {
    backendLanguage = body.data?.language === 'go' ? 'go' : 'python';
    listeners.forEach((fn) => fn());
    return backendLanguage;
  })
  .catch(() => {
    backendLanguage = 'python';
    listeners.forEach((fn) => fn());
    return 'python';
  });

export const fetchBackendLanguage = (): Promise<string> => promise;

export const getBackendLanguage = (): string | null => backendLanguage;

export const subscribeBackendLanguage = (listener: Listener): (() => void) => {
  listeners.add(listener);
  return () => {
    listeners.delete(listener);
  };
};