/** Share only concurrent work. Completed values and failures are never cached. */
export function singleFlight<Key, Value>(load: (key: Key) => PromiseLike<Value>) {
  const pending = new Map<Key, Promise<Value>>();
  return (key: Key): Promise<Value> => {
    const existing = pending.get(key);
    if (existing) return existing;
    const request = Promise.resolve().then(() => load(key)).finally(() => {
      if (pending.get(key) === request) pending.delete(key);
    });
    pending.set(key, request);
    return request;
  };
}