export type SingleFlight<K, V> = {
  run(key: K, create: () => Promise<V>): Promise<V>;
  has(key: K): boolean;
  activeCount(): number;
};

/** Coalesce concurrent work for one key without caching its settled result. */
export function createSingleFlight<K, V>(): SingleFlight<K, V> {
  const active = new Map<K, Promise<V>>();

  return {
    run(key, create) {
      const existing = active.get(key);
      if (existing) return existing;

      const pending = Promise.resolve().then(create);
      active.set(key, pending);
      const clear = () => {
        if (active.get(key) === pending) active.delete(key);
      };
      void pending.then(clear, clear);
      return pending;
    },
    has(key) {
      return active.has(key);
    },
    activeCount() {
      return active.size;
    },
  };
}