/**
 * Tiny in-process TTL cache with request coalescing (spec §59). Used for aggregates only
 * (stats, trending, pulse, radar, rankings) — never for the live feed itself.
 */
const store = new Map<string, { at: number; ttl: number; value: unknown; pending?: Promise<unknown> }>();

export async function cached<T>(key: string, ttlMs: number, fn: () => Promise<T>): Promise<T> {
  const now = Date.now();
  const hit = store.get(key);
  if (hit && now - hit.at < hit.ttl) return hit.value as T;
  if (hit?.pending) return hit.pending as Promise<T>;
  const pending = fn()
    .then((value) => {
      store.set(key, { at: Date.now(), ttl: ttlMs, value });
      return value;
    })
    .catch((e) => {
      // serve stale on failure when we have anything at all
      if (hit) {
        store.set(key, { ...hit, pending: undefined });
        return hit.value as T;
      }
      store.delete(key);
      throw e;
    });
  store.set(key, { at: hit?.at ?? 0, ttl: hit?.ttl ?? ttlMs, value: hit?.value, pending });
  return pending;
}

export function cacheStats(): { entries: number; keys: string[] } {
  return { entries: store.size, keys: [...store.keys()].slice(0, 50) };
}

export function invalidate(prefix: string): void {
  for (const k of store.keys()) if (k.startsWith(prefix)) store.delete(k);
}