export type Bucket = { count: number; resetAt: number };

export type RateLimitStore = Pick<
  Map<string, Bucket>,
  "get" | "set" | "delete" | "size"
> & {
  [Symbol.iterator](): IterableIterator<[string, Bucket]>;
};

export type RateLimitDeps = {
  now?: () => number;
  store?: RateLimitStore;
  ipOf?: (request: Request) => string;
};

const defaultStore: RateLimitStore = new Map<string, Bucket>();

function defaultIpOf(request: Request): string {
  const fwd = request.headers.get("x-forwarded-for");
  if (fwd) return fwd.split(",")[0].trim();
  const real = request.headers.get("x-real-ip");
  if (real) return real.trim();
  return "unknown";
}

export function checkRateLimit(
  request: Request,
  opts: { key: string; limit: number; windowMs: number },
  deps: RateLimitDeps = {}
): Response | null {
  const now = (deps.now ?? Date.now)();
  const store = deps.store ?? defaultStore;
  const ip = (deps.ipOf ?? defaultIpOf)(request);
  const bucketKey = `${opts.key}:${ip}`;
  const bucket = store.get(bucketKey);

  if (!bucket || now >= bucket.resetAt) {
    store.set(bucketKey, { count: 1, resetAt: now + opts.windowMs });
    if (store.size > 10000) {
      for (const [k, b] of store) {
        if (b.resetAt <= now) store.delete(k);
      }
    }
    return null;
  }

  if (bucket.count >= opts.limit) {
    const retryAfter = Math.ceil((bucket.resetAt - now) / 1000);
    return Response.json(
      { error: "Too many requests" },
      {
        status: 429,
        headers: { "Retry-After": String(retryAfter) },
      }
    );
  }

  bucket.count += 1;
  return null;
}