/**
 * @typedef {object} SlowDownConfig
 * @property {import('../rate-limit/index.js').RateLimitStore} store
 * @property {string | number} window   Duration string ('1m', '30s') or ms.
 * @property {number} delayAfter        First `delayAfter` requests are free;
 *                                     subsequent ones get progressively delayed.
 * @property {number} delayMs           Base per-request delay after the free window.
 * @property {number} [maxDelayMs=20000]  Cap the per-request delay.
 * @property {'linear' | 'exponential'} [growth='linear']
 *   `linear`: delay = delayMs * excess.
 *   `exponential`: delay = delayMs * 2^(excess - 1) (capped by maxDelayMs).
 */
/**
 * Progressive-delay throttle — a "soft" rate limiter that never rejects but
 * slows abusers down. Sits nicely IN FRONT of a hard limiter: `slowDown`
 * catches naive scrapers cheaply, the limiter catches the persistent ones.
 *
 * The returned object exposes `.check({ key })` matching the rate-limit
 * contract, so `multi({ limiters: [slowDown(...), rateLimit.fixed(...)] })`
 * composes them.
 *
 * **DoS caveat.** The delay is implemented by holding the request open on
 * `setTimeout` — each slowed request continues to occupy a socket + an
 * event-loop slot for the duration of its delay. An attacker who does not
 * care about latency can trivially exhaust concurrent connections. Always
 * pair with (a) a hard `maxConnections` on `http.Server` or a `limit_conn`
 * at your reverse proxy, and (b) a real rejecting limiter downstream (fed
 * by `multi({ limiters: [slowDown(...), rateLimit.sliding(...)] })`) so the
 * abuser eventually gets a `429` and the socket is released.
 *
 * @param {SlowDownConfig} config
 * @returns {{ check: (input: { key: string }) => Promise<{
 *   allowed: boolean, remaining: number, reset: Date | null,
 *   retryAfter: number | null, delayMs: number,
 * }> }}
 */
export function slowDown(config: SlowDownConfig): {
    check: (input: {
        key: string;
    }) => Promise<{
        allowed: boolean;
        remaining: number;
        reset: Date | null;
        retryAfter: number | null;
        delayMs: number;
    }>;
};
export type SlowDownConfig = {
    store: import("../rate-limit/index.js").RateLimitStore;
    /**
     * Duration string ('1m', '30s') or ms.
     */
    window: string | number;
    /**
     * First `delayAfter` requests are free;
     * subsequent ones get progressively delayed.
     */
    delayAfter: number;
    /**
     * Base per-request delay after the free window.
     */
    delayMs: number;
    /**
     * Cap the per-request delay.
     */
    maxDelayMs?: number | undefined;
    /**
     * `linear`: delay = delayMs * excess.
     * `exponential`: delay = delayMs * 2^(excess - 1) (capped by maxDelayMs).
     */
    growth?: "linear" | "exponential" | undefined;
};