/**
 * Rate limiter module for API calls
 *
 * Uses factory + lazy initialization singleton pattern:
 * - RateLimiter class: Delay-based rate limiting with configurable requests/second
 * - getRateLimiter(api, config): Returns singleton per API type
 * - resetRateLimiters(): Clears singletons for test isolation
 */
/**
 * API types supported by the rate limiter
 */
export type ApiType = "pubmed" | "crossref" | "isbn" | "arxiv";
/**
 * Configuration for rate limiter
 */
export interface RateLimiterConfig {
    pubmedApiKey?: string;
}
/**
 * Rate limiter interface
 */
export interface RateLimiter {
    readonly requestsPerSecond: number;
    readonly intervalMs: number;
    readonly lastRequestTime: number;
    acquire(): Promise<void>;
}
/**
 * Create a new rate limiter instance
 */
export declare function createRateLimiter(options: {
    requestsPerSecond: number;
}): RateLimiter;
/**
 * Get singleton rate limiter for the specified API type
 *
 * Note: Configuration is only used on first call. Subsequent calls
 * return the existing singleton regardless of config changes.
 */
export declare function getRateLimiter(api: ApiType, config: RateLimiterConfig): RateLimiter;
/**
 * Reset all rate limiter singletons (for test isolation)
 */
export declare function resetRateLimiters(): void;
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