export declare const BASE_62_DIGITS = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
export declare function validateOrderKey(key: string): void;
export declare function isValidOrderKey(key: string): boolean;
/**
 * @param {string | null} a
 * @param {string | null} b
 * @param {string=} digits
 * @return {string}
 */
export declare function generateKeyBetween(a: string | null, b: string | null, digits?: string): string;
/**
 * same preconditions as generateKeysBetween.
 * n >= 0.
 * Returns an array of n distinct keys in sorted order.
 * If a and b are both null, returns [a0, a1, ...]
 * If one or the other is null, returns consecutive "integer"
 * keys.  Otherwise, returns relatively short keys between
 * a and b.
 */
export declare function generateNKeysBetween(a: string | null, b: string | null, n: number, digits?: string): string[];