/** Delay between reveal frames. */
export declare const TYPEWRITER_FRAME_MS = 22;
/**
 * Ceiling on frames for one reply, whatever its length.
 *
 * Without it a long answer reveals one character per frame and takes minutes.
 * The step widens instead, so a 5,000-character answer and a 200-character one
 * both finish in about the same time.
 */
export declare const TYPEWRITER_MAX_FRAMES = 260;
export interface TypewriterMessage {
    id: string;
    text: string;
    displayText?: string;
    typing?: boolean;
    footprints?: unknown;
}
export interface TypewriterHost<M extends TypewriterMessage> {
    messages(): M[];
    setMessages(next: M[]): void;
    /** Injectable for tests; defaults to the browser's. */
    setTimeout?(fn: () => void, ms: number): number;
    clearTimeout?(id: number): void;
    /** Injectable for tests; defaults to the media query. */
    prefersReducedMotion?(): boolean;
}
/**
 * How many units to reveal per frame so the whole text fits in the frame budget.
 * Never below one, or a text longer than the budget would never finish.
 */
export declare function revealStep(totalUnits: number, maxFrames?: number): number;
export declare function createTypewriter<M extends TypewriterMessage>(host: TypewriterHost<M>): {
    appendToken: (messageId: string, delta: string) => void;
    shouldAnimate: (text: string) => boolean;
    addReply: (message: M) => void;
    clear: (messageId: string) => void;
    clearAll: () => void;
    /** Messages still revealing. Exposed for tests and leak checks. */
    readonly pending: number;
};