/** Spread indices that should be mounted around `current`, clamped to the book. */
export declare function mountWindow(current: number, spreadCount: number, radius: number): number[];
export interface VirtualizerDelta {
    toMount: number[];
    toEvict: number[];
}
/**
 * Tracks which spreads are mounted and, on each move, reports the difference:
 * spreads entering the ±radius window (`toMount`) and those leaving it
 * (`toEvict`). Default radius 2 = current spread ±2.
 */
export declare class Virtualizer {
    #private;
    constructor(radius?: number);
    get mounted(): number[];
    /** Recenter on `current`; mounts/evicts to match the window and returns the delta. */
    update(current: number, spreadCount: number): VirtualizerDelta;
}