/**
 * Fixed simulation timestep (seconds), consumed through an accumulator
 * ("fix your timestep"). Same trajectory at 60/120/144 Hz, testable step
 * by step. Lower it if very stiff springs ever prove unstable - the API
 * does not change.
 */
export declare const SIMULATION_TIMESTEP_S: number;
export interface SimulationState {
    position: number;
    /** units/s */
    velocity: number;
}
/**
 * Every physics mode (spring, decay, inertia...) reduces to an acceleration
 * function plus a rest condition over a (position, velocity) state. This is
 * what makes retargeting and handoff free: the state survives, only the
 * simulation behind it changes.
 */
export interface Simulation {
    /** units/s^2 */
    acceleration(position: number, velocity: number): number;
    /** Settled position when the state qualifies as rest, null while moving. */
    rest(position: number, velocity: number): number | null;
}
/** One semi-implicit (symplectic) Euler step: velocity first, then position. */
export declare function stepSimulation(simulation: Simulation, state: SimulationState, timestepS: number): SimulationState;
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