/* Damped springs for values that retarget mid-flight, keeping their velocity, as a
   dragged or redirected lens does. A spring is an angular frequency (rad/s) and a
   damping ratio; the caller steps them in fixed substeps, which keeps stiff springs
   stable when frames drop. */

export type SpringConfig = { omega: number; zeta: number };
export type Spring = { value: number; velocity: number };

/** The substep, in seconds. */
export const STEP = 1 / 240;

// iOS's measurements are in points of a 54 pt lens; glass converts them by its
// own thickness.
export const LENS_PT = 54;
// Pressed, glass swells on a bouncy spring, and springs back on release...
export const SWELL: SpringConfig = { omega: 17.6, zeta: 0.59 };
// ...to this much longer, and as much wider in proportion.
export const SWELL_PT = 14.6;

export function spring(value = 0): Spring {
  return { value, velocity: 0 };
}

export function stepSpring(spring: Spring, target: number, { omega, zeta }: SpringConfig, dt: number) {
  spring.velocity += (omega * omega * (target - spring.value) - 2 * zeta * omega * spring.velocity) * dt;
  spring.value += spring.velocity * dt;
}