public class Collision implements Comparable<Collision>
{
    double timestep;
    Ball ball1, ball2;
    int idx;

    Collision(double t, Ball b1, Ball b2)
    {
	timestep = t;
	ball1 = b1;
	ball2 = b2;
	idx = -1;
    }
    Collision(double t, Ball b1, int ndx)
    {
	timestep = t;
	ball1 = b1;
	ball2 = null;
	idx = ndx;
    }

    public String toString()
    {
	String str = "timestep " + timestep;
	if (idx<0) str += " BALL";
	return str;
    }

    public int compareTo(Collision other)
    {
	//Collision c = (Collision) other;
	if (timestep>other.timestep) return 1;
	if (timestep<other.timestep) return -1;
	return 0;
    }

    void update_velocity()
    {
	if (idx<0) ball_collision();
	else wall_collision();
    }

    void wall_collision()
    {
	if (idx<2) ball1.vx = -ball1.vx;
	else ball1.vy = -ball1.vy;
    }

    void ball_collision()
    {
	double eta = 1.0; // coefficient of restitution
	// relative position
	double px = ball2.px - ball1.px;
	double py = ball2.py - ball1.py;
	// unit vector along line of centers
	double vnorm = Math.hypot(px,py);
	px = px/vnorm;
	py = py/vnorm;
	double m1 = ball1.mass;
	double m2 = ball2.mass;
	double mt = m1+m2;
	// velocity components (normal)
	double v1 = ball1.vx*px+ball1.vy*py;
	double v2 = ball2.vx*px+ball2.vy*py;
	// velocity components (transverse)
	double qx = py;
	double qy = -px;
	double u1 = ball1.vx*qx+ball1.vy*qy;
	double u2 = ball2.vx*qx+ball2.vy*qy;
	// calculate changed velocity
	double v1f = ((eta+1.0)*m2*v2+v1*(m1-eta*m2))/mt;
	double v2f = ((eta+1.0)*m1*v1+v2*(m2-eta*m1))/mt;
	// back to original coordinates.
	ball1.vx = v1f*px+u1*qx;
	ball1.vy = v1f*py+u1*qy;
	ball2.vx = v2f*px+u2*qx;
	ball2.vy = v2f*py+u2*qy;
    }
}