Collision.java

public class Collision implements Comparable<Collision>
{
    double timestep;
    Ball ball, ball2;
    WallSeg seg = null;
    int ndx;

    Collision(double t, Ball b, Ball b2)
    {
	timestep = t;
	ball = b;
	ball2 = b2;
	ndx = -1;
    }
    Collision(double t, Ball b, int ndx)
    {
	timestep = t;
	ball = b;
	ball2 = null;
	this.ndx = ndx;
    }
    Collision(double t, Ball b, WallSeg seg)
    {
        timestep = t;
	ball = b;
	ball2 = null;
	this.seg = seg;
        ndx = 5;
    }

    public String toString()
    {
	String str = String.format("dt %6.2f ",timestep);
	if (ndx==-1) str += " BALL  ";
	else if (ndx==-2) str += " PT  ";
	else if (ndx<4) str += " WIN ";
	else if (ndx==5) str += " SEG  " + seg;
	str += ball;
	if (ndx==-1) str += "vs" + ball2;
	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 (ndx==-1) ball_collision();
	else if (ndx==-2) pt_collision();
	else if (ndx==5) seg_collision();
	else wall_collision();
    }

    void seg_collision()
    {
	double kx = seg.kx;
	double ky = seg.ky;
	double px = ball.px - seg.pt0.px;
	double py = ball.py - seg.pt0.py;
	double dist;
	// distance to line
	dist = (kx*py - ky*px);
	//System.out.format("distance: %g%n", dist);
	//System.out.format("kx %g ky %g%n",kx,ky);
	//System.out.format("before: vx %g vy %g%n",ball.vx,ball.vy);
	// velocity components
	double vperp = kx*ball.vy - ky*ball.vx;
	double vtran = kx*ball.vx + ky*ball.vy;
	vperp = -vperp;
	ball.vx = kx*vtran - ky*vperp;
	ball.vy = kx*vperp + ky*vtran; 
	//System.out.format("after: vx %g vy %g%n",ball.vx,ball.vy);
    }

// ndx == (2,3) vertical wall, ndx == (0,1) horizontal wall
    void wall_collision()
    {
	if (ndx<2) ball.vx = -ball.vx;
	else ball.vy = -ball.vy;
    }

    void pt_collision()
    {
	// relative position
	double px = ball.px - ball2.px;
	double py = ball.py - ball2.py;
	// unit vector along line of centers
	double vnorm = Math.hypot(px,py);
	px = px/vnorm;
	py = py/vnorm;
	// velocity component (normal)
	double v1 = ball.vx*px+ball.vy*py;
	// velocity component (transverse)
	double qx = py;
	double qy = -px;
	double u1 = ball.vx*qx+ball.vy*qy;
	// calculate changed velocity
	double v1f = -v1;
	// back to original coordinates.
	ball.vx = v1f*px+u1*qx;
	ball.vy = v1f*py+u1*qy;
   }

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