import java.util.*;

public class Ball extends PhysicsElement  {
   private static int id=0;  // Ball identification number
   private final double mass;
   private final double radius;
   private double pos_t;     // current position at time t
   private double pos_tPlusDelta;  // next position at t + delta
   private double speed_t;   // speed at time t
   private double speed_tPlusDelta;   // speed at t + delta
   
   private Ball(){   // nobody can create a ball without state
     super(id++);
     mass=0;
     radius=0;
   }
   public Ball(double mass, double r, double position, double speed){
      super(id++);
      this.mass = mass;
      radius = r;
      pos_t = position;
      speed_t = speed;
   }
   public double getMass() {
      return mass;
   }
   public double getRadius() {
      return radius;
   }
   public double getPosition() {
      return pos_t;
   }
   public double getSpeed() {
      return speed_t;
   }
   public boolean collide(Ball b){
      return Math.abs(b.getPosition()-pos_t)<(b.getRadius()+radius);
   }
   public void computeNextState(double delta_t, MyWorld w) {
     double speed=speed_t;
     PhysicsElement pe;
     if ((pe=w.getCollidingElement(this))!= null){ /* elastic collision */
        if (pe instanceof Ball) {
           Ball b =(Ball) pe;
           speed=(speed_t*(mass-b.getMass())+2*b.getMass()*b.getSpeed())/(mass+b.getMass());
        } else {
           // case Container
        }
     } 
    
     speed_tPlusDelta = speed;
     pos_tPlusDelta = pos_t + speed*delta_t;
   }
   public void updateState(){
     pos_t = pos_tPlusDelta;
     speed_t = speed_tPlusDelta;
   }
   public String getDescription() {
      return "Ball_" + getId()+": x ";
   }
   public String getState() {
      return getPosition()+" ";
   }
}