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 in delta time in future
   private double speed_t;   // speed at time t
   private double speed_tPlusDelta;   // speed in delta time in future
   private double a_t;    // acceleration at time t
   private double a_tMinusDelta;  // acceleration delta time ago;
   private ArrayList<Spring> springs;  // ArrayList can grow, arrays cannot. 

   private Ball(){   // nobody can create a block without state
     this(1.0,0.1,0,0);
   }
   public Ball(double mass, double radius, double position, double speed){
      super(id++);
      this.mass = mass;
      pos_t = position;
      speed_t = speed;
   }
   public double getRadius() {
      return radius;
   }
   public double getSpeed() {
      return speed_t;
   }
   private double getNetForce() {
    // to be coded by you
   }

   public void computeNextState(double delta_t, MyWorld world) {
     Ball b;  // Assumption: on collision we only change speed.   
     if ((b=world.findCollidingBall(this))!= null){ /* elastic collision */
        speed_tPlusDelta=(speed_t*(mass-b.getMass())+2*b.getMass()*b.getSpeed())/(mass+b.getMass());
        pos_tPlusDelta = pos_t;
        a_t= getNetForce()/mass;
        pos_tPlusDelta = pos_t;
     } else {
         // to be coded by you        
     }
   }
   public boolean collide(Ball b) {
   // to be coded by you
    }
   public void updateState(){
     pos_t = pos_tPlusDelta;
     speed_t = speed_tPlusDelta;
   }
   
   public String getDescription() {
   // to be coded by you
   }
}