import java.util.*;
public class Block extends PhysicsElement  {
   private static int id=0;
   private final float mass;
   protected Vector2D pos_t;
   private Vector2D pos_tPlusDelta;
   private Vector2D speed_t;
   private Vector2D speed_tPlusDelta;
   private Vector2D a_t, a_tMinusDelta;
   private Vector<Spring> springs;  // This Vector is a growable array of objects
                            // it has nothing to do with Vector2D.
   private Block(){ // nobody can create a block without state
      super(id++);
      mass=0;
   }  
   public Block(float mass, Vector2D position, Vector2D speed){
      super(id++);
      this.mass = mass;
      pos_t = position;
      this.speed_t = speed;
      a_tMinusDelta = new Vector2D();  // it is not quite right, aproximation
      springs = new Vector<Spring>();
   }
   public void attachSpring (Spring spring) {
     springs.add(spring);
   }
   public void detachSpring (Spring spring) {
     springs.remove(spring);
   }
   public Vector2D getPosition() {
      return pos_t;
   }
   public void computeNextState(double delta_t) {
     Vector2D force = new Vector2D();
     Spring spring;
     for (int i=0; i<springs.size(); i++) {
        spring = springs.get(i);
        if (spring != null) force = force.plus(spring.getForce(this));
     }
     a_t = force.times(1/mass);
     pos_tPlusDelta = pos_t.plus(speed_t.times(delta_t)).plus(a_t.times(delta_t*delta_t/2));
     Vector2D jerk_times_delta_t = a_t.minus(a_tMinusDelta); 
     speed_tPlusDelta = speed_t.plus(a_t.times(delta_t).plus(jerk_times_delta_t.times(delta_t/2)));
   }
   public void updateState(){
     pos_t = pos_tPlusDelta;
     speed_t = speed_tPlusDelta;
     a_tMinusDelta = a_t;
   }
   public String getDescription() {
      return "Block#" + getId() + ": "+getPosition().getDescription()+",";
   }
   public String getState() {
         return getPosition()+",";
   }
}