Spring.java

import java.awt.*;

public class Spring extends PhysicsElement implements Simulateable, Elastic {
	private static int id=0;  // Spring identification
	private final double restLength;
	private final double stiffness;
	private SpringAttachable a_end, b_end;
	private double aLoosePosition, bLoosePosition;

	private double currentLength = 0;
	private double delta_x = 0;
	
	private SpringView view;

	private Spring() {   // nobody can create a block without state
		this(0,0);
	}
	public Spring(double restLength, double stiffness) {
		super(id++);
		this.restLength = restLength;
		this.stiffness = stiffness;
		a_end = b_end = null;
		aLoosePosition=0;
		bLoosePosition = restLength;
		currentLength = restLength;
		view = new SpringView(this);
	}
	
	public boolean isAattachedTo(SpringAttachable sa){
		return (sa == this.a_end);
	}
	
	public boolean isBattachedTo(SpringAttachable sa){
		return (sa == this.b_end);
	}
	
	public void attachAend(SpringAttachable sa) {  // note: we attach a spring to a ball, 
		if(a_end == null){                            // not the other way around.
		  //a_end.detachSpring(this);
			a_end = sa;
			sa.attachSpring(this);
		}
	}
	public void attachBend(SpringAttachable sa) {  // note: we attach a spring to a ball, 
		if (b_end == null){                           // not the other way around.
			//b_end.detachSpring(this);
			b_end = sa;
			sa.attachSpring(this);
		}
	}
	public void detachAend() {
		if (a_end == null)
			return;

		a_end.detachSpring(this);
		aLoosePosition = a_end.getPosition();
		a_end=null;
	}
	public void detachBend() {
		if (b_end == null)
			return;

		b_end.detachSpring(this);
		bLoosePosition = b_end.getPosition();
		b_end=null;
	}

	public double getAendPosition() {
		if (a_end != null)
			return a_end.getPosition();
		else 
			return aLoosePosition;
	}
	public double getBendPosition() {
		if (b_end != null)
			return b_end.getPosition();
		else
			return bLoosePosition;
	}
	public double getRestLength() {
		return restLength;
	}
	public double getForce(SpringAttachable ball) {
		double force = 0;
		if ((a_end == null) || (b_end == null))
			return force;
		if ((ball != a_end) && (ball != b_end))
			return force;
		double a_pos = getAendPosition();
		double b_pos = getBendPosition();
		double stretch = Math.abs(b_pos-a_pos)-restLength;
		force = stretch*stiffness;
		if ((ball==a_end)^(a_pos<b_pos)) return -force;
		return force;
	}
	public void updateView (Graphics2D g){
		view.updateView(g);
	}
	public void setSelected(){
		view.setSelected();
	}
	public void setReleased(){
		view.setReleased();
	}
	public boolean contains(double x, double y){
		return view.contains(x,y);
	}
	public String getDescription() {
		return "Spring_"+ getId()+":a_end\tb_end";
	}
	public String getState() {
		String s = getAendPosition() + "\t" + getBendPosition();
		return s;
	}

	public void computeNextState(double delta_t, MyWorld w) {
		//currentLength = Math.abs(b_end.getPosition() - a_end.getPosition());
		if ((b_end == null) || (a_end == null))
			delta_x = 0;
		else {
			currentLength = b_end.getPosition() - a_end.getPosition();
			delta_x = currentLength - restLength;
		}
	}
	
	public void updateState() {
	}

	public void dragTo(double x) {

		if ((a_end == null) && (b_end == null)) {
			aLoosePosition = x - currentLength/2;
			bLoosePosition = currentLength/2 + x;
		} else if ((a_end != null) && (b_end == null)) {
			currentLength = Math.abs(a_end.getPosition() - bLoosePosition);
			((PhysicsElement)a_end).dragTo(x - currentLength/2);
			bLoosePosition = currentLength/2 + x;
		} else if ((a_end == null) && (b_end != null)) {
			currentLength = Math.abs(aLoosePosition - b_end.getPosition());
			aLoosePosition = x - currentLength/2;
			((PhysicsElement)b_end).dragTo(currentLength/2 + x);
		} else if ((a_end != null) && (b_end != null)) {
			currentLength = Math.abs(a_end.getPosition() - b_end.getPosition());
			((PhysicsElement)a_end).dragTo(x - currentLength/2);
			((PhysicsElement)b_end).dragTo(currentLength/2 + x);
		}

	}
}