// GravField
// 5/20/1999
// (c) 1999 steven r. loomis <srl@monkey.sbay.org> 
// http://www.monkey.sbay.org/~srl
// shows gravity field strength and vectors for a given set of masses.

package us.ca.sj.monkey.srl;

import java.applet.*;
import java.awt.*;
import java.util.*;
import java.lang.*;

class Mass
{
	public double x;
	public double y;
	public double dx;
	public double dy;
	public double m;
	public boolean fixed;

	public Mass(double xx, double yy, double dxx, double dyy, double mm,boolean ff)
	{
		x = xx;
		y = yy;
		m = mm;
		dx = dxx;
		dy = dyy;
		fixed = ff;
	}

	public void Draw(Graphics g)
	{
		int r = (int)(m);
		if(fixed == true)
			g.setColor(Color.black);
		else	
			g.setColor(Color.blue);
		g.fillOval((int) (x-(r/2)), (int)(y-(r/2)), r,r);

		if(fixed == false)
		{
			g.setColor(Color.red);
			g.drawLine(  (int) (x), (int)(y) ,  (int) (x+dx), (int)(y+dy)  );
		}
	}

	public void Erase(Graphics g)
	{
		int r = (int)(m);
		g.setColor(Color.white);

		g.fillRectl((int) (x-(r/2)), (int)(y-(r/2)), r,r);
	}

	public void doIdle()
	{
		x += dx;
		y += dy;
	}
};

public class GSim1 extends Applet implements Runnable 
{
	//public static final double G = 0.0000000000667; // real life
	public static final double G = 20; // :)  !!

	private TextField massField;

	private TextField dxField;

	private TextField dyField;
	private Vector	  masses;
	private Thread	fThread = null;

	// GRRRRRRRRRRRRRRRRRRRRRRRR
				double gx = 0;
				double gy = 0;
				double gnet = 0;

	public void run()
	{
		for(;;)
		{

		Mass m = null; 
		Mass massar[] = new Mass[masses.size()];
		int i = 0;
		int k;
		int numMasses;

		for ( Enumeration e = masses.elements() ;
				 e.hasMoreElements() ; 
				  )
		{
			m = (Mass)e.nextElement();
		
			m.doIdle();

			massar[i++] = m;
		}
		numMasses = i;


		for(i=0;i<numMasses;i++)
		{
		    if(!massar[i].fixed)
		    {
					/* m1= m[k] will affect m2= m[i].   */
					gx=0;
					gy=0;	
					gnet=0; /* probably unused */

			for(k=0;k<numMasses;k++)
			{
				if(k != i)
				{
					calculateForce(massar[k],massar[i]);
				}
			}
	
			massar[i].dx += (gx/massar[i].m);
			massar[i].dy += (gy/massar[i].m);
		    }
		}



			repaint();

	//		System.out.println("Thread awoke");
			try
			{
				Thread.sleep(50);
			}
			catch(Throwable t)
			{
				System.out.println(t.toString());
				return;
			}
		}
	}
	
	public void start()
	{
		fThread.start();
	}

	public void stop()
	{
		fThread.stop();
	}

	public void init()
	{
		this.setBackground(Color.white);
	
		massField = new TextField("40",10);
		this.add(new Label("Mass: "));
		this.add(massField);
		dxField = new TextField("0",10);
		this.add(new Label("dx: "));
		this.add(dxField);
		dyField = new TextField("0",10);
		this.add(new Label("dy: "));
		this.add(dyField);

		masses = new Vector();
		fThread = new Thread(this);
	}

	public boolean mouseDown(Event e, int x, int y)
	{
		try
		{
			masses.addElement(
				new Mass(x,y,new Integer(dxField.getText()).intValue(),new Integer(dyField.getText()).intValue(),
				new Integer(massField.getText()).intValue(), (e.modifiers & Event.SHIFT_MASK) > 0));
			repaint();
		}
		catch ( Throwable t )
		{
			System.out.println("click failed: " + t.toString());
		}

		return true;
	}

	int n = 4;

	public void paint(Graphics gr)
	{
		//System.out.println("hello 1");

		Rectangle rect = this.bounds();
		gr.setColor(this.getBackground());
		gr.fillRect(rect.x,rect.y,rect.width,rect.height);

		gr.setColor(Color.blue);
		
		Mass m = null; 

		for ( Enumeration e = masses.elements() ;
				 e.hasMoreElements() ; 
				  )
		{
			m = (Mass)e.nextElement();
			
			m.Draw(gr);
		}


	}

	private void calculateForce(Mass m1, Mass m2 ) /* force of m1 on m2 */
	{
		// RETURNS it's result by adding to gnet/gx/gy

					double rsquared = ((m1.x-m2.x)*(m1.x-m2.x)) + 
						      ((m1.y-m2.y)*(m1.y-m2.y));
					double r =  /* distance */
						java.lang.Math.sqrt( rsquared );

					double rhatx = m2.x-m1.x;  /* x and y components of rhat  [point p->m[i] ] */
					double rhaty = m2.y-m1.y;

					rhatx /= r;
					rhaty /= r;
					
					double g = - (G * m1.m * m2.m)/rsquared;

					gnet += g; /* add up net g */
					gx += (rhatx * g);
					gy += (rhaty * g);
	}

};