// 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.

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

class Mass
{
	public int x;
	public int y;
	public int m;

	public Mass(int xx, int yy, int mm)
	{
		x = xx;
		y = yy;
		m = mm;
	}
};

public class GravField extends Applet
{
	//public static final double G = 0.0000000000667; // real life
	public static final double G = 1; // :)  !!

	private TextField massField;
	private Vector	  masses;

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

	

	public void init()
	{
		this.setBackground(Color.white);
	
		massField = new TextField("8000",10);
		this.add(new Label("Mass: "));
		this.add(massField);

		masses = new Vector();
	}

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

		return true;
	}

	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);
			//System.out.println("hello 2");
		
		Mass m = null; 
		Mass massar[] = new Mass[masses.size()];
		int i = 0;
		int numMasses;

		for ( Enumeration e = masses.elements() ;
				 e.hasMoreElements() ; 
				  )
		{
			m = (Mass)e.nextElement();
			//System.out.println("hello in enum");
			gr.fillRect(m.x,m.y,3,3);
			//System.out.println("done with enum");
			massar[i++] = m;
		}
		numMasses = i;

		gr.setColor(Color.red);

		int h,v;

		double gmin = 999999999;
		double gmax = 0;

		for(v=40;v<rect.height;v += 1)
		{
			for(h=0;h<rect.width;h += 1)
			{
				gx = 0;
				gy = 0;
				gnet = 0;
				
				for(i=0;i<numMasses;i++)
				{
					m = massar[i];
				
					calculateForce(m,h,v);
				}
	
				if ( ! Double.isInfinite( gnet ) )
				{ /* don't count the infinite values! */
					if ( gnet > gmax )
					{
						gmax = gnet;	
					}
	
					if ( gnet < gmin )
					{
						gmin = gnet;
					}					
				}
			}
		}

		System.out.println("Gmax = " + gmax + ", Gmin = " + gmin);


		/* now that we know min and max, let's paint! */ 

		double logmax = Math.log(gmax);
		double logmin = Math.log(gmin);

		double logspread = logmax - logmin;
		if (( gmax == 0) || ( logspread == 0))
		{
			gr.drawString("Even field", 10,20);
		}
		else
		{

		// first the field
		for(v=40;v<rect.height;v += 1)
		{
			for(h=0;h<rect.width;h += 1)
			{
				gx = 0;
				gy = 0;
				gnet = 0;
				
				for(i=0;i<numMasses;i++)
				{
					m = massar[i];
				
					calculateForce(m,h,v);
				}
	
				if ( ! Double.isInfinite( gnet ) )
				{ /* don't count the infinite values! */
					float intensity = (float)((Math.log(gnet)-logmin)/logspread);	

					gr.setColor(new Color(intensity,intensity,intensity));
					gr.drawRect(h,v,1,1);	
				}
			}
		}

		// now the vectors

		for(v=40;v<rect.height;v += 1)
		{
			for(h=0;h<rect.width;h += 1)
			{
				gx = 0;
				gy = 0;
				gnet = 0;
				
				for(i=0;i<numMasses;i++)
				{
					m = massar[i];
					calculateForce(m,h,v);
				}
	
				if ( ! Double.isInfinite( gnet ) )
				{ /* don't count the infinite values! */
					int ARSZ = 30;

					if ( ((h % ARSZ)==0) && ((v % ARSZ)==0) )
					{
						// draw arrows

						double rx = (gx/gnet);
						double ry = (gy/gnet);

						// length should be from 0-(ARSZ*.75)
						double arlen = (gnet/gmax) * 1000;
						gr.setColor(Color.green);
						gr.drawLine(h,v,(int)(h+(rx*arlen)),(int)(v+(ry*arlen)));
						gr.setColor(Color.blue);
						gr.drawRect((int)(h+(rx*arlen)),(int)(v+(ry*arlen)),1,1);

					}
				}
			}
		}


		} // end 'else'

	}

	private void calculateForce(Mass m, int h, int v)
	{
		// RETURNS it's result by adding to gnet/gx/gy

					double rsquared = ((h-m.x)*(h-m.x)) + 
						      ((v-m.y)*(v-m.y));
					double r =  /* distance */
						java.lang.Math.sqrt( rsquared );

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

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

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

};