/*
   Copyright F. P. Marin ( May 22 16:18:46 1997 )
   E-mail: felix@bloch.ciens.ucv.ve
   
   	Permission to use, copy, modify and distribute this software and
   its documentation for NON-COMERCIAL purposes and without fee is hereby
   granted provided that this copyright notice appears in all copies.
   	F. P. Marin makes no representations or warranties about the
   suitability of the software, either express or implied, including but
   no limited to the implied warranties of merchantability, fitness for a
   particular purpose, or non-infringement.
   	F. P. Marin shall not be liable for any damage suffered by license
   as a result of using, modifying or distributing this software or its
   derivatives.

   HTML code is:

   <applet archive="IdealGas.jar"
              code="IdealGas.class"
            height=300
             width=300>
   <param name=delay    value="42">
   <param name=maxv     value="3">
   <param name=npoints  value="20">
   <param name=radius   value="2">
   </applet>
*/
import java.awt.*;


public class IdealGas extends java.applet.Applet implements Runnable
{         
       double X,Y;
       Graphics gr;
       int delay,diam,dx,dy,ip,maxv,np,rad,x[],y[],z[],vx[],vy[];
       Image img;
       Thread point=null;
       

       public void init()
       {
        int t;
        String s=getParameter("npoints");

        np=( s!=null ) ? Integer.parseInt(s):10;
        s=getParameter("maxv");
        maxv=( s!=null ) ? Integer.parseInt(s):3;
        s=getParameter("radius");
        rad=( s!=null ) ? Integer.parseInt(s):2;
        diam=2*rad;
        s=getParameter("delay");
        delay=( s!=null ) ? Integer.parseInt(s):42;
        x=new int[np];
        y=new int[np];
        vx=new int[np];
        vy=new int[np];
        dx=size().width - diam -1;
        dy=size().height - diam -1;
        for ( int i=0 ; i<np ; ++i ) {
            do {
                x[i]=(int)(rad + Math.floor(dx*Math.random()));
                y[i]=(int)(rad + Math.floor(dy*Math.random()));
                for ( t=0 ; t<i ; ++t ) 
                    if ( they_crash(i,t) ) break;
            } while ( t<i );
            vx[i]=( Math.random()<0.5 ) ? maxv:(-maxv);
            vy[i]=( Math.random()<0.5 ) ? maxv:(-maxv);
        }
        dx+=rad;
        dy+=rad;
        img=createImage(size().width,size().height);
        gr=img.getGraphics();
        setBackground(Color.blue);
       }


       public void start()
       {
        if ( point==null ) {
           point=new Thread(this);
           point.start();
        }
       }


       public void run()
       {
        boolean insidex,insidey;
        int j,oldx,oldy;

        repaint();
        try { Thread.sleep(delay); }
        catch (InterruptedException e) {}
        while ( true ) {
              for ( ip=0 ; ip<np ; ++ip ) { 
                  oldx=x[ip];
                  oldy=y[ip];
                  x[ip]+=vx[ip];
                  y[ip]+=vy[ip];
                  insidex=(  ( rad<=x[ip] ) && ( x[ip]<dx )  );
                  insidey=(  ( rad<=y[ip] ) && ( y[ip]<dy )  );
                  if (  ( insidex ) && ( insidey )  ) {
                     for ( j=0 ; j<ip ; ++j )
                         if ( they_crash(ip,j) ) break;
                     if ( j==ip ) {
                        while ( ++j<np ) if ( they_crash(ip,j) ) break;
                     }
                     if ( j<np ) {       
                        x[ip]=oldx;
                        y[ip]=oldy;
                        oldx=vx[ip];
                        vx[ip]=vx[j];
                        vx[j]=oldx;
                        oldx=vy[ip];
                        vy[ip]=vy[j];
                        vy[j]=oldx;
                     }
                        
                  } else { 
                     x[ip]=oldx;
                     y[ip]=oldy;
                     if ( !insidex ) vx[ip]=-vx[ip];
                     if ( !insidey ) vy[ip]=-vy[ip];
                  }
              }
              repaint();
              try { Thread.sleep(delay); }
              catch (InterruptedException e) {}
        }
       }


       public boolean they_crash(int i0,int i1)
       {
        X=Math.abs(x[i0] - x[i1]);
        Y=Math.abs(y[i0] - y[i1]);

               if ( X<Y ) {
                  X/=Y;
                  X=Y*Math.sqrt(1.0 + X*X);
        } else if ( X>0.0 ) {
           Y/=X;
           X*=Math.sqrt(1.0 + Y*Y);
        } 
        return ( X<=diam );
       }


       public void update(Graphics g)
       {
        paint(g);
       }

    
       public void paint(Graphics g)
       {
        gr.setColor(Color.blue);
        gr.fillRect(0,0,size().width,size().height);
        gr.setColor(Color.yellow);
        for ( int i=0 ; i<np ; ++i )
            gr.fillOval(x[i] - rad,y[i] - rad,diam,diam);
        g.drawImage(img,0,0,this);
       }

       
       public void stop()
       {
        if ( point!=null ) {
           point.stop();
           point=null;
        }
       }
}