//      ******  The Java AWT Pendulum Annimation Applet *****
//  **** Version 2 uses image double-buffering and time correction *****

import java.awt.Image;
import java.awt.Button;
import java.awt.Event;
import java.awt.FlowLayout;
import java.awt.TextField;
import java.awt.Graphics;
import java.awt.Insets;
import java.awt.Label;
import java.awt.Font;
import java.awt.Color;
import java.awt.Rectangle;
import java.awt.FontMetrics;  

public class penduldb extends java.applet.Applet implements Runnable {
   Image offscreenIm;
   Graphics offscreenGr;
   Thread runner;
   final float pi = 3.141592653f ;
   int xtie, xp, yp, xold, yold; //width and height of plot.    
   int pendWrite = 110; //vertical height of pendulum region to paint.
   int wApp, hApp, dispDelay;
   public float grav, leng, omega, phase, thetao=pi/10; // pendulum parameters.
   String inParm;
   Color bgnd = Color.white;
   String slabel = "Length = ";
   Label l1;
   long timeNow, tRef;

public void init() {                            // get speed parameter from html file.
             wApp = this.size().width;
             hApp = this.size().height;              
            offscreenIm = createImage(wApp, pendWrite);
            offscreenGr = offscreenIm.getGraphics();
 
            Font f = new Font("TimesRoman", Font.PLAIN, 10);
            FontMetrics fm = getFontMetrics(f);
            setFont(f) ;

     inParm=getParameter("gravity") ;
        if (inParm==null)                         //  if no user input in html file.
          grav=9.80f;                              //    m/sec^2
        else  grav = Float.valueOf(inParm).floatValue();
     
     inParm=getParameter("length") ;
        if (inParm==null)                       //  if no user input in html file.
          leng=1.0f;                                  //    meters.
        else leng =  Float.valueOf(inParm).floatValue();
        
    
     inParm=getParameter("displayDelay") ;
        if (inParm==null)                      //  if no user input in html file.
          dispDelay=500;                       //half a second default.
        else dispDelay = Integer.parseInt(inParm);
          
           setLayout(new FlowLayout());
           Button fast = new Button("Faster");
           Button slow = new Button("Slower");
           l1 = new Label(slabel+(int)(leng*100)+"cm");         
           add(l1);           
           add(slow);
           add(fast);
        


 }     // end of init.


public Insets insets() {
 return new Insets(pendWrite+2,2,2,2);
  }

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

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

 
 public void pause(long time) {
            try { Thread.sleep(time);}  // wait for time milliseconds.
            catch (InterruptedException e) { }
       }



  public void run() {
              omega =(float) (1E-3*Math.sqrt(grav/leng) );
             // System.out.println("\n-------- Applet  Data --------");
             // System.out.println("Applet Width = "+wApp);
             // System.out.println("Applet Height = "+hApp);
             // System.out.println("-------- Pendulum Data --------");              
             // System.out.println("Pendulum Length = " + leng + " meters");
             // System.out.println("Gravity = " + grav + " m/sec^2");
             // System.out.println("Expected period = " + 2*pi/(omega*1000) +" secs");
              xtie=wApp/2 ; // center applet tie point.
          timeNow = tRef = System.currentTimeMillis();  //start time.
                 while (true) {
                    omega =(float) (1E-3*Math.sqrt(grav/leng) );
                    phase = (float) (thetao*Math.sin(omega*(timeNow-tRef)) );  
                 xp =(int) (xtie - 100*Math.sin(phase) );   //current values
                 yp =(int) ( 100*Math.cos(phase) ); 
                   repaint();
                   timeNow  += dispDelay ;    
           pause(Math.max(0, timeNow - System.currentTimeMillis()));
     //System.out.println("Time: " +System.currentTimeMillis()) ;   
           }
  }

 public void update(Graphics g) {         //override update method.
     paint(g);
  }
 

 public void paint(Graphics g) {           
               if (timeNow > dispDelay) {                //don't draw at start time.
                offscreenGr.setColor(bgnd);
                offscreenGr.fillRect(0,0,wApp,pendWrite);     // remove anything from other interrupts. 
                offscreenGr.setColor(Color.black) ;
                offscreenGr.drawLine(xtie,0,xp,yp) ;
                offscreenGr.setColor(Color.red) ;
                offscreenGr.fillOval(xp-5, yp-5, 10, 10) ;  //draw circle centered at end point of pendulum.
               }
    else   {
     offscreenGr.setColor(bgnd);   //for viewers that don't automatically set the backgnd.
     offscreenGr.fillRect(0,0,wApp,pendWrite);    
           
         }   
 
  g.drawImage(offscreenIm,0,0,this);  //write buffered image to screen.

 } 


 public boolean action(Event evt, Object arg) {   //intercept AWT action.
      if (evt.target instanceof Button) 
         adjustSpeed((String)arg);
       return true;
     }


// ------------ Handler for Button Press ----------------

   void adjustSpeed( String buttonName) {
       if (buttonName.equals("Faster")){ 
            leng = leng/1.5f; //increase speed by 50% each click.
            if (leng*1000<1) leng = 1;       // set shortest pendulum. 
            l1.setText(slabel + (int)(leng*100)+"cm"); // display in cm.
            }  
       if (buttonName.equals("Slower")) {
            leng =leng*1.5f;           
            l1.setText(slabel +(int)(leng*100)+"cm");
           }
     
     }



 
   public String getAppletInfo() {
   return "Copyright 1996 Michel I. Gallant";
  } 



}