\input TeX4ht.sty
\Preamble{html,sty}      \EndPreamble

\input  ProTex.sty
\AlProTex{java,<<<>>>,list,title,`}


\HCode{<APPLET CODE="turtle.class" WIDTH="300" HEIGHT="300"
> </APPLET>}


\HPage{code}

A mini-turtle program.

\<turtle\><<<
import java.applet.*;
import java.awt.*;
public class turtle extends Applet
{

   Graphics g;
   `<vars`>
   `<turtle functions`>
   public void init()
   {
     `<initial state`>
     `<draw interface`>
   }
   public void paint(Graphics g) {
     this.g = g;
     `<execute commands`>
   }
   public boolean action(Event e,Object o)
   {
     `<get command`>
     repaint();
     return true;
   }

}
>>>


The turtle can move forward without trace, move forward
while tracing its path, and rotate.


\<turtle functions\><<<
   void Move(int x, int y){ this.x += x;  this.y -= y; }

   void Move(int r){ x += (int) (r * Math.cos(d));
                     y -= (int) (r * Math.sin(d)); }

   void Line(int x, int y){
      g.drawLine( this.x, this.y,  this.x+x, this.y-y );
      Move(x,y);
   }
   void Line(int r){
    Line(   (new Double(r * Math.cos(d))).intValue(),
            (new Double( r * Math.sin(d) )).intValue() ); }

   void Rotate(int d){
      this.d += d * 3.14  / 180;
      while( 6.28 < this.d ){ this.d -= 6.28; }
      while( this.d < 0 ){ this.d += 6.28; }
   }

>>>


The text field may be modified by the users.

\<vars\><<<
Button moveButton,
       lineButton,
       clearButton,
       rotateButton;
TextField in;
>>>


\<draw interface\><<<
clearButton  = new Button("Clear");   add(clearButton);
moveButton   = new Button("Move");    add(moveButton);
lineButton   = new Button("Line");    add(lineButton);
rotateButton = new Button("Rotate");  add(rotateButton);
in           = new TextField(3);      add(in);
in.setText( "50" );
>>>


\<execute commands\><<<
      int i;
  x = 150; y = 150; d = 0;
  for( i=0; i<N; i+=2 ){

     switch( command[i] ) {

case   `<Line op`>:{    Line( command[i+1] ); break; }
case   `<Move op`>:{    Move( command[i+1] ); break; }
case `<Rotate op`>:{  Rotate( command[i+1] ); break; }

}
}
>>>

\<get command\><<<
if("Clear".equals(o)) {  `<initial state`>  }
else{

   if(  "Line".equals(o)){  command[N]=`<Line op`>;   }
   if(  "Move".equals(o)){  command[N]=`<Move op`>;   }
   if("Rotate".equals(o)){  command[N]=`<Rotate op`>; }

   String s =  new String( in.getText() );
   Integer i = new  Integer(     s  );
   command[N+1]=   i.intValue();
   N+=2;

}
>>>

\<vars\><<<
int command[] = new int[300];
>>>

\<Line op\><<<
1
>>>

\<Move op\><<<
2
>>>

\<Rotate op\><<<
3
>>>


\<initial state\><<<
N = 0;
>>>

\<vars\><<<
int N;
int x, y;
double d;
>>>


\OutputCode\<turtle\>

\EndHPage{}

\bye