/* lucaslehmerapplet implements the Lucas-Lehmer test of primality
    for Mersenne  primes               
                                      M. Gallant    01/99       */

import java.awt.*;
import java.applet.*;
import java.io.*;
import java.util.*;
import java.math.BigInteger;

public class lucaslehmerapplet extends Applet
    {
      public TextArea textArea = new TextArea (15, 50);
      public TextField tf1 = new TextField("3", 5) ;
      public TextField tf2 = new TextField("101", 5) ;
      Button testprimes = new Button("Test for Primes") ;
      Button clearbutton = new Button("Clear Text") ;
      Button printvalue = new Button("Print") ;
       BigInteger zero=new BigInteger("0") ;
       BigInteger one =new BigInteger("1") ;
       BigInteger two =new BigInteger("2") ;

    public void init ()
        {
       testprimes.setBackground(Color.red) ;
       clearbutton.setBackground(Color.white) ;

            add (clearbutton) ;
            add (tf1) ;
            add (tf2) ;
            add (testprimes) ;
            add (printvalue) ;
            add (textArea);
        }       

   public boolean action(Event evt, Object arg) {     //intercept AWT action.
    if (evt.target == clearbutton) {
     textArea.setText("") ;
     return true ;
     }
    else
      if (evt.target == testprimes) {
         getPrimes() ;
         return true;
       }              // end if
    else
      if (evt.target == printvalue) {
       int p = Integer.parseInt(tf2.getText()) ;
       textArea.append(" 2^"+p+"-1 = " + two.pow(p).subtract(one) + "\n") ;    
       return true;
      }

     else return false;
   }                 // end action handler


   private void getPrimes() {
     textArea.setText("") ;
     //BigInteger zero=new BigInteger("0") ;
     //BigInteger one =new BigInteger("1") ;
     //BigInteger two =new BigInteger("2") ;

     int p1 = Integer.parseInt(tf1.getText()) ;  // get command-line exponent  
     int p2 = Integer.parseInt(tf2.getText()) ;

     if(p1%2 == 0) { 
       textArea.append("--- Exponents must be odd ----") ;
       return ;
     }

    if(p2<p1)
     p2 = p1 ;

    for(int p=p1; p<=p2; p+=2) {  //only use odd exponents.
      BigInteger mersenne = two.pow(p).subtract(one) ;

      BigInteger lucas = new BigInteger("4") ;  //initialize
       for(int i=3; i<=p; i++)
        lucas = lucas.pow(2).subtract(two).mod(mersenne) ;

      if(lucas.compareTo(zero) == 0){
       textArea.append("***** 2^"+p+"-1  is prime: " + mersenne +"\n") ;
       }
      else
       textArea.append(" 2^"+p+"-1  is composite.\n") ;
 
    }
   } // end getPrimes()

 }