package org.mersenne;

import java.util.Scanner;
import java.math.BigInteger;
import java.io.*;
/**
* Java implementation of LL.
**/

class SimpleLucas {
  public static void main(String args[]) {
   BigInteger sumBig = BigInteger.valueOf(4);

   System.out.println("Enter exponent: ");

   Scanner sc = new Scanner(System.in);
   int iIteration = sc.nextInt();

   System.out.print("Start calculating ... ");
   if (isPrime(iIteration)==false) {
     System.out.println("Exponent is not prime!");
     return;
     }
   System.out.println("Your exponent "+iIteration+" is prime!");

   BigInteger bi1, bi2, MB, minusTwo;
   bi1 = new BigInteger("2");
   bi2 = bi1.pow(iIteration);
   MB = bi2.add(BigInteger.valueOf(-1));
   minusTwo = new BigInteger("-2");

   long startCounter = System.currentTimeMillis();
   BigInteger powBig;

   System.out.println("Variables is init");

    for (int x=1;x<=iIteration-2;x++) {
       long startIt = System.currentTimeMillis();
       powBig = sumBig.pow(2);
       powBig = powBig.add(minusTwo);
       sumBig = powBig.mod(MB);
       if (x==1){
         long stopCounter = System.currentTimeMillis() - startCounter;
         System.out.println("time for first iteration, ms: "+stopCounter);
       }
       long stopIt = System.currentTimeMillis() - startIt;
       if (x%1000==0) System.out.println("iteration: "+x+" time: "+stopIt);
     }

  if (sumBig.equals(BigInteger.valueOf(0)))
     System.out.println(iIteration + " is prime!");
  else {
     String resString = sumBig.toString(16).toUpperCase();
     int lenghtResString = resString.length();
     if (lenghtResString>16) resString=resString.substring(lenghtResString-16,lenghtResString);
     System.out.println(iIteration + " is not prime! Res64:" + resString);

     try (FileOutputStream results = new FileOutputStream("results.txt")) {
          for (int k=0;k<=15;k++)
            results.write(resString.charAt(k));
       } catch (IOException e) {
         System.out.println("Error to write file:" + e );
       }
  }
  }
/**
 * Is it prime?
 */
    private static boolean isPrime(int N) {
        if (N < 2) return false;
        for (int i = 2; i*i <= N; i++)
            if (N % i == 0) return false;
        return true;
    }
}