package Crypto;
 
import java.util.Scanner;
 
/**
 *
 * @author Михайл Скиба <michael.skiba@rub.de>
 */
public class Crypto {
 
    public static void main(String args[]) {
        while (true) {
            int a, p, remainder = 0, counter = 0;
            System.out.println("Please enter a");
            Scanner scanner = new Scanner(System.in);
            a = scanner.nextInt();
            System.out.println("Please enter p (must be prime)");
            scanner = new Scanner(System.in);
            p = scanner.nextInt();
 
            if (!isPrime(p)) {
                System.out.println(p + " is not a prime number!");
                System.exit(0);
            }
 
            do {
                counter++;
                if (counter == 1) {
                    remainder = a;
                } else {
                    remainder = (remainder * a) % p;
                }
                /* Debug-Line: System.out.printf("%d^%d = %d mod %d\n", a, counter, remainder, p); */
            } while (remainder != 1);
            System.out.printf("ord(%d)= %d", a, counter);
            if (counter == (p - 1)) {
                System.out.printf(" (%d is a primitiv element)", a);
            }
 
            System.out.printf("\nContinue y?\n");
            scanner = new Scanner(System.in);
            if (!scanner.nextLine().equalsIgnoreCase("y")) {
                System.exit(0);
            }
        }
    }
 
// Shamelessly stolen from the interwebs ...
// http://stackoverflow.com/questions/2385909/what-would-be-the-fastest-method-to-test-for-primality-in-java
    public static boolean isPrime(int n) {
        return !new String(new char[n]).matches(".?|(..+?)\\1+");
    }
}