import java.util.Scanner;

public class Encryption {
    /* File: Encryption.java
    * -------------------------------
    * This program will read in a four-digit number and encrypt/decrypt it based on a set algorithm.
    * 1. Replace each digit by (the sum of that digit plus 7) modulus 10.
    * 2. Swap the first digit with the third, and swap the second digit with the forth.
    */

    public static void main ( String[] args )
    {

        Scanner scan = new Scanner( System.in );

        while(true) {
            System.out.printf("SUPER Encryption! Enter a four-digit number and we will encrypt or decrypt it for you.\n");

            //These lines will ask the user if he/she wants to encrypt a number or decrypt a number.
            System.out.printf("Would you like to encrypt, or decrypt today? Enter enc or dec.\n");

            String EncTest = scan.nextLine();

            //Provides error checking in case the user enters invalid input.
            while(EncTest.equalsIgnoreCase("enc") != true && EncTest.equalsIgnoreCase("dec") != true)   {

                System.out.printf("Please answer either enc or dec.\n");
                EncTest=scan.nextLine();
            }

            //This line will ask the user for their input, as an int. Ptext stands for plain text.
            System.out.printf("\nWhat number are you wishing to encrypt or decrypt? Please enter only one.\n");
            String ptext = scan.nextLine();

            //Errorchecking if entered number is different than 4.
            while(ptext.length() != 4) {
                System.out.printf("Please enter a four-digit number.\n");
                ptext = scan.nextLine();
            }

            //Will run the encrypt or decrypt functions.
            if (EncTest.equalsIgnoreCase("enc")){
                    System.out.printf("Your encrypted number is: %s\n", encrypt(ptext));
            }

            else{
                System.out.printf("Your decrypted number is: %s\n", decrypt(ptext));
            }

            //Asks the user whether he/she wants to rerun the program.
            System.out.printf("Do you wish to run this program again? Answer yes or no.\n");
            String rerun = scan.nextLine();

            //Provides error checking in case the user enters invalid input.
            while(rerun.equalsIgnoreCase("yes") != true && rerun.equalsIgnoreCase("no") != true)    {
                System.out.printf("Please answer either yes or no.\n");
                rerun=scan.nextLine();
            }

            if(rerun == "no")System.exit(0);
        }
    }


    /* Method: encrypt
     * This method will help encrypt a four-digit number using the algorithm stated above.
     * Parameters: String to be encrypted.
     * Return: Encrypted String
     */

public static String encrypt (String ptext){

    //Makes a char array to represent the string ptext.
    char[] etext = ptext.toCharArray();

    //Step one of the algorithm. Replace each digit by (the sum of that digit plus 7) modulus 10.
    for(int i = 0; i < ptext.length(); i++) {

        //Changes etext[i] to a string, then parses to int, then applies the math.
        //The + 48 is required at the end to parse the values back into decimal integers instead of ASCII values.
        etext[i] =  (char) (((Integer.parseInt (Character.toString (etext[i])) + 7) % 10) + 48);

    }

    //Step two. To replace the first digit with the third, and swap the second digit with the forth.
    char temp;

    temp = etext[0];
    etext[0] = etext[2];
    etext[2] = temp;

    temp = etext[2];
    etext[1] = etext[3];
    etext[3] = temp;

    return new String(etext);
}


public static String decrypt (String ptext){

    //Makes a char array to represent the string ptext.
        char[] etext = ptext.toCharArray();

        //Step one of the algorithm in reverse. Replace each digit by (the sum of that digit plus 3) modulus 10.
        for(int i = 0; i < ptext.length(); i++) {

            //Changes etext[i] to a string, then parses to int, then applies the math.
            //The + 48 is required at the end to parse the values back into decimal integers instead of ASCII values.
            etext[i] =  (char) (((Integer.parseInt (Character.toString (etext[i])) + 3) % 10) + 48);

        }

        //Step two. To replace the first digit with the third, and swap the second digit with the forth.
        char temp;

        temp = etext[0];
        etext[0] = etext[2];
        etext[2] = temp;

        temp = etext[2];
        etext[1] = etext[3];
        etext[3] = temp;

        return new String(etext);
    }

}