// Tyler Rigsby, CSE 142
// Allows encoding and decoding of secret messages via a Caesar Cipher
import java.util.*;

public class CaesarCipher {
    public static void main(String[] args) {
        System.out.println("Welcome to the CSE 142 Caesar Cipher program!");
        
        Scanner console = new Scanner(System.in);
        
        System.out.println("Enter your message to encode or decode: ");
        String message = console.nextLine();
        message = message.toLowerCase();
        
        System.out.print("Please enter the key: ");
        int key = console.nextInt();
        
        System.out.print("Do you want to (E)ncode or (D)ecode? ");
        String answer = console.next().toLowerCase();
        
        if (answer.startsWith("e")) {   
            String encoded = shift(message, key);
            System.out.println("The encoded message is: " + encoded);
        } else {
            String decoded = shift(message, -key);
            System.out.println("The decoded message is: " + decoded);
        }   
    }
    
    // Returns a String with all the letters of message in the a-z range shifted
    // by the value of key. e.g. "foo" with a key of 2 becomes "hqq"
    public static String shift(String message, int key) {
        String result = "";
        for (int i = 0; i < message.length(); i++) {
            char letter = message.charAt(i);
            
            // only want to apply shift if the character is between a-z
            if (letter >= 'a' && letter <= 'z') {
                letter = (char) (letter + key); // or, letter += key

                // if that shifts it outside the a-z range, subtract or add 26 to
                // loop back around
                if (letter > 'z') {
                    letter -= 26;
                } else if (letter < 'a') {
                    letter += 26;
                }
            }
            result = result + letter; // or, result += letter
        }   
        return result;
    }
}