// Caesar.java: implement the Caesar cipher
//  This carries out a simple rotation of lower-case letters,
//  and does nothing to all other characters, making the decryption
//  process even easier, because caps and punctuation marks survive unchanged.
//  Usage: java Caesar (-d | -e) key
//  Above, option "-d" is for decryption, while "-e" is for encryption
import java.io.*;
public class Caesar {
  private Reader in; // standard input stream for message
  private int key; // (en|de)cryption key

  // Caesar: constructor, opens standard input, passes key
  public Caesar(int k) {
   // open file
   in = new InputStreamReader(System.in);
   key = k;
  }
  // (en|de)crypt: just feed in opposite parameters
  public void encrypt() { translate(key); }
  public void decrypt() { translate(-key); }

  // translate: input message, translate
  private void translate(int k) {
   char c;
   while ((byte)(c = getNextChar()) != -1) {
     if (Character.isLowerCase(c)) {
      c = rotate(c, k);        
     }
     System.out.print(c);
   }
  }

  // getNextChar: fetches next char.
  public char getNextChar() {
   char ch = ' '; // = ' ' to keep compiler happy
   try {
     ch = (char)in.read();
   } catch (IOException e) {
     System.out.println("Exception reading character");
   }
   return ch;
  }
  // rotate: translate using rotation, version with table lookup
  public char rotate(char c, int key) { // c must be lowercase
   String s = "abcdefghijklmnopqrstuvwxyz";
   int i = 0;
   while (i < 26) {
     // extra +26 below because key might be negative
     if (c == s.charAt(i)) return s.charAt((i + key + 26)%26);
       i++;
   }
   return c;
  }


  // main: check command, (en|de)crypt, feed in key value
  public static void main(String[] args) {
   if (args.length != 2) {
     System.out.println("Usage: java Caesar (-d | -e) key");
     System.exit(1);
   }
   Caesar cipher = new Caesar(Integer.parseInt(args[1]));
   if (args[0].equals("-e")) cipher.encrypt();
   else if (args[0].equals("-d")) cipher.decrypt();
   else {
     System.out.println("Usage: java Caesar (-d | -e) key");
     System.exit(1);
   }
  }
}