/**
  * Vigenerova sifra - demonstrace
  * @author Pavel Micka
  */
 public class VigenereCipher {
     /**
      * Zasifruje text Vigenerovou sifrou
      * @param s otevreny text
      * @param key klic (jen velka pismena)
      * @return zasifrovany text (jen velka pismena)
      */
     public static String encipher(String s, String key){
         StringBuilder builder = new StringBuilder();
         for(int i = 0; i < s.length(); i ++){
             if(s.charAt(i) < 65 || s.charAt(i) > 90){ //znak v ASCII
                 throw new IllegalArgumentException("" +
                         "Sifrovany retezec neobsahuje jen velka pismena");
             }
             //modularne pricteme shift
             char encyphered = s.charAt(i) + getShift(key, i) > 90 ? (char)((s.charAt(i) + getShift(key, i)) - 26) : (char)(s.charAt(i) + getShift(key, i));
             builder.append(encyphered);
         }
         return builder.toString();
     }
     /**
      * Desifruje dany zasiforvany text Vigenerovou sifrou
      * @param s zasifrovany text
      * @param key klic
      * @return otevreny text
      */
     public static String decipher(String s, String key){
         StringBuilder builder = new StringBuilder();
         for(int i = 0; i < s.length(); i ++){
             if(s.charAt(i) < 65 || s.charAt(i) > 90){ //znak v ASCII
                 throw new IllegalArgumentException("" +
                         "Desifrovany retezec neobsahuje jen velka pismena");
             }
             //modularne odecteme shift
             char decyphered = s.charAt(i) - getShift(key, i) < 65 ? (char)((s.charAt(i) - getShift(key, i)) + 26) : (char)(s.charAt(i) - getShift(key, i));
             builder.append(decyphered);
         }
         return builder.toString();
     }
     /**
      * Zjisti shift
      * @param key klic
      * @param i pozice v textu
      * @return shift
      */
     private static int getShift(String key, int i) {
             if(key.charAt(i % key.length()) < 65 || key.charAt(i % key.length()) > 90){ //znak v ASCII
                 throw new IllegalArgumentException("" +
                         "Klic retezec neobsahuje jen velka pismena");
             }
         return ((int)key.charAt(i % key.length())) - 65;
     }
     
     public static void main(String[] args){
         String text = "KULTURNIATASEJESPION";
         String key = "PES";
         String enciphered = encipher(text, key);
         System.out.println(enciphered);
         System.out.println(decipher(enciphered, key));
     }
 }