public class Vigenère_Cipher {

    public static String encode(String key, String message) {
        StringBuilder newKey;
        key = key.toUpperCase();
        message = message.toUpperCase();
        if (key.length() < message.length()) {
            int i = 0;
            newKey = new StringBuilder();
            while (newKey.length() < message.length()) {
                if (i >= key.length()) {
                    i = 0;
                    newKey.append(key.charAt(i));
                }
                else {
                    newKey.append(key.charAt(i));
                }
                i++;
            }
        }
        else if (key.length() > message.length()) {
            newKey = new StringBuilder(key.substring(0, message.length()));
        }
        else {
            newKey = new StringBuilder(key);
        }

        StringBuilder encodedMessage = new StringBuilder();
        String alphabets = "abcdefghijklmnopqrstuvwxyz".toUpperCase();
        for (int i=0;i<message.length();i++) {
            int index1 = alphabets.indexOf(message.charAt(i));
            int index2 = alphabets.indexOf(newKey.charAt(i));
            encodedMessage.append(alphabets.charAt((index1 + index2) % 26));
        }


        return encodedMessage.toString();
    }

    public static void main(String[] args) {
        System.out.println(encode("REDDIT", "TODAYISMYBIRTHDAY"));
    }
}