public class Encryption {
    public static final String ALPHABET = "abcdefghijklmnopqrstuvwxyz0123456789";
    private int shift;

    public Encryption(int shift) {
        this.shift = shift;
    }

    public String encrypt(String plaintext) {
        String ciphertext = "";
        for (int i = 0; i < plaintext.length(); i++) {
            char c = plaintext.charAt(i);
            if (Character.isLetter(c) || Character.isDigit(c)) {
                // encrypt character c
                int index = ALPHABET.indexOf(c);
                int shiftedIndex = (index + this.shift) % ALPHABET.length();
                c = ALPHABET.charAt(shiftedIndex);
            }
            ciphertext += c;
            updateShift(c);
        }
        return ciphertext;
    }

    private void updateShift(char c) {
        String previousCipher = "" + c + this.shift;
        int hash = previousCipher.hashCode();
        this.shift = Math.abs(hash) % ALPHABET.length();
    }

}