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

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

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

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

}