public long[] decode(String hash) {
        if (hash.isEmpty()) {
            return new long[0];
        }

        String validChars = this.alphabet + this.guards + this.seps;
        for (int i = 0; i < hash.length(); i++) {
            if (validChars.indexOf(hash.charAt(i)) == -1) {
                return new long[0];
            }
        }

        return this._decode(hash, this.alphabet);
    }
@Test
    public void test_wrong_decoding() {
        final Hashids a = new Hashids("this is my pepper");
        final long[]  b = a.decode("NkK9");
        Assert.assertEquals(b.length, 0);
    }