private static byte[] decodabetFor(char[] chars) {
      byte[] decodabet = new byte[Ascii.MAX + 1];
      Arrays.fill(decodabet, (byte) -1);
      for (int i = 0; i < chars.length; i++) {
        char c = chars[i];
        checkArgument(c < decodabet.length, "Non-ASCII character: %s", c);
        checkArgument(decodabet[c] == -1, "Duplicate character: %s", c);
        decodabet[c] = (byte) i;
      }
      return decodabet;
    }