/** Helper to manually escape a 7-bit ascii character */
  private String escapeAscii(char c) {
    Preconditions.checkArgument(c < 128);
    String hex = "0123456789ABCDEF";
    return "%" + hex.charAt((c >> 4) & 0xf) + hex.charAt(c & 0xf);
  }

  /** Helper to manually escape a 7-bit ascii character */
  private String escapeAscii(char c) {
    Preconditions.checkArgument(c < 128);
    String hex = "0123456789ABCDEF";
    return "%" + hex.charAt((c >> 4) & 0xf) + hex.charAt(c & 0xf);
  }

      super(alphabet, null);
      checkArgument(alphabet.chars.length == 16);
      for (int i = 0; i < 256; ++i) {
        encoding[i] = alphabet.encode(i >>> 4);
        encoding[i | 0x100] = alphabet.encode(i & 0xF);
      }
    }

    @Override
    void encodeTo(Appendable target, byte[] bytes, int off, int len) throws IOException {
      checkNotNull(target);
      checkPositionIndexes(off, off + len, bytes.length);