Traverser<Character> traverser = Traverser.forGraph(DIAMOND_GRAPH);
    assertEqualCharNodes(traverser.breadthFirst(charactersOf("")), "");
    assertEqualCharNodes(traverser.breadthFirst(charactersOf("bc")), "bcd");
    assertEqualCharNodes(traverser.breadthFirst(charactersOf("a")), "abcd");
    assertEqualCharNodes(traverser.breadthFirst(charactersOf("acdb")), "acdb");

	rb.Read(buf)
	if rb.Length() != 3 {
		t.Fatalf("expect len 3 bytes but got %d. r.w=%d, r.r=%d", rb.Length(), rb.w, rb.r)
	}
	rb.Write([]byte(strings.Repeat("abcd", 15)))

	if !bytes.Equal(rb.Bytes(nil), []byte("bcd"+strings.Repeat("abcd", 15))) {
		t.Fatalf("expect 63 ... but got %s. r.w=%d, r.r=%d", rb.Bytes(nil), rb.w, rb.r)
	}

	rb.Reset()
	n, err = rb.Write([]byte(strings.Repeat("abcd", 16)))
	if err != nil {

    // These have a combination of invalid length, unrecognized characters and wrong padding.
    assertFailsToDecode(base64(), "AB=C", "Unrecognized character: =");
    assertFailsToDecode(base64(), "A=BCD", "Invalid input length 5");
    assertFailsToDecode(base64(), "?", "Invalid input length 1");
  }

  public void testBase64CannotUpperCase() {
    assertThrows(IllegalStateException.class, () -> base64().upperCase());