// The highest index we've seen so far.
  private int max = -1;

  /** Construct a new sparse array builder. */
  public CharEscaperBuilder() {
    this.map = new HashMap<>();
  }

  /** Add a new mapping from an index to an object to the escaping. */
  @CanIgnoreReturnValue
  public CharEscaperBuilder addEscape(char c, String r) {
    map.put(c, checkNotNull(r));
    if (c > max) {
      max = c;
    }
    return this;