last--;
    }

    return (first == 0 && last == len - 1)
        ? collapseFrom(sequence, replacement)
        : finishCollapseFrom(
            sequence, first, last + 1, replacement, new StringBuilder(last + 1 - first), false);
  }

  private String finishCollapseFrom(
      CharSequence sequence,
      int start,
      int end,
      char replacement,
      StringBuilder builder,