You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: diffviewer.rb
    path of file: ./repos/narou/lib
    
    the code of the file until where you have to start completion: # frozen_string_literal: true

#
# Copyright 2013 whiteleaf. All rights reserved.
#

require "diff/lcs"

#
# 指定されたソースの差分を作成する
#
# 書式はなんちゃってu
    
  def output_differing_line_and_before_and_behind(index, force = false)
    last = index + 1
    return last if @buffer[index] || index < 0
    event = @events[index] or return last
    if %w(! + -).include?(event.action)
      @buffer[index] = [event, decorate_event(event)]
      ((index - 3)..(index + 3)).each do |i|
        output_differing_line_and_before_and_behind(i, true)
      end
      last = index + 3 + 1
    elsif force
      @buffer[index] = [event, " #{event.old_element}"]
    end