class Test::Unit::Diff::ReadableDiffer

def compute_width(line, start, _end)
  if line.respond_to?(:encoding) and
      Encoding.compatible?(Encoding::UTF_8, line.encoding)
    utf8_line = line[start..._end].encode(Encoding::UTF_8)
    width = 0
    utf8_line.each_codepoint do |unicode_codepoint|
      if UTF8Line.wide_character?(unicode_codepoint)
        width += 2
      else
        width += 1
      end
    end
    width
  elsif line.is_a?(UTF8Line)
    line.compute_width(start, _end)
  else
    _end - start
  end
end