def get_change_extent(str1, str2)
        start = 0
        limit = [str1.size, str2.size].sort.first
        while start < limit and str1[start, 1] == str2[start, 1]
          start += 1
        end
        ending = -1
        limit -= start
        while -ending <= limit and str1[ending, 1] == str2[ending, 1]
          ending -= 1
        end

        return [start, ending + 1]
      end
def get_change_extent(str1, str2)
        start = 0
        limit = [str1.size, str2.size].sort.first
        while start < limit and str1[start, 1] == str2[start, 1]
          start += 1
        end
        ending = -1
        limit -= start
        while -ending <= limit and str1[ending, 1] == str2[ending, 1]
          ending -= 1
        end

        return [start, ending + 1]
      end
Determines the extent of differences between two string. Returns
 an array containing the offset at which changes start, and then
 negative offset at which the chnages end. If the two strings have
 neither a common prefix nor a common suffic, [0, 0] is returned.