diff between two files based on lcs

diff.rb

class Diff
  def initialize(_old,_new)
    @old = _old
    @new = _new
  end

  def results
     @global_index = {}
     @old.each_with_index{|v,i| @global_index[v] = i}
     @new.each_with_index{|v,i| @global_index[v] = i}
     result = diff(@old, @new).group_by(&:first).map{|k, v| {k => v.map(&:last).flatten}}.reduce Hash.new, :merge
     both = @old | @new
     ignore = []
     delta = 0
     both.each_with_index do |val, i|
       if ignore.include? val
         delta = delta + 1
         next
       end
       variant = result.select{|k,v| v.include?(val)}
       show_both = variant.values.flatten.map{|v| @global_index[v]}.uniq.count == 1
       p "#{i+1-delta} #{variant.keys.first} #{show_both ? variant.values.flatten.map(&:strip).join('|') : val.strip} "
       if show_both
         variant.values.flatten.each do |x|
           ignore.push x
         end
       end
     end
     result
  end

  def diff(_old, _new)
    _old ||= []
    _new ||= []

    old_index = {}
    _old.each_with_index do |val, i|
      (old_index[val] ||= []).push(i)
    end

    conjunction = {}

    start_old = 0
    start_new = 0
    length = 0

    _new.each_with_index do |val,i|
      _conjunction = {}
      Array(old_index[val]).each do |j|
        _conjunction[j] = (conjunction[j-1] || 0) + 1
        if (_conjunction[j] > length)
          length = _conjunction[j]
          start_old = j - length + 1
          start_new = i - length + 1
        end
      end
      conjunction = _conjunction
    end

    if length == 0
      modified = []
      _old.each{|i| _new.each{|j| modified.push(_old.delete(i),_new.delete(j)) if @global_index[i]==@global_index[j] }}
      return [[_old.empty? ? [] : ['-',_old]],[_new.empty? ? [] : ['+',_new]], [['*',modified]]].flatten(1).reject { |c| c.empty? }
    else
      return diff(_old[0,start_old],_new[0,start_new]) + [['=',_new[start_new, start_new + length -1]]] + diff(_old[start_old+length .. -1],_new[start_new+length..-1])
    end
  end
end

@file1 = File.readlines "file1.txt"
@file2 = File.readlines "file2.txt"
Diff.new(@file1,@file2).results

file1.txt

Some
Simple
Text
File

file2.txt

Another
Text
File
With
Additional
Lines