class Song
  def initialize(name, artist, duration)
    @name = name
    @artist = artist
    @duration = duration
  end
end

class SongList
  def initialize
    @songs = Array.new
  end

  def append(song)
    @songs.push(song)
    self
  end

  def delete_first
    @songs.shift
  end

  def delete_last
    @songs.pop
  end

  def [](index)
    @songs[index]
  end
end

require 'test/unit'
class TestSongList < Test::Unit::TestCase
  def test_delete
    list = SongList.new
    s1 = Song.new('title1', 'artist1', 1)
    s2 = Song.new('title2', 'artist2', 2)
    s3 = Song.new('title3', 'artist3', 3)
    s4 = Song.new('title4', 'artist4', 4)
    
    list.append(s1).append(s2).append(s3).append(s4)
    
    assert_equal(s1, list[0])
    assert_equal(s3, list[2])
    assert_nil(list[9])
    
    assert_equal(s1, list.delete_first)
    assert_equal(s2, list.delete_first)
    assert_equal(s4, list.delete_last)
    assert_equal(s3, list.delete_last)
    assert_nil(list.delete_last)
  end
end