99solution.rb

class Bottles
  def song
    verses(99, 0)
  end

  def verses(first, last)
    first.downto(last).map { |n| verse(n) }.join("\n")
  end

  def verse(n)
    VerseFactory.make(n).call
  end
end

class VerseFactory
  def self.make(n)
    return RunningOutVerse.new if n == 2
    return LastOneVerse.new if n == 1
    return AllOutVerse.new if n == 0
    StandardVerse.new(n)
  end
end

class StandardVerse
  def initialize(n)
    @n = n
  end

  def call
    <<-VERSE
#{@n} bottles of beer on the wall, #{@n} bottles of beer.
Take one down and pass it around, #{@n-1} bottles of beer on the wall.
    VERSE
  end
end

class RunningOutVerse
  def call
    <<-VERSE
2 bottles of beer on the wall, 2 bottles of beer.
Take one down and pass it around, 1 bottle of beer on the wall.
    VERSE
  end
end

class LastOneVerse
  def call
    <<-VERSE
1 bottle of beer on the wall, 1 bottle of beer.
Take it down and pass it around, no more bottles of beer on the wall.
    VERSE
  end
end

class AllOutVerse
  def call
    <<-VERSE
No more bottles of beer on the wall, no more bottles of beer.
Go to the store and buy some more, 99 bottles of beer on the wall.
    VERSE
  end
end