My 99bottles solution

bottles.rb

class Bottles
  attr_reader :bottle_number

  def verse(bottle_number)
    @bottle_number = bottle_number
    return plural if bottle_number > 2
    return two if bottle_number == 2
    return one if bottle_number == 1
    return zero if bottle_number == 0
  end

  def verses(first_bottle, last_bottle)
    result = (last_bottle..first_bottle).to_a.reverse.map do |num|
      verse(num)
    end.join("\n")
    result
  end

  def song
    verses(99, 0)
  end

  private

    def plural
      first = "#{bottle_number} bottles of beer on the wall, #{bottle_number} bottles of beer.\n"
      second = "Take one down and pass it around, #{next_num} bottles of beer on the wall.\n"
      [first, second].join('')
    end

    def two
      first = "2 bottles of beer on the wall, 2 bottles of beer.\n"
      second = "Take one down and pass it around, 1 bottle of beer on the wall.\n"
      [first, second].join('')
    end

    def one
      first = "1 bottle of beer on the wall, 1 bottle of beer.\n"
      second = "Take it down and pass it around, no more bottles of beer on the wall.\n"
      [first, second].join('')
    end

    def zero
      first = "No more bottles of beer on the wall, no more bottles of beer.\n"
      second = "Go to the store and buy some more, 99 bottles of beer on the wall.\n"
      [first, second].join('')
    end

    def next_num
      bottle_number - 1
    end
end