99bottles.rb

class Bottles

  def song
    verses(99, 0)
  end

  def verses(upper_bound, lower_bound)
    upper_bound.downto(lower_bound).map { |i| verse(i) }.join("\n")
  end

  def verse(verse_number)
    current_bn = create_bottle_number(verse_number)
    next_bn    = create_bottle_number(current_bn.next)

    "#{current_bn.quantity.capitalize} #{current_bn.container} of beer on the wall, " +
    "#{current_bn.quantity} #{current_bn.container} of beer.\n" +
    "#{current_bn.action}, " +
    "#{next_bn.quantity} #{next_bn.container} of beer on the wall.\n"
  end

  def create_bottle_number(number)
    BottleNumber.for(number)
  end

end

class BottleNumber
  attr_reader :number

  def self.for(number)
    bottle_number = available_bottle_numbers.find { |subclass| subclass.handles?(number) }
    bottle_number.new(number)
  end

  def initialize(number)
    @number = number
  end

  def container
    'bottles'
  end

  def pronoun
    'one'
  end

  def quantity
    number.to_s
  end

  def action
    "Take #{pronoun} down and pass it around"
  end

  def next
    number - 1
  end

  private
  def self.handles?(number)
    true
  end

  def self.available_bottle_numbers
    available_bottle_numbers = subclasses
    available_bottle_numbers << self
  end
end

class BottleNumber1 < BottleNumber
  def self.handles?(number)
    number == 1
  end

  def pronoun
    'it'
  end

  def container
    'bottle'
  end
end

class BottleNumber0 < BottleNumber
  def self.handles?(number)
    number.zero?
  end

  def next
    99
  end

  def action
    "Go to the store and buy some more"
  end

  def quantity
    'no more'
  end
end

class SixPack < BottleNumber
  def self.handles?(number)
    number == 6
  end

  def container
    'six-pack'
  end

  def quantity
    '1'
  end
end

module Subclasses
  # return a list of the subclasses of a class
  def subclasses(direct = false)
    classes = []
    if direct
      ObjectSpace.each_object(Class) do |c|
        next unless c.superclass == self
        classes << c
      end
    else
      ObjectSpace.each_object(Class) do |c|
        next unless c.ancestors.include?(self) and (c != self)
        classes << c
      end
    end
    classes
  end
end

Object.send(:include, Subclasses)