foodchain.rb

class FoodChain
  VERSION = 2

  def self.song
    0.upto(7).map { |n| FoodChainStanza.nth(n) }.join("\n")
  end
end

class FoodChainStanza
  ANIMALS = %w(fly spider bird cat dog goat cow horse)

  SENTENCES = [
    "",
    "It wriggled and jiggled and tickled inside her.\n",
    "How absurd to swallow a bird!\n",
    "Imagine that, to swallow a cat!\n",
    "What a hog, to swallow a dog!\n",
    "Just opened her throat and swallowed a goat!\n",
    "I don't know how she swallowed a cow!\n",
    "She's dead, of course!\n"
  ]

  def self.nth(n)
    self.new(n).to_s
  end

  def initialize(n)
    @n = n
  end

  def to_s
    result = intro
    result << continuation if @n < 7
    result
  end

  private

  def intro
    result = "I know an old lady who swallowed a #{ANIMALS[@n]}.\n"
    result << SENTENCES[@n]
  end

  def continuation
    result = @n.downto(1).map { |i| middle_sentence(i) }.join
    result << ending
  end

  def middle_sentence(i)
    result = "She swallowed the #{ANIMALS[i]} to catch the #{ANIMALS[i - 1]}"
    result << ' that wriggled and jiggled and tickled inside her' if i == 2
    result << ".\n"
  end

  def ending
    "I don't know why she swallowed the fly. Perhaps she'll die.\n"
  end
end