Bottles of Beer, for Sandi Metz

beer_song.rb

module Beer

  # Beer::Number represents a number of bottles on the wall. It uses
  # MODULUS 100 arithmetic and has special knowledge of verses,
  # plurals and other song related phrases.
  class Number
    def initialize(n)
      @value = n
    end

    def pred
      Number.new(@value-1)
    end

    def to_s
      @value.to_s
    end

    def verse
      result = ""
      result << on_the_wall.capitalize << ", "
      result << of_beer << ".\n"
      result << action << ", "
      result << pred.on_the_wall << ".\n"
      result
    end

    protected

    def on_the_wall
      "#{of_beer} on the wall"
    end

    private

    def of_beer
      "#{self} #{bottles} of beer"
    end

    def bottles
      "bottles"
    end

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

    def pronoun
      "one"
    end

    class SpecialNumber < Number
      def self.new
        super(:ignore)
      end
    end

    class One < SpecialNumber
      def initialize(_)
        super(1)
      end

      def bottles
        "bottle"
      end

      def pronoun
        "it"
      end
    end

    class Zero < SpecialNumber
      def initialize(_)
        super(0)
      end

      def pred
        Number.new(99)
      end

      def to_s
        "no more"
      end

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

    CACHE = {
      0 => Zero.new,
      1 => One.new,
    }

    def self.new(number)
      CACHE[number] ||= super
    end
  end

  # Beer::Song is the 99 bottles of beer song.
  class Song
    def verse(n)
      Number.new(n).verse
    end

    def verses(start=99, finish=0)
      start.downto(finish).map { |i| verse(i) + "\n" }.join
    end
  end
end

if $0 == __FILE__
  puts Beer::Song.new.verses
end

beer_song_spec.rb

require 'rspec/given'
require 'beer_song'

module Beer
  V99 =
    "99 bottles of beer on the wall, 99 bottles of beer.\n" +
    "Take one down and pass it around, 98 bottles of beer on the wall.\n"

  V98 =
    "98 bottles of beer on the wall, 98 bottles of beer.\n" +
    "Take one down and pass it around, 97 bottles of beer on the wall.\n"

  V2 =
    "2 bottles of beer on the wall, 2 bottles of beer.\n" +
    "Take one down and pass it around, 1 bottle of beer on the wall.\n"

  V1 =
    "1 bottle of beer on the wall, 1 bottle of beer.\n" +
    "Take it down and pass it around, no more bottles of beer on the wall.\n"

  V0 =
    "No more bottles of beer on the wall, no more bottles of beer.\n" +
    "Go to the store and buy some more, 99 bottles of beer on the wall.\n"

  describe Number do
    Given(:bn) { Number.new(n) }

    Invariant { bn.pred == Number.new((n+99) % 100) }

    context "with 99" do
      Given(:n) { 99 }
      Then { bn.to_s == '99' }
      Then { bn.verse == V99 }
    end

    context "with 98" do
      Given(:n) { 98 }
      Then { bn.to_s == '98' }
      Then { bn.verse == V98 }
    end

    context "with 2" do
      Given(:n) { 2 }
      Then { bn.to_s == '2' }
      Then { bn.verse == V2 }
    end

    context "with 1" do
      Given(:n) { 1 }
      Then { bn.to_s == '1' }
      Then { bn.verse == V1 }
    end

    context "with 0" do
      Given(:n) { 0 }
      Then { bn.to_s == 'no more' }
      Then { bn.verse == V0 }
    end
  end

  describe Song do
    Given(:bottle) { Song.new }

    def count(string, pattern)
      string.scan(Regexp.new(Regexp.quote(pattern))).size
    end

    context "one verse" do
      When(:result) { bottle.verse(2) }
      Then { result == V2 }
    end

    context "entire song" do
      When(:result) { bottle.verses(99) }
      Then { count(result, V99 + "\n") == 1 }
      Then { count(result, V98 + "\n") == 1 }
      Then { count(result, V2 + "\n") == 1 }
      Then { count(result, V1 + "\n") == 1 }
      Then { count(result, V0 + "\n") == 1 }
    end
  end
end

beer_song_test.rb

require 'minitest/autorun'
require 'minitest/pride'
require './beer_song'

class BeerSongTest < Minitest::Test

  def beer_song
    @beer_song = Beer::Song.new
  end

  def teardown
    @beer_song = nil
  end

  def test_a_typical_verse
    expected =
      "8 bottles of beer on the wall, 8 bottles of beer.\n" +
      "Take one down and pass it around, 7 bottles of beer on the wall.\n"
    assert_equal expected, beer_song.verse(8)
  end

  def test_another_typical_verse
    expected =
      "3 bottles of beer on the wall, 3 bottles of beer.\n" +
      "Take one down and pass it around, 2 bottles of beer on the wall.\n"
    assert_equal expected, beer_song.verse(3)
  end
  def test_verse_1
    expected =
      "1 bottle of beer on the wall, 1 bottle of beer.\n" +
      "Take it down and pass it around, no more bottles of beer on the wall.\n"
    assert_equal expected, beer_song.verse(1)
  end

  def test_verse_2
    expected =
      "2 bottles of beer on the wall, 2 bottles of beer.\n" +
      "Take one down and pass it around, 1 bottle of beer on the wall.\n"
    assert_equal expected, beer_song.verse(2)
  end

  def test_verse_0
    expected =
      "No more bottles of beer on the wall, no more bottles of beer.\n" +
      "Go to the store and buy some more, 99 bottles of beer on the wall.\n"
    assert_equal expected, beer_song.verse(0)
  end

  def test_several_verses
    expected =
      "8 bottles of beer on the wall, 8 bottles of beer.\n" +
      "Take one down and pass it around, 7 bottles of beer on the wall.\n\n" +
      "7 bottles of beer on the wall, 7 bottles of beer.\n" +
      "Take one down and pass it around, 6 bottles of beer on the wall.\n\n" +
      "6 bottles of beer on the wall, 6 bottles of beer.\n" +
      "Take one down and pass it around, 5 bottles of beer on the wall.\n\n"
    assert_equal expected, beer_song.verses(8, 6)
  end

  def test_all_the_rest_of_the_verses
    expected =
      "3 bottles of beer on the wall, 3 bottles of beer.\n" +
      "Take one down and pass it around, 2 bottles of beer on the wall.\n" +
      "\n" +
      "2 bottles of beer on the wall, 2 bottles of beer.\n" +
      "Take one down and pass it around, 1 bottle of beer on the wall.\n" +
      "\n" +
      "1 bottle of beer on the wall, 1 bottle of beer.\n" +
      "Take it down and pass it around, no more bottles of beer on the wall.\n" +
      "\n" +
      "No more bottles of beer on the wall, no more bottles of beer.\n" +
      "Go to the store and buy some more, 99 bottles of beer on the wall.\n" +
      "\n"
    assert_equal expected, beer_song.verses(3)
  end
end

Rakefile

#!/usr/bin/env ruby

require 'rake/clean'
require 'rake/testtask'

task :default => [:spec, :test]

task :spec do
  sh "rspec ."
end

task :test do
  ruby "beer_song_test.rb"
end