PatrickMcSweeny's solution

Output the lyrics to 'The Twelve Days of Christmas'.

On the first day of Christmas my true love gave to me, a Partridge in a Pear Tree.

On the second day of Christmas my true love gave to me, two Turtle Doves, and a Partridge in a Pear Tree.

On the third day of Christmas my true love gave to me, three French Hens, two Turtle Doves, and a Partridge in a Pear Tree.

On the fourth day of Christmas my true love gave to me, four Calling Birds, three French Hens, two Turtle Doves, and a Partridge in a Pear Tree.

On the fifth day of Christmas my true love gave to me, five Gold Rings, four Calling Birds, three French Hens, two Turtle Doves, and a Partridge in a Pear Tree.

On the sixth day of Christmas my true love gave to me, six Geese-a-Laying, five Gold Rings, four Calling Birds, three French Hens, two Turtle Doves, and a Partridge in a Pear Tree.

On the seventh day of Christmas my true love gave to me, seven Swans-a-Swimming, six Geese-a-Laying, five Gold Rings, four Calling Birds, three French Hens, two Turtle Doves, and a Partridge in a Pear Tree.

On the eighth day of Christmas my true love gave to me, eight Maids-a-Milking, seven Swans-a-Swimming, six Geese-a-Laying, five Gold Rings, four Calling Birds, three French Hens, two Turtle Doves, and a Partridge in a Pear Tree.

On the ninth day of Christmas my true love gave to me, nine Ladies Dancing, eight Maids-a-Milking, seven Swans-a-Swimming, six Geese-a-Laying, five Gold Rings, four Calling Birds, three French Hens, two Turtle Doves, and a Partridge in a Pear Tree.

On the tenth day of Christmas my true love gave to me, ten Lords-a-Leaping, nine Ladies Dancing, eight Maids-a-Milking, seven Swans-a-Swimming, six Geese-a-Laying, five Gold Rings, four Calling Birds, three French Hens, two Turtle Doves, and a Partridge in a Pear Tree.

On the eleventh day of Christmas my true love gave to me, eleven Pipers Piping, ten Lords-a-Leaping, nine Ladies Dancing, eight Maids-a-Milking, seven Swans-a-Swimming, six Geese-a-Laying, five Gold Rings, four Calling Birds, three French Hens, two Turtle Doves, and a Partridge in a Pear Tree.

On the twelfth day of Christmas my true love gave to me, twelve Drummers Drumming, eleven Pipers Piping, ten Lords-a-Leaping, nine Ladies Dancing, eight Maids-a-Milking, seven Swans-a-Swimming, six Geese-a-Laying, five Gold Rings, four Calling Birds, three French Hens, two Turtle Doves, and a Partridge in a Pear Tree.

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For installation and learning resources, refer to the exercism help page.

For running the tests provided, you will need the Minitest gem. Open a terminal window and run the following command to install minitest:

gem install minitest

If you would like color output, you can require 'minitest/pride' in the test file, or note the alternative instruction, below, for running the test file.

Run the tests from the exercise directory using the following command:

ruby twelve_days_test.rb

To include color from the command line:

ruby -r minitest/pride twelve_days_test.rb

Source

Wikipedia http://en.wikipedia.org/wiki/The_Twelve_Days_of_Christmas_(song)

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

require 'minitest/autorun'
require_relative 'twelve_days'

class TwelveDaysTest < Minitest::Test
  # This test is an acceptance test.
  #
  # If you find it difficult to work the problem with so much
  # output, go ahead and add a `skip`, and write whatever
  # unit tests will help you. Then unskip it again
  # to make sure you got it right.
  # There's no need to submit the tests you write, unless you
  # specifically want feedback on them.
  def test_the_whole_song
    song_file = File.expand_path('../song.txt', __FILE__)
    expected = IO.read(song_file)
    assert_equal expected, TwelveDays.song
  end
end

class TwelveDays
  GIFTS = {
    'first': "a Partridge in a Pear Tree.\n",
    'second': 'two Turtle Doves',
    'third': 'three French Hens',
    'fourth': 'four Calling Birds',
    'fifth': 'five Gold Rings',
    'sixth': 'six Geese-a-Laying',
    'seventh': 'seven Swans-a-Swimming',
    'eighth': 'eight Maids-a-Milking',
    'ninth': 'nine Ladies Dancing',
    'tenth': 'ten Lords-a-Leaping',
    'eleventh': 'eleven Pipers Piping',
    'twelfth': 'twelve Drummers Drumming'
  }.freeze

  def self.song
    new.sing
  end

  def sing
    day_names
      .map do |day_name|
        state_day(day_name) + list_gifts(day_name)
        .push(first_day(day_name))
        .join(', ')
      end
      .join("\n")
  end

  def day_names
    GIFTS.keys
  end

  def state_day(day)
    "On the #{day} day of Christmas my true love gave to me, "
  end

  def list_gifts(day_name)
    day_names[1..day_names.find_index(day_name)]
      .reverse
      .map { |d| GIFTS[d] }
  end

  def first_day(day_name)
    ('and ' unless day_name == :first).to_s + GIFTS[:first]
  end
end