defmodule TwelveDays do
  @doc """
  Given a `number`, return the song's verse for that specific day, including
  all gifts for previous days in the same line.
  """
  @spec verse(number :: integer) :: String.t()
  def verse(1), do: "On the first day of Christmas my true love gave to me: a Partridge in a Pear Tree."
  def verse(2), do: "On the second day of Christmas my true love gave to me: two Turtle Doves, and a Partridge in a Pear Tree."
  def verse(3), do: "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."
  def verse(4), do: "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."
  def verse(5), do: "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."
  def verse(6), do: "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."
  def verse(7), do: "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."
  def verse(8), do: "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."
  def verse(9), do: "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."
  def verse(10), do: "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."
  def verse(11), do: "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."
  def verse(12), do: "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."

  @doc """
  Given a `starting_verse` and an `ending_verse`, return the verses for each
  included day, one per line.
  """
  @spec verses(starting_verse :: integer, ending_verse :: integer) :: String.t()
  def verses(starting_verse, ending_verse) do
    starting_verse..ending_verse
    |> Enum.map(&verse/1)
    |> Enum.join("\n")
  end

  @doc """
  Sing all 12 verses, in order, one verse per line.
  """
  @spec sing() :: String.t()
  def sing(), do: verses(1, 12)

end