defmodule NumToWordsList do
  def say(0), do: "zero"
  def say(number) when is_integer(number), do: [to_word(number)] |> List.flatten |> Enum.reject(&Kernel.is_nil/1) |> Enum.join(" ")
  def say(_), do: IO.puts "That's not a number"

  defp to_word(n) when n < 0, do: ["negative", to_word(-n)]
  defp to_word(0), do: nil
  defp to_word(1), do: "one"
  defp to_word(2), do: "two"
  defp to_word(3), do: "three"
  defp to_word(4), do: "four"
  defp to_word(5), do: "five"
  defp to_word(6), do: "six"
  defp to_word(7), do: "seven"
  defp to_word(8), do: "eight"
  defp to_word(9), do: "nine"
  defp to_word(10), do: "ten"
  defp to_word(11), do: "eleven"
  defp to_word(12), do: "twelve"
  defp to_word(13), do: "thirteen"
  defp to_word(14), do: "fourteen"
  defp to_word(15), do: "fifteen"
  defp to_word(16), do: "sixteen"
  defp to_word(17), do: "seventeen"
  defp to_word(18), do: "eighteen"
  defp to_word(19), do: "nineteen"
  defp to_word(20), do: "twenty"
  defp to_word(30), do: "thirty"
  defp to_word(40), do: "forty"
  defp to_word(50), do: "fifty"
  defp to_word(60), do: "sixty"
  defp to_word(70), do: "seventy"
  defp to_word(80), do: "eighty"
  defp to_word(90), do: "ninety"
  defp to_word(n) when n < 100, do: [to_word(div(n,10)*10), to_word(rem(n, 10))]
  defp to_word(n) when n < 1_000, do: [to_word(div(n,100)), "hundred", to_word(rem(n, 100))]
  defp to_word(n) when n < 1_000_000, do: [to_word(div(n,1_000)), "thousand", to_word(rem(n, 1_000))]
  defp to_word(n) when n < 1_000_000_000, do: [to_word(div(n,1_000_000)), "million", to_word(rem(n, 1_000_000))]
  defp to_word(n) when n < 1_000_000_000_000, do: [to_word(div(n,1_000_000_000)), "billion", to_word(rem(n, 1_000_000_000))]
  defp to_word(_), do: "I'm sorry, I can't count that high."
end