gistfile1.ex

defmodule Generator do
  # Generates a method like the following
  # def to_word(n) when n >= 1_000 do
  #   [to_word(div(n, 1_000)), "thousand,", to_word(rem n, 1_000)]
  # end
  defmacro gen_method(input) do
    quote bind_quoted: [input: input] do
      def to_word(n) when n >= unquote(input[:num]) do
        [to_word(div(n, unquote(input[:num]))), "#{unquote(input[:text])},", to_word(rem n, unquote(input[:num]))]
      end
    end
  end
end

defmodule NumFun do
  require Generator

  def word(n) do
    to_word(n)
      |> List.flatten
      |> Enum.filter(&(&1))
      |> Enum.join(" ")
  end

  [
    [num: 1_000, text: "thousand"],
    [num: 1_000_000, text: "million"],
    [num: 1_000_000_000, text: "billion"],
    [num: 1_000_000_000_000, text: "trillion"],
    [num: 1_000_000_000_000_000, text: "quadrillion"],
    [num: 1_000_000_000_000_000_000, text: "quintillion"]
  ] |> Enum.reverse 
  |> Enum.map(&Generator.gen_method(&1))

  def to_word(0), do: nil
  def to_word(1), do: "one"
  def to_word(2), do: "two"
  def to_word(3), do: "three"
  def to_word(4), do: "four"
  def to_word(5), do: "five"
  def to_word(6), do: "six"
  def to_word(7), do: "seven"
  def to_word(8), do: "eight"
  def to_word(9), do: "nine"
  def to_word(10), do: "ten"
  def to_word(11), do: "eleven"
  def to_word(12), do: "twelve"
  def to_word(13), do: "thirteen"
  def to_word(15), do: "fifteen"
  def to_word(20), do: "twenty"
  def to_word(30), do: "thirty"
  def to_word(40), do: "forty"
  def to_word(50), do: "fifty"
  def to_word(80), do: "eighty"

  def to_word(n) when n >= 100 do
    [to_word(div(n, 100)), "hundred", with_and?(to_word(rem n, 100))]
  end

  def to_word(n) when n > 13 and n < 20 do
    to_word(rem n, 10) <> "teen"
  end

  def to_word(n) when n < 100 and rem(n, 10) == 0 do
    to_word(div n, 10) <> "ty"
  end

  def to_word(n) when n < 100 and rem(n, 10) != 0 do
    [to_word(div(n, 10) * 10),  to_word(rem n, 10)]
  end

  def with_and?(n) when n != nil do
    ["and", n]
  end

  def with_and?(n), do: nil
end