roman.exs

defmodule Roman do
  @numerals %{1 => "I",
              5 => "V",
             10 => "X",
             50 => "L",
            100 => "C",
            500 => "D",
           1000 => "M"}

  def numerals(number) do
    Integer.digits(number)
    |> Enum.reverse
    |> Stream.with_index
    |> Enum.map(&numerals_for_digit/1)
    |> Enum.reverse
    |> Enum.join
  end

  def magnitude(decimal_place) do
    :math.pow(10, decimal_place) |> round
  end

  def numerals_for_digit(digit_with_index) do
    {digit, index} = digit_with_index
    magnitude = magnitude(index)

    template(digit, @numerals[1 * magnitude],
                    @numerals[5 * magnitude],
                    @numerals[10 * magnitude])
  end

  def template(digit, one, five, ten) do
    case digit do
      1 -> one
      2 -> one  <> one
      3 -> one  <> one <> one
      4 -> one  <> five
      5 -> five
      6 -> five <> one
      7 -> five <> one <> one
      8 -> five <> one <> one <> one
      9 -> one  <> ten
      _ -> ""
    end
  end
end