One of the Exercism exercises on the Elixir track is to take an integer (0 to ~3000) and convert it to a Roman numeral. My solution was a little “pipe-happy,” and I have subsequently discovered better approaches. In the course of doing all that piping, I used Enum.reverse() twice. I’ve read that sometimes list reversal is O(n) and sometimes O(1) with a pointer swap, but I don’t know which applies in Elixir. Should I even care? Or perhaps a better question, at what length of list should I start caring?

For reference, my solution:

defmodule RomanNumerals do
  @doc """
  Convert the number to a roman number.
  """
  @spec numeral(pos_integer) :: String.t()
  def numeral(number) when is_integer(number) do
    number
    |> Integer.digits()
    |> Enum.reverse()
    |> Enum.with_index()
    |> Enum.map(&get_roman/1)
    |> Enum.reverse()
    |> List.to_string()
  end

  def get_roman({_number, _index} = pair) do
    case pair do
      {0, _} -> ""
      {1, 0} -> "I"
      {2, 0} -> "II"
      {3, 0} -> "III"
      {4, 0} -> "IV"
      {5, 0} -> "V"
      {6, 0} -> "VI"
      {7, 0} -> "VII"
      {8, 0} -> "VIII"
      {9, 0} -> "IX"
      {1, 1} -> "X"
      {2, 1} -> "XX"
      {3, 1} -> "XXX"
      {4, 1} -> "XL"
      {5, 1} -> "L"
      {6, 1} -> "LX"
      {7, 1} -> "LXX"
      {8, 1} -> "LXXX"
      {9, 1} -> "XC"
      {1, 2} -> "C"
      {2, 2} -> "CC"
      {3, 2} -> "CCC"
      {4, 2} -> "CD"
      {5, 2} -> "D"
      {6, 2} -> "DC"
      {7, 2} -> "DCC"
      {8, 2} -> "DCCC"
      {9, 2} -> "CM"
      {1, 3} -> "M"
      {2, 3} -> "MM"
      {3, 3} -> "MMM"
      _ -> "Too High!"
    end
  end
end

And a more elegant solution by one of the other students:

defmodule RomanNumerals do
  @doc """
  Convert the number to a roman number.
  """
  @spec numeral(pos_integer) :: String.t()
  def numeral(n) do
    cond do
      n >= 1000 -> "M"  <> numeral(n - 1000)
      n >= 900  -> "CM" <> numeral(n - 900)
      n >= 500  -> "D"  <> numeral(n - 500)
      n >= 400  -> "CD" <> numeral(n - 400)
      n >= 100  -> "C"  <> numeral(n - 100)
      n >= 90   -> "XC" <> numeral(n - 90)
      n >= 50   -> "L"  <> numeral(n - 50)
      n >= 40   -> "XL" <> numeral(n - 40)
      n >= 10   -> "X"  <> numeral(n - 10)
      n >= 9    -> "IX" <> numeral(n - 9)
      n >= 5    -> "V"  <> numeral(n - 5)
      n >= 4    -> "IV" <> numeral(n - 4)
      n >= 1    -> "I"  <> numeral(n - 1)
      true      -> ""
    end
  end
end