defmodule RomanNumerals do
@spec to_roman(integer) :: String.t()
def to_roman(number) when is_integer(number) and number > 0 do
to_roman_recursive(number, "")
end
def to_roman(_), do: ""
defp to_roman_recursive(0, acc), do: acc
defp to_roman_recursive(number, acc) when number >= 1000, do: to_roman_recursive(number - 1000, acc <> "M")
defp to_roman_recursive(number, acc) when number >= 900, do: to_roman_recursive(number - 900, acc <> "CM")
defp to_roman_recursive(number, acc) when number >= 500, do: to_roman_recursive(number - 500, acc <> "D")
defp to_roman_recursive(number, acc) when number >= 400, do: to_roman_recursive(number - 400, acc <> "CD")
defp to_roman_recursive(number, acc) when number >= 100, do: to_roman_recursive(number - 100, acc <> "C")
defp to_roman_recursive(number, acc) when number >= 90, do: to_roman_recursive(number - 90, acc <> "XC")
defp to_roman_recursive(number, acc) when number >= 50, do: to_roman_recursive(number - 50, acc <> "L")
defp to_roman_recursive(number, acc) when number >= 40, do: to_roman_recursive(number - 40, acc <> "XL")
defp to_roman_recursive(number, acc) when number >= 10, do: to_roman_recursive(number - 10, acc <> "X")
defp to_roman_recursive(number, acc) when number >= 9, do: to_roman_recursive(number - 9, acc <> "IX")
defp to_roman_recursive(number, acc) when number >= 5, do: to_roman_recursive(number - 5, acc <> "V")
defp to_roman_recursive(number, acc) when number >= 4, do: to_roman_recursive(number - 4, acc <> "IV")
defp to_roman_recursive(number, acc) when number >= 1, do: to_roman_recursive(number - 1, acc <> "I")
end
