The roman numeral representation of an Int.

RomanNumerals.swift

extension Int {
    func toRoman(uppercase: Bool = true) -> String? {
        guard self > 0, self < 4000 else { return nil }
        let romanDigits =
            [["", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"],
             ["", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"],
             ["", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"],
             ["", "M", "MM", "MMM"]]
        let romanNumber = String(self).reversed().enumerated()
            .map { romanDigits[$0.offset][Int(String($0.element))!] }
            .reversed().joined()
        return uppercase ? romanNumber : romanNumber.lowercased()
    }
}