RomanNumeralGenerator.swift

extension String {
  func occurrencesOfCharacter(_ character: Character) -> Int {
        var count = 0
        for char in self {
            if char == character {
                count += 1
            }
        }
        return count
    }
}

class RomanNumeralAnalyzer {
    
    private static let decimals = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1]
    private static let numerals = ["M", "CM", "D", "CD", "C", "XC", "L", "XL", "X", "IX", "V", "IV", "I"]
    
    static func romanNumeralForInt(_ number: Int) -> String {
        var output = ""
        var number = number

        while number > 0 {
            for (index, decimal) in decimals.enumerated() {
                if number - decimal >= 0 {
                    number -= decimal
                    output += numerals[index]
                    break
                }
            }
        }

        return output
    }

    static func numberOfXsBetween1And(_ number: Int) -> Int {
        guard number > 0 else {
            return 0
        }
        let xChar = Character("X")
        let numerals = (1...number).compactMap { romanNumeralForInt($0) }
        return numerals.map { (numeral) -> Int in numeral.occurrencesOfCharacter(xChar) }
                       .reduce(into: 0, +=)
    }
    
}