Part of RomanCalculator

roman.kt

fun add(first: String, second: String): String {
    return intToRoman(romanToInt(first) + romanToInt(second))
}

// It's important to keep it descending order
private val romanMap = linkedMapOf(
        "M" to 1000,
        "CM" to 900,
        "D" to 500,
        "CD" to 400,
        "C" to 100,
        "XC" to 90,
        "L" to 50,
        "XL" to 40,
        "X" to 10,
        "IX" to 9,
        "V" to 5,
        "IV" to 4,
        "I" to 1
)

private fun romanToInt(roman: String): Int {
    var sum = 0
    var previous = 0

    for (char in roman.reversed()) {
        val numValue = romanMap[char.toString()] ?: 0
        sum += if (numValue < previous) -numValue else numValue
        previous = numValue
    }

    return sum
}

private fun intToRoman(n: Int): String {
    var rest = n
    var result = ""

    while (rest > 0) {
        val biggestRoman = romanMap.entries.first { it.value <= rest }
        result += biggestRoman.key
        rest -= biggestRoman.value
    }

    return result
}