LeetCode/kt/roman-to-integer.kt

class Solution {
    /**
     * Holds fixed mapping of roman numerals to their respective numeric value.
     */
    private val values = mapOf(
        'I' to 1, 'V' to 5, 'X' to 10, 'L' to 50, 'C' to 100, 'D' to 500, 'M' to 1000
    )

    /**
     * Represents an accumulator for right fold when turning roman numeral into an integer value.
     * @property sum Holds the current sum of the accumulator.
     * @property next Holds the value of the previous roman numeral digit.
     */
    private data class Accumulator(val sum: Int = 0, val next: Int = 0) {
        /**
         * @param it Current roman numeral digit.
         * @return Sign that is to be used when modifying sum of the accumulator.
         */
        private fun sign(it: Int): Int = when (next > it) {
            true -> -1
            false -> 1
        }

        /**
         * @param it Currently processed roman numeral digit.
         * @return Correctly updated accumulator.
         */
        fun update(it: Int): Accumulator = copy(sum = sum + sign(it) * it, next = it)
    }

    fun romanToInt(s: String): Int = s
        .map { values[it]!! }
        .foldRight(Accumulator()) { it, acc ->
            acc.update(it)
        }
        .sum
}

fun main() {
    val s = Solution()

    val tests = mapOf(
        "I" to 1,
        "II" to 2,
        "III" to 3,
        "IV" to 4,
        "V" to 5,
        "VI" to 6,
        "VII" to 7,
        "VIII" to 8,
        "IX" to 9,
        "X" to 10,
        "LVIII" to 58,
        "MCMXCIV" to 1994
    )

    tests.forEach { (num, i) ->
        check(s.romanToInt(num) == i) {
            "$num: ${s.romanToInt(num)} == $i"
        }
    }
}