Used to demonstrate test driven refactoring in https://failedtofunction.com/test-driven-refactoring/

RomanNumeral.kt

package com.failedtofunction.examples.numerals

class RomanNumeral {

    private  val value: Int
    private val representation: String

    private constructor(value: Int, representation: String) {
        this.value = value
        this.representation = representation
    }

    fun toInt(): Int = value

    override fun toString(): String = representation

    inline operator fun plus(another: RomanNumeral): RomanNumeral = fromInt(toInt() + another.toInt())

    override fun equals(other: Any?): Boolean {
        return if (other is RomanNumeral) {
            value == other.value
        } else {
            false
        }
    }

    override fun hashCode(): Int {
        return value.hashCode()
    }

    companion object {
        private val descendingMap: Map<String, Int> = listOf(
            "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
        ).toMap()

        private fun toRomanNumeral(value: Int): String {
            var num = value
            var result = ""
            for ((numeral, value) in descendingMap.entries) {
                while (num >= value) {
                    num -= value
                    result += numeral
                }
            }
            return result
        }

        fun fromString(rep: String): RomanNumeral {
            val value = rep
                .toCharArray()
                .reversed()
                .map { descendingMap[it.toString()]!! }
                .fold(Pair(0, 0)) { state, item ->
                    val maxVal = kotlin.math.max(state.first, item)
                    val runningTotal = if (item >= state.first) {
                        state.second + item
                    } else {
                        state.second - item
                    }
                    Pair(maxVal, runningTotal)
                }.second
            return RomanNumeral(value, rep)
        }

        fun fromInt(value: Int): RomanNumeral = RomanNumeral(value, toRomanNumeral(value))
    }
}

RomanNumeralTest.kt

package com.failedtofunction.examples.numerals

import kotlin.test.Test
import kotlin.test.assertEquals

class RomanNumeralTest {

    @Test
    fun `should add two Roman Numerals together showing the result as a Roman Numeral`() {
        val four = RomanNumeral.fromString("IV")
        val eleven = RomanNumeral.fromString("XI")
        val expected = RomanNumeral.fromString("XV")
        val result = four + eleven
        assertEquals(expected, result)
    }

    @Test
    fun `should be able to access the Int value of a Roman Numeral created from the String representation`() {
        val input = RomanNumeral.fromString("IV")
        val expected = 4
        val result = input.toInt()
        assertEquals(expected, result)
    }

    @Test
    fun `should show the Roman Numeral representation when calling toString()`() {
        val input = RomanNumeral.fromInt(4)
        val expected = "IV"
        val result = input.toString()
        assertEquals(expected, result)
    }

    @Test
    fun `should verify that equivalent Roman Numerals created from Int and String are equal`() {
        val input = 4
        val expected = RomanNumeral.fromString("IV")
        val result = RomanNumeral.fromInt(input)
        assertEquals(expected, result)
    }

    @Test
    fun `should verify that Roman numbers with equivalent values created from either Int or String have the same hash`() {
        val first = RomanNumeral.fromString("XX")
        val second = RomanNumeral.fromInt(20)
        assertEquals(first.hashCode(), second.hashCode())
    }

    @Test
    fun `should convert integers from 1 to 1000 to Roman Numerals and back`() {
        for (i in 1..1000) {
            val asRomanNumeral = RomanNumeral.fromInt(i)
            val asInteger = RomanNumeral.fromString(asRomanNumeral.toString())
            assertEquals(i, asInteger.toInt(), "Incorrect conversion of $i either to Roman Numeral $asRomanNumeral or back to $asInteger")
        }
    }
}