Examples from How to Start Test Driven Development on failedtofunction.com

RomanNumeralExtensions.kt

package com.failedtofunction.examples.numerals

fun Char.romanNumeralValue(): Int =
    when (this) {
        'I' -> 1
        'V' -> 5
        'X' -> 10
        'L' -> 50
        'C' -> 100
        'D' -> 500
        'M' -> 1000
        else -> throw Exception("'$this' is not a valid Roman Numeral character.")
    }

fun String.fromRomanNumeral(): Int =
    this
        .toCharArray()
        .reversed()
        .map { it.romanNumeralValue() }
        .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


fun String.addRomanNumeral(other: String): String {
    val thisValue = this.fromRomanNumeral()
    val otherValue = other.fromRomanNumeral()
    val sum = thisValue + otherValue
    return sum.toRomanNumeral()
}

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

RomanNumeralExtensionsTest.kt

package com.failedtofunction.examples.numerals

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

class RomanNumeralExtensionsTest {

    @Test
    fun `should convert a Roman Numeral to an integer`() {
        val input = "IV"
        val expected = 4
        val result = input.fromRomanNumeral()
        assertEquals(expected, result)
    }

    @Test
    fun `should convert the year in Roman Numerals to an integer`() {
        val input = "MMXXIII"
        val expected = 2023
        val result = input.fromRomanNumeral()
        assertEquals(expected, result)
    }

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

    @Test
    fun `should convert an Int to an Roman Numeral`() {
        val input = 4
        val expected = "IV"
        val result = input.toRomanNumeral()
        assertEquals(expected, result)
    }

    @Test
    fun `should convert the year at time of writing to the correct Roman Numeral`() {
        val input = 2023
        val expected = "MMXXIII"
        val result = input.toRomanNumeral()
        assertEquals(expected, result)
    }

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