Coding Dojo Pardubice #4 – Functional approach

Dojo.cs

using System;
using System.Collections.Generic;
using System.Linq;
using Xunit;
using Xunit.Extensions;

namespace CodingDojo
{
    public class Dojo
    {
        internal struct ArabicRomanPair
        {
            internal readonly int Arabic;
            internal readonly string Roman;

            public ArabicRomanPair(int arabic, string roman)
            {
                Arabic = arabic;
                Roman = roman;
            }
        }

        private static readonly IEnumerable<ArabicRomanPair> RomanNumbers = new List<ArabicRomanPair>
                                                                                {
            new ArabicRomanPair(1000, "M"),
            new ArabicRomanPair(900, "CM"),
            new ArabicRomanPair(500, "D"),
            new ArabicRomanPair(400, "CD"),
            new ArabicRomanPair(100, "C"), 
            new ArabicRomanPair(90, "XC"), 
            new ArabicRomanPair(50, "L"), 
            new ArabicRomanPair(40, "XL"), 
            new ArabicRomanPair(10, "X"), 
            new ArabicRomanPair(9, "IX"), 
            new ArabicRomanPair(5, "V"),
            new ArabicRomanPair(4, "IV"),
            new ArabicRomanPair(1, "I"),
        };

        public static int RomanToArabic(string roman)
        {
            var highestMatch = RomanNumbers.FirstOrDefault(n => roman.StartsWith(n.Roman));
            return String.IsNullOrEmpty(roman)
                ? 0
                : highestMatch.Arabic + RomanToArabic(roman.Substring(highestMatch.Roman.Length));
        }

        public static string ArabicToRoman(int arabic)
        {
            var highestMatch = RomanNumbers.FirstOrDefault(n => arabic >= n.Arabic);
            return arabic == 0
                ? ""
                : highestMatch.Roman + ArabicToRoman(arabic - highestMatch.Arabic);
        }

        public static IEnumerable<object[]> RomanToArabicTestData
        {
            get
            {
                yield return new object[] {"I", 1};
                yield return new object[] { "I", 1 };
                yield return new object[] { "II", 2 };
                yield return new object[] { "III", 3 };
                yield return new object[] { "V", 5 };
                yield return new object[] { "VI", 6 };
                yield return new object[] { "XII", 12 };
                yield return new object[] { "LX", 60 };
                yield return new object[] { "XL", 40 };
                yield return new object[] { "XX", 20 };
                yield return new object[] { "IV", 4 };
                yield return new object[] { "MCMXCIX", 1999 };
                yield return new object[] { "DCLXVI", 666 };
                yield return new object[] { "CDLIX", 459 };
                yield return new object[] { "", 0 };
            }
        }

        [Theory]
        [PropertyData("RomanToArabicTestData")]
        public void ForArabicNumberReturnRomanRepresentation(string romanNumber, int arabicNumber)
        {
            Assert.Equal(romanNumber, ArabicToRoman(arabicNumber));
        }

        [Theory]
        [PropertyData("RomanToArabicTestData")]
        public void ForRomanRepresentationReturnArabicNumber(string romanNumber, int arabicNumber)
        {
            Assert.Equal(arabicNumber, RomanToArabic(romanNumber));
        }
    }
}