An implementation of the BlackJack Helper coding challenge on CoderRadio

BlackJackHelper.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using NUnit.Framework;

/* Slightly more logic to implement the implicit rules
 * 
 * Program expects a single string as the input
 * The first character is the dealers card the rest are the players cards
 *
 * Example: BlackJack Helper.exe 43T5
 *   Dealer has a 4
 *   Player has a 3 and a Ten and a Five
 */
 
namespace Blackjack_Helper {
    class Program {
        public const string Hit = "Hit";
        public const string Stand = "Stand";
        public const string Double = "Double if allowed, otherwise Hit";
        public const string DoubleStand = "Double if allowed, otherwise Stand";
        public const string Split = "Split";
        public const string SurrenderOrHit = "Surrender if allowed, otherwise Hit";
        public const string SurrenderOrSplit = "Surrender if allowed, otherwise Split";
        public const string SurrenderOrStand = "Surrender if allowed, otherwise Stand";
        public const string HelpText = "[Dealer card][Players cards]. Cards = 2,3,4,5,6,7,8,9,T,A";

        static void Main(string[] args)
        {
            new Program(Console.In, Console.Out);
        }

        public Program(TextReader input, TextWriter output)
        {
            var cards = input.ReadLine();
            if (cards == null || cards.Length < 3) {
                output.Write(HelpText);
                return;
            }

            var dealersIndex = new List<char> { '2', '3', '4', '5', '6', '7', '8', '9', 'T', 'A' }.IndexOf(cards.First());
            var playersHand = new Hand(cards.Substring(1));

            var rule = FindRule(playersHand, dealersIndex);
            output.Write(rule);
        }

        private string FindRule(Hand playersHand, int dealersIndex)
        {
            if (playersHand.CanSplit)
                return FindSplitRule(playersHand, dealersIndex);
            if (playersHand.IsSoft)
                return FindSoftRule(playersHand, dealersIndex);
            return FindNormalRule(playersHand, dealersIndex);
        }

        private static string FindSplitRule(Hand playersHand, int dealersIndex)
        {
            return new Dictionary<int, string[]>()
            {
                [2] = new[] { Split, Split, Split, Split, Split, Split, Split, Split, Split, Split },
                [4] = new[] { Split, Split, Split, Split, Split, Split, Hit, Hit, Hit, Hit },
                [6] = new[] { Split, Split, Split, Split, Split, Split, Hit, Hit, Hit, Hit },
                [8] = new[] { Hit, Hit, Hit, Split, Split, Hit, Hit, Hit, Hit, Hit },
                [10] = new[] { Double, Double, Double, Double, Double, Double, Double, Double, Hit, Hit },
                [12] = new[] { Split, Split, Split, Split, Split, Hit, Hit, Hit, Hit, Hit },
                [14] = new[] { Split, Split, Split, Split, Split, Split, Hit, Hit, Hit, Hit },
                [16] = new[] { Split, Split, Split, Split, Split, Split, Split, Split, Split, SurrenderOrSplit },
                [19] = new[] { Split, Split, Split, Split, Split, Stand, Split, Split, Stand, Stand },
                [20] = new[] { Stand, Stand, Stand, Stand, Stand, Stand, Stand, Stand, Stand, Stand }
            }[playersHand.Value][dealersIndex];
        }

        private static string FindSoftRule(Hand playersHand, int dealersIndex)
        {
            return new Dictionary<int, string[]>
            {
                [3] = new[] { Hit, Hit, Hit, Double, Double, Hit, Hit, Hit, Hit, Hit },
                [4] = new[] { Hit, Hit, Hit, Double, Double, Hit, Hit, Hit, Hit, Hit },
                [5] = new[] { Hit, Hit, Double, Double, Double, Hit, Hit, Hit, Hit, Hit },
                [6] = new[] { Hit, Hit, Double, Double, Double, Hit, Hit, Hit, Hit, Hit },
                [7] = new[] { Hit, Double, Double, Double, Double, Hit, Hit, Hit, Hit, Hit },
                [8] = new[] { Stand, DoubleStand, DoubleStand, DoubleStand, DoubleStand, Stand, Stand, Hit, Hit, Hit }
            }[playersHand.Value][dealersIndex];
        }

        private static string FindNormalRule(Hand playersHand, int dealersIndex)
        {
            if (playersHand.Value < 8) return Hit;
            if (playersHand.Value >= 17) return Stand;

            return new Dictionary<int, string[]>
            {
                [8] = new[] { Hit, Hit, Hit, Hit, Hit, Hit, Hit, Hit, Hit, Hit },
                [9] = new[] { Hit, Double, Double, Double, Double, Hit, Hit, Hit, Hit, Hit },
                [10] = new[] { Double, Double, Double, Double, Double, Double, Double, Double, Hit, Hit },
                [11] = new[] { Double, Double, Double, Double, Double, Double, Double, Double, Double, Double },
                [12] = new[] { Hit, Hit, Stand, Stand, Stand, Hit, Hit, Hit, Hit, Hit },
                [13] = new[] { Stand, Stand, Stand, Stand, Stand, Hit, Hit, Hit, Hit, Hit },
                [14] = new[] { Stand, Stand, Stand, Stand, Stand, Hit, Hit, Hit, Hit, Hit },
                [15] = new[] { Stand, Stand, Stand, Stand, Stand, Hit, Hit, Hit, SurrenderOrHit, SurrenderOrHit },
                [16] = new[] { Stand, Stand, Stand, Stand, Stand, Hit, Hit, SurrenderOrHit, SurrenderOrHit, SurrenderOrHit },
                [17] = new[] { Stand, Stand, Stand, Stand, Stand, Stand, Stand, Stand, Stand, SurrenderOrStand }
            }[playersHand.Value][dealersIndex];
        }
    }

    class Hand {
        private readonly string _hand;
        private readonly int _softValue;
        private readonly int _hardValue;

        public Hand(string hand)
        {
            _hand = hand.ToUpper();

            var values = CalculateHandValue(hand);
            _softValue = values.Item1;
            _hardValue = values.Item2;
        }

        private Tuple<int, int> CalculateHandValue(string hand)
        {
            var aces = hand.Count(m => m == 'A');

            var hard = hand.ToCharArray().Sum(c => {
                if (c == 'A') return 11;
                if (c == 'T') return 10;
                return (int)char.GetNumericValue(c);
            });

            var soft = hard - (aces * 10);

            while (hard > 21 && aces-- > 0)
                hard -= 10;

            return Tuple.Create(soft, hard);
        }

        public bool CanSplit => _hand.Length == 2 && _hand.All(card => card == _hand.First());

        public bool IsSoft => _softValue <= 8 && _softValue != _hardValue;

        public int Value => IsSoft ? _softValue : _hardValue;
    }


    public class Tests {
        // 12
        [TestCase("248", Program.Hit)]
        [TestCase("557", Program.Stand)]

        // 15
        [TestCase("TT5", Program.SurrenderOrHit)]
        [TestCase("T5T", Program.SurrenderOrHit)]

        // Soft
        [TestCase("3A7", Program.DoubleStand)]
        [TestCase("3A34", Program.DoubleStand)]
        [TestCase("342AA", Program.DoubleStand)]
        [TestCase("37A", Program.DoubleStand)]
        [TestCase("38A", Program.Stand)]

        // Split (12)
        [TestCase("566", Program.Split)]

        public void HappyPath(string input, string expectedOutput)
        {
            TextReader inputStream = new StringReader(input);
            TextWriter outputStream = new StringWriter(new StringBuilder());
            new Program(inputStream, outputStream);
            Assert.That(outputStream.ToString(), Is.EqualTo(expectedOutput));
        }

        [TestCase("")]
        [TestCase("AA")]
        [TestCase("/?")]
        public void IncorrectInput(string input)
        {
            HappyPath(input, Program.HelpText);
        }
    }
}