diff --git a/advent-of-code-2021.csproj b/advent-of-code-2021.csproj
index d2794ff..5706de0 100644
--- a/advent-of-code-2021.csproj
+++ b/advent-of-code-2021.csproj
@@ -78,6 +78,9 @@
 	  <None Update="inputs\20.txt">
 	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
 	  </None>
+	  <None Update="inputs\21.txt">
+	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+	  </None>
 	</ItemGroup>
 
 </Project>
diff --git a/inputs/21.txt b/inputs/21.txt
new file mode 100644
index 0000000..f0848de
--- /dev/null
+++ b/inputs/21.txt
@@ -0,0 +1,2 @@
+Player 1 starting position: 4
+Player 2 starting position: 8
\ No newline at end of file
diff --git a/src/21.cs b/src/21.cs
new file mode 100644
index 0000000..00a9149
--- /dev/null
+++ b/src/21.cs
@@ -0,0 +1,134 @@
+namespace aoc2021;
+
+internal class Day21 : Day
+{
+    private record struct GameState((int p1, int p2) Positions, int Turn, (int p1, int p2) Score, (long p1, long p2) Wins, int RollVal, int TotalRolls, int TotalRounds)
+    {
+        public override int GetHashCode() => HashCode.Combine(Positions.p1, Positions.p2, Turn, RollVal, TotalRounds);
+    }
+
+    internal override void Go()
+    {
+        var lines = Util.ReadAllLines("inputs/21.txt");
+        var player1Pos = int.Parse(lines.ElementAt(0).Split(": ")[1]);
+        var player2Pos = int.Parse(lines.ElementAt(1).Split(": ")[1]);
+        Part1(player1Pos, player2Pos);
+        Part2(player1Pos, player2Pos);
+    }
+
+    private static void Part1(int player1Pos, int player2Pos)
+    {
+        using var t = new Timer();
+
+        var playerPos = new int[2]
+        {
+            player1Pos,
+            player2Pos,
+        };
+        var (playerScore, numRolls) = PlayGame(playerPos, 1000, 10);
+
+        t.Stop();
+        Logger.Log($"<+black>> part1: <+white>{numRolls * playerScore.Min()}<r>");
+    }
+
+    private static void Part2(int player1Pos, int player2Pos)
+    {
+        using var t = new Timer();
+
+        var playerPos = new int[2]
+        {
+            player1Pos,
+            player2Pos,
+        };
+        var (p1wins, p2wins) = PlayQuantumGame(new List<int>(playerPos), 21);
+
+        t.Stop();
+        Logger.Log($"<+black>> part2: p1: {p1wins:N0}, p2: {p2wins:N0} -> <+white>{Math.Max(p1wins, p2wins)}<r>");
+    }
+
+    private static (long[] scores, long numRolls) PlayGame(int[] playerPos, int maxScore, int dieSides)
+    {
+        var playerScore = new long[2]
+        {
+            0,
+            0,
+        };
+
+        int dieVal = 1;
+        int turn = 0;
+        long numRolls = 0;
+        while (!playerScore.Any(x => x >= maxScore))
+        {
+            for (int i = 0; i < 3; i++)
+            {
+                playerPos[turn] = PlayOneRoll(playerPos[turn], dieVal);
+                dieVal = (dieVal + 1) % dieSides;
+                numRolls++;
+            }
+
+            playerScore[turn] += playerPos[turn];
+            turn = 1 - turn;
+        }
+
+        return (playerScore, numRolls);
+    }
+
+    private static int PlayOneRoll(int playerPos, int dieVal)
+    {
+        return ((playerPos + dieVal - 1) % 10) + 1;
+    }
+
+    private static readonly Dictionary<int, GameState> cachedWinCases = new();
+
+    private static (long, long) PlayQuantumGame(List<int> playerPos, int maxScore, List<int>? playerScores = null, int turn = 0, int rollNum = 0, int rollVal = 0, int totalRounds = 0, int totalRolls = 0)
+    {
+        playerScores ??= new List<int> { 0, 0 };
+        if (cachedWinCases.TryGetValue(HashCode.Combine(playerPos[0], playerPos[1], turn, rollVal, totalRounds), out GameState winState))
+        {
+            return winState.Wins;
+        }
+
+        var wins = (0L, 0L);
+        while (true)
+        {
+            totalRounds++;
+            for (int i = rollNum; i < 3; i++)
+            {
+                totalRolls++;
+                var twoWins = PlayQuantumGame(new List<int>(playerPos), maxScore, new List<int>(playerScores), turn, i + 1, 2, totalRounds, totalRolls);
+                var threeWins = PlayQuantumGame(new List<int>(playerPos), maxScore, new List<int>(playerScores), turn, i + 1, 3, totalRounds, totalRolls);
+                wins = (wins.Item1 + twoWins.Item1 + threeWins.Item1, wins.Item2 + twoWins.Item2 + threeWins.Item2);
+                playerPos[turn] = PlayOneRoll(playerPos[turn], 1);
+                rollVal = 1;
+            }
+            if (rollNum == 3)
+            {
+                playerPos[turn] = PlayOneRoll(playerPos[turn], rollVal);
+            }
+
+            playerScores[turn] += playerPos[turn];
+            if (playerScores[turn] >= maxScore)
+            {
+                if (turn == 0)
+                {
+                    wins.Item1++;
+                }
+                else
+                {
+                    wins.Item2++;
+                }
+
+                GameState state = new((playerPos[0], playerPos[1]), turn, (playerScores[0], playerScores[1]), wins, rollVal, totalRolls, totalRounds);
+                cachedWinCases[state.GetHashCode()] = state;
+
+                break;
+            }
+
+            turn = 1 - turn;
+            rollNum = 0;
+            rollVal = 0;
+        }
+
+        return wins;
+    }
+}
diff --git a/src/main.cs b/src/main.cs
index cdbdfa9..472cb60 100644
--- a/src/main.cs
+++ b/src/main.cs
@@ -1,14 +1,14 @@
 using aoc2021;
 
-var arg = args.FirstOrDefault();
-if (arg == "all")
-{
-    var types = System.Reflection.Assembly
+var types = System.Reflection.Assembly
              .GetExecutingAssembly()
              .GetTypes()
              .Where(t => t.IsSubclassOf(typeof(Day)) && !t.IsAbstract && t.Name != "DayTemplate")
              .OrderBy(t => t.Name);
 
+var arg = args.FirstOrDefault();
+if (arg == "all")
+{
     foreach (var type in types)
     {
         using var day = (Day)Activator.CreateInstance(type)!;
@@ -17,28 +17,22 @@ if (arg == "all")
 }
 else
 {
-    using Day day = arg switch
+    Day? day = null;
+    if (string.IsNullOrEmpty(arg))
     {
-        "1" => new Day01(),
-        "2" => new Day02(),
-        "3" => new Day03(),
-        //"4" => new Day04(),
-        "5" => new Day05(),
-        "6" => new Day06(),
-        "7" => new Day07(),
-        "8" => new Day08(),
-        "9" => new Day09(),
-        "10" => new Day10(),
-        "11" => new Day11(),
-        "12" => new Day12(),
-        "13" => new Day13(),
-        "14" => new Day14(),
-        "15" => new Day15(),
-        "16" => new Day16(),
-        "17"=> new Day17(),
-        "18" => new Day18(),
-        "19" => new Day19(),
-        _ => new Day20(),
-    };
-    day.Go();
+        day = new Day21();
+    }
+    else
+    {
+        var type = types.FirstOrDefault(x => x.Name == $"Day{arg?.PadLeft(2, '0')}");
+        if (type == null)
+        {
+            Logger.Log($"Unknown day <cyan>{arg}<r>");
+        }
+        else
+        {
+            day = (Day?)Activator.CreateInstance(type);
+        }
+    }
+    day?.Go();
 }