diff --git a/AOC.Test/Test2015.cs b/AOC.Test/Test2015.cs
index 239d057..0d73fa4 100644
--- a/AOC.Test/Test2015.cs
+++ b/AOC.Test/Test2015.cs
@@ -26,6 +26,7 @@ public class Test2015
     [DataRow(typeof(Day18), "1061", "1006")]
     [DataRow(typeof(Day19), "576", "207")]
     [DataRow(typeof(Day20), "665280", "705600")]
+    [DataRow(typeof(Day21), "78", "148")]
     public void CheckAllDays(Type dayType, string part1, string part2)
     {
         Common.CheckDay(dayType, part1, part2);
diff --git a/AOC2015/Day21.cs b/AOC2015/Day21.cs
index e645348..4f48829 100644
--- a/AOC2015/Day21.cs
+++ b/AOC2015/Day21.cs
@@ -5,11 +5,79 @@ namespace AOC2015;
 /// </summary>
 public sealed class Day21() : Day(2015, 21, "RPG Simulator 20XX")
 {
-    public override void ProcessInput()
+    private const int PlayerHp = 100;
+    private IEnumerable<Combination>? _combinations;
+    private Dictionary<string, int>? _boss;
+
+    private record Combination(Item Weapon, Item Armor, Item Ring1, Item Ring2)
     {
+        public int TotalDamage => Weapon.Damage + Ring1.Damage + Ring2.Damage;
+        public int TotalArmor => Armor.Armor + Ring1.Armor + Ring2.Armor;
+        public int TotalCost => Weapon.Cost + Armor.Cost + Ring1.Cost + Ring2.Cost;
     }
 
-    public override object Part1() => "";
+    private record Item(string Name, int Cost = 0, int Damage = 0, int Armor = 0);
 
-    public override object Part2() => "";
-}
+    public override void ProcessInput()
+    {
+        _boss = Input.ToDictionary(k => k.Split(": ")[0], v => int.Parse(v.Split(": ")[1]));
+
+        var weapons = new Item[]
+        {
+            new(Name: "Dagger",     Cost: 8,  Damage: 4),
+            new(Name: "Shortsword", Cost: 10, Damage: 5),
+            new(Name: "Warhammer",  Cost: 25, Damage: 6),
+            new(Name: "Longsword",  Cost: 40, Damage: 7),
+            new(Name: "Greataxe",   Cost: 74, Damage: 8)
+        };
+
+        var armor = new Item[]
+        {
+            new(Name: "No armor",   Cost: 0,   Armor: 0),
+            new(Name: "Leather",    Cost: 13,  Armor: 1),
+            new(Name: "Chainmail",  Cost: 31,  Armor: 2),
+            new(Name: "Splintmail", Cost: 53,  Armor: 3),
+            new(Name: "Bandedmail", Cost: 75,  Armor: 4),
+            new(Name: "Platemail",  Cost: 102, Armor: 5)
+        };
+
+        var rings = new Item[]
+        {
+            new(Name: "No ring",    Cost: 0,   Damage: 0, Armor: 0),
+            new(Name: "Damage +1",  Cost: 25,  Damage: 1, Armor: 0),
+            new(Name: "Damage +2",  Cost: 50,  Damage: 2, Armor: 0),
+            new(Name: "Damage +3",  Cost: 100, Damage: 3, Armor: 0),
+            new(Name: "Defense +1", Cost: 20,  Damage: 0, Armor: 1),
+            new(Name: "Defense +2", Cost: 40,  Damage: 0, Armor: 2),
+            new(Name: "Defense +3", Cost: 80,  Damage: 0, Armor: 3)
+        };
+
+        _combinations =
+            from w in weapons
+            from a in armor
+            from ring1 in rings
+            from ring2 in rings
+            where ring1.Cost == 0 || ring1.Cost != ring2.Cost
+            select new Combination(w, a, ring1, ring2);
+    }
+
+    private bool StillAlive(Combination combination)
+    {
+        var myDamage          = Math.Max(combination.TotalDamage - _boss!["Armor"], 1);
+        var bossDamagePerTurn = Math.Max(_boss["Damage"] - combination.TotalArmor, 1);
+
+        var turnsToLose = PlayerHp / bossDamagePerTurn;
+        if (PlayerHp % bossDamagePerTurn > 0) turnsToLose++;
+        
+        var turnsToKillBoss = _boss["Hit Points"] / myDamage;
+        if (_boss["Hit Points"] % myDamage > 0) turnsToKillBoss++;
+
+        return turnsToLose >= turnsToKillBoss;
+    }
+
+    public override object Part1() =>
+        _combinations!.Where(StillAlive).Min(c => c.TotalCost);
+
+    public override object Part2() =>
+        _combinations!.Where(c => !StillAlive(c)).Max(c => c.TotalCost);
+}
\ No newline at end of file