diff --git a/2015/README.md b/2015/README.md
index 1bbb4bf..a0ee495 100644
--- a/2015/README.md
+++ b/2015/README.md
@@ -17,7 +17,7 @@
 |      |      | [1]  | [2] | [3]  | [4]  | [5]  |
 |  [6] | [7]  | [8]  | [9] | [10] | [11] | [12] |
 | [13] | [14] | [15] | [16]| [17] | [18] | [19] |
-| [20] | 21   | 22   | 23  | 24   | 25   |      |
+| [20] | [21] | 22   | 23  | 24   | 25   |      |
 
 [1]: ./lib/2015/1.ex
 [2]: ./lib/2015/2.ex
@@ -39,3 +39,4 @@
 [18]: ./lib/2015/18.ex
 [19]: ./lib/2015/19.ex
 [20]: ./lib/2015/20.ex
+[21]: ./lib/2015/21.ex
diff --git a/2015/lib/2015/21.ex b/2015/lib/2015/21.ex
new file mode 100644
index 0000000..5af6938
--- /dev/null
+++ b/2015/lib/2015/21.ex
@@ -0,0 +1,192 @@
+import AOC
+
+aoc 2015, 21 do
+  defmodule Weapon do
+    defstruct [:damage, :cost]
+    def damage(%Weapon{damage: damage}), do: damage
+    def damage(nil), do: 0
+    def cost(%Weapon{cost: cost}), do: cost
+    def cost(nil), do: 0
+  end
+
+  defmodule Armor do
+    defstruct [:defence, :cost]
+    def defence(%Armor{defence: defence}), do: defence
+    def defence(nil), do: 0
+    def cost(%Armor{cost: cost}), do: cost
+    def cost(nil), do: 0
+  end
+
+  defmodule Ring do
+    defstruct damage: 0, defence: 0, cost: 0
+    def defence(%Ring{defence: defence}), do: defence || 0
+    def defence(nil), do: 0
+    def damage(%Ring{damage: damage}), do: damage || 0
+    def damage(nil), do: 0
+    def cost(%Ring{cost: cost}), do: cost
+    def cost(nil), do: 0
+  end
+
+  defmodule Character do
+    defstruct [:hp, :weapon, :armor, rings: []]
+
+    def cost(%Character{} = char) do
+      Weapon.cost(char.weapon) + Armor.cost(char.armor) +
+        (Enum.map(char.rings, &Ring.cost/1) |> Enum.sum())
+    end
+
+    def damage(%Character{rings: rings} = char) do
+      Weapon.damage(char.weapon) + (Enum.map(rings, &Ring.damage/1) |> Enum.sum())
+    end
+
+    def defence(%Character{rings: rings} = char) do
+      Armor.defence(char.armor) + (Enum.map(rings, &Ring.defence/1) |> Enum.sum())
+    end
+
+    def equip(%Character{weapon: nil} = char, %Weapon{} = weapon) do
+      %Character{char | weapon: weapon}
+    end
+
+    def equip(%Character{armor: nil} = char, %Armor{} = armor) do
+      %Character{char | armor: armor}
+    end
+
+    def equip(%Character{rings: []} = char, %Ring{} = ring) do
+      %Character{char | rings: [ring]}
+    end
+
+    def equip(%Character{rings: [ring_1]} = char, %Ring{} = ring_2) do
+      %Character{char | rings: [ring_1, ring_2]}
+    end
+
+    def equip(%Character{} = char, nil), do: char
+
+    def attack(%Character{} = attacker, %Character{} = defender) do
+      damage = damage(attacker)
+      defence = defence(defender)
+      %Character{defender | hp: defender.hp - max(1, damage - defence)}
+    end
+
+    def defeated?(%Character{hp: hp}), do: hp <= 0
+  end
+
+  def p1 do
+    boss = boss()
+
+    for weapon <- weapons(),
+        armor <- [nil | armor()],
+        ring_1 <- [nil | rings()],
+        ring_2 <- [nil | rings() -- [ring_1]],
+        reduce: :infinity do
+      min_cost ->
+        player =
+          %Character{hp: 100}
+          |> Character.equip(weapon)
+          |> Character.equip(armor)
+          |> Character.equip(ring_1)
+          |> Character.equip(ring_2)
+
+        curr_cost = Character.cost(player)
+
+        if curr_cost < min_cost do
+          case battle(player, boss) do
+            {:win, _} -> curr_cost
+            {:lose, _} -> min_cost
+          end
+        else
+          min_cost
+        end
+    end
+  end
+
+  def p2 do
+    boss = boss()
+
+    for weapon <- weapons(),
+        armor <- [nil | armor()],
+        ring_1 <- [nil | rings()],
+        ring_2 <- [nil | rings() -- [ring_1]],
+        reduce: 0 do
+      max_cost ->
+        player =
+          %Character{hp: 100}
+          |> Character.equip(weapon)
+          |> Character.equip(armor)
+          |> Character.equip(ring_1)
+          |> Character.equip(ring_2)
+
+        curr_cost = Character.cost(player)
+
+        if max_cost < curr_cost do
+          case battle(player, boss) do
+            {:win, _} -> max_cost
+            {:lose, _} -> curr_cost
+          end
+        else
+          max_cost
+        end
+    end
+  end
+
+  defp battle(player, boss) do
+    player_attack = fn {player, boss} ->
+      boss = Character.attack(player, boss)
+
+      if Character.defeated?(boss) do
+        {:halt, {:win, player}}
+      else
+        {:cont, {player, boss}}
+      end
+    end
+
+    boss_attack = fn {player, boss} ->
+      player = Character.attack(boss, player)
+
+      if Character.defeated?(player) do
+        {:halt, {:lose, player}}
+      else
+        {:cont, {player, boss}}
+      end
+    end
+
+    Stream.cycle([player_attack, boss_attack])
+    |> Enum.reduce_while({player, boss}, fn turn, characters ->
+      turn.(characters)
+    end)
+  end
+
+  defp boss do
+    %Character{hp: 103, weapon: %Weapon{damage: 9}, armor: %Armor{defence: 2}}
+  end
+
+  defp weapons do
+    [
+      %Weapon{cost: 8, damage: 4},
+      %Weapon{cost: 10, damage: 5},
+      %Weapon{cost: 25, damage: 6},
+      %Weapon{cost: 40, damage: 7},
+      %Weapon{cost: 74, damage: 8}
+    ]
+  end
+
+  defp armor do
+    [
+      %Armor{cost: 13, defence: 1},
+      %Armor{cost: 31, defence: 2},
+      %Armor{cost: 53, defence: 3},
+      %Armor{cost: 75, defence: 4},
+      %Armor{cost: 102, defence: 5}
+    ]
+  end
+
+  defp rings do
+    [
+      %Ring{cost: 25, damage: 1},
+      %Ring{cost: 50, damage: 2},
+      %Ring{cost: 100, damage: 3},
+      %Ring{cost: 20, defence: 1},
+      %Ring{cost: 40, defence: 2},
+      %Ring{cost: 80, defence: 3}
+    ]
+  end
+end
diff --git a/README.md b/README.md
index 8b652f5..cced555 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@
 
 | Year | Stars | Languages |
 | - | - | - |
-| [2015] | **40/50** 🌟 | Elixir |
+| [2015] | **42/50** 🌟 | Elixir |
 | 2016 | | |
 | [2017] | **18/50** 🌟 | Haskell |
 | 2018 | | |