diff --git a/2021/lib/2021/21.ex b/2021/lib/2021/21.ex
new file mode 100644
index 0000000..498f2bd
--- /dev/null
+++ b/2021/lib/2021/21.ex
@@ -0,0 +1,56 @@
+import AOC
+
+
+aoc 2021, 21 do
+
+  def p1 do
+    {total_rolls, {{_, losing_score}, _}} = input_players()
+    |> play_game(deterministic_die())
+
+    total_rolls * losing_score
+  end
+
+  def p2 do
+  end
+
+  def play_game(players, die, winning_score \\ 1000) do
+    Enum.reduce_while(rolls(die), {0, players}, fn
+      roll, {roll_count, {player, other_player}} ->
+        {_, score} = player = move(player, roll)
+
+        action = if score >= winning_score, do: :halt, else: :cont
+
+        {action, {roll_count + 3, {other_player, player}}}
+    end)
+  end
+
+  def move({position, score}, roll) do
+    position = (position + Enum.sum(roll)) |> to_board_position()
+    score = score + position
+
+    {position, score}
+  end
+
+  def to_board_position(n) do
+    1..10 |> Enum.at(Integer.mod(n, 10) - 1)
+  end
+
+  def rolls(die, roll_size \\ 3), do: Stream.chunk_every(die, roll_size)
+
+  def deterministic_die() do
+    Stream.iterate(1, fn
+      100 -> 1
+      n -> n + 1
+    end)
+  end
+
+  def input_players() do
+    input_stream()
+    |> Enum.map(fn line ->
+      [position] = Regex.run(~r/Player \d starting position: (\d+)/, line, capture: :all_but_first)
+
+      {String.to_integer(position), 0}
+    end)
+    |> List.to_tuple()
+  end
+end