From ac5fd19a42e1a364301e645998d06b7d695d0b9f Mon Sep 17 00:00:00 2001
From: Sloane Perrault <sloane.perrault@gmail.com>
Date: Wed, 21 Sep 2022 09:19:54 -0400
Subject: [PATCH] 2021 day 21

---
 2021/README.md      |  4 +++-
 2021/lib/2021/21.ex | 47 ++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/2021/README.md b/2021/README.md
index df2fac9..a82dfd9 100644
--- a/2021/README.md
+++ b/2021/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  |
+| [19]  | 20  | [21]  | 22  | 23  | 24   | 25  |
 
 <!-- links -->
 
@@ -40,3 +40,5 @@
 [17]: ./lib/2021/17.ex
 [18]: ./lib/2021/18.ex
 [19]: ./lib/2021/19.ex
+
+[21]: ./lib/2021/21.ex
diff --git a/2021/lib/2021/21.ex b/2021/lib/2021/21.ex
index 498f2bd..6cc9c52 100644
--- a/2021/lib/2021/21.ex
+++ b/2021/lib/2021/21.ex
@@ -11,8 +11,52 @@ aoc 2021, 21 do
   end
 
   def p2 do
+    input_players()
+    |> run_dirac_dice_game()
+    |> Tuple.to_list()
+    |> Enum.max()
   end
 
+  @dirac_dice_winning_score 21
+  @quantum_rolls %{ 3 => 1, 4 => 3, 5 => 6, 6 => 7, 7 => 6, 8 => 3, 9 => 1 }
+
+  def run_dirac_dice_game(players, up_to_play \\ :player_1)
+  def run_dirac_dice_game({{_, player_1_score}, _}, :player_2) when player_1_score >= @dirac_dice_winning_score, do: {1, 0}
+  def run_dirac_dice_game({_, {_, player_2_score}}, :player_1) when player_2_score >= @dirac_dice_winning_score, do: {0, 1}
+
+  def run_dirac_dice_game({player_1, player_2}, :player_1) do
+    for {player_1, freq} <- quantum_move(player_1) do
+      run_dirac_dice_game({player_1, player_2}, :player_2)
+      |> multiply_by(freq)
+      |> compact()
+    end
+    |> compact()
+  end
+
+  def run_dirac_dice_game({player_1, player_2}, :player_2) do
+    for {player_2, freq} <- quantum_move(player_2) do
+      run_dirac_dice_game({player_1, player_2}, :player_1)
+      |> multiply_by(freq)
+      |> compact()
+    end
+    |> compact()
+  end
+
+  def quantum_move(player) do
+    for {roll, freq} <- @quantum_rolls do
+      {move(player, roll), freq}
+    end
+  end
+
+  def multiply_by(xs, c) when is_list(xs), do: Enum.map(xs, &multiply_by(&1, c))
+  def multiply_by({a, b}, c), do: {a * c, b * c}
+
+  def compact(xs) when is_list(xs) do
+    {as, bs} = Enum.unzip(xs)
+    {Enum.sum(as), Enum.sum(bs)}
+  end
+  def compact(tp) when is_tuple(tp), do: tp
+
   def play_game(players, die, winning_score \\ 1000) do
     Enum.reduce_while(rolls(die), {0, players}, fn
       roll, {roll_count, {player, other_player}} ->
@@ -24,8 +68,9 @@ aoc 2021, 21 do
     end)
   end
 
+  def move(player, rolls) when is_list(rolls), do: move(player, Enum.sum(rolls))
   def move({position, score}, roll) do
-    position = (position + Enum.sum(roll)) |> to_board_position()
+    position = to_board_position(position + roll)
     score = score + position
 
     {position, score}