2024 Day 6 part 2

2024/day6.exs

@@ -2,6 +2,7 @@
 defmodule Patrol do
   defstruct obstacles: MapSet.new(),
             visited: MapSet.new(),
+            path: MapSet.new(),
             x: 0,
             y: 0,
             max_x: nil,
@@ -11,20 +12,32 @@ defmodule Patrol do
 end
 
 defmodule Day6 do
-  def part1(%Patrol{} = patrol) when patrol.x == patrol.max_x or patrol.y == patrol.max_y do
-    MapSet.size(patrol.visited)
+  def part1(patrol), do: MapSet.size(patrol(patrol).visited)
+
+  def patrol(%Patrol{} = patrol) when patrol.x in [-1, patrol.max_x] or patrol.y in [-1, patrol.max_y] do
+    patrol
   end
 
-  def part1(%Patrol{} = patrol) do
+  def patrol(%Patrol{} = patrol) do
     next_x = patrol.x + patrol.dir_x
     next_y = patrol.y + patrol.dir_y
 
+    patrol =
       if MapSet.member?(patrol.obstacles, {next_x, next_y}) do
         {dir_x, dir_y} = turn_right(patrol.dir_x, patrol.dir_y)
-      part1(%Patrol{patrol | dir_x: dir_x, dir_y: dir_y})
+        %Patrol{patrol | dir_x: dir_x, dir_y: dir_y}
       else
         visited = MapSet.put(patrol.visited, {patrol.x, patrol.y})
-      part1(%Patrol{patrol | x: next_x, y: next_y, visited: visited})
+        %Patrol{patrol | x: next_x, y: next_y, visited: visited}
+      end
+
+    next_path = {patrol.x, patrol.y, patrol.dir_x, patrol.dir_y}
+
+    if MapSet.member?(patrol.path, next_path) do
+      :loop
+    else
+      patrol = %Patrol{patrol | path: MapSet.put(patrol.path, next_path)}
+      patrol(patrol)
     end
   end
 
@@ -33,9 +46,20 @@ defmodule Day6 do
   def turn_right(0, 1), do: {-1, 0}
   def turn_right(-1, 0), do: {0, -1}
 
-  def part2(_input) do
-    # hmmmmmmm probably need completely different solution
-    :ok
+  def part2(patrol) do
+    completed_patrol = patrol(patrol)
+
+    candidate_obstructions =
+      Enum.reduce(completed_patrol.visited, MapSet.new(), fn {x, y}, candidate_obstructions ->
+        Enum.into([{x - 1, y}, {x + 1, y}, {x, y - 1}, {x, y + 1}], candidate_obstructions)
+      end)
+
+    Enum.count(candidate_obstructions, fn {x, y} ->
+      new_obstacles = MapSet.put(patrol.obstacles, {x, y})
+      new_patrol = %Patrol{patrol | obstacles: new_obstacles}
+
+      patrol(new_patrol) == :loop
+    end)
   end
 
   def input do