2024 Day 15 Part 2 😵

2024/day15.exs

@@ -2,10 +2,8 @@
 defmodule Day15 do
   def part1({warehouse, robot, moves}) do
     attempt_moves(moves, robot, warehouse)
-    |> Enum.map(fn
-      {{x, y}, :box} -> x + y * 100
-      _ -> 0
-    end)
+    |> Enum.filter(&match?({_, :box}, &1))
+    |> Enum.map(fn {{x, y}, :box} -> x + y * 100 end)
     |> Enum.sum()
   end
 
@@ -47,8 +45,140 @@ defmodule Day15 do
     end
   end
 
-  def part2(_input) do
-    :boring_skipped
+  def part2({warehouse, {x, y}, moves}) do
+    bigger_warehouse =
+      Enum.reduce(warehouse, %{}, fn {{x, y}, v}, bigger ->
+        {left, right} =
+          case v do
+            :box -> {{:box, :left}, {:box, :right}}
+            :wall -> {:wall, :wall}
+          end
+
+        bigger |> Map.put({x * 2, y}, left) |> Map.put({x * 2 + 1, y}, right)
+      end)
+
+    robot = {x * 2, y}
+
+    attempt_moves2(moves, robot, bigger_warehouse)
+    |> Enum.filter(&match?({_, {:box, :left}}, &1))
+    |> Enum.map(fn {{x, y}, {:box, :left}} -> x + y * 100 end)
+    |> Enum.sum()
+  end
+
+  def attempt_moves2([], _robot, warehouse), do: warehouse
+
+  def attempt_moves2([move | next_moves], robot, warehouse) do
+    next = move.(robot)
+
+    case Map.fetch(warehouse, next) do
+      :error ->
+        attempt_moves2(next_moves, next, warehouse)
+
+      {:ok, :wall} ->
+        attempt_moves2(next_moves, robot, warehouse)
+
+      {:ok, {:box, side}} ->
+        case get_pushable_boxes2(next, side, move, warehouse, [{next, side}]) do
+          :wall ->
+            attempt_moves2(next_moves, robot, warehouse)
+
+          boxes ->
+            new_boxes = Map.new(boxes, fn {box, side} -> {move.(box), {:box, side}} end)
+
+            warehouse =
+              warehouse
+              |> Map.drop(Enum.map(boxes, &elem(&1, 0)))
+              |> Map.merge(new_boxes)
+
+            attempt_moves2(next_moves, next, warehouse)
+        end
+    end
+  end
+
+  # need to account for 2-width boxes when pushing vertically
+  #        @<--pushing down moves down all RHS of stack
+  # [][][][]
+  #  [][][]      @
+  #   [][]       []<--make sure RHS moves too
+  #    []
+  #     @<- pushing up pushes all
+  def get_pushable_boxes2(box, side, move, warehouse, pushable_boxes) do
+    next = move.(box)
+
+    case abs(elem(next, 1) - elem(box, 1)) do
+      # horizontal
+      0 ->
+        case Map.fetch(warehouse, next) do
+          :error ->
+            pushable_boxes
+
+          {:ok, :wall} ->
+            :wall
+
+          {:ok, {:box, next_side}} ->
+            get_pushable_boxes2(next, next_side, move, warehouse, [
+              {next, next_side} | pushable_boxes
+            ])
+        end
+
+      # vertical
+      1 ->
+        {x, y} = box
+
+        {other_box, other_side} =
+          case side do
+            :left -> {{x + 1, y}, :right}
+            :right -> {{x - 1, y}, :left}
+          end
+
+        other_next = move.(other_box)
+
+        pushable_boxes = [{other_box, other_side} | pushable_boxes]
+
+        side
+        |> case do
+          :left ->
+            {{Map.fetch(warehouse, next), Map.fetch(warehouse, other_next)}, next, other_next}
+
+          :right ->
+            {{Map.fetch(warehouse, other_next), Map.fetch(warehouse, next)}, other_next, next}
+        end
+        |> case do
+          {{:error, :error}, _, _} ->
+            pushable_boxes
+
+          {{{:ok, :wall}, _}, _, _} ->
+            :wall
+
+          {{_, {:ok, :wall}}, _, _} ->
+            :wall
+
+          {{{:ok, {:box, side}}, :error}, left, _right} ->
+            get_pushable_boxes2(left, side, move, warehouse, [{left, side} | pushable_boxes])
+
+          {{:error, {:ok, {:box, side}}}, _left, right} ->
+            get_pushable_boxes2(right, side, move, warehouse, [{right, side} | pushable_boxes])
+
+          {{{:ok, {:box, :left}}, {:ok, {:box, :right}}}, left, _} ->
+            # if left-right, just one box, only need to check one side
+            # []
+            # []
+            get_pushable_boxes2(left, :left, move, warehouse, [{left, :left} | pushable_boxes])
+
+          {{{:ok, {:box, :right}}, {:ok, {:box, :left}}}, left, right} ->
+            # if right-left, two boxes, so need to check both...
+            # [][]
+            #  []
+            left = get_pushable_boxes2(left, :right, move, warehouse, [{left, :right}])
+            right = get_pushable_boxes2(right, :left, move, warehouse, [{right, :left}])
+
+            if left == :wall or right == :wall do
+              :wall
+            else
+              pushable_boxes ++ left ++ right
+            end
+        end
+    end
   end
 
   def input do