From 1972ad56aeedd049a1a8eb34dbd13897134b4e38 Mon Sep 17 00:00:00 2001
From: Adam Millerchip <adam@millerchip.net>
Date: Tue, 17 Dec 2024 09:59:22 +0900
Subject: [PATCH] refactor day15 a bit

Remove need to duplicate first value of pushable_boxes/2
Fail fast when left branch produces :wall in part 2
---
 2024/day15.exs | 25 +++++++++++--------------
 1 file changed, 11 insertions(+), 14 deletions(-)

diff --git a/2024/day15.exs b/2024/day15.exs
index b5dad91..34d159a 100755
--- a/2024/day15.exs
+++ b/2024/day15.exs
@@ -78,7 +78,7 @@ defmodule Day15 do
         attempt_moves2(next_moves, robot, warehouse)
 
       {:ok, {:box, side}} ->
-        case get_pushable_boxes2(next, side, move, warehouse, [{next, side}]) do
+        case get_pushable_boxes2(next, side, move, warehouse) do
           :wall ->
             attempt_moves2(next_moves, robot, warehouse)
 
@@ -102,8 +102,9 @@ defmodule Day15 do
   #   [][]       []<--make sure RHS moves too
   #    []
   #     @<- pushing up pushes all
-  def get_pushable_boxes2(box, side, move, warehouse, pushable_boxes) do
+  def get_pushable_boxes2(box, side, move, warehouse, pushable_boxes \\ []) do
     next = move.(box)
+    pushable_boxes = [{box, side} | pushable_boxes]
 
     case abs(elem(next, 1) - elem(box, 1)) do
       # horizontal
@@ -116,9 +117,7 @@ defmodule Day15 do
             :wall
 
           {:ok, {:box, next_side}} ->
-            get_pushable_boxes2(next, next_side, move, warehouse, [
-              {next, next_side} | pushable_boxes
-            ])
+            get_pushable_boxes2(next, next_side, move, warehouse, pushable_boxes)
         end
 
       # vertical
@@ -154,28 +153,26 @@ defmodule Day15 do
             :wall
 
           {{{:ok, {:box, side}}, :error}, left, _right} ->
-            get_pushable_boxes2(left, side, move, warehouse, [{left, side} | pushable_boxes])
+            get_pushable_boxes2(left, side, move, warehouse, pushable_boxes)
 
           {{:error, {:ok, {:box, side}}}, _left, right} ->
-            get_pushable_boxes2(right, side, move, warehouse, [{right, side} | pushable_boxes])
+            get_pushable_boxes2(right, side, move, warehouse, 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])
+            get_pushable_boxes2(left, :left, move, warehouse, 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
+            with left when left != :wall <- get_pushable_boxes2(left, :right, move, warehouse, []),
+                 right when right != :wall <- get_pushable_boxes2(right, :left, move, warehouse, []) do
               pushable_boxes ++ left ++ right
+            else
+              :wall -> :wall
             end
         end
     end