Oh I’d hate to accidentally delete my code for this one… I usually use reduce_while/3 if I need to return early. Tons of it in this problem.

I thought it was going to be tedious at first but in the end, just a couple modifications from part one are enough. This runs in 15ms:

https://github.com/lud/adventofcode/blob/main/lib/solutions/2024/day15.ex

For example, in the following case

######
#...##
#[][]#
#.[].#
#.@..#
######

^

In this case, I try to push the box on the 4th row upward, which in requires pushing the left box on the 3rd row upward (ok), then the right box on the 3rd row upward (failed). In traditional programming language, you need to pull back the left box (backtracking), but in Elixir, you just need to use the state before pushing.