Yes I thought it would be slow because for part 2 I modify the free blocks list not only at the head, but lower in the linked list.

But I guess while slow, linked lists are highly optimized in Erlang.

I’d suggest you run my solution on your machine though. I know that @bjorng uses the time reported by mix test for instance. I use :timer.tc in my helper commands so it’s going to report lower times.

It took me a long time to find an off-by-one error in part 2. The combined runtime for both parts is 0.4 seconds.

https://github.com/bjorng/advent-of-code/blob/main/2024/day09/lib/day09.ex

Glad it helped! I’ll also recommend @bjorng’s solution (or anyone else who used a map instead of a list to track state). Map-based solutions often end up being more performant. I’m trying to write really terse solutions this year so I end up sacrificing speed for LOC.