Fast Parse VS aoc2021

Perhaps use Li Haoyi's fastparse instead? https://github.com/com-lihaoyi/fastparse

You might want to adapt Li Haoyi’s XML parser for fastparse.

Mostly a mess of pattern matching. I really need to make some generic tree utilities. Haven't been able to find a decent parser combinator that works in Scala 3 (I usually use fastparse which depends heavily on Scala 2 macros, and scala-parser-combinators works in Scala 3, but I've had a lot of trouble getting it to not be too greedy), so I used the state monad from cats to parse at the bottom of the file, which I think turned out fairly nice.

Agreed! I would say that parser combinators are the sweet spot and the right choice in most cases.

Scala has them as well, e.g.: https://com-lihaoyi.github.io/fastparse/

And the good thing is, you don't have to learn a completely new language/syntax, you can use the host language's syntax and you have full IDE support as well.

(Here's my original version that used the uint64 encoding: https://github.com/danvk/aoc2021/blob/e795c78090c35c0b9880fd692ba1677fae31e044/day23/day23.go)

Here's my previous solution, which didn't use any mutation: https://github.com/danvk/aoc2021/blob/92b5352806e0e854ebc36e6c9804dc13b5262349/day18/day18.go