Fast Parse VS Advent-of-code

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.

Wow, almost identical to mine: https://github.com/MichalMarsalek/Advent-of-code/blob/master/2022/Nim/day6.nim

This year I was writing two sets of solutions in Nim. The first one focuses on idiomatic, nice and short and readable Nim. The other focuses purely on speed. The combined running times of the fast solutions is 130 ms. Please let me know if you have any tips on how to make my solutions more simple and/or idiomatic.

Beautiful Nim! My solution is actually very similar.