Using Scala 3 with lots of pattern matching. So far I have not had to use any type annotations outside of method signatures, import statements in problem objects, while loops, var declarations, or scala.collection.mutable except for PriorityQueue.

My setup is just the standard library, a text editor with autocomplete, and a terminal with sbt ~run. The @main method uses a macro to only compile one solution at a time. So far I only added two helper methods. The first is to make a Range like x to y in the case that x>y. This may be done with an explicit call of by -1 already, so it may be added with a simple inline extension. The second is a non-comparative median using the bisection method to minimize absolute deviations, inspired by day seven and used again in day ten.

So far all the solutions performed well until day twenty, then my approach to iterative closest point was way too slow. This might be improved using the median absolute deviation heuristic, and being more confident to find triangles with matching side lengths instead of points with similar cube shells.

Twenty-one is pretty slow too, and it would be nice to make it work for the example input as well as the actual input.