This was my initial solution I used to submit the puzzle answer. Runtime for part 2 on my input set was around a second, so now I'm going to find optimization targets. I'm sure there's a lot I could be doing much better.
Pretty happy with this one overall. I tripped myself up on part 2 and caused a huge headache by not realizing that I was passing the already-modified array from part 1 into part 2. I left some code to copy the 2d array because that was my original solution (and it's not straightforward to copy jagged arrays), but then realized that the synchronized flash doesn't happen until after step 100 (at least for my input and the demo input) so we can just account for that in part 2 and pick up where part 1 left off.
As always, plenty of room for improvement, but I'm reasonably satisfied with the core implementation. I'm honestly not sure why the Distinct is needed in the GetBasinSize() result as it's supposed to not even add the point if the point is already in there, but it's late so I'll check that out later.
I hate almost everything about this solution. It's all awful and a product of constantly adjusting how I was solving the problem to try and get the correct answer, so there are vestigial pieces of each road I headed down. Then I ended up using some combination of all of those pieces to get the actual answer, and...well it just needs to be overhauled dramatically. But it works, so I guess mission accomplished.
I'm reasonably happy with this one. I'm sure there's an easier way to determine the smallest distance, at least for part 1, but this works fast enough for me.
There is a lot to be disliked here, starting with the Line Length calculation, which I'm sure I'm just totally overthinking or otherwise being dumb about, to the way I'm picking the next point while moving along the line.
Also, TIL how much of a difference a good GetHashCode() override can make when comparing/equating custom struct instances. Not implementing GetHashCode() on this solution resulted in ~250ms runtime on my PC. Implementing a "bad" GetHashCode() (which was `return x.GetHashCode() | y.GetHashCode();` which seemed fine to me ¯\_(ツ)_/¯) resulted in >1s runtime on my PC. Using this implementation (HashCode.Combine()) results in ~25ms runtime on my PC. Those numbers are with nothing changing other than the implementation of GetHashCode() on struct Point.
I also disabled nullability in my quest to improve performance here. The compiler was complaining that my Equals() override wasn't equivalent because of nullability, so it had to be `object? obj` which I didn't care for since this was a value type and not a reference type.
