Day 5: Hydrothermal Venture
Haskell: Part 1 (00:28:39, rank 3916), Part 2 (00:52:45, rank 4469)
This was a fun problem that seems well-suited to good old mutation, but I decided to stick with plain old immutable Data.Map which turned out to be computationally okay.
This time parsing was super easy, and it just took me a while to wrap my head around how to generate the individual points from the segments. I missed the part about considering only straight lines but it didn't take too long to fix that.
Today's part 2 seemed like a tiny step beyond what I was already doing -- in fact I expected it to work just by removing the straight line filtering step from part 1, but it turned out I was generating the points incorrectly.
All I wanted was to say "give me all points (x,y) where x is in the range (x1..x2) and y is in the range (y1..y2)", but in Haskell you need to specify a range in the form [x1, next number on the way to x2 .. x2], and they have to be different numbers or you get an infinite list.
So you have to check if x1 == x2 && y1 == y2 and draw the steps from [0] in that case; otherwise the steps are from zero to the maximum absolute change in either dimension, which then gets multiplied by the direction we're moving in (i.e. x could be going down and y could be going up).
I'm certain that there's a really simple, elegant way to express all of that, but with 4 hours' sleep it hasn't occurred to me. If you're reading this and can see a nicer way, I'd love it if you would open an issue (or PR adding an improved version). In the meantime I'll scour a few other repos and see how other people did it.