Octopath Traveller 0
Looks so nice!
Not quite dead yet.
\o/ I get to learn more functions from you 
My day 9 part 2 was absolutely disastrous! I got it somewhat working after reading up on polygon intersection on Wikipedia, but it was still wrong for many hours and many tries.
Then, I realized I would need powerful debugging tools, since everything was passing the test/sample but failing with the real input.
The simplest way I thought of doing so was to manually (vim-ing) convert the input to an SVG using <path>, and overlay the solutions my program was outputting with a simple <rect>.
This led to a big aha! moment...
Big spoilers! Problem visualization!
In my solution I was only checking the corners of the solution were inside the polygon. But I needed to check the edges didn't intersect anything as well!
After some fixups, tada! 
And it's very ironic to me I overlooked this... because I have written a lot about this exact kind of thing!