this post was submitted on 10 Dec 2024
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Day 12:
P1
Ok. I have been traumatised by computational geometry before, so I was initially spiralling. Luckily, there wasn't too much comp geo stuff to recall.My solution was a lot simpler than I initially thought: no need for union-find, accounting for regions inside regions, etc. Just check every square for a given region, and if it touches a square in the same region that you've seen before, subtract the common edge. This is linear in the area of the problem, so it's fast enough.
P2
It took a moment to figure out that I could modify the above perimeter counting to mark the squares containing the perimeter and walk along it afterwards, counting each edge. This is also linear in area.