As with previous attempts, this one was absolutely screaming to be visualised. The challenge was “At what step do the Octopus Flashes become synchronised”; it sounded like a combinatorial trap but it wasn’t, it was a reasonable range and once that happens it’s begging to be turned into an animation. Here’s a heat-map style animation with lighter colours being closer to a flash and the big black screen at the end being the result of all the squares flashing at once.

What I find most interesting about this is how little structure is visible in the pattern of flashes until nearly the very end when all of a sudden there’s clear movement to “mop up” and, of course, once it’s in-step it’ll stay in step.

The code used to generate it (but not the data) is here: python/day11part2_extracredit.py

(If that doesn’t work, Click here to view the video directly)