My solution is here.

Part 1 was pretty straightforward. I built a map of rules, and then used a bit of binary pattern matching to move to the next state.

In part 2 it really helped me that I did AoC last year. As soon as I saw it, I knew that there had to be a cycle. I did a quick experiment, and found out that the plants converge to the stable state pretty soon. In that state, the plants relative positioning doesn’t change, but they gradually move in one direction.

I briefly thought about solving this special case, but I wasn’t quite happy with that, because I thought there might exist a more general situation with larger cycles. Therefore, I wrote a more complex recursion which caches pattern transitions, and can handle the cycle of any length.

Here’s my solution to Day 12:

https://github.com/JEG2/advent_of_code_2018/blob/master/day_12/subterranean_sustainability.exs

I also found the cycle and quit there.

Here’s a video of the creation of this code (good for 14 days):

Thanks for sharing your solutions, I am learning tons by reading your code! On line 10 of the solution here, it looks like a declaration of the function without any body. Why is this separate and why not put this in the definition at line 14 itself? Thanks in advance