bug: Fixing day 12 solution

[truggeri]

Summary

My original solution from #13 did not solve Day 12 correctly. This change fixes that. Rather than maintaining lists of patches, we now have a single reference to the "garden" (data semantics of a reference) and iterate over it. The new approach is as follows,

1. Parse the input characters into plots
2. Update each plot to use the correct fences
3. Assign region ids to each plot from top down and left to right
4. Look for regions that can be merged from left to right

• Always merge from higher id into lower id which removes the possibility of loops
• Only merge into a patch once per iteration
• Continue this iteration until no potential merges are found

5. Iterate over the plots to collect the area and perimeter stats on each patch
6. Calculate cost of each patch

The merge algorithm took the longest to get right. Having logic that detected when prior merges affected the current merge was tricky to get right, but simplifying the amount of changes per iteration and increasing the number of iterations fixed things.