Day 18 of Advent of Code 2021 took rather a long time to get right. Half of the problem was that there were a couple of gotchas in the requirements and the choice of data structure hugely impacted how easy it was to come up with a solution. Just like Day 17 I was using TDD and this helped me root out some blind alleys nicely. The problem The story was roughly as follows: In order to continue on the Advent of Code submarine journey, we had to solve snailfish number problems.
