Another year of Advent of Code draws to a close. This year, I made a post detailing my solution to each puzzle. Some were simple, some very hard, but all, as ever, were interesting problems.
From the start, this year was harder than the last. Day 1’s puzzle set the stage. Part 1 is a straightforward regex problem, but part 2 requires you to read numbers written as both numerals (1) and words (one). This isn’t hard in itself, but the input contains overlapping matches like oneight. I figured this out quickly, but an inexperienced player might have been stuck for a long time.
The early puzzles kept up the difficulty, with several containing special cases to trip up the unwary. Day 10 in particular caused me a lot of trouble. Part 2 requires you to find the area enclosed by the pipes, allowing that you can “squeeze through” pipes (e.g., you can move horizontally along --- but not vertically). This rules out the obvious approach of flood-filling the interior area. I eventually figured out you could expand each tile into nine, marking the tiles blocked by pipes, compute the area, and divide by nine.
I kept up until around day 20, when the toughest puzzles began to appear. I came back to these one by one in early January, often consulting the subreddit for help. In time, I solved them all.
My favorites included day 19 (requiring a tricky form of depth-first search) and day 24 (finally, a puzzle that emphasized matrix algebra). Overall, the puzzles were much tougher than last year’s, in what I suspect was an intentional difficulty spike. Veteran players should be kept on their toes, after all. I wonder what new twists we’ll see next year, which will be the tenth (!) edition of Advent of Code.
I write this in February, when it’s hard to look forward to next winter. But it’s not all bad. Each November, I find myself wondering how the summer passed so quickly and knowing the cold, drizzly weather will only get colder. But soon I’ll have some puzzles to solve.