Since 2020, every year I solve the Advent of Code puzzles. This post, like those of previous years, is an experience report of my 25+ day journey in December.

**Spoiler alert**: If you’re still working or planning to work on the puzzles
stop reading now.

# Puzzles

Between December 1 and December 25, a programming puzzle appears every day on the Advent of Code website. Each problem has two parts, the second part unlocks after you complete the first. To understand each day’s puzzle, go to the website and read the full description (for example Day 1: Trebuchet?!).

# Setup

This year again, now the third time, I wrote my solutions in Clojure. Every year I understand more of the standard library and I feel I write more concise and more idiomatic code than in the previous years.

Usually I solved the day’s problem before going to work, sometimes I coded a bit in the evening to polish my solution.

My Day 14 and Day 22 solutions are slow: they run for about a hundred seconds to find the solution. I suspect that these programs would be more efficient if I wrote them as a loop updating some local mutable state. Next year, I want to study profiling and optimizing Clojure code.

# Highlights

On Day 12 I was proud to find a recursive solution using dynamic programming. In previous years, for example Day 10 in 2020, I struggled with similar problems.

I solved Day 14 and Day 20 by finding cycles in a periodic solution. This is also something I was more comfortable this year than before.

It was difficult to wrap my head around the interval arithmetics required to solve Day 5 and Day 19. Finally, I brute forced Day 5 and put aside the second part of Day 19 for a few days.

The second parts of Day 18, Day 21, Day 23 and Day 24 were the hardest to crack. I couldn’t solve these problems on my own and I used some help from Reddit.

On Day 18 I was on the right track, but I didn’t figure out how to apply Pick’s theorem to find the solution.

Day 21 was just too hard for me: I had some ideas, but nothing worked. I failed to recognize the hidden patterns in the input data. A typical Advent of Code puzzle.

Day 23 asked to find the longest path in a labyrinth. This is, as I learned this year, a much harder problem than finding the shortest path. I understood how to reduce the size of the search space, but my code is messy and I made a lot of mistakes during the implementation.

Day 24 was about line intersections. I understand well the underlying mathematical models but I failed to find a set of linear equations that yielded the solution. During the fourth week, I was tired and impatient to do symbolic math.

Day 25 asked to remove three vertices from a graph to create two isolated groups of nodes. Instead of writing code, I solved this problem graphically: I visualized the graph with the Graphviz and manually selected the nodes to remove.

# Summary

2023 Advent of Code started well and I had a few small victories on some harder days. The second parts of Day 18, Day 21, Day 23 and Day 24 were too hard for me and I used external help. As always, on these tough days I learned the most.

As usual, after solving the puzzles I like to see how others approached the same problem. Here are some repositories I regularly checked:

My solutions are available on GitHub.

# Acknowledgments

Thanks Eric Wastl for creating and running Advent of Code.

I enjoyed exchanging ideas and analyzing solutions with my friend Kornél.