Table of contents

• Day 1 - Parentheses
• Day 2 - The gift that keeps on giving (errors)
• Interlude 1 - Small steps
• Day 3, part 1 - Visiting pointers
• Day 3, part 2 - Extracting the set
• Day 4 - Pointless hashing
• Day 5 - Just a string
• Day 6, part 1 - Lights out
• Day 6, part 2 - Brighter
• Day 7, part 1 - Wires
• Day 7, part 2 - Solving (but not the puzzle)
• Day 7, part 3 - Solutions?
• Day 7, part 4 - Caution: refactoring needed
• Day 8 - Counting characters
• Interlude 2 - Lessons learned
• Day 9, part 1 - A short trip

I started Advent of Code in 2024, and did the whole thing in 2025 as well, both in Python. Python in my go-to language for basically anything, so that made sense. But Advent of Code exists since 2015, so there are still lots of puzzles available to me. I think there are some good opportunities for learning new things there, but probably not that much if I keep inside my comfort area.

So let’s try some C.

I haven’t done C programming since I was an engineering student, and I didn’t do much in C back then. A little bit of C++ for my master thesis. But since then, mostly Python, with a bit of Java, PHP, and Javascript on the side. But C is the backbone of most modern programming language. Python is written in C (at least the most commonly used version). You can’t get away from C. I know, I know – Rust is right there. I’ll get to it. Maybe for the 2016 edition.

In the meantime – C.

My personal rules are:

• Use standard library tools when available, learning them is part of the fun.
• Don’t use any AI. The goal is to learn, and AI is very bad for learning (and in general, but also for learning).
• Try to learn good practices along the way, as much as possible.

All my solutions will be on Codeberg here: https://codeberg.org/adfoucart/aocode15. Eric Wastl asks not to share the puzzle inputs, so if you want them you’ll have to create your own account, and get your own!