Well I did it! For the first time since 2015 I managed to complete all the problems within 24 hours of them being published. It was a nice practice in my common lisp skills though many days were solved in a rush due to all other things that happen in my hectic life.

Here follows some comments on the last nine days of Advent of Code.

## day 17

OK, this was quite easy for me since I had already played around with Conway's game of life and the problem of day 17 was a three dimensional and then a four dimensional implementation. By understanding that one does not need to keep the state of every cell in the game, since only the active cells can cause changes, it is easy to create an efficient solution.

## day 18

For this day I was lucky to think of the shunting-yard algorithm. I parsed the input into an rpn expression and then evaluated that recursively. The first part was without operator precedence (i.e. all operations have the same weight), and the second part needed additions to happen before multiplications.

## day 19

The problems of day 19 were tricky. They had to do with trees of rules, that could be constructed with a grammar parser and all that jazz. It was one of the day that troubled me the most.

## day 20

Oh, day 20. This was probably the most difficult day of all. The problem was to construct a jigsaw of images, that where in random rotations. It wasn't so much that it needed some novel idea to solve but it needed a lot of micro-management of brute force and backtracking to produce the answer.

## day 21

In day 21 the problem was to map ingredients to allergens from a list of unknown products. This was mostly a set problem and was fast and elegant to solve around.

## day 22

A game of combat with recursive sub-games was the gist of the day. It wasn't that much of a challenge but it was fun!

## day 23

Another game for day 23, this time a game of rotations in a deck of cups(?). It had a lot of edge-cases so was quite bothersome to implement.

## day 24

Hexagonal grids and game of life. Once again an easy day!

## day 25

Day 25 requested the implementation of a Diffie-Hellman-Merkle. It was interesting and fun as a challenge for Christmas day.

## epilogue

Challenges like this are fun, and I feel nice that I tried to finish the whole ordeal this year. Will I do again next year? Well, that depends mostly on how busy I will be. If you see this post and feel interested in any of my solutions feel free to ask me anything. Bye!