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Advent of Code 2025

It’s back! (Advent of Code)!

It’s been ten years (of advent of code, I haven’t done them all (yet)) and oh what a ten years it’s been. This time around, there will be only 12 days instead of 25, but honestly, that means I’m not working on these right up until Christmas. So I’m okay with this.

Once again, Rust! But this time, I won’t be using cargo-aoc, instead I wrote my own proc macros. Mostly to see if I could. 😄 See this section for more information.

Full solutions will once again be posted to GitHub (including previous years and possibly some I haven’t written up yet): jpverkamp/advent-of-code

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AoC 2024 Day 25: Christmas Lockpickinator

Source: Day 25: Code Chronicle

Full solution for today (spoilers!).

Part 1

You are given a series of locks and keys (see below). How many unique (lock, key) are there that do not overlap (they do not have to fit perfectly).

A lock starts from the top. The entire top row is # and the entire bottom row is ..

#####
.####
.####
.####
.#.#.
.#...
.....

A key is the opposite:

.....
.....
.....
#....
#.#..
#.#.#
#####

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AoC 2024 Day 23: LAN Partinator

Source: Day 23: LAN Party

Full solution for today (spoilers!).

Part 1

You are given the edges of an undirected graph. Count how many complete subgraphs of size three exist that contain one or more starting with the letter t.

Aside: Games with local (but not hotseat) multiplayer have gotten rather rarer over the years… how many people still know what a LAN party is/was?

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AoC 2024 Day 22: Xorshiftinator

Source: Day 22: Monkey Market

Full solution for today (spoilers!).

Part 1

Implement a PRNG with the following update function:

  1. Multiply by 64, xor with the previous value, modulo 16777216
  2. Divide by 32, xor with the previous value (from step 1), modulo 16777216
  3. Multiply by 2048, xor with the previous value (from step 2), module 16777216

For each of a series of seeds, sum the 2000th generated number.

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AoC 2024 Day 21: Busy Workinator

Source: Day 21: Keypad Conundrum

Full solution for today (spoilers!).

Part 1

You are trying to type a code on a keypad:

+---+---+---+
| 7 | 8 | 9 |
+---+---+---+
| 4 | 5 | 6 |
+---+---+---+
| 1 | 2 | 3 |
+---+---+---+
    | 0 | A |
    +---+---+

But you cannot type directly. Instead, you can control a pointer on the keypad with arrow keys:

    +---+---+
    | ^ | A |
+---+---+---+
| < | v | > |
+---+---+---+

Whenever you type a ^ on the arrow keys, the pointer on the keypad will move up one, etc. When you type A, then the pointer on the keypad will type whatever it is pointing at.

But that’s not enough either. Add a second keypad. And then a third, that is the one you are actually controlling.

For each output sequence multiple the length of the minimum input sequence needed to generate it by the numeric value of the input sequence (ignoring any A); sum these.

Note: Moving off any keypad or into the blank spaces is an error.

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AoC 2024 Day 20: Shadow Catinator

Source: Day 20: Race Condition

Full solution for today (spoilers!).

Part 1

Given a maze with exactly one path, find how many single walls you can walk through (remove) that shorten the best path by at least 100 units.

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