Advent of Code 2021

All my Advent of Code repos:

• AoC 2015 in Nim, Python
• AoC 2016 in Python, Clojure (+ visualizations)
• AoC 2017 in Nim, OCaml, Python
• AoC 2018 in Nim, Python, Racket
• AoC 2019 in OCaml, Python, Clojure
• AoC 2020 in Nim, one liner-y Python, Racket
• AoC 2021 in Python, Racket (this repo)
• AoC 2022 in Python, Clojure
• AoC 2023 in Clojure
• AoC 2024 in Clojure (Clerk notebooks), Python, Elixir
• AoC 2025 in Clojure (Clerk notebooks)

 

This year it is back to roots - Python!

I'll probably add solutions in other languages on some later date.

EDIT, November 2022: And I did - I've solved initial days in Racket too.

EDIT, November 2025: Days 19, 22, 23 were solved in Clojure.

Solutions

Task	Python Solution	Racket Solution	Comment for Python solution
Day 00: Helper file	aoc.py	aoc.rkt	Utilities to use for solving the tasks.
Day 01: Sonar Sweep	day01.py	day01.rkt	(a + b + c) < (b + c + d) --> a < d
Day 02: Dive!	day02.py	day02.rkt	"Up, up, down, down, left, right, left, right..."
Day 03: Binary Diagnostic	day03.py	day03.rkt	from collections import Counter
Day 04: Giant Squid	day04.py	day04.rkt	Set operations (<=, -) to the rescue.
Day 05: Hydrothermal Venture	day05.py	day05.rkt	TIL defaultdict is faster than Counter.
Day 06: Lanternfish	day06.py	day06.rkt	Recursion with memoization.
Day 07: The Treachery of Whales	day07.py	day07.rkt	Mean mean!
Day 08: Seven Segment Search	day08.py	day08.rkt	Using Python 3.10 and pattern matching.
Day 09: Smoke Basin	day09.py	day09.rkt	Recursive DFS.
Day 10: Syntax Scoring	day10.py	day10.rkt	dict(zip(...)) for easier typing
Day 11: Dumbo Octopus	day11.py	day11.rkt	Iterative DFS.
Day 12: Passage Pathing	day12.py	day12.rkt	Recursive DFS generator with yield from.
Day 13: Transparent Origami	day13.py	day13.rkt	Fold. Not fold.
Day 14: Extended Polymerization	day14.py		Counter to the rescue.
Day 15: Chiton	day15.py		Dijkstra.
Day 16: Packet Decoder	day16.py		iter for some elegant stream implementation.
Day 17: Trick Shot	day17.py		Analytical + brute force.
Day 18: Snailfish	day18.py		My first ever eval in 7 instances of AoC.
Day 19: Beacon Scanner	day19.py	day19.clj
Day 20: Trench Map	day20.py		I had 9-neighbours helper ready.
Day 21: Dirac Dice	day21.py		Lanternfish says hi!
Day 22: Reactor Reboot	day22.py	day22.clj
Day 23: Amphipod	day23.py	day23.clj
Day 24: Arithmetic Logic Unit	day24.py
Day 25: Sea Cucumber	day25.py