🎄 Advent of Code 2021 🎄

This repository contains my solutions for Advent of Code 2021, the first year I ever participated.

What is Advent of Code?

Advent of Code is a series of small programming puzzles created by Eric Wastl. Every day from December 1st to 25th, a puzzle is released alongside an engaging fictional Christmas story. Each puzzle consists of two parts, the second of which usually contains some interesting twist or changing requirements and is only unlocked after completing the first one. The objective is to solve all parts and collect fifty stars ⭐ until December 25th to save Christmas.

Many users compete on the global leaderboard by solving the puzzles in an unbelievably fast way in order to get some extra points. Personally, I see Advent of Code as a fun exercise to do during the Advent season while waiting for Christmas. I often use it to learn a new programming language (like I did in 2021 with C#) or some advanced programming concepts. I can only encourage you to participate as well - of course in a way that you find fun. Just get started and learn more about Advent of Code here.

About This Project

The solutions for Advent of Code 2021 were originally developed using .NET 6 and C# 10 at the time. Since then I have taken some time to update them to more recent versions (.NET 9 and C# 13), which allowed me to take advantage of new language features and modern data structures that either did not exist yet or I did not know about (as I was just starting to learn the whole ecosystem). These include expression bodies (=>), collection expressions ([...]), target-typed new(), Span<T> and ReadOnlySpan<T>, types of the System.Collections.Immutable or System.Collections.Frozen namespaces and much more.

For this project and in general when developing software, I strive to produce readable and well documented source code. However, I also enjoy benchmarking and optimizing my code, which is why I sometimes implement a less idiomatic, yet more efficient solution at the expense of readability. In those situations, I try to document my design choices with analogies, possible alternative solutions and sometimes little sketches to better illustrate the way a piece of code works.

The general structure of this project is as follows:

Day 1 - Sonar Sweep/
    Resources/
        .gitkeep
    Source/
        Benchmark.cs
        SonarSweep.cs
    Day 1 - Sonar Sweep.csproj
Day 2 - Dive!/
    Resources/
        .gitkeep
    Source/
        Benchmark.cs
        Dive.cs
    Day 2 - Dive!.csproj
...
Day 25 - Sea Cucumber/
    ...
.gitignore
Advent of Code 2021.sln
LICENSE
README.md

On the top level, the solution file contains 25 standalone projects for the days of the Advent calendar, organized into separate directories. Each one provides a corresponding .csproj file that can be opened in Visual Studio. In addition, there are the usual Source and Resources directories, the latter of which contains the puzzle description and my personal input for that day. However, as requested by the creator of Advent of Code, these are only present in my own private copy of the repository and therefore not publicly available.

If you're posting a code repository somewhere, please don't include parts of Advent of Code like the puzzle text or your inputs.

As a consequence, you will have to provide your own inputs for the days, as described in more detail in the following section.

Dependencies And Usage

If you want to try out one of my solutions, simply follow these steps below:

0. Make sure you have .NET 9 or a later version installed on your machine.

1. Clone the repository (or download the source code) to a directory of your choice.

   git clone https://github.com/Piwimau/Advent-of-Code-2021 "./Advent of Code 2021"
   cd "./Advent of Code 2021"

2. Put your input for the day in a file called input.txt and copy it to the appropriate resources directory. You can get all inputs from the official website if you have not downloaded them already.

   cp input.txt "./Day 1 - Sonar Sweep/Resources"

3. Nagivate into the appropriate day's directory.

   cd "./Day 1 - Sonar Sweep"

4. Finally, run the code in release mode to take advantage of all optimizations and achieve the best performance.

   dotnet run -c Release

   Optionally, specify an additional flag --benchmark to benchmark the relevant day on your machine. Note that in this mode no output for the results of the solved puzzle is produced.

   dotnet run -c Release --benchmark

If you have Visual Studio installed on your machine, you may also just open the provided solution file and proceed from there.

Benchmarks

Finally, here are some (non-scientific) benchmarks I created using the fantastic BenchmarkDotNet package and my main machine (Intel Core i9-13900HX, 32GB DDR5-5600 RAM) running Windows 11 24H2. All benchmarks include the time spent for reading the input from disk, as well as printing the puzzle results (although the output is written to TextWriter.Null when benchmarking, which is effectively a no-op and rather fast).

Day	Min	Max	Mean	Median	Standard Deviation
Day 1 - Sonar Sweep	0.054 ms	0.057 ms	0.055 ms	0.055 ms	0.001 ms
Day 2 - Dive!	0.101 ms	0.105 ms	0.103 ms	0.102 ms	0.001 ms
Day 3 - Binary Diagnostic	0.076 ms	0.077 ms	0.076 ms	0.076 ms	0.000 ms
Day 4 - Giant Squid	1.459 ms	1.516 ms	1.490 ms	1.492 ms	0.016 ms
Day 5 - Hydrothermal Venture	10.650 ms	11.047 ms	10.867 ms	10.905 ms	0.112 ms
Day 6 - Lanternfish	0.019 ms	0.020 ms	0.020 ms	0.020 ms	0.000 ms
Day 7 - The Treachery of Whales	3.567 ms	3.673 ms	3.624 ms	3.639 ms	0.037 ms
Day 8 - Seven Segment Search	0.175 ms	0.182 ms	0.177 ms	0.177 ms	0.002 ms
Day 9 - Smoke Basin	0.994 ms	1.036 ms	1.008 ms	1.004 ms	0.013 ms
Day 10 - Syntax Scoring	0.131 ms	0.135 ms	0.133 ms	0.133 ms	0.001 ms
Day 11 - Dumbo Octopus	1.027 ms	1.059 ms	1.039 ms	1.039 ms	0.009 ms
Day 12 - Passage Pathing	32.794 ms	34.190 ms	33.348 ms	33.262 ms	0.402 ms
Day 13 - Transparent Origami	0.132 ms	0.137 ms	0.135 ms	0.135 ms	0.002 ms
Day 14 - Extended Polymerization	0.208 ms	0.222 ms	0.213 ms	0.212 ms	0.004 ms
Day 15 - Chiton	31.869 ms	32.248 ms	32.028 ms	32.014 ms	0.108 ms
Day 16 - Packet Decoder	0.036 ms	0.039 ms	0.038 ms	0.037 ms	0.001 ms
Day 17 - Trick Shot	0.467 ms	0.475 ms	0.470 ms	0.470 ms	0.002 ms
Day 18 - Snailfish	15.981 ms	16.463 ms	16.186 ms	16.187 ms	0.137 ms
Day 19 - Beacon Scanner	234.062 ms	241.932 ms	237.632 ms	237.508 ms	2.211 ms
Day 20 - Trench Map	22.148 ms	22.750 ms	22.511 ms	22.535 ms	0.152 ms
Day 21 - Dirac Dice	2.759 ms	2.919 ms	2.836 ms	2.840 ms	0.050 ms
Day 22 - Reactor Reboot	34.651 ms	36.186 ms	35.877 ms	36.039 ms	0.401 ms
Day 23 - Amphipod	270.380 ms	297.666 ms	280.041 ms	280.234 ms	7.234 ms
Day 24 - Arithmetic Logic Unit	115.883 ms	121.170 ms	118.048 ms	118.097 ms	1.429 ms
Day 25 - Sea Cucumber	53.427 ms	54.866 ms	54.009 ms	53.951 ms	0.395 ms
Total	833.050 ms	880.170 ms	851.964 ms	852.163 ms	12.720 ms

License

This project is licensed under the MIT License. Feel free to experiment with the code, adapt it to your own preferences, and share it with others.