Advent of Code F# – Day 21

You can become a serverless blackbelt. Enrol to my 4-week online workshop Production-Ready Serverless and gain hands-on experience building something from scratch using serverless technologies. At the end of the workshop, you should have a broader view of the challenges you will face as your serverless architecture matures and expands. You should also have a firm grasp on when serverless is a good fit for your system as well as common pitfalls you need to avoid. Sign up now and get 15% discount with the code yanprs15!

The source code for this post (both Part 1 and Part 2) is available here and you can click here to see my solutions for the other Advent of Code challenges.

Description for today’s challenge is here.


Modelling and simulating a RPG, this challenge totally got my nerd brain into overdrive, which is probably why I went totally overboard with modelling the domain! (you’ll see soon enough..)


First we have characters in the game (you and the boss) with hit points (HP), damage and armor scores:


Then we have the equipments, which have costs, as well as damage and armor stats:


Then we also have shops, who sell these equipments:



Initially, you have 100 Hit Points, but no damage or armor stats (these come from equipments).

The state of the boss comes from your input:


We also need to configure the shop’s inventory according to the description of the challenge:



From the description, we know that:

  • you have to pick one weapon
  • you can have 0-1 armor
  • you can have 0-2 rings

So, to make combining equipments easier later on (we want your selection of equipments to be a simple Equipment[]), let’s create a few sequences of Equipment[] to represent your choices of weapons, armors and rings.

For weapons and armors, this is pretty straightforward:


For rings, it’s slightly more involved:


Because the ordering of the rings are not important – [| ring1; ring2 |] is the same as [| ring2; ring1 |] – so here we work out all combinations of 2 rings from the available 6.

Next, let’s work out all possible equipment combinations we can buy from the shop:


We’ll also need to be able to simulate a game between you and the boss:


Notice in the inner recursive loop above, we swap the role of the attacker and defender on every iteration (whilst also deducting the defender’s HP) and return the name of the winner when the defender’s HP reaches 0<HP>.

Now that all the pieces are in place, we can answer the challenge by iterating over all equipment combos, and sending you in battle with the boss with each. For the combos that results in you winning, we simply work out the cost and find the min:


job done!


Part 2

Only a minor tweak is required here – change the winner to “boss” and look for max cost instead of min:


Liked this article? Support me on Patreon and get direct help from me via a private Slack channel or 1-2-1 mentoring.
Subscribe to my newsletter

Hi, I’m Yan. I’m an AWS Serverless Hero and I help companies go faster for less by adopting serverless technologies successfully.

Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.

Hire me.

Skill up your serverless game with this hands-on workshop.

My 4-week Production-Ready Serverless online workshop is back!

This course takes you through building a production-ready serverless web application from testing, deployment, security, all the way through to observability. The motivation for this course is to give you hands-on experience building something with serverless technologies while giving you a broader view of the challenges you will face as the architecture matures and expands.

We will start at the basics and give you a firm introduction to Lambda and all the relevant concepts and service features (including the latest announcements in 2020). And then gradually ramping up and cover a wide array of topics such as API security, testing strategies, CI/CD, secret management, and operational best practices for monitoring and troubleshooting.

If you enrol now you can also get 15% OFF with the promo code “yanprs15”.

Enrol now and SAVE 15%.

Check out my new podcast Real-World Serverless where I talk with engineers who are building amazing things with serverless technologies and discuss the real-world use cases and challenges they face. If you’re interested in what people are actually doing with serverless and what it’s really like to be working with serverless day-to-day, then this is the podcast for you.

Check out my new course, Learn you some Lambda best practice for great good! In this course, you will learn best practices for working with AWS Lambda in terms of performance, cost, security, scalability, resilience and observability. We will also cover latest features from re:Invent 2019 such as Provisioned Concurrency and Lambda Destinations. Enrol now and start learning!

Check out my video course, Complete Guide to AWS Step Functions. In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. There is something for everyone from beginners to more advanced users looking for design patterns and best practices. Enrol now and start learning!