Advent of Code F# – Day 14

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.


This challenge involves a lot of calculations with km, seconds, km/s, etc. so let’s define these units:


Since a reindeer can be either resting, or flying, let’s capture that as its status:


and finally, let’s capture other important info such as speed, how long a reindeer can fly/rest for, etc:


The input to this challenge looks like this:

Rudolph can fly 22 km/s for 8 seconds, but then must rest for 165 seconds.

Cupid can fly 8 km/s for 17 seconds, but then must rest for 114 seconds.

Prancer can fly 18 km/s for 6 seconds, but then must rest for 103 seconds.

so let’s parse each line into a new State for a reindeer:


couple of things to note from the above:

  1. Distance is initialized at 0<km>
  2. Status is initialized as Flying 0<s>, as in ‘has flown for 0 seconds’

Next, let’s simulate how the state of a reindeer transitions from one second to the next:


this should seem pretty straightforward, the only thing worth pointing out is that when Status transitions from Resting => Flying and Flying => Resting, the new status is associated with 1<s> rather than 0<s>. That’s because in the second that has just passed (between the start of the function and the end), we have actually been in the new status.

e.g. at the start of the function, we deemed that we have already flown for the max amount of time possible, so for the duration of this second, we’ll be resting, which is why our Distance hasn’t changed and why by the end of the function, we’ll have rested for 1 second.

Finally, let’s run this simulation for 2503 seconds and find the furthest Distance of all the reindeer:



Part 2

Part 2 introduces a new concept of Score, so let’s update our model:


and at the end of each round of simulation, we’ll need to award the reindeer that has flown the furthest with a point each:


and finally, we’re looking for the highest Score instead of the furthest Distance:


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!