Advent of Code F# – Day 9

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.

 

The input for Day 9 looks like this:

AlphaCentauri to Snowdin = 66

AlphaCentauri to Tambi = 28

AlphaCentauri to Faerun = 60

Snowdin to Tambi = 22

Snowdin to Faerun = 12

there are similarities between this challenge and Day 7 in that they’re essentially a graph problem. In that case, let’s model this problem as a graph:

day09_01

in our model of this problem, we have a set of cities, and the connections between them (where the key is the city to travel from, and the value is a map of the cities that you can travel to and the distance between the pair).

Like we did in Day 7, let’s add a helper function to help connect two cities – a and b – together, along with the distance between them.

day09_02

Armed with the above, we can now go back to our input, parse each line into two cities and their distance, and then fold over them to build up a graph:

day09_03v2

notice that the information we get from the input list is actually bi-directional, which is why in the fold we have to add the direction from a to b, as well as from b to a.

Next, starting with each cities, we want to find the routes that’ll visit each city once and only once.

day09_04

aside: in hindsight, both the model and the path finding algorithm here can be simplified, I simply didn’t realise the distance data is bi-directional (it was implied through the example which I skimmed over and got started writing code…). For instance, the data is set up such that every city is connected to every other city, so the steps I have taken above to determine ‘which cities can I travel from here’ is redundant.

That said, I decided to keep the solution as it is because:

a) it can solve a more general problem where cities do not form a mesh network

b) it works, and pretty easy to understand, so why change 


The last building block we need to compose our final solution is the ability to calculate the total distance we have to travel in a route:

day09_05

and to answer the challenge:

day09_06

in hindsight, it might be better to write the above as:

    graph |> findPaths |> Seq.map calcDistance |> Seq.min

 

Part 2

To work out the longest distance one can travel, simply flap the last line to look for the max distance instead, simple:

day09_07

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!