Advent of Code F# – Day 15

Yan Cui

I help clients go faster for less using serverless technologies.

This article is brought to you by

Is your CI build step taking too long? Try Depot for free today and experience up to 40x faster build speed!

Unlock faster CI for FREE

ps. look out for all my other solutions for Advent of Code challenges here.

 

Day 15

See details of the challenge here.

Today’s challenge is pretty straight forward, and a brute force approach would have suffice – try every t from 0 to inifinity and return the first t that satisfies this equation for all the discs:

        (t + disc.Number – (disc.Positions – disc.Time0Position)) % disc.Positions = 0

That said, there’s a simple optimization we can apply by restricting ourselves to values of t that gets you through the first slot. Give my input for the challenge:

Disc #1 has 13 positions; at time=0, it is at position 11.
Disc #2 has 5 positions; at time=0, it is at position 0.
Disc #3 has 17 positions; at time=0, it is at position 11.
Disc #4 has 3 positions; at time=0, it is at position 0.
Disc #5 has 7 positions; at time=0, it is at position 2.
Disc #6 has 19 positions; at time=0, it is at position 17.

The first t that will get us through disc #1 is t=1 where disc #1 reaches position 0 on t=2 (which is 1s away from t). From there, we only need to check every 13s, ie t=14, t=27, t=40, …

 

Part 2

After getting the first capsule (it contained a star! what great fortune!), the
machine detects your success and begins to rearrange itself.

When it’s done, the discs are back in their original configuration as if it were
time=0 again, but a new disc with 11 positions and starting at position 0 has
appeared exactly one second below the previously-bottom disc.

With this new disc, and counting again starting from time=0 with the
configuration in your puzzle input, what is the first time you can press the
button to get another capsule?

 

Links

Whenever you’re ready, here are 4 ways I can help you:

  1. Production-Ready Serverless: Join 20+ AWS Heroes & Community Builders and 1000+ other students in levelling up your serverless game. This is your one-stop shop for quickly levelling up your serverless skills.
  2. Do you want to know how to test serverless architectures with a fast dev & test loop? Check out my latest course, Testing Serverless Architectures and learn the smart way to test serverless.
  3. I help clients launch product ideas, improve their development processes and upskill their teams. If you’d like to work together, then let’s get in touch.
  4. Join my community on Discord, ask questions, and join the discussion on all things AWS and Serverless.

Leave a Comment

Your email address will not be published. Required fields are marked *