Advent of Code F# – Day 15

Yan Cui

I help clients go faster for less using serverless technologies.

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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?