**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?