If somebody is missing Stream solution of the part 2 today, here it is.

Here is my solution.

I solved part 1 in the morning, but my program needed 17 seconds to find the answer.

That started my day of premature optimization. In hindsight, the problem with my original solution was probably that I used streams in a non-efficient way. I reimplemented part 1 with my own implementation of lazy lists. I hoped that it would be useful for part 2. It wasn’t.

When I saw that all examples and my input had offsets in second half of the input, I knew how to solve it. Unfortunately I did some more premature optimization and didn’t get it to work. I then took a look in the forum and saw all my premature optimizations were totally unnecessary. I wrote my own solution inspired by a quick look at @sasajuric’s solution.

My solution solves part 1 in 0.2 seconds and part 2 in about 3 seconds.

Day 16 really sucked.
I didn’t figure it out. I wouldn’t have figured it out. I just don’t know or didn’t recall the magical matrix properties I’d have needed to figure out from this specific input to make it work, so I spent a couple of hours floundering to get you this useless video that will teach nothing and that you should probably not watch:

I’m still bitter about this day being a damn loss of all my free time, it felt like failing a shitty whiteboard programming interview.

This is my solution for day 16. I’m not happy with part 2, because the solution only works if the requested sequence is in the second half. On the upside, part 1 is reasonably fast (about 400 ms on my machine).

What if the input starts with 0000000?

370ms part 1
2.1s part 2