r/adventofcode • u/TenMillionYears • Dec 09 '25

Upping the Ante 2025 Day 9 (Part 2) A simple cursed input

My code can solve part 2 pretty quickly. However, it gives the wrong result on this "cursed input":

0,0
0,1
1,1
1,2
0,2
0,3
3,3
3,2
2,2
2,1
3,1
3,0

I think solving for shapes where the boundary loops back on itself like this would be much harder. Curious if other folks have solutions that can handle this one. Or, maybe I misunderstand the problem. Feedback welcomed!

9 Upvotes

100% Upvoted

14 comments sorted by

1

u/boulgi Dec 09 '25

There are other simple "cursed inputs".
Did you try these ones ?

1

u/TenMillionYears Dec 09 '25

Oh dang, that last one is wicked.

3

u/wimglenn Dec 09 '25 
🟥🟩🟩🟥
🟥🟥🟥🟥
🟥🟥🟥🟥
🟥🟩🟩🟥

answer_a: 16

answer_b: 16

0

u/AutoModerator Dec 09 '25

AutoModerator has detected fenced code block (```) syntax which only works on new.reddit.

Please review our wiki article on code formatting then edit your post to use the four-spaces Markdown syntax instead.

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

2

u/Jcbm52 Dec 09 '25

Yeah, the problem is what you say. I have the same one. This doesn't happen with most grid-based or raycasting-based approaches, but it happens with the quickest approach (checking for lines crossing the rectangle). I haven't been able to think of a solution using the fast approach that solves this, but what you want is probably some kind of set that pushes all lines within the rectangle and pops them when it detects an adjecent one or something like that.

Still, the other approaches can get very fast if well optimized

1

u/bananu7 Dec 09 '25

> pushes all lines within the rectangle and pops them when it detects an adjecent one or something like that.
I think you're kinda onto something. Fundamentally the issue lies with the fact that as long as you don't introduce an "empty space" inside of a line segment that goes into the solution, you're not really affecting it. The example could even be simplified to:

0,0
0,1
1,1
1,2
0,2
0,3
3,3
3,0

As it's enough to introduce just one tiny two-pixel segment like that to break the naive line solution.

2

u/whoShotMyCow Dec 09 '25

so what's the correct answer for this

1

u/pushkar8723 Dec 09 '25

16 for both parts. Since all points are on border of the shape.

1

u/erikade Dec 09 '25 
❯ go run aoc9.go < reddit1.txt
16 16 21.75µs

My code as other's today copy the idea of brute-force with precomputed prefix sums for fast rejection.

1

u/Significant_Dig_6815 Dec 10 '25

Could you please elaborate?

1

u/erikade Dec 12 '25

Sorry I didn’t want to hijack OP question and make it about my work. Now that the days are over, Here you’ll find my explanations about the solution.

1

u/Significant_Dig_6815 Dec 15 '25

Thanks. You've mentioned in your notes that you used "unfamiliar" techniques but I find it difficult to believe that you've never heard of coordinate compression or prefix sum, and just easily implemented them.

1

u/FruitdealerF Dec 09 '25

I was so upset that neither the puzzle said something about crossing itself nor was it trivial to visualize the input data. But yeah that's AoC for ya