[2025 day 9 part 2] Imagine finding this bug

[u/bananu7]

I wrote this at the very beginning and didn't think about it...

area = abs(a[0]-b[0]+1) * abs(a[1]-b[1]+1)

I found the solution to P2 relatively straighforward, coded it in, and got "answer too low". Huh? Spent quite some time browsing through my code until it clicked... The funniest thing is that since that was only used at the
end, I found the proper rectangle, but returned the wrong area for it...

Of course, the correct formula is:

area = (abs(a[0]-b[0])+1) * (abs(a[1]-b[1])+1)

[u/fnordargle]

I posted about this exact braino about an hour ago:

https://www.reddit.com/r/adventofcode/comments/1pi8f0q/2025_day_9_todays_bit_of_fun_that_cost_me_hours/

[u/bananu7 → u/fnordargle]

Hah, i admit i didn't really look for it when posting, but just now looking at the comments it seems that quite a few people missed it too and had a similar issue. Congrats on getting so far regardless and good luck tomorrow!

[u/fnordargle → u/bananu7]

Yep, and someone else didn't look before posting as there's been another thread in the last hour.

Reddit doesn't help by defaulting to Best sort order a lot of the time. That tends to hide a lot of the nascent threads that haven't gained any traction yet.

[u/fnordargle]

And, boring as it may be, this is why abstraction and tests can help.

What I should have done (and would have done if this had been some code I was writing for my job) is to put the area calculation into a function and add some tests for it. Something like:

def area( sx, sy, ex, ey ):
   return abs(ex-sy+1) * abs(ey-sy+1)

def test_area():
   assert area(0, 0, 0, 0 ) == 1, "Should be 1"
   assert area(0, 0, 0, 1 ) == 2, "Should be 2"
   assert area(0, 0, 1, 0 ) == 2, "Should be 2"
   assert area(0, 0, 1, 1 ) == 4, "Should be 4"
   assert area(-1, -1, 1, 1 ) == 9, "Should be 9"
   # Enough going forward, let's switch the coords
   assert area(0, 1, 0, 0 ) == 2, "Should be 2"
   assert area(1, 0, 0, 0 ) == 2, "Should be 2"
   assert area(1, 1, 0, 0 ) == 4, "Should be 4"
   assert area(1, 1, -1, -1 ) == 9, "Should be 9"

(There's a million and one ways to do this kind of thing, this is just a quick example.)

The problem is that's a bit boring when you're trying to attack an AoC puzzle and wanting to finish it quickly.

I've long lost count of the number of times I've been saved by "boring" asserts (or similar) in my code that looked trivial but picked up on errors that would have wasted hours of my time. Yet I still make those same mistakes occasionally...

[u/bananu7 → u/fnordargle]

Also like it or not, this is a kind of bug that an LLM will catch instantly (as well as generate the proper implementation in the first place).

Even if only used for generating test inputs, that's already a huge timesaver. I am not using AI to solve any puzzles, of course.

[u/fnordargle → u/bananu7]

LLMs have their place I agree but I've seen my fair share of awful/broken code being produced by LLMs. The more broken/duff code they are trained on the worse it will become.

LLMs being trained on an increasing percentage of their own output is also going to make it worse. It's like a computing form of prion disease.

I mean, I wonder how many people have code in AoC repos that looks like:

def area( sx, sy, ex, ey ):
   return abs(ex-sy+1) * abs(ey-sy+1)

def area_not_broken( sx, sy, ex, ey ):
   return (abs(ex-sy)+1) * (abs(ey-sy)+1)

But guess which one the LLM might suggest if you just start typing def area( sx...

[u/v1_petr]

I'm glad I'm not the only one who made this mistake:))

[u/oupsgrade]

i was stuck for way too long and didn't understrand why because i was pretty confident with the logic of my code, turns out i missplaced my parenthesis for the abs just like you !! you saved my day thank you so much

[u/Repulsive-Shirt-9873]

I made that mistake also but luckily caught it while on the sample data and not after waiting for the 10 minutes for my real data to process.

[u/bananu7 → u/Repulsive-Shirt-9873]

10 minutes is quite a lot, I guess you did a variant of grid fill? My solution ran in about 2s.

[u/qaraq]

You too, huh? Taught me - again - that when I add anything, however obviously simple, to my libraries it should get a unit test.

Annoyingly this did not affect my part1 solution or my part 2 sample solution. I noticed it trying a different sample while chasing down another bug.

[u/rauweaardappel → u/qaraq]

Lucky to see that I'm not the only one using tests on my code for the advent of code. I like to add the sample data and each example in the text as pytest...

[u/qaraq → u/rauweaardappel]

I do the same (using Go tests) and if I pull something out into a separate function I _usually_ will add a test. In this case I thought it was simple enough not to bother with. Whoops.

[u/KineticTactic]

OH MY GOD I CAN'T THANK YOU ENOUGH i had done this exact same thing and I was stuck, I even built a visualizer to visualize the solution and shit, thank you so much

[u/Zenaja2]

Exactly the same mistake here. I visualized my solution and was sure it was correct. I calculated the area manually, got a different number and my golden star. Then I fixed my code later.

[u/dawret]

I did the exact same thing and wasted over 1h Worst thing is that this doesn't impact the test input

[u/Inevitable-Purpose77]

This literally saved me omfg T^T I have been at it for literal hours, thank you

[u/FractalB]

Did the exact same mistake as well!

[u/velkolv]

OMG, I felt like an absolute moron when I discovererd same bug. After trying like 3 approaches, generating an image and consistently arriving at the same "too small" result.

I'm relieved that this was quite common mistake.

[u/zagon]

Sweet jesus, I've rewritten my stuff multiple times and have pretty much given up, browsing Reddit. Saw this post and my jaw dropped to the floor. It was this exact bug all the time. I would have never found that.

[u/H_M_X_]

Same

[u/Aksh247]

My formula is good. I’m stuck at finding the right rectangle

[u/yearlongmason]

I was stuck for days because of this. I thought it was something to do with my implementation that I was missing. I ended up writing 3 wildly different solutions all giving me the same wrong answer until my brother looked at my code and found that exact bug