My favorite is the 366-byte C program emulator that can run Linux and Doom [0]. The VM implements an OISC - a One Instruction Set Computer [1].
[0] https://github.com/ioccc-src/winner/blob/master/2025/cable/p...
[1] https://github.com/ioccc-src/winner/blob/master/2025/cable/R...
The GameBoy emulator's code also looks like the GameBoy. Slow clap this is insane, definitely my favorite entry.
https://github.com/ioccc-src/winner/blob/master/2025/ncw1/pr...
The author, Nick Craig-Wood, is the creator of rclone!
Glad you enjoyed it :-)
If you want to see how the sausage was made, here is the source:
https://github.com/ncw/ioccc-gameboy
You'll find an unobfuscated version (kind-of) there too. This the the one I actually worked on then I had a program squash all the variable names and squeeze it into the gameboy shape
The size limit for the entry was the killer. You are allowed 2503 non white space characters (a simplification - the rules are complicated) in IOCCC entries and 4K total code size. This isn't a lot to fit a Z80 processor and a GameBoy hardware emulator in!
I first wrote a full Gameboy emulator in C. It started out at about 6000 non white space characters. I then spent about about 100 hours work trying to get it to fit into the 2503 limit. For a long time I wasn't sure it was going to fit.
I decided making the emulator play Tetris (which is a fairly simple game) was the target so I stripped out features like the half carry flag in the Z80 emulator and the windowing system in the Gameboy emulation which Tetris didn't need. I also abused the C code terribly doing things with implicit int I can never un-see. I also got creative with the IOCCC rules which are implemented in a C program which checks your source and I spent some time reverse engineering that looking for loopholes! I discovered that the operators defined in <iso646.h> only count for one token which was very useful.
Once I had it small enough I had to supply some games to run with it. I created 4, a test program written in z80 assembler, a pi calculator (written in assembler), a 3d tic tac toe game (written in C with gbdk-2020) and a chess program also written in C. I discovered that quite a few open source games ran on the emulator too so I added a downloader for those where I could. Apparently not many games use BCD arithmetic - who would have thought!
It was a fun project.
Uh... is this supposed to be valid standalone C?
GCC says there are a bunch of undefined symbols, first one being "R" right in the beginning:
typedef unsigned char u;
u w,X,T,D[1<<16],t[]=R,U=255;That's defined in Makefile
That is correct. It is cheating, but the judges let a small amount of it slide, especially if you come up with an amusing enough justification. I could not get it to fit otherwise!
Damn, that is cool! Looking at me typing css & php...
In case anyone was wondering, the IOCCC specifically permits LLM use in their guidelines.
"The IOCCC has a rich history of remarkable winning entries created by authors who skillfully employed various techniques (often their own tools) to develop their code."
I'm in the no-AI camp, but for this case, I find it interesting, especially since there's little obfuscated C online, and LLMs cannot infer intention from the actual code. Did you spot any entries with LLM support?
Also, the reverse is interesting: how well can they guess the function of the obfuscated code?
Rule 7 would be self-contradictory if "tools" include AI.
https://www.ioccc.org/2025/rules.html
It seems to refer to custom code generators. Why would they mean AI if they explicitly talk about a "rich history" (when AI wasn't available)?
So it turned into an LLM-gymnastics competition?
The website itself is obfuscated, it’s not easy to find the C sources at all!
The first sentence links to the section that lists the winning entries, each of which have a “C code” link in the top right.
Can jump straight to https://www.ioccc.org/2025/#inventory
It's really hard to navigate. I cannot work out what the contest is; it seems to assume you know what it is already.
I with the Underhanded C Contest would come back, it was far more interesting to me. No disrespect intended to the Obfuscated C competitors.
I love the submissions of IOCCC generally, but the schedule and submission process looks like a mess, is that part of the joke? Or is it just because being consistent and designing a simple process is hard¡
I'm not sure this kind of competition is still meaningful, given that LLM can easily convert a program clearly written in any programming language to the most obfuscated C code, and can still easily verify it's correctness in an automated way.
Do I miss anything?
Yes, you haven’t tried it. LLMs are actually awesome at deobfuscation, but terrible at obfuscation. They just can’t do it yet.
They also lack the creativity needed for those entries. Obfuscation is only one part of it. Coming up with the idea is another. Many entries also have special qualities that make them true works of art.
> They just can’t do it yet.
Have you tried it or are you guessing this?
Yes, I’ve tried it. For example, this was my winning entry from a year ago [0]. The LLM only performs trivial obfuscation, not advanced transformations.
For example:
if (x == 1 || x == 2) { ...
if (!(2+x*x-3*x)) { ...
[0] https://github.com/ioccc-src/winner/blob/master/2024/macke/p...
I checked this in an online C-compiler and it is not equivalent to x==1 | x==2.
this statement is equivalent to x==2 | x==3.
For example, x=3, 3/2 = 1 then 1-1 = 0 so that !(0) is 1 or true. Also for x=1, 1/2 = 0 then 0-1 = -1 and !(-1) = 0 or false.
I agree with your point in general though about size constraints.
It's pretty easy to see what that does though, even if it is shorter. Wouldn't the other approach be more obfuscated?
extrano84 already found some errors but also 0 will fail and if x is int (instead of unsigned int) all negative numbers will also fail (but so will the original s-macke obfuscation as well).
Not sure what you mean by advanced transformations but I got these versions from ChatGPT without explicit instructions.
if(x-1<2&&x)...
if((1<<x)&6)...
if(x<3&x)...
if(3%x&&x<3)...
if(!((x-1)*(x-2)))
if(!(x^1|x^2))...
if(!(x*x-3*x+2))Just two months ago I tried to write a short K code with Claude Opus 4.6, only to find that while it had sufficient knowledge about K vocabularies it didn't try to make good use of them. K is, while slightly obscure and obfuscated, a real programming language and certainly better known than obfuscated programming. I don't have high hope for IOCCC-grade obfuscation.
They should train on some of my colleagues code - they have an inate skill of turning anything simple and clear into a mindboggling mess
Lol you're so much smarter, nice
> Do I miss anything?
That human art is worth the humanity in the art.
As soon as anything is automated, it's worth nothing.
Chess?
You are missing that the IOCCC isn’t just about obfuscation, that the judges have taste.
yes llm can do it but i think competitions have more to do with developing scientific temperament, competitive mindset and complex problem solving skills. that's why i think they are still relevant and will be relevant for a long time.
Where's the fun in that?
> Do I miss anything?
School ? /s
Tis a pity to not have LLMs compete, given level of obfuscation they be capable of.
In my experience LLMs were pretty good at deobfuscating many entries (including mine) but very awful at generating any significantly obfuscated code. So obfuscation can be regarded as a truly humane art---at least for now.
well, at some point I (like after 5 hours of CC) I can swear im looking at pure random coming out. so perhaps the ability of LLMs to make complex stuff even more complex, would be beneficial. obfuscation, after all, is not about having performant code, but an art form, right?
/curious though given the very nice conversation we're having here, why the parent topic gets down-voted. is neither off-topic, nor rude.../
I think the "obfuscation" is actually two very different acts: the apparent obfuscation, that is concerned with randomly looking output, and the information-theoretic obfuscation, that takes computational effort to undo it. Commercial obfuscators are mostly the former, making undo much more annoying but easy to undo if you have a right tool. The obfuscation in IOCCC is much more the latter, requiring the heavy logic and deduction to see through that. In my experience LLMs have been capable of doing the former and undoing the latter but not doing the latter, presumably because any obfuscated program still has to run somehow. Given that this form of obfuscation is not common (and that LLMs tend to work well with established things), your initial statement was I believe quite far-fetched.
I pointed Claude Opus 4.7 at my Forth entry ( https://github.com/ioccc-src/winner/blob/master/2025/ncw3/pr... ) and it absolutely refused to have anything to do with it citing security violations and accusing me of creating malware :-)
Claude seems OK with it now, so I don't know whether that was a glitch but it was quite funny.
I mean they can generate 1 million lines of code, it's obfuscation by brute force, how are you going to read that if not with another LLM
IOCCC disallows such entries for the obvious reason ;-)
LLMs are allowed. [0]
> You are free to use whatever tools you wish to write your code. This includes tools that are AI based, LLM (large language model), Virtual coding assistants, code generators, or similar tools, as well as your own tools. The IOCCC judges do not discriminate on the basis of the tools used to write obfuscated C code so long as you are the ultimate author of the code you submit.
It's a bit annoying getting frontier models to even work on IOCCC code because of "safety", but even if you get around that LLMs just aren't very good at it. Obfuscating code involves a level of creativity and deviousness that LLMs struggle to meet.
Maybe, but it can't make the gameboy code look like a gameboy too, unless asked to, and that probably would not work even. That's the difference between an LLM and a human, and that's what make this competition worthwhile.
So like at a film festival, 90% of the entries won a price, but unlike a film festival there's not a single best. Weird, like modern education.
While this has been downvoted to the death, it is fun to guess how many entries are submitted to each IOCCC. My best guess is around 10^2.5, i.e. 3--400. Rationales:
- The number of winning entries and losing entries that get revealed later in public suggests that this number should be at least 50.
- The number of judging rounds, as the FAQ says, is at least 3 and possibly more. If each judging round eliminates about a half of entries, we should expect at least 10 submissions per each winning entries. I personally think the actual elimination rate can be as low as 1--20% at the end, but at least first few rounds should be easy so I think this is a good minimum guess: 1--200.
- The current number of individual judges is just enough for the three-digit number of submissions. It has a striking resemblance with typical academic conferences with typical acceptance rate, by the way! If there were thousands of submissions (like today's AI conferences...) there ought to be much more judges, and more importantly, more levels of judges so that each judge can do just enough work throughout the entire process. So this establishes the maximum guess: 1,000.
- My best guess is simply a geometric mean of two extrema.
Wow! And it also implements a very interesting variant of SUBLEQ that is turing complete.
>This VM implements an OISC - a One Instruction Set Computer. That instruction takes three signed 32-bit operands, a, b and c, and runs a program from memory m[] as follows:
1 PC (program counter) starts at 0
2 Fetch the next instruction (32-bit signed operands a, b and c)
3 If the low bit on any operand is set, remove it, and replace that operand with m[operand] i.e., a dereference of that address
4 Set m[b] = m[b] - m[a]
5 If m[b] is 0 or negative, set the PC to c, otherwise increment PC by 3 words
6 Go to step 2
Here is the video:
https://www.youtube.com/live/MoWCwZx1Swc?si=eIOlRsKWNKRVRZeB...