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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.

Damn, that is cool! Looking at me typing css & php...
It's an overdone trope in obfuscated programming contests - to make the code look like a picture.
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...

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

I've spent the past few weeks coming up with my own simple programming language, which compiles to linux/amd64 assembly.

I could have gone all out writing standard library routines for opening files, running shell commands, coding strstr, strcpy, and similar. And to be honest I did implement some things I didn't need as part of the learning process (for example print(getenv("HOME")) works). But I soon realized I needed some example programs to test things and show off.

So of course the first real program I implemented was a brainfuck interpreter. Which means my language is now, indirectly, turing complete!

My early versions took 9 minutes to output the famous mandelbrot program, so I had to make a bunch of optimizations, and later implemented support for switch/case statements to speed things up. Now I can generate the same output in two minutes - so room for improvement, but also a good bit of progress!

Cheating by implementing another language in my own was very very satisfying. Though of course this is all for fun/learning and not intended to be used seriously by anybody, not even myself!

https://github.com/skx/s-lang

I think I like this idea, but the linked-to Eternal Software Initiative [1] is a bit confusing. There are several different versions of the instructions to decode this, all conflicting.

There's the one here: Set m[b] = m[b] - m[a]

Then it links to the reference implementation on github [2] which says you just need the napkin notes [3], which is dividing everything read by 4, which is corroborated by the reference implmentation [4], but it's not clear why 4 is chosen here rather than 2, as it seems to waste a bit. Was this bit needed, or is it reserved for future expansion?

I presume the original implementation didn't do the divide by 4 and it was added later, but I don't see why it was needed, other than perhaps just making LLVM code gen a little easier. I'd need to work through lots of examples to work out if the system as described is impossible without dividing by 4 (although you'd presumably only be able to access even addresses, and the PC increases by 3 each time, so it would definitely be annoying to refer to code locations).

Then the reference implementation starts doing magic when location 64 is accessed, overwriting locations 64-67 with the current time, which is mentioned in the napkin description, but not the description on the main page.

Both descriptions mention the magic -1 address, so it seems strange that the very implementation-dependent UTC clock isn't also implemented with -ve addresses rather than trashing memory that is otherwise free for the implementation to use as desired.

Both descriptions also mention the regular timer interrupt process, which also seems disappointing, reusing address 0 as the interrupt handler location and 1 as the saved PC, which means that you have to overwrite the initial entry point at location 0 as soon as the program starts.

[1] https://eternal-software.org/

[2] https://github.com/adriancable/eternal

[3] https://github.com/adriancable/eternal/blob/main/docs/napkin...

[4] https://github.com/adriancable/eternal/blob/main/vm/vm.c

I downloaded and built this, and I feel confident in stating that this is the most impressive thing I have ever seen.
What I find fascinating is the fact that you can implement those few lines in an esoteric language such as FRACTRAN or Game of Life and even boot Linux on them. Seems doable now. In theory.
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?

This primarily affects the judges who are opening themselves up to potentially a flood of shoddy code, but given the nature of the contest, I suspect they are very good at differentiating interesting code from low quality code.

I think it's great that IOCCC accepts code that might have been built with machine assistance, because it makes the purely handcrafted winners seem even more valuable.

So it turned into an LLM-gymnastics competition?
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)?

The website itself is obfuscated, it’s not easy to find the C sources at all!
Can jump straight to https://www.ioccc.org/2025/#inventory
The first sentence links to the section that lists the winning entries, each of which have a “C code” link in the top right.
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 miss it too, and think of it every year. This is also very nice, though.
There's a Frieren [1] reference in there! https://www.ioccc.org/2025/yang2/index.html

One of the main characters is called Fern, and she almost exclusively uses the common offensive magic of Zoltraak.

[1] https://en.wikipedia.org/wiki/Frieren

There are two Frieren references, the other one is https://www.ioccc.org/2025/yang1/index.html
OMG, my game boy game of life implementation is included in one of the winning entries!
After I made the emulator I trawled GitHub to try to find games it could play within the 32K limit. I found yours - thank you :-) - and the ./try.sh script has an option to download it from GitHub for the user to test.
Back in 2000 I was being interviewed for my first internship, to join a team of C programmers. They showed me one of the winner entries of the prior years, asked me to review the code and left the room. About 5 minutes later they came back:

– And?

– I'm sorry I wasted your time. I just can't understand it.

They burst into laughs and asked me to start the joining process.

I wonder if people still make fun of interns. I still have a good laugh when I remember myself freaking out.

Wait, I kind of don't get it.

So Obfuscated C Code Contest works but Capture the Flag doesn't? Because of AI?

https://twit.tv/posts/tech/ai-disrupts-capture-flag-what-mea...

Capture the flag has clear objectives while obfuscated C contest does not. I understand improvements in AI for goal-orientated contests, I am not sure what would be considered improvements in open-ended contests with artistic flair.

Maybe you are asking "can't someone think up a clever idea and ask the AI to implement it according to IOCCC constraints?" And I believe current AI tools are still unable do that at a level that the human judges find worthy.

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¡
Some comments suggested the idea, have there been any IOCCC entries written in pre-64bit DEBUG.COM? (which could <read a file as input)
Is it zero-indexed, or are they really behind in choosing winners, or am I loosing my mind... Isn't it 2026 not 2025??
> We plan to open IOCCC30 towards the end of 2026 and have the contest run for a similar amount of time, closing sometime towards the end of Q1 2027.

Based on that it Looks like the contest starts late and runs into the next year. the 2025 rules were finalized in December.

OOOOOH! IOCCC is back!

All my love <3 <3 <3 to organizers, thank you for continuing the IOCCC context, please never go away again!

There's another contest called Underhanded C that I enjoyed a lot reading, but it has been inactive for a decade at this point...
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.

The competition never was about actual obfuscation. If you really just want obfuscation, you are actually writing a DRM system, like Denuvo.

If anything, it is closer to code golf, the main obfuscation is often a result of all the trickery needed to do something impressive in a small amount of code. Of course, minification techniques are used, like renaming variables to single character and messing with the formatting, but that's the boring part, no one is going to win because of that.

Another aspect is being clever and unique, and abusing the rules is often rewarded... once. LLMs are not good at that. The judges are human, the code needs to a appeal to a human, not just be hard to understand.

> Do I miss anything?

That human art is worth the humanity in the art.

As soon as anything is automated, it's worth nothing.

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.
You are missing that the IOCCC isn’t just about obfuscation, that the judges have taste.
Tis a pity to not have LLMs compete, given level of obfuscation they be capable of.
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.