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!
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!
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
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 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.
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.
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) { ...
can be transformed into:
if (!(2+x*x-3*x)) { ...
An LLM will do this if you explicitly ask it to, but not on its own.
One of the main instruments of obfuscation (and the way to get more out of the size constraints) is making the code as short as possible, so in that example you'd prefer
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.
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¡
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.
> 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.
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:
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...
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
Here is the video:
https://www.youtube.com/live/MoWCwZx1Swc?si=eIOlRsKWNKRVRZeB...
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."
So it turned into an LLM-gymnastics competition?
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
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'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:
can be transformed into: An LLM will do this if you explicitly ask it to, but not on its own.[0] https://github.com/ioccc-src/winner/blob/master/2024/macke/p...
One of the main instruments of obfuscation (and the way to get more out of the size constraints) is making the code as short as possible, so in that example you'd prefer
It's pretty easy to see what that does though, even if it is shorter. Wouldn't the other approach be more obfuscated?
But then we all know that LLM has come a long way since one year ago.
Are you sure they still can't do it?
They should train on some of my colleagues code - they have an inate skill of turning anything simple and clear into a mindboggling mess
> 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?
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.
> Do I miss anything?
School ? /s
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¡
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.
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.
[0] https://www.ioccc.org/faq.html#ai
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.