nnzip 1.1.2

Cross-platform text compression that uses a local GPT-2 (via llama.cpp) as the probability model.

nnzip — neural-network text compression

A cross-platform CLI that compresses English text using a local GPT-2 as a probability model. On natural prose it gets around 15-25% of the original size — typically 3-5× better than gzip.

pip install nnzip

compress book.txt              # produces book.txt.nnz
decompress book.txt.nnz        # restores book.txt

Works on macOS, Linux, and Windows. No GPU required; uses llama.cpp under the hood so it picks up Metal on Apple Silicon and AVX/CUDA elsewhere automatically.

What it actually does

When you compress a file, nnzip walks through it one token at a time. At each position it asks GPT-2: given everything before, what's your probability distribution over the next token? Then it spends -log₂(P(actual token)) bits encoding it with arithmetic coding.

If GPT-2 is 90% sure about the next token (the very common case in fluent English), encoding costs about 0.15 bits. If GPT-2 is totally surprised (1-in-50,000), it costs ~16 bits. The average across natural English ends up around 4-5 bits per token instead of the ~32 bits each token would take if stored naively.

Decompression runs the same forward passes in the same order. Because GPT-2 is deterministic with greedy inference, both sides see identical probability distributions and the arithmetic coder unwinds back to the exact original token stream. The decompressed file is bit-identical to the original.

The compressed .nnz file contains zero model weights — just MAGIC + version + token_count + arithmetic-coded payload. Both ends rely on the same pinned GGUF model from Hugging Face, downloaded once to ~/.cache/huggingface/ on first use (~252 MB).

Quick demo

$ printf 'The morning rain pattered against the windows of the small cottage.' > demo.txt
$ wc -c demo.txt
67 demo.txt

$ compress demo.txt
compressing demo.txt -> demo.txt.nnz
loading openai-gpt2-124M-F16.gguf...
loaded in 0.3s (vocab 50257, threads 9)
  encoded 13/13 tokens (47.6 tok/s)

original:    67 bytes
compressed:  21 bytes (31.3% of original)

$ decompress demo.txt.nnz
recovered 67 chars -> demo.txt

A 50 KB chunk of Pride and Prejudice lands at about 23% of the original (~11.5 KB). For comparison, gzip -9 on the same input gets ~57%.

Performance and limits, plainly

• Speed. Around 50 tokens/sec on an Apple M1 Max CPU. A 100 KB English file takes a couple of minutes to compress and another couple to decompress. This is slower than gzip by orders of magnitude. It is not a tool you'd use to compress your downloads folder.
• English-only is its sweet spot. GPT-2 was trained on English internet text. Random binary, source code, non-English, base64-encoded blobs — these typically compress to 100%+ of the original (nnzip gives up and emits its escape encoding for everything). Don't use this on data that doesn't look like English prose.
• Lossless. Provably. Arithmetic coding with a deterministic probability source round-trips bit-for-bit.
• GPT-2 has a 1024-token context window. Past that, nnzip uses a sliding window of the last 512 tokens to predict the next one. Long-range compression suffers a little after the first ~1000 tokens, but it still works on arbitrarily large files.
• Cross-platform install, same-machine round-trip. llama.cpp uses platform-specific acceleration (Metal on Mac, AVX on Linux/Windows, CUDA if available), and these can produce floating-point logits that differ in the last few bits across machines. Same machine that compressed should decompress. Same OS / CPU family is usually fine.
• No encryption. Anyone with the same nnzip version can decompress a .nnz file. Use a real encryption tool on top if you need privacy.

Why GPT-2 is a great compressor for English

Shannon's source coding theorem says you can't compress data below its entropy — the average number of bits needed per symbol given perfect prediction. For English text, the entropy is somewhere around 1.0-1.3 bits per character. Most classical compressors (gzip, bzip2, xz) approximate the entropy using simple statistical models — adjacent character frequencies, run-length, Lempel-Ziv pattern matching. Their best on plain English is around 25-30% of original.

GPT-2 is a much smarter model. It's seen billions of words and learned what's plausible at a phrase, sentence, and paragraph level. So when it predicts the next token, its distribution is sharper — closer to the data's true entropy. Sharper predictions mean fewer bits per symbol via arithmetic coding. That's all the trick is.

Bigger models compress better still. DeepMind showed in Language Modeling Is Compression (2024) that Chinchilla 70B compresses Wikipedia to ~8% of original, beating every classical codec. The trade-off is obvious: bigger model, more compute. GPT-2 small (124M params, 252 MB) is a practical sweet spot — fast enough to actually use, small enough to ship via pip.

Optional tunables

Env var	Effect
NNZIP_MODEL_PATH=/path/to/your.gguf	Use a different GGUF model (any llama.cpp-compatible GPT-2 variant). Both sides need to use the same one.

What's in this repo

The nnzip CLI is the current thing in this project. The repo also includes a multi-stage experiment that led here — the kind of journey that goes from "wrong idea" to "right idea." If you only care about the tool, skip the rest.

The actual tool (stages 7-8 of the journey)

File	What it does
nnzip/__init__.py	The whole package: model loading, arithmetic coding, CLI entry points
pyproject.toml	Declares the compress, decompress, and nnzip CLI commands plus dependencies
arithmetic_coder.py	A standalone portable arithmetic coder (used by the HTML self-extractor below; nnzip itself uses constriction)
api_compress.py	An earlier OpenAI-API-based experiment: same idea but uses OpenAI's API as the probability model instead of a local one. Slower and pay-per-use; left in for reference.
template.html	A self-extracting HTML wrapper for the API version — the .nnz payload bakes into a single HTML file the recipient can open in any browser

The hash brute-forcing detour (stages 1-5)

Before landing on real compression, the project spent stage 1-5 trying to brute-force decompress files from just their SHA-256 hash + length. That doesn't actually work (the pigeonhole principle is a wall), but it's an entertaining way to learn why and to push hardware to its limits.

File	Role	Best result
compress.py / decompress.py	Python brute forcer	~0.6 M hashes/s
compress_index.py / decompress_index.py	"Deterministic ordering" variant that makes the failure visible	proves the size wall
brute.c	C version with CommonCrypto + pthreads	~45 M H/s, ~75× Python
brute_neon.c	ARMv8 SHA-2 hardware intrinsics	~380 M H/s, ~635× Python
brute_mb.c	4-way multi-buffer SIMD SHA-256 — an instructive failure (slower than hardware SHA on M1)	~80 M H/s
brute_metal.m	Metal compute shader on M1 Max's GPU (32 cores, 4096 ALU lanes)	~1.0 GH/s
brute_combined.m	CPU NEON-HW and GPU running concurrently on different parts of the search space	~1.4 GH/s (~2300× Python)

Build them with clang -O3 -Wall -Wno-deprecated-declarations -o brute brute.c etc. They're not part of the pip package — they're standalone executables for stress-testing.

The journey, summarized

Stage	Idea	Outcome
1	"Just send the SHA-256 hash and brute-force decompress"	Doesn't work — pigeonhole guarantees collisions
2	C + threads	Faster brute force, same impossibility
3	NEON hardware SHA	Faster still
4	M1 Max GPU compute shader	1 GH/s
5	CPU + GPU concurrent	1.4 GH/s
6	"Use a deterministic generator and send the index"	Mathematically equivalent to storing the file as a giant integer — the index is the same size as the file
7	Local GPT-2 + arithmetic coding	Actually compresses
8	API and HTML variants	Same idea, different deployment models

The lesson behind 1-6 is the pigeonhole principle: there are more N-byte inputs than there are shorter outputs, so no scheme can compress every input. Real compression escapes by giving up on compressing arbitrary data and instead exploiting the patterns in the data we actually have. nnzip takes that to its modern extreme — the "pattern" is everything GPT-2 learned about English from billions of words of internet text.

Inspirations and prior art

• Witten, Neal, Cleary, "Arithmetic Coding for Data Compression" (1987) — the core algorithm.
• DeepMind, "Language Modeling Is Compression" (2024) — showed that big LLMs are state-of-the-art compressors.
• Fabrice Bellard's ts_zip (2023) — production LLM compression with a custom model.

License

MIT.