# ans
[](https://crates.io/crates/ans)
[](https://docs.rs/ans)
[](https://github.com/arclabs561/ans/actions/workflows/ci.yml)
rANS entropy coding with bits-back primitives. `no_std`, zero dependencies.
## Batch API
```rust
use ans::{decode, encode, FrequencyTable};
let counts = [10u32, 20, 70]; // A, B, C
let table = FrequencyTable::from_counts(&counts, 14)?;
let message = [0u32, 2, 1, 2, 2, 0];
let bytes = encode(&message, &table)?;
let back = decode(&bytes, &table, message.len())?;
assert_eq!(back, message);
# Ok::<(), ans::AnsError>(())
```
## Streaming API
Symbol-at-a-time encoding/decoding. Required for bits-back coding (BB-ANS, ROC).
```rust
use ans::{RansEncoder, RansDecoder, FrequencyTable};
let table = FrequencyTable::from_counts(&[3, 7], 12)?;
let message = [0u32, 1, 1, 0, 1];
// Encode in reverse order (rANS requirement).
let mut enc = RansEncoder::new();
for &sym in message.iter().rev() {
enc.put(sym, &table)?;
}
let bytes = enc.finish();
// Decode in forward order.
let mut dec = RansDecoder::new(&bytes)?;
let mut decoded = Vec::new();
for _ in 0..message.len() {
decoded.push(dec.get(&table)?);
}
assert_eq!(decoded, message);
# Ok::<(), ans::AnsError>(())
```
### Bits-back primitives
`RansDecoder::peek` and `RansDecoder::advance` allow inspecting the decoded slot
before advancing state, which is the key operation for bits-back coding:
```rust
# use ans::{RansEncoder, RansDecoder, FrequencyTable};
# let table = FrequencyTable::from_counts(&[3, 7], 12)?;
# let bytes = ans::encode(&[0u32, 1], &table)?;
let mut dec = RansDecoder::new(&bytes)?;
let sym = dec.peek(&table); // look at slot without advancing
dec.advance(sym, &table)?; // advance after external logic
# Ok::<(), ans::AnsError>(())
```
## Building frequency tables
From integer counts (quantized internally):
```rust
use ans::FrequencyTable;
let table = FrequencyTable::from_counts(&[10, 20, 70], 14)?;
# Ok::<(), ans::AnsError>(())
```
From floating-point probabilities (e.g. neural network output):
```rust
use ans::FrequencyTable;
let table = FrequencyTable::from_float_probs(&[0.1, 0.2, 0.7], 14)?;
# Ok::<(), ans::AnsError>(())
```
From pre-normalized frequencies (skip internal quantization):
```rust
use ans::FrequencyTable;
// Frequencies must sum to exactly 2^precision_bits.
let table = FrequencyTable::from_normalized(&[1024, 1024, 2048], 12)?;
# Ok::<(), ans::AnsError>(())
```
## 64-bit variant
`Rans64Encoder`/`Rans64Decoder` (and batch helpers `encode64`/`decode64`) use a
64-bit state and emit 32-bit words during renormalization. This gives finer
frequency resolution and fewer renormalization steps per symbol -- the variant
used by production codecs (JPEG XL, LZFSE).
```rust
use ans::{encode64, decode64, FrequencyTable};
let table = FrequencyTable::from_counts(&[3, 7], 14)?;
let bytes = encode64(&[0u32, 1, 1, 0], &table)?;
let back = decode64(&bytes, &table, 4)?;
assert_eq!(back, &[0, 1, 1, 0]);
# Ok::<(), ans::AnsError>(())
```
## `no_std`
Zero dependencies. `no_std`-compatible (requires `alloc`). Builds on `wasm32-unknown-unknown`.
## Notes
- Encoding returns a byte vector in **stack format**: decoding consumes bytes from the end.
- `precision_bits` sets the frequency resolution ($T = 2^p$). Typical range: 12-16.
The table allocates ~$4 \cdot 2^p$ bytes for the slot lookup.
## License
MIT OR Apache-2.0