Asymmetric Numeral Systems (rANS) entropy coding primitives.

This crate provides a small, dependency-light implementation of byte-oriented rANS.

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>(())

• Encoding returns a byte vector in a stack format: decoding consumes bytes from the end.
• This crate is focused on correctness and integration simplicity (not maximum throughput).

FrequencyTable::from_counts(counts, precision_bits) builds a model with total mass (T = 2^{precision_bits}). Practical guidance:

• Larger precision_bits approximates the empirical distribution more closely (less quantization), but increases memory and can slow decoding.
• Typical ranges are ~12–16 for small alphabets.

The table stores sym_by_slot of length (T), mapping each slot to a symbol. This dominates:

• Approx size (\approx 4 \cdot 2^{precision_bits}) bytes (u32 per slot), plus cdf/freqs.
• Example: precision_bits = 14 → (2^{14} = 16384) slots → ~64 KiB for sym_by_slot.

This is an entropy coder, not encryption. Do not treat it as a cryptographic primitive.

MIT OR Apache-2.0