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//! Multi-symbol rANS decoder.
//!
//! [`RAnsSymbolDecoder`] reconstructs symbols from a probability table and a
//! lookup table built during initialization. Precision is stored at runtime
//! (rather than as a const generic) to avoid monomorphization bloat while
//! keeping shift/mask-based decoding. Port of Draco's `rans_symbol_decoder.h`.
use crate::ans::AnsDecoder;
use crate::decoder_buffer::DecoderBuffer;
use crate::rans_symbol_coding::RAnsSymbol;
/// RAnsSymbolDecoder with runtime precision to avoid monomorphization bloat.
/// Instead of const generics, we store the precision bits at runtime.
/// Performance is preserved by storing `rans_precision_bits` and using bit
/// operations (shift/mask) instead of division/modulo.
pub struct RAnsSymbolDecoder<'a> {
pub ans: AnsDecoder<'a>,
probability_table: Vec<RAnsSymbol>,
lut: Vec<u32>,
num_symbols: usize,
rans_precision_bits: u32, // Store bits for shift operations
rans_precision_mask: u32, // (1 << bits) - 1 for fast modulo
rans_precision: u32,
l_rans_base: u32,
}
impl<'a> RAnsSymbolDecoder<'a> {
pub fn new(rans_precision_bits: u32) -> Self {
let rans_precision = 1u32 << rans_precision_bits;
let l_rans_base = rans_precision * 4;
Self {
ans: AnsDecoder::new(&[]),
probability_table: Vec::new(),
lut: Vec::new(),
num_symbols: 0,
rans_precision_bits,
rans_precision_mask: rans_precision - 1,
rans_precision,
l_rans_base,
}
}
pub fn create(&mut self, buffer: &mut DecoderBuffer) -> bool {
if !self.decode_table(buffer) {
return false;
}
true
}
fn decode_table(&mut self, buffer: &mut DecoderBuffer) -> bool {
let _start_pos = buffer.position();
let bitstream_version = buffer.bitstream_version();
let num_symbols = if bitstream_version < 0x0200 {
#[cfg(not(feature = "legacy_bitstream_decode"))]
{
return false;
}
#[cfg(feature = "legacy_bitstream_decode")]
match buffer.decode_u32() {
Ok(v) => v as usize,
Err(_) => return false,
}
} else {
match buffer.decode_varint() {
Ok(v) => v as usize,
Err(_) => return false,
}
};
self.num_symbols = num_symbols;
if num_symbols == 0 {
return true;
}
// Each probability-table entry consumes at least one input byte while it
// is decoded below, and a single byte can cover at most 64 entries (a
// zero-frequency run encodes up to 63 extra symbols). A count beyond that
// bound cannot be backed by the remaining input, so reject it before
// resizing instead of allocating gigabytes for a malformed varint. This
// is a relative input-consistency check on a cold path, not a fixed cap.
if num_symbols > buffer.remaining_size().saturating_mul(64) {
return false;
}
self.probability_table
.resize(num_symbols, RAnsSymbol::default());
// NOTE: C++ only early-returns for num_symbols == 0.
// For num_symbols == 1, it still reads the probability table byte.
// We must do the same to stay in sync with the buffer!
let mut i = 0;
while i < num_symbols {
let b = match buffer.decode_u8() {
Ok(v) => v,
Err(_) => return false,
};
let mode = b & 3;
if mode == 3 {
// Zero frequency offset
let offset = (b >> 2) as usize;
for j in 0..=offset {
if i + j >= num_symbols {
return false;
}
self.probability_table[i + j].prob = 0;
}
i += offset;
} else {
let num_extra_bytes = mode as usize;
let mut prob = (b >> 2) as u32;
for b_idx in 0..num_extra_bytes {
let extra = match buffer.decode_u8() {
Ok(v) => v,
Err(_) => return false,
};
prob |= (extra as u32) << (8 * (b_idx + 1) - 2);
}
self.probability_table[i].prob = prob;
}
i += 1;
}
// Compute cumulative probabilities and LUT
self.lut.resize(self.rans_precision as usize, 0);
let mut cum_prob: u32 = 0;
for i in 0..num_symbols {
let prob = self.probability_table[i].prob;
self.probability_table[i].cum_prob = cum_prob;
// Bounds check: ensure we don't write past the LUT
let end_idx = cum_prob.saturating_add(prob);
if end_idx > self.rans_precision {
// Malformed probability table - probabilities exceed precision
return false;
}
self.lut[cum_prob as usize..end_idx as usize].fill(i as u32);
cum_prob = end_idx;
}
if cum_prob != self.rans_precision {
return false;
}
true
}
pub fn start_decoding(&mut self, buffer: &mut DecoderBuffer<'a>) -> bool {
// Draco advances the buffer past the encoded rANS data regardless of the
// number of symbols (the encoded size prefix is always present).
// C++: v < 2.0 uses fixed u64, v >= 2.0 uses varint u64.
let bitstream_version = buffer.bitstream_version();
let bytes_to_read = if bitstream_version < 0x0200 {
#[cfg(not(feature = "legacy_bitstream_decode"))]
{
return false;
}
#[cfg(feature = "legacy_bitstream_decode")]
match buffer.decode::<u64>() {
Ok(v) => v as usize,
Err(_) => return false,
}
} else {
match buffer.decode_varint() {
Ok(v) => v as usize,
Err(_) => return false,
}
};
if self.num_symbols <= 1 {
// Still need to advance the buffer past the encoded bytes.
if buffer.try_advance(bytes_to_read).is_err() {
return false;
}
return true;
}
let data = buffer.remaining_data();
if data.len() < bytes_to_read {
return false;
}
let rans_data = &data[..bytes_to_read];
self.ans = AnsDecoder::new(rans_data);
// Multi-symbol rANS may use the 4-byte (0xC0) final-state encoding.
if !self.ans.read_init(self.l_rans_base, true) {
return false;
}
if buffer.try_advance(bytes_to_read).is_err() {
return false;
}
true
}
#[inline(always)]
pub fn decode_symbol(&mut self) -> u32 {
self.try_decode_symbol().unwrap_or(0)
}
#[inline(always)]
pub fn try_decode_symbol(&mut self) -> Option<u32> {
if self.num_symbols <= 1 {
return Some(0);
}
// Match Draco C++ (ans.h) rans_read(): normalize first, then use
// bit operations for division/modulo by rans_precision (power of two).
// Using shift/mask is equivalent to div/mod but much faster.
self.ans.read_normalize();
let quo = self.ans.state >> self.rans_precision_bits; // Fast division
let rem = self.ans.state & self.rans_precision_mask; // Fast modulo
let symbol_id = *self.lut.get(rem as usize)?;
let sym = self.probability_table.get(symbol_id as usize)?;
let state_base = quo.checked_mul(sym.prob)?;
let state_offset = rem.checked_sub(sym.cum_prob)?;
self.ans.state = state_base.checked_add(state_offset)?;
Some(symbol_id)
}
}
#[cfg(test)]
mod tests {
use super::RAnsSymbolDecoder;
use crate::rans_symbol_coding::RAnsSymbol;
#[test]
fn try_decode_symbol_rejects_invalid_lut_symbol_id() {
let mut decoder = RAnsSymbolDecoder::new(1);
decoder.num_symbols = 2;
decoder.lut = vec![99, 99];
decoder.probability_table = vec![RAnsSymbol::default(); 2];
decoder.ans.state = decoder.l_rans_base;
assert_eq!(decoder.try_decode_symbol(), None);
}
#[test]
fn try_decode_symbol_rejects_inconsistent_cumulative_probability() {
let mut decoder = RAnsSymbolDecoder::new(1);
decoder.num_symbols = 2;
decoder.lut = vec![0, 0];
decoder.probability_table = vec![
RAnsSymbol {
prob: 1,
cum_prob: 1,
},
RAnsSymbol::default(),
];
decoder.ans.state = decoder.l_rans_base;
assert_eq!(decoder.try_decode_symbol(), None);
}
}