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//! Streaming LZFSE decoder.
//!
//! Buffers input until a whole block (magic + header + payload) can be
//! decoded, then drains the decoded payload into the caller's output slice
//! across as many `decode` calls as the caller needs.
use alloc::vec::Vec;
use crate::error::Error;
use crate::lzfse::{lzfse_v2, lzvn};
use crate::traits::{RawDecoder, RawProgress};
/// 4-byte block magics.
const MAGIC_UNCOMPRESSED: [u8; 4] = *b"bvx-";
const MAGIC_LZVN: [u8; 4] = *b"bvxn";
const MAGIC_V1: [u8; 4] = *b"bvx1";
const MAGIC_V2: [u8; 4] = *b"bvx2";
const MAGIC_EOS: [u8; 4] = *b"bvx$";
/// Streaming decoder state machine.
pub struct Decoder {
/// Bytes the caller has fed us that we haven't yet consumed.
input_buf: Vec<u8>,
/// Decoded bytes pending delivery to the caller.
output_buf: Vec<u8>,
/// Read cursor into `output_buf`. We keep the buffer around so we don't
/// have to shift bytes on every partial drain; once `output_pos ==
/// output_buf.len()`, we clear both.
output_pos: usize,
/// State.
state: State,
/// Once we hit the end-of-stream marker (or have signalled it once), we
/// short-circuit further calls.
eos: bool,
/// Set on any decode error so callers don't accidentally resume.
poisoned: bool,
}
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum State {
/// Waiting for the next 4-byte block magic.
AwaitMagic,
/// Read magic; waiting for the block-specific header bytes.
AwaitHeader(BlockKind),
/// Header parsed; waiting for the rest of the payload, then decode + drain.
AwaitPayload {
kind: BlockKind,
/// For uncompressed blocks: bytes to copy. For LZVN: compressed bytes
/// to decode.
payload_len: usize,
/// For LZVN: expected decoded size from the header.
decoded_size: usize,
},
/// Stream is finished.
Done,
}
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum BlockKind {
Uncompressed,
Lzvn,
/// `bvx2` (LZFSE v2): FSE + LZ77. Decoded by [`lzfse_v2::decode_block`]
/// once the whole block (variable-length header + both payload streams)
/// is buffered.
V2,
/// `bvx1` (LZFSE v1, uncompressed-freq variant): not emitted by modern
/// encoders; returns [`Error::Unsupported`].
V1,
}
impl Decoder {
pub fn new() -> Self {
Self {
input_buf: Vec::new(),
output_buf: Vec::new(),
output_pos: 0,
state: State::AwaitMagic,
eos: false,
poisoned: false,
}
}
fn raw_decode_inner(&mut self, input: &[u8], output: &mut [u8]) -> Result<RawProgress, Error> {
if self.poisoned {
return Err(Error::Corrupt);
}
let mut consumed = 0usize;
let mut written = 0usize;
loop {
// 1. Drain any pending decoded output first.
if self.output_pos < self.output_buf.len() {
let want = (self.output_buf.len() - self.output_pos).min(output.len() - written);
output[written..written + want]
.copy_from_slice(&self.output_buf[self.output_pos..self.output_pos + want]);
self.output_pos += want;
written += want;
if self.output_pos == self.output_buf.len() {
// Fully drained; reset.
self.output_buf.clear();
self.output_pos = 0;
}
if written == output.len() {
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
// If we just drained and output still has room, loop to
// try to make more progress.
}
// 2. If we've already hit end-of-stream, signal done.
if self.eos {
return Ok(RawProgress {
consumed,
written,
done: true,
});
}
// 3. Pull from caller's `input` into `input_buf`. We pull lazily:
// only as much as the current state needs.
if consumed < input.len() {
self.input_buf.extend_from_slice(&input[consumed..]);
consumed = input.len();
}
// 4. Advance the state machine.
match self.state {
State::AwaitMagic => {
if self.input_buf.len() < 4 {
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
let mut magic = [0u8; 4];
magic.copy_from_slice(&self.input_buf[..4]);
// Drop the magic.
self.input_buf.drain(..4);
match magic {
MAGIC_EOS => {
self.state = State::Done;
self.eos = true;
// loop to emit done on next iteration
}
MAGIC_UNCOMPRESSED => {
self.state = State::AwaitHeader(BlockKind::Uncompressed);
}
MAGIC_LZVN => {
self.state = State::AwaitHeader(BlockKind::Lzvn);
}
MAGIC_V1 => {
self.state = State::AwaitHeader(BlockKind::V1);
}
MAGIC_V2 => {
self.state = State::AwaitHeader(BlockKind::V2);
}
_ => {
self.poisoned = true;
return Err(Error::BadHeader);
}
}
}
State::AwaitHeader(kind) => match kind {
BlockKind::Uncompressed => {
// 4-byte LE n_raw_bytes.
if self.input_buf.len() < 4 {
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
let n_raw = u32::from_le_bytes([
self.input_buf[0],
self.input_buf[1],
self.input_buf[2],
self.input_buf[3],
]) as usize;
self.input_buf.drain(..4);
self.state = State::AwaitPayload {
kind: BlockKind::Uncompressed,
payload_len: n_raw,
decoded_size: n_raw,
};
}
BlockKind::Lzvn => {
// 8-byte header: n_raw_bytes (u32 LE) + n_payload_bytes (u32 LE).
if self.input_buf.len() < 8 {
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
let n_raw = u32::from_le_bytes([
self.input_buf[0],
self.input_buf[1],
self.input_buf[2],
self.input_buf[3],
]) as usize;
let n_payload = u32::from_le_bytes([
self.input_buf[4],
self.input_buf[5],
self.input_buf[6],
self.input_buf[7],
]) as usize;
self.input_buf.drain(..8);
self.state = State::AwaitPayload {
kind: BlockKind::Lzvn,
payload_len: n_payload,
decoded_size: n_raw,
};
}
BlockKind::V2 => {
// The v2 header is variable-length (FSE frequency
// tables follow the fixed packed fields). Buffer the
// fixed 28 bytes (post-magic: n_raw + three u64 words)
// first so we can read `header_size` and the payload
// sizes, then arrange to buffer the whole block (header
// + payload) before decoding it in one shot.
let fixed = lzfse_v2::V2_HEADER_FIXED_BYTES;
if self.input_buf.len() < fixed {
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
let header_size = match lzfse_v2::parse_header_size(&self.input_buf) {
Ok(h) => h as usize,
Err(e) => {
self.poisoned = true;
return Err(e);
}
};
let n_payload = match lzfse_v2::parse_payload_size(&self.input_buf) {
Ok(n) => n as usize,
Err(e) => {
self.poisoned = true;
return Err(e);
}
};
// `header_size` includes the 4-byte magic we already
// dropped; remaining block bytes after the magic are
// `header_size - 4 + n_payload`.
let header_len = match header_size.checked_sub(4) {
Some(h) if h >= fixed => h,
_ => {
self.poisoned = true;
return Err(Error::Corrupt);
}
};
let block_len = match header_len.checked_add(n_payload) {
Some(b) => b,
None => {
self.poisoned = true;
return Err(Error::Corrupt);
}
};
self.state = State::AwaitPayload {
kind: BlockKind::V2,
payload_len: block_len,
decoded_size: 0,
};
}
BlockKind::V1 => {
self.poisoned = true;
return Err(Error::Unsupported);
}
},
State::AwaitPayload {
kind,
payload_len,
decoded_size,
} => {
if self.input_buf.len() < payload_len {
return Ok(RawProgress {
consumed,
written,
done: false,
});
}
match kind {
BlockKind::Uncompressed => {
// Copy payload_len bytes into output_buf for drain.
self.output_buf
.extend_from_slice(&self.input_buf[..payload_len]);
self.input_buf.drain(..payload_len);
self.state = State::AwaitMagic;
}
BlockKind::Lzvn => {
// Decode in one shot into output_buf.
//
// Bound the capacity hint by what the payload could
// plausibly produce so an attacker-controlled
// `decoded_size` (n_raw_bytes) cannot force a huge
// up-front allocation (DoS / OOM): a single 1-byte
// LZVN opcode expands to at most ~16 output bytes.
// `decode_block` still enforces the real output size
// against `decoded_size`, so under-hinting only makes
// the Vec grow as actual bytes are produced.
let capacity_hint =
decoded_size.min(payload_len.saturating_mul(16).saturating_add(64));
let mut block_out = Vec::with_capacity(capacity_hint);
if let Err(e) = lzvn::decode_block(
&self.input_buf[..payload_len],
payload_len,
decoded_size,
&mut block_out,
) {
self.poisoned = true;
return Err(e);
}
self.output_buf.append(&mut block_out);
self.input_buf.drain(..payload_len);
self.state = State::AwaitMagic;
}
BlockKind::V2 => {
// The whole block (header + both payload streams)
// is now buffered in `payload_len` bytes. Decode in
// one shot. Bound the up-front output reservation by
// a payload-derived hint (an FSE block can expand
// more than LZVN, but is still bounded; the decoder
// enforces the exact `n_raw_bytes` internally).
let cap_hint = payload_len.saturating_mul(32).saturating_add(1 << 16);
let mut block_out = Vec::new();
match lzfse_v2::decode_block(
&self.input_buf[..payload_len],
&mut block_out,
cap_hint,
) {
Ok(consumed_block) => {
debug_assert_eq!(consumed_block, payload_len);
}
Err(e) => {
self.poisoned = true;
return Err(e);
}
}
self.output_buf.append(&mut block_out);
self.input_buf.drain(..payload_len);
self.state = State::AwaitMagic;
}
BlockKind::V1 => {
// Unreachable — header step would have errored.
self.poisoned = true;
return Err(Error::Unsupported);
}
}
}
State::Done => {
self.eos = true;
return Ok(RawProgress {
consumed,
written,
done: true,
});
}
}
}
}
}
impl Default for Decoder {
fn default() -> Self {
Self::new()
}
}
impl RawDecoder for Decoder {
fn raw_decode(&mut self, input: &[u8], output: &mut [u8]) -> Result<RawProgress, Error> {
self.raw_decode_inner(input, output)
}
fn raw_finish(&mut self, output: &mut [u8]) -> Result<RawProgress, Error> {
// finish drains any pending output and, if the stream has reached
// the end-of-stream marker, returns `done`. Otherwise we surface
// an UnexpectedEnd to signal truncation.
if self.poisoned {
return Err(Error::Corrupt);
}
let p = self.raw_decode_inner(&[], output)?;
if p.done {
return Ok(p);
}
// No more input is coming. If we haven't seen the EOS marker but
// we have nothing buffered and nothing pending, treat as
// unexpected-end. If we still have decoded bytes to drain, signal
// OutputFull-style (done=false, written>0).
if p.written > 0 || !self.output_buf.is_empty() {
return Ok(p);
}
if self.state == State::AwaitMagic && self.input_buf.is_empty() {
// No partial block in flight. Empty input followed by finish on
// a fresh decoder is fine — return StreamEnd.
self.eos = true;
return Ok(RawProgress {
consumed: 0,
written: 0,
done: true,
});
}
// Mid-block at EOI — truncated.
self.poisoned = true;
Err(Error::UnexpectedEnd)
}
fn raw_reset(&mut self) {
self.input_buf.clear();
self.output_buf.clear();
self.output_pos = 0;
self.state = State::AwaitMagic;
self.eos = false;
self.poisoned = false;
}
}