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//! The Open Bitstream Unit layer (AV1 spec §5.3).
//!
//! An AV1 stream — whether the configuration OBUs in an `av1C` box or the coded
//! frame in an item's payload — is a sequence of OBUs, each a one- or two-byte
//! header followed by an optional `leb128` size and a payload. This module
//! splits a byte buffer into borrowed OBUs without interpreting their contents;
//! [`super::seq`] and [`super::frame`] interpret the ones that matter.
//!
//! AVIF mandates the *low-overhead* framing, in which every OBU carries its own
//! size field. This reader accepts a sizeless final OBU as well — it extends to
//! the end of the buffer — because that is the one unambiguous case and
//! rejecting it would turn a legal-enough stream into a spurious error.
use super::bits::BitReader;
use otf_pixels_core::{PixelsError, Result};
/// The kind of an OBU (`obu_type`, §6.2.2).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ObuType {
/// `OBU_SEQUENCE_HEADER` (1).
SequenceHeader,
/// `OBU_TEMPORAL_DELIMITER` (2).
TemporalDelimiter,
/// `OBU_FRAME_HEADER` (3).
FrameHeader,
/// `OBU_TILE_GROUP` (4).
TileGroup,
/// `OBU_METADATA` (5).
Metadata,
/// `OBU_FRAME` (6) — a frame header immediately followed by tile data.
Frame,
/// `OBU_REDUNDANT_FRAME_HEADER` (7).
RedundantFrameHeader,
/// `OBU_TILE_LIST` (8).
TileList,
/// `OBU_PADDING` (15).
Padding,
/// Any reserved or unknown type, carried through so a stream that uses one
/// is skipped rather than rejected.
Reserved(u8),
}
impl ObuType {
fn from_bits(value: u32) -> Self {
match value {
1 => Self::SequenceHeader,
2 => Self::TemporalDelimiter,
3 => Self::FrameHeader,
4 => Self::TileGroup,
5 => Self::Metadata,
6 => Self::Frame,
7 => Self::RedundantFrameHeader,
8 => Self::TileList,
15 => Self::Padding,
other => Self::Reserved(other as u8),
}
}
}
/// A parsed OBU header (§5.3.2).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct ObuHeader {
/// The OBU type.
pub kind: ObuType,
/// `temporal_id` from the extension header, or 0 when absent.
pub temporal_id: u8,
/// `spatial_id` from the extension header, or 0 when absent.
pub spatial_id: u8,
}
/// A single OBU: its header and a borrow of its payload bytes.
#[derive(Debug, Clone, Copy)]
pub struct Obu<'a> {
/// The parsed header.
pub header: ObuHeader,
/// The payload, exactly `obu_size` bytes, ready for a fresh [`BitReader`].
pub payload: &'a [u8],
/// The byte offset of `payload` within the buffer `parse_stream` was given,
/// so a caller can locate tile data absolutely without pointer arithmetic.
pub payload_start: usize,
}
impl<'a> Obu<'a> {
/// Split a byte buffer into its OBUs.
///
/// Stops cleanly at the end of the buffer. Rejects a header whose forbidden
/// bit is set and a size that would run past the buffer — the two ways OBU
/// framing is used to point a decoder outside its input.
pub fn parse_stream(data: &'a [u8]) -> Result<Vec<Obu<'a>>> {
let mut obus = Vec::new();
let mut pos = 0;
while pos < data.len() {
let Some(rest) = data.get(pos..) else {
break;
};
let mut reader = BitReader::new(rest);
if reader.f(1)? != 0 {
return Err(PixelsError::malformed(
"avif",
"an AV1 OBU forbidden bit was set",
));
}
let kind = ObuType::from_bits(reader.f(4)?);
let extension_flag = reader.flag()?;
let has_size_field = reader.flag()?;
let _obu_reserved_1bit = reader.f(1)?;
let (temporal_id, spatial_id) = if extension_flag {
let temporal_id = reader.f(3)? as u8;
let spatial_id = reader.f(2)? as u8;
let _extension_reserved_3bits = reader.f(3)?;
(temporal_id, spatial_id)
} else {
(0, 0)
};
let payload_len = if has_size_field {
usize::try_from(reader.leb128()?).map_err(|_| {
PixelsError::malformed("avif", "an AV1 OBU size exceeds this platform's usize")
})?
} else {
// A sizeless OBU runs to the end of the buffer; only the last
// OBU can legally be sizeless.
rest.len().saturating_sub(reader.byte_position())
};
let payload_start = pos + reader.byte_position();
let payload_end = payload_start
.checked_add(payload_len)
.filter(|&e| e <= data.len());
let Some(payload_end) = payload_end else {
return Err(PixelsError::malformed(
"avif",
"an AV1 OBU declares more bytes than the stream holds",
));
};
let Some(payload) = data.get(payload_start..payload_end) else {
return Err(PixelsError::malformed(
"avif",
"an AV1 OBU payload range is invalid",
));
};
obus.push(Obu {
header: ObuHeader {
kind,
temporal_id,
spatial_id,
},
payload,
payload_start,
});
pos = payload_end;
}
Ok(obus)
}
}
#[cfg(test)]
#[allow(
clippy::unwrap_used,
clippy::indexing_slicing,
clippy::panic,
clippy::unusual_byte_groupings,
reason = "tests operate on known-good values and assert shapes directly"
)]
mod tests {
use super::*;
use otf_pixels_core::ErrorCode;
/// Build a low-overhead OBU: header byte with the size-field flag set,
/// a single-byte leb128 length, then the payload.
fn sized_obu(kind: u8, payload: &[u8]) -> Vec<u8> {
let mut out = Vec::new();
// forbidden=0, type=kind, ext=0, has_size=1, reserved=0
let header = (kind << 3) | 0b0000_0010;
out.push(header);
assert!(payload.len() < 128, "test payloads stay single-byte leb128");
out.push(payload.len() as u8);
out.extend_from_slice(payload);
out
}
#[test]
fn splits_a_stream_into_sized_obus() {
let mut stream = sized_obu(2, &[]); // temporal delimiter
stream.extend(sized_obu(1, &[0xAA, 0xBB])); // sequence header
stream.extend(sized_obu(6, &[0xCC])); // frame
let obus = Obu::parse_stream(&stream).unwrap();
assert_eq!(obus.len(), 3);
assert_eq!(obus[0].header.kind, ObuType::TemporalDelimiter);
assert_eq!(obus[0].payload, &[] as &[u8]);
assert_eq!(obus[1].header.kind, ObuType::SequenceHeader);
assert_eq!(obus[1].payload, &[0xAA, 0xBB]);
assert_eq!(obus[2].header.kind, ObuType::Frame);
assert_eq!(obus[2].payload, &[0xCC]);
}
#[test]
fn reads_the_extension_header_ids() {
// forbidden=0 type=1 ext=1 has_size=1 reserved=0 -> 0b0_0001_1_1_0
let mut stream = vec![0b0000_1110];
// extension byte: temporal_id=3 (011) spatial_id=2 (10) reserved=0(000)
stream.push(0b011_10_000);
stream.push(0x01); // leb128 size = 1
stream.push(0x55); // payload
let obus = Obu::parse_stream(&stream).unwrap();
assert_eq!(obus.len(), 1);
assert_eq!(obus[0].header.temporal_id, 3);
assert_eq!(obus[0].header.spatial_id, 2);
assert_eq!(obus[0].payload, &[0x55]);
}
#[test]
fn a_sizeless_final_obu_takes_the_rest_of_the_buffer() {
// header: type=6 ext=0 has_size=0 -> 0b0_0110_0_0_0
let mut stream = vec![0b0011_0000];
stream.extend_from_slice(&[1, 2, 3, 4]);
let obus = Obu::parse_stream(&stream).unwrap();
assert_eq!(obus.len(), 1);
assert_eq!(obus[0].header.kind, ObuType::Frame);
assert_eq!(obus[0].payload, &[1, 2, 3, 4]);
}
#[test]
fn a_size_past_the_buffer_is_rejected() {
// has_size=1, size=200, but only a few bytes follow.
let stream = vec![0b0011_0010, 200, 1, 2, 3];
let err = Obu::parse_stream(&stream).unwrap_err();
assert_eq!(err.code(), ErrorCode::Malformed);
}
#[test]
fn a_set_forbidden_bit_is_rejected() {
let stream = vec![0b1011_0010, 0x00];
let err = Obu::parse_stream(&stream).unwrap_err();
assert_eq!(err.code(), ErrorCode::Malformed);
}
#[test]
fn an_unknown_type_is_carried_through_not_rejected() {
let stream = sized_obu(9, &[0x01]);
let obus = Obu::parse_stream(&stream).unwrap();
assert_eq!(obus[0].header.kind, ObuType::Reserved(9));
}
}