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//! Handler for lacing and delacing operations on frame data.
use crate::{Error, base::VInt64, functional::Encode, io::blocking_impl::ReadFrom};
// https://www.matroska.org/technical/notes.html
/// Handler for lacing and delacing operations on frame data.
pub enum Lacer {
/// Xiph lacing (variable-size frames with size prefixes)
///
/// The Xiph lacing uses the same coding of size as found in the Ogg container \[@?RFC3533\]. The bits 5-6 of the Block Header flags are set to 01.
/// The Block data with laced frames is stored as follows:
/// Lacing Head on 1 Octet: Number of frames in the lace minus 1.
/// Lacing size of each frame except the last one.
/// Binary data of each frame consecutively.
/// The lacing size is split into 255 values, stored as unsigned octets – for example, 500 is coded 255;245 or [0xFF 0xF5]. A frame with a size multiple of 255 is coded with a 0 at the end of the size – for example, 765 is coded 255;255;255;0 or [0xFF 0xFF 0xFF 0x00].
/// The size of the last frame is deduced from the size remaining in the Block after the other frames.
Xiph,
/// Fixed-size lacing (all frames have the same size)
FixedSize,
/// EBML lacing (variable-size frames with EBML-encoded sizes)
///
/// The EBML lacing encodes the frame size with an EBML-like encoding \[@!RFC8794\]. The bits 5-6 of the Block Header flags are set to 11.
///
/// The Block data with laced frames is stored as follows:
/// Lacing Head on 1 Octet: Number of frames in the lace minus 1.
/// Lacing size of each frame except the last one.
/// Binary data of each frame consecutively.
///
/// The first frame size is encoded as an EBML Variable-Size Integer value, also known as VINT in \[@!RFC8794\].
/// The remaining frame sizes are encoded as signed values using the difference between the frame size and the previous frame size.
/// These signed values are encoded as VINT, with a mapping from signed to unsigned numbers.
/// Decoding the unsigned number stored in the VINT to a signed number is done by subtracting 2^((7*n)-1)-1, where n is the octet size of the VINT.
Ebml,
}
impl Lacer {
/// Encode multiple frames into a single laced block
pub fn lace(&self, frames: &[&[u8]]) -> Vec<u8> {
if frames.is_empty() {
return vec![];
}
let num_frames = frames.len();
let mut output = vec![];
output.push((num_frames - 1) as u8); // Number of frames - 1
match self {
Lacer::Xiph => {
for frame in &frames[..num_frames - 1] {
let mut size = frame.len();
while size >= 0xFF {
output.push(0xFF);
size -= 0xFF;
}
output.push(size as u8);
}
for frame in frames {
output.extend_from_slice(frame);
}
output
}
Lacer::FixedSize => {
let frame_size = frames[0].len();
if let Some((idx, bad_frame)) = frames
.iter()
.enumerate()
.find(|(_, f)| f.len() != frame_size)
{
panic!(
"All frames must have the same size for FixedSize lacing: expected size {}, but frame at index {} has size {}",
frame_size,
idx,
bad_frame.len()
);
}
for frame in frames {
output.extend_from_slice(frame);
}
output
}
Lacer::Ebml => {
if num_frames == 1 {
output.extend_from_slice(frames[0]);
return output;
}
let sizes = frames.iter().map(|f| f.len() as u64).collect::<Vec<_>>();
// except first size, other sizes are stored as diffs to the previous size
let diff_sizes = std::iter::once(
// first
VInt64::new(sizes[0]),
)
.chain(sizes.windows(2).map(|w| {
let diff = w[1] as i64 - w[0] as i64;
//-(2^6^-1) to 2^6^
let n = if diff > -(2i64.pow(6) - 1) && diff < (2i64.pow(6)) {
1
} else if diff > -(2i64.pow(13) - 1) && diff < (2i64.pow(13)) {
2
} else if diff > -(2i64.pow(20) - 1) && diff < (2i64.pow(20)) {
3
} else if diff > -(2i64.pow(27) - 1) && diff < (2i64.pow(27)) {
4
} else if diff > -(2i64.pow(34) - 1) && diff < (2i64.pow(34)) {
5
} else if diff > -(2i64.pow(41) - 1) && diff < (2i64.pow(41)) {
6
} else if diff > -(2i64.pow(48) - 1) && diff < (2i64.pow(48)) {
7
} else {
panic!("Frame size diff too large for EBML lacing: diff = {}", diff);
};
// map to unsigned
let diff_unsigned = diff + (2i64.pow(7 * n as u32 - 1) - 1);
VInt64::new(diff_unsigned as u64)
}))
// dont include last size, it is deduced from remaining data
.take(num_frames - 1);
for size in diff_sizes {
size.encode(&mut output).unwrap();
}
for frame in frames {
output.extend_from_slice(frame);
}
output
}
}
}
/// Decode a laced block into individual frames
pub fn delace<'a>(&self, data: &'a [u8]) -> crate::Result<Vec<&'a [u8]>> {
// TODO(perf): avoid heap allocations ideally
// we should be able to return a `impl Iterator<Item = crate::Result<&'a [u8]>>` here
// can make it work using nightly features like `generators`.
// but not sure how to do that with the current stable Rust.
if data.is_empty() {
return Ok(vec![]);
}
let num_frames = data[0] as usize + 1;
if num_frames == 1 {
return Ok(vec![&data[1..]]);
}
match self {
Lacer::Xiph => {
let mut out = Vec::with_capacity(num_frames);
let data_start_pos = data
.iter()
.enumerate()
.skip(1)
.filter(|(_, b)| **b != 0xFF)
.nth(num_frames - 2)
.map(|(i, _)| i)
.ok_or(Error::MalformedLacingData)?
+ 1;
let laced_data = data
.get(data_start_pos..)
.ok_or(Error::MalformedLacingData)?;
let mut start = 0;
for size in data[1..data_start_pos]
.split_inclusive(|b| *b != 0xFF)
.map(|chunk| chunk.iter().map(|b| *b as usize).sum::<usize>())
{
out.push(
laced_data
.get(start..start + size)
.ok_or(Error::MalformedLacingData)?,
);
start += size;
}
out.push(laced_data.get(start..).ok_or(Error::MalformedLacingData)?);
Ok(out)
}
Lacer::FixedSize => {
let data_len = data.len() - 1;
// all frames must have the same size
if !data_len.is_multiple_of(num_frames) {
return Err(Error::MalformedLacingData);
}
Ok(data[1..].chunks(data_len / num_frames).collect())
}
Lacer::Ebml => {
let mut data_buf = &data[1..];
let mut out_sizes = Vec::with_capacity(num_frames - 1);
let first_size = VInt64::read_from(&mut data_buf)?;
out_sizes.push(*first_size as usize);
for _ in 1..(num_frames - 1) {
let oct_size = data_buf
.first()
.ok_or(Error::MalformedLacingData)?
.leading_zeros()
+ 1;
let current_encoded_vint = VInt64::read_from(&mut data_buf)?;
// unsigned to signed
let diff = *current_encoded_vint as i64 - (2i64.pow(7 * oct_size - 1) - 1);
let new_size = out_sizes
.last()
.unwrap()
.checked_add_signed(diff as isize)
.ok_or(Error::MalformedLacingData)?;
out_sizes.push(new_size);
}
let mut out = Vec::with_capacity(num_frames);
let mut start = 0;
for size in out_sizes {
out.push(
data_buf
.get(start..start + size)
.ok_or(Error::MalformedLacingData)?,
);
start += size;
}
out.push(data_buf.get(start..).ok_or(Error::MalformedLacingData)?);
Ok(out)
}
}
}
}
#[cfg(test)]
mod lacer_tests {
use super::*;
#[test]
fn test_xiph_lacing() {
// 0 frames
let laced = Lacer::Xiph.lace(&[]);
assert_eq!(laced, vec![]);
let frames: Vec<_> = Lacer::Xiph.delace(&[]).unwrap();
assert_eq!(frames.len(), 0);
// 4 frames, sizes: 255, 256, 1, remaining
let len = vec![0x03, 0xFF, 0x00, 0xFF, 0x1, 0x1];
let frame0 = vec![2u8; 255];
let frame1 = vec![42u8; 256];
let frame2 = vec![38u8; 1];
let frame3 = vec![100u8; 1];
let laced = Lacer::Xiph.lace(&[&frame0, &frame1, &frame2, &frame3]);
let data = [len, frame0, frame1, frame2, frame3].concat();
assert_eq!(laced, data);
let frames: Vec<_> = Lacer::Xiph.delace(&data).unwrap();
assert_eq!(frames.len(), 4);
assert_eq!(frames[0], &[2u8; 255]);
assert_eq!(frames[1], &[42u8; 256]);
assert_eq!(frames[2], &[38u8; 1]);
assert_eq!(frames[3], &[100u8; 1]);
// 1 frame, size: remaining
let len = vec![0x00];
let frame0 = vec![2u8; 255];
let laced = Lacer::Xiph.lace(&[&frame0]);
let data = [len, frame0].concat();
assert_eq!(laced, data);
let frames: Vec<_> = Lacer::Xiph.delace(&data).unwrap();
assert_eq!(frames.len(), 1);
assert_eq!(frames[0], &[2u8; 255]);
// 2 frames, sizes: 32, remaining
let len = vec![0x01, 0x20];
let frame0 = vec![2u8; 32];
let frame1 = vec![42u8; 256];
let laced = Lacer::Xiph.lace(&[&frame0, &frame1]);
let data = [len, frame0, frame1].concat();
assert_eq!(laced, data);
let frames: Vec<_> = Lacer::Xiph.delace(&data).unwrap();
assert_eq!(frames.len(), 2);
assert_eq!(frames[0], &[2u8; 32]);
assert_eq!(frames[1], &[42u8; 256]);
// 4 frames, sizes: 600, 3, 520, remaining
let len = vec![0x03, 0xFF, 0xFF, 0x5A, 0x3, 0xFF, 0xFF, 0xA];
assert_eq!(0xff + 0xff + 0x5A, 600);
assert_eq!(0xff + 0xff + 0xA, 520);
let frame0 = vec![2u8; 600];
let frame1 = vec![42u8; 3];
let frame2 = vec![38u8; 520];
let frame3 = vec![100u8; 1];
let laced = Lacer::Xiph.lace(&[&frame0, &frame1, &frame2, &frame3]);
let data = [len, frame0, frame1, frame2, frame3].concat();
assert_eq!(laced, data);
let frames: Vec<_> = Lacer::Xiph.delace(&data).unwrap();
assert_eq!(frames.len(), 4);
assert_eq!(frames[0], &[2u8; 600]);
assert_eq!(frames[1], &[42u8; 3]);
assert_eq!(frames[2], &[38u8; 520]);
assert_eq!(frames[3], &[100u8; 1]);
}
#[test]
fn test_ebml_lacing() {
// 0 frames
let laced = Lacer::Ebml.lace(&[]);
assert_eq!(laced, vec![]);
let frames: Vec<_> = Lacer::Ebml.delace(&[]).unwrap();
assert_eq!(frames.len(), 0);
// 3 frames, sizes: 800, 500, remaining(1000)
// store as size diffs: 800, -300
// offset = 2**(7*n - 1) - 1
// n = 2 -> 2**13 - 1 = 8191
// convert to uint: 800, 7891(-300+8191)
// encode as VInt:
// 0x4320(800), 0x5ED3(7891)
let len = vec![0x02, 0x43, 0x20, 0x5E, 0xD3];
let frame0 = vec![2u8; 800];
let frame1 = vec![42u8; 500];
let frame2 = vec![38u8; 1000];
let laced = Lacer::Ebml.lace(&[&frame0, &frame1, &frame2]);
let data = [len, frame0, frame1, frame2].concat();
assert_eq!(laced, data);
let frames: Vec<_> = Lacer::Ebml.delace(&data).unwrap();
assert_eq!(frames.len(), 3);
assert_eq!(frames[0], &[2u8; 800]);
assert_eq!(frames[1], &[42u8; 500]);
assert_eq!(frames[2], &[38u8; 1000]);
// 7 frames, sizes 2, 5000, 4980, 400, 20, 2000, remaining(300)
// store as size diffs: 2, 4998, -20, -4580, -380, 1980
let len = vec![
0x06, 0x82, 0x73, 0x85, 0xAB, 0x4E, 0x1B, 0x5E, 0x83, 0x67, 0xBB,
];
let frame0 = vec![2u8; 2];
let frame1 = vec![42u8; 5000];
let frame2 = vec![38u8; 4980];
let frame3 = vec![100u8; 400];
let frame4 = vec![7u8; 20];
let frame5 = vec![8u8; 2000];
let frame6 = vec![9u8; 300];
let laced = Lacer::Ebml.lace(&[
&frame0, &frame1, &frame2, &frame3, &frame4, &frame5, &frame6,
]);
let data = [len, frame0, frame1, frame2, frame3, frame4, frame5, frame6].concat();
assert_eq!(laced, data);
let frames: Vec<_> = Lacer::Ebml.delace(&data).unwrap();
assert_eq!(frames.len(), 7);
assert_eq!(frames[0], &[2u8; 2]);
assert_eq!(frames[1], &[42u8; 5000]);
assert_eq!(frames[2], &[38u8; 4980]);
assert_eq!(frames[3], &[100u8; 400]);
assert_eq!(frames[4], &[7u8; 20]);
assert_eq!(frames[5], &[8u8; 2000]);
assert_eq!(frames[6], &[9u8; 300]);
}
#[test]
fn test_fixed_size_lacing() {
// 0 frames
let laced = Lacer::FixedSize.lace(&[]);
assert_eq!(laced, vec![]);
let frames: Vec<_> = Lacer::FixedSize.delace(&[]).unwrap();
assert_eq!(frames.len(), 0);
// 3 frames, sizes: 500, 500, 500
let len = vec![0x02];
let frame0 = vec![2u8; 500];
let frame1 = vec![42u8; 500];
let frame2 = vec![38u8; 500];
let laced = Lacer::FixedSize.lace(&[&frame0, &frame1, &frame2]);
let data = [len, frame0, frame1, frame2].concat();
assert_eq!(laced, data);
let frames: Vec<_> = Lacer::FixedSize.delace(&data).unwrap();
assert_eq!(frames.len(), 3);
assert_eq!(frames[0], &[2u8; 500]);
assert_eq!(frames[1], &[42u8; 500]);
assert_eq!(frames[2], &[38u8; 500]);
}
}