moq_audio/decode/

decoder.rs

//! Audio decoder front end.
//!
//! Mirror of [`encode::Encoder`](crate::encode::Encoder): dispatches over the
//! catalog codec and produces interleaved `f32` PCM.

use std::time::Duration;

use unsafe_libopus::{OPUS_OK, OpusDecoder, opus_decode_float, opus_decoder_create, opus_decoder_destroy};

use crate::opus;
use crate::pcm;
use crate::{Error, Format};

/// Opus packets cap at 120 ms (RFC 6716 §2.1.4).
const MAX_FRAME_MS: usize = 120;

/// Decoder configuration: the PCM layout to emit, plus the subscription's
/// latency budget.
///
/// The mirror of [`encode::Config`](crate::encode::Config): it describes the
/// output, since the codec's own shape is read from the catalog.
///
/// `#[non_exhaustive]`: build via [`Config::new`] (or `default()`) and set the
/// optional fields, so future knobs don't break callers.
#[derive(Clone, Debug, Default)]
#[non_exhaustive]
pub struct Config {
	/// How to pack samples in each emitted frame.
	pub format: Format,
	/// Sample rate to emit at. `None` uses the codec's native rate from the
	/// catalog; anything else resamples.
	pub sample_rate: Option<u32>,
	/// Channel count to emit. `None` uses the codec's native count; anything
	/// else remixes mono and stereo at the decode boundary.
	pub channels: Option<u32>,
	/// Upper bound on buffering before skipping a stalled group.
	///
	/// Forwarded to [`moq_mux::container::Consumer::with_latency`]: if a group is
	/// stuck and a newer group is more than this far ahead, the consumer skips.
	/// `None` keeps the moq-mux default of zero, which skips aggressively. Set it
	/// to the playout buffer you can tolerate (typically tens to a few hundred ms)
	/// for the best congestion-vs-quality trade-off. The `_max` suffix is a
	/// reminder that we never *add* latency here: the consumer skips only when
	/// newer data is already this far ahead. A companion `latency_min` for
	/// jitter-buffer padding will land in a follow-up.
	pub latency_max: Option<Duration>,
}

impl Config {
	/// A default config: the codec's native rate and channel count, interleaved
	/// `f32`, and the moq-mux default latency.
	pub fn new() -> Self {
		Self::default()
	}
}

/// Decodes codec packets into interleaved `f32` PCM.
///
/// The bring-your-own-payload layer under [`Consumer`](super::Consumer): use it
/// when the packets don't come from a plain track subscription.
pub struct Decoder {
	backend: Backend,
	sample_rate: u32,
	channel_count: u32,
}

enum Backend {
	Opus(Opus),
	Pcm { bytes_per_frame: usize },
}

struct Opus {
	inner: *mut OpusDecoder,
	pre_skip_remaining: usize,
	max_frame_size: usize,
}

// SAFETY: see Encoder.
unsafe impl Send for Opus {}

impl Decoder {
	/// Build a decoder from a catalog [`AudioConfig`](hang::catalog::AudioConfig).
	///
	/// Parses the OpusHead `description` if present; falls back to the catalog's
	/// declared sample rate / channel count. PCM uses those catalog fields
	/// directly and requires an absent `description`.
	pub fn new(catalog: &hang::catalog::AudioConfig) -> Result<Self, Error> {
		match &catalog.codec {
			hang::catalog::AudioCodec::Opus => Self::new_opus(catalog),
			hang::catalog::AudioCodec::Pcm => Self::new_pcm(catalog),
			codec => Err(Error::Unsupported(format!("unsupported audio codec: {codec}"))),
		}
	}

	fn new_opus(catalog: &hang::catalog::AudioConfig) -> Result<Self, Error> {
		let (sample_rate, channel_count, pre_skip) = if let Some(desc) = &catalog.description {
			let mut buf = desc.as_ref();
			match moq_mux::codec::opus::Config::parse(&mut buf) {
				Ok(head) => (head.sample_rate, head.channel_count, head.pre_skip),
				Err(_) => (catalog.sample_rate, catalog.channel_count, 0),
			}
		} else {
			(catalog.sample_rate, catalog.channel_count, 0)
		};

		opus::validate_rate(sample_rate)?;
		let channels = opus::validate_channels(channel_count)?;

		let mut err = 0i32;
		// SAFETY: out-pointer is valid; inner is checked for null below.
		let inner = unsafe { opus_decoder_create(sample_rate as i32, channels, &mut err) };
		if err != OPUS_OK || inner.is_null() {
			return Err(opus::error(err, "opus_decoder_create"));
		}

		let max_frame_size = (sample_rate as usize * MAX_FRAME_MS) / 1000;
		let pre_skip_remaining = (pre_skip as usize * sample_rate as usize) / 48_000;

		Ok(Self {
			backend: Backend::Opus(Opus {
				inner,
				pre_skip_remaining,
				max_frame_size,
			}),
			sample_rate,
			channel_count,
		})
	}

	fn new_pcm(catalog: &hang::catalog::AudioConfig) -> Result<Self, Error> {
		if catalog.sample_rate == 0 {
			return Err(Error::Unsupported("pcm sample rate must be greater than zero".into()));
		}
		if catalog.channel_count == 0 {
			return Err(Error::Unsupported("pcm channel count must be greater than zero".into()));
		}
		if catalog.description.is_some() {
			return Err(Error::Unsupported("pcm catalog description must be absent".into()));
		}
		let bitrate = pcm::bitrate(catalog.sample_rate, catalog.channel_count)?;
		if catalog.bitrate.is_some_and(|declared| declared != bitrate) {
			return Err(Error::Unsupported(format!(
				"pcm catalog bitrate must be {bitrate} bits per second"
			)));
		}
		let bytes_per_frame = pcm::frame_bytes(1, catalog.channel_count)?;

		Ok(Self {
			backend: Backend::Pcm { bytes_per_frame },
			sample_rate: catalog.sample_rate,
			channel_count: catalog.channel_count,
		})
	}

	/// The rate the codec decodes at, read from the catalog.
	pub fn sample_rate(&self) -> u32 {
		self.sample_rate
	}

	/// The channel count the codec decodes at, read from the catalog.
	pub fn channel_count(&self) -> u32 {
		self.channel_count
	}

	/// Decode one packet into interleaved `f32` PCM.
	pub fn decode(&mut self, packet: &[u8]) -> Result<Vec<f32>, Error> {
		match &mut self.backend {
			Backend::Opus(opus) => {
				let mut out = vec![0.0f32; opus.max_frame_size * self.channel_count as usize];
				// SAFETY: `inner` owns a live OpusDecoder; packet/out slices are
				// bounded by the lengths we pass.
				let samples = unsafe {
					opus_decode_float(
						&mut *opus.inner,
						packet.as_ptr(),
						packet.len() as i32,
						out.as_mut_ptr(),
						opus.max_frame_size as i32,
						0,
					)
				};
				if samples < 0 {
					return Err(crate::opus::error(samples, "opus_decode_float"));
				}
				out.truncate(samples as usize * self.channel_count as usize);
				let trim_frames = opus.pre_skip_remaining.min(samples as usize);
				if trim_frames > 0 {
					let trim_samples = trim_frames * self.channel_count as usize;
					out.copy_within(trim_samples.., 0);
					out.truncate(out.len() - trim_samples);
					opus.pre_skip_remaining -= trim_frames;
				}
				Ok(out)
			}
			Backend::Pcm { bytes_per_frame } => {
				if packet.is_empty() || !packet.len().is_multiple_of(*bytes_per_frame) {
					return Err(Error::Misaligned {
						got: packet.len(),
						expected: packet.len().max(1).next_multiple_of(*bytes_per_frame),
					});
				}

				Ok(packet
					.chunks_exact(pcm::BYTES_PER_SAMPLE)
					.map(|sample| f32::from_le_bytes([sample[0], sample[1], sample[2], sample[3]]))
					.collect())
			}
		}
	}
}

impl Drop for Opus {
	fn drop(&mut self) {
		// SAFETY: `inner` is a live OpusDecoder that nothing else aliases.
		unsafe { opus_decoder_destroy(self.inner) };
	}
}

#[cfg(test)]
mod tests {
	use super::*;

	#[test]
	fn pcm_rejects_incomplete_channel_frame() {
		let catalog = hang::catalog::AudioConfig::new(hang::catalog::AudioCodec::Pcm, 48_000, 2);
		let mut decoder = Decoder::new(&catalog).unwrap();

		assert!(matches!(
			decoder.decode(&[]),
			Err(Error::Misaligned { got: 0, expected: 8 })
		));
		assert!(matches!(
			decoder.decode(&[0; 4]),
			Err(Error::Misaligned { got: 4, expected: 8 })
		));
	}

	#[test]
	fn decoder_rejects_unknown_codec() {
		let catalog = hang::catalog::AudioConfig::new(hang::catalog::AudioCodec::Unknown("future".into()), 48_000, 2);

		assert!(matches!(Decoder::new(&catalog), Err(Error::Unsupported(_))));
	}

	#[test]
	fn pcm_rejects_incorrect_catalog_bitrate() {
		let mut catalog = hang::catalog::AudioConfig::new(hang::catalog::AudioCodec::Pcm, 48_000, 2);
		catalog.bitrate = Some(1);

		assert!(matches!(Decoder::new(&catalog), Err(Error::Unsupported(_))));
	}
}