Decoding opus files

opus.rs

fn decode_opus(data: &[u8]) -> Result<Arc<oddio::Frames<f32>>> {
    let _span = tracy_client::span!("opus decode");
    let mut ogg = ogg::PacketReader::new(std::io::Cursor::new(data));
    let mut decoder = opus::Decoder::new(SAMPLE_RATE, opus::Channels::Mono).unwrap();
    let mut buffer = Vec::new();
    let mut serial = None;
    let mut pre_skip = 0;
    let mut scale = 1.0;
    let mut seen_comment = false;
    while let Some(packet) = ogg.read_packet().context("reading ogg packet")? {
        if serial.is_none() && packet.first_in_stream() {
            // Is this an Opus stream?
            if !packet.data.starts_with(ID_MAGIC) || packet.data[8] & 0xF0 != 0 {
                continue;
            }
            // Is it sensible?
            let channels = packet.data[9];
            if channels != 1 {
                bail!("unsupported channel count: {}", channels);
            }
            serial = Some(packet.stream_serial());
            pre_skip = usize::from(u16::from_le_bytes(packet.data[10..12].try_into().unwrap()));
            let output_gain = i16::from_le_bytes(packet.data[16..18].try_into().unwrap());
            scale = 10.0f32.powf(f32::from(output_gain) / (20.0 * 256.0));
            let channel_mapping_family = packet.data[18];
            if channel_mapping_family != 0 {
                bail!(
                    "unsupported channel mapping family {}",
                    channel_mapping_family
                );
            }
            continue;
        }
        // Is this part of the Opus stream?
        if Some(packet.stream_serial()) != serial {
            continue;
        }
        // Have we passed the comment header yet?
        if !seen_comment {
            seen_comment = true;
            if packet.data.starts_with(COMMENT_MAGIC) {
                continue;
            } else {
                error!("missing comment header");
            }
        }
        // Decode samples
        let n = decoder
            .get_nb_samples(&packet.data)
            .context("decoding opus packet")?;
        let start = buffer.len();
        buffer.resize(start + n, 0.0);
        decoder
            .decode_float(&packet.data, &mut buffer[start..], false)
            .context("decoding opus")?;
        if pre_skip > 0 {
            let skip = pre_skip.min(buffer.len());
            buffer.drain(0..skip);
            pre_skip -= skip;
        }
    }
    for sample in &mut buffer {
        *sample *= scale;
    }
    trace!(
        "loaded {:.02} seconds of audio",
        buffer.len() as f32 / SAMPLE_RATE as f32
    );
    Ok(oddio::Frames::from_slice(SAMPLE_RATE, &buffer))
}