snapcast_client/decoder/
opus.rs

1//! Opus decoder using opus-decoder (pure Rust, no C dependencies).
2//!
3//! The snapserver sends a custom "pseudo header":
4//!   4 bytes: ID (0x4F505553 as LE u32)
5//!   4 bytes: sample rate (LE u32)
6//!   2 bytes: bits per sample (LE u16)
7//!   2 bytes: channels (LE u16)
8
9use anyhow::{Result, bail};
10use opus_decoder::OpusDecoder as OpusDec;
11use snapcast_proto::SampleFormat;
12use snapcast_proto::message::codec_header::CodecHeader;
13
14use crate::decoder::Decoder;
15
16const OPUS_ID: u32 = 0x4F50_5553;
17const MAX_FRAME_SIZE: usize = 2880;
18
19/// Parse the Opus pseudo header.
20fn parse_opus_header(payload: &[u8]) -> Result<SampleFormat> {
21    if payload.len() < 12 {
22        bail!(
23            "Opus header too small ({} bytes, need >= 12)",
24            payload.len()
25        );
26    }
27    let id = u32::from_le_bytes(payload[0..4].try_into().unwrap());
28    if id != OPUS_ID {
29        bail!("not an Opus header (expected 0x{OPUS_ID:08X}, got 0x{id:08X})");
30    }
31    let rate = u32::from_le_bytes(payload[4..8].try_into().unwrap());
32    let bits = u16::from_le_bytes(payload[8..10].try_into().unwrap());
33    let channels = u16::from_le_bytes(payload[10..12].try_into().unwrap());
34    Ok(SampleFormat::new(rate, bits, channels))
35}
36
37/// Opus audio decoder (pure Rust).
38pub struct OpusDecoder {
39    decoder: OpusDec,
40    sample_format: SampleFormat,
41    pcm_buf: Vec<i16>,
42}
43
44impl Decoder for OpusDecoder {
45    fn set_header(&mut self, header: &CodecHeader) -> Result<SampleFormat> {
46        let new = create(header)?;
47        *self = new;
48        Ok(self.sample_format)
49    }
50
51    fn decode(&mut self, data: &mut Vec<u8>) -> Result<bool> {
52        if data.is_empty() {
53            return Ok(false);
54        }
55        tracing::trace!(codec = "opus", input_bytes = data.len(), "decode");
56        let decoded_samples = match self
57            .decoder
58            .decode(data.as_slice(), &mut self.pcm_buf, false)
59        {
60            Ok(n) => n,
61            Err(e) => {
62                tracing::warn!(codec = "opus", error = %e, "decode failed");
63                return Ok(false);
64            }
65        };
66        let total_samples = decoded_samples * self.sample_format.channels() as usize;
67        let mut out = Vec::with_capacity(total_samples * 2);
68        for &s in &self.pcm_buf[..total_samples] {
69            out.extend_from_slice(&s.to_le_bytes());
70        }
71        *data = out;
72        Ok(true)
73    }
74}
75
76/// Create an OpusDecoder from a CodecHeader.
77pub fn create(header: &CodecHeader) -> Result<OpusDecoder> {
78    let sf = parse_opus_header(&header.payload)?;
79    let dec = OpusDec::new(sf.rate(), sf.channels() as usize)
80        .map_err(|e| anyhow::anyhow!("failed to create Opus decoder: {e}"))?;
81    Ok(OpusDecoder {
82        decoder: dec,
83        sample_format: sf,
84        pcm_buf: vec![0i16; MAX_FRAME_SIZE * sf.channels() as usize],
85    })
86}
87#[cfg(test)]
88mod tests {
89    use super::*;
90
91    fn opus_header(rate: u32, bits: u16, channels: u16) -> Vec<u8> {
92        let mut h = Vec::new();
93        h.extend_from_slice(&OPUS_ID.to_le_bytes());
94        h.extend_from_slice(&rate.to_le_bytes());
95        h.extend_from_slice(&bits.to_le_bytes());
96        h.extend_from_slice(&channels.to_le_bytes());
97        h
98    }
99
100    #[test]
101    fn parse_header_48000_16_2() {
102        let sf = parse_opus_header(&opus_header(48000, 16, 2)).unwrap();
103        assert_eq!(sf.rate(), 48000);
104        assert_eq!(sf.bits(), 16);
105        assert_eq!(sf.channels(), 2);
106    }
107
108    #[test]
109    fn parse_header_too_small() {
110        assert!(parse_opus_header(&[0; 8]).is_err());
111    }
112
113    #[test]
114    fn parse_header_bad_magic() {
115        let mut h = opus_header(48000, 16, 2);
116        h[0] = 0xFF;
117        assert!(parse_opus_header(&h).is_err());
118    }
119
120    #[test]
121    fn create_decoder_48000_16_2() {
122        let header = CodecHeader {
123            codec: "opus".into(),
124            payload: opus_header(48000, 16, 2),
125        };
126        let dec = create(&header);
127        assert!(dec.is_ok());
128    }
129}
snapcast_client/decoder/opus.rs

snapcast_client/decoder/
opus.rs
