snapcast_client/decoder/
flac.rs

1//! FLAC decoder using symphonia.
2//!
3//! The snapserver sends a FLAC stream header (fLaC + STREAMINFO) as the CodecHeader,
4//! then individual FLAC frames as WireChunk payloads.
5
6use anyhow::{Result, bail};
7use snapcast_proto::SampleFormat;
8use snapcast_proto::message::codec_header::CodecHeader;
9use symphonia::core::audio::SampleBuffer;
10use symphonia::core::codecs::{CODEC_TYPE_FLAC, CodecParameters, DecoderOptions};
11use symphonia::core::formats::Packet;
12
13use crate::decoder::Decoder;
14
15/// Parse FLAC STREAMINFO from a codec header payload.
16///
17/// Layout: "fLaC" (4) + block_header (4) + STREAMINFO (34)
18/// STREAMINFO bytes 10-13 contain: sample_rate (20 bits) | channels-1 (3 bits) | bps-1 (5 bits)
19fn parse_streaminfo(payload: &[u8]) -> Result<(SampleFormat, CodecParameters)> {
20    if payload.len() < 42 {
21        bail!(
22            "FLAC header too small ({} bytes, need >= 42)",
23            payload.len()
24        );
25    }
26    if &payload[0..4] != b"fLaC" {
27        bail!("not a FLAC header (missing fLaC magic)");
28    }
29
30    // STREAMINFO starts at offset 8 (after 4-byte magic + 4-byte block header)
31    let si = &payload[8..];
32
33    // Bytes 10-13 of STREAMINFO (offsets 10..14 from si start):
34    // 20 bits sample rate | 3 bits (channels-1) | 5 bits (bps-1) | 36 bits total samples
35    let b10 = si[10] as u32;
36    let b11 = si[11] as u32;
37    let b12 = si[12] as u32;
38
39    let sample_rate = (b10 << 12) | (b11 << 4) | (b12 >> 4);
40    let channels = ((b12 >> 1) & 0x07) + 1;
41    let bits_per_sample = (((b12 & 0x01) << 4) | (si[13] as u32 >> 4)) + 1;
42
43    let sf = SampleFormat::new(sample_rate, bits_per_sample as u16, channels as u16);
44
45    let mut params = CodecParameters::new();
46    params
47        .for_codec(CODEC_TYPE_FLAC)
48        .with_sample_rate(sample_rate)
49        .with_bits_per_sample(bits_per_sample)
50        .with_channels(
51            symphonia::core::audio::Channels::from_bits(((1u64 << channels) - 1) as u32)
52                .unwrap_or(symphonia::core::audio::Channels::FRONT_LEFT),
53        )
54        .with_extra_data(payload[8..42].to_vec().into_boxed_slice());
55
56    Ok((sf, params))
57}
58
59/// FLAC audio decoder using symphonia.
60pub struct FlacDecoder {
61    decoder: Box<dyn symphonia::core::codecs::Decoder>,
62    sample_format: SampleFormat,
63    packet_id: u64,
64}
65
66impl FlacDecoder {
67    fn new_from_params(sf: SampleFormat, params: &CodecParameters) -> Result<Self> {
68        let decoder = symphonia::default::get_codecs()
69            .make(params, &DecoderOptions::default())
70            .map_err(|e| anyhow::anyhow!("failed to create FLAC decoder: {e}"))?;
71        Ok(Self {
72            decoder,
73            sample_format: sf,
74            packet_id: 0,
75        })
76    }
77}
78
79impl Decoder for FlacDecoder {
80    fn set_header(&mut self, header: &CodecHeader) -> Result<SampleFormat> {
81        tracing::trace!(
82            codec = "flac",
83            payload_len = header.payload.len(),
84            "set_header"
85        );
86        let (sf, params) = parse_streaminfo(&header.payload)?;
87        *self = Self::new_from_params(sf, &params)?;
88        Ok(self.sample_format)
89    }
90
91    fn decode(&mut self, data: &mut Vec<u8>) -> Result<bool> {
92        if data.is_empty() {
93            return Ok(false);
94        }
95
96        tracing::trace!(
97            codec = "flac",
98            input_bytes = data.len(),
99            packet_id = self.packet_id,
100            "decode"
101        );
102
103        let packet = Packet::new_from_slice(0, self.packet_id, 0, data);
104        self.packet_id += 1;
105
106        let decoded = match self.decoder.decode(&packet) {
107            Ok(buf) => buf,
108            Err(e) => {
109                tracing::warn!(codec = "flac", error = %e, "decode failed");
110                return Ok(false);
111            }
112        };
113
114        let spec = *decoded.spec();
115        let frames = decoded.frames() as u64;
116
117        let mut sample_buf = SampleBuffer::<i16>::new(frames, spec);
118        sample_buf.copy_interleaved_ref(decoded);
119        let samples = sample_buf.samples();
120
121        let mut out = Vec::with_capacity(samples.len() * 2);
122        for &s in samples {
123            out.extend_from_slice(&s.to_le_bytes());
124        }
125
126        *data = out;
127        Ok(true)
128    }
129}
130
131/// Create a FlacDecoder from a CodecHeader (convenience for factory use).
132pub fn create(header: &CodecHeader) -> Result<FlacDecoder> {
133    let (sf, params) = parse_streaminfo(&header.payload)?;
134    FlacDecoder::new_from_params(sf, &params)
135}
136
137#[cfg(test)]
138mod tests {
139    use super::*;
140
141    /// Build a minimal valid FLAC header: fLaC + STREAMINFO block
142    /// for 44100 Hz, 16-bit, 2 channels
143    fn flac_header_44100_16_2() -> Vec<u8> {
144        let mut h = Vec::new();
145        h.extend_from_slice(b"fLaC");
146        // Block header: type=0 (STREAMINFO), last=1 → 0x80, length=34
147        h.push(0x80);
148        h.extend_from_slice(&[0x00, 0x00, 0x22]); // length = 34
149
150        // STREAMINFO (34 bytes), all multi-byte fields are BIG-endian:
151        h.extend_from_slice(&[0x00, 0x10]); // min block size = 16
152        h.extend_from_slice(&[0x10, 0x00]); // max block size = 4096
153        h.extend_from_slice(&[0x00, 0x00, 0x00]); // min frame size
154        h.extend_from_slice(&[0x00, 0x00, 0x00]); // max frame size
155
156        // Bytes 10-13: sample_rate(20) | channels-1(3) | bps-1(5) | total_samples high 4 bits
157        // 44100 = 0xAC44
158        // byte10 = 0xAC44 >> 12 = 0x0A
159        // byte11 = (0xAC44 >> 4) & 0xFF = 0xC4
160        // byte12 = ((0xAC44 & 0x0F) << 4) | ((2-1) << 1) | ((16-1) >> 4)
161        //        = (0x04 << 4) | (1 << 1) | (15 >> 4) = 0x40 | 0x02 | 0x00 = 0x42
162        // byte13 = ((16-1) & 0x0F) << 4 = 0xF0
163        h.push(0x0A);
164        h.push(0xC4);
165        h.push(0x42);
166        h.push(0xF0);
167
168        // Remaining 4 bytes of total_samples + 16 bytes MD5
169        h.extend_from_slice(&[0; 20]);
170
171        assert_eq!(h.len(), 42);
172        h
173    }
174
175    #[test]
176    fn parse_streaminfo_44100_16_2() {
177        let payload = flac_header_44100_16_2();
178        let (sf, _params) = parse_streaminfo(&payload).unwrap();
179        assert_eq!(sf.rate(), 44100);
180        assert_eq!(sf.bits(), 16);
181        assert_eq!(sf.channels(), 2);
182    }
183
184    #[test]
185    fn parse_streaminfo_48000_24_2() {
186        let mut h = Vec::new();
187        h.extend_from_slice(b"fLaC");
188        h.push(0x80);
189        h.extend_from_slice(&[0x00, 0x00, 0x22]);
190        h.extend_from_slice(&[0x10, 0x00]); // min block
191        h.extend_from_slice(&[0x10, 0x00]); // max block
192        h.extend_from_slice(&[0; 6]); // frame sizes
193
194        // 48000 = 0xBB80
195        // byte10 = 0x0B, byte11 = 0xB8, byte12 = 0x02 | (1<<1) | (23>>4) = 0x02|0x02|0x01 = 0x03
196        // Wait: channels-1=1, bps-1=23
197        // byte12 = (0x00 << 4) | (1 << 1) | (23 >> 4) = 0x00 | 0x02 | 0x01 = 0x03
198        // byte13 = (23 & 0x0F) << 4 = 0x70
199        h.push(0x0B);
200        h.push(0xB8);
201        h.push(0x03);
202        h.push(0x70);
203        h.extend_from_slice(&[0; 20]);
204
205        let (sf, _) = parse_streaminfo(&h).unwrap();
206        assert_eq!(sf.rate(), 48000);
207        assert_eq!(sf.bits(), 24);
208        assert_eq!(sf.channels(), 2);
209    }
210
211    #[test]
212    fn parse_streaminfo_too_small() {
213        assert!(parse_streaminfo(&[0; 10]).is_err());
214    }
215
216    #[test]
217    fn parse_streaminfo_bad_magic() {
218        let mut h = vec![0u8; 42];
219        h[0..4].copy_from_slice(b"NOPE");
220        assert!(parse_streaminfo(&h).is_err());
221    }
222
223    #[test]
224    fn create_decoder_from_header() {
225        let header = CodecHeader {
226            codec: "flac".into(),
227            payload: flac_header_44100_16_2(),
228        };
229        let dec = create(&header);
230        if let Err(e) = &dec {
231            eprintln!("Error: {e:?}");
232        }
233        let dec = dec.unwrap();
234        assert_eq!(dec.sample_format.rate(), 44100);
235    }
236}
snapcast_client/decoder/flac.rs

snapcast_client/decoder/
flac.rs
