ingest: add native mp3 decoder

go.mod

@@ -4,4 +4,6 @@ go 1.22
 
 require github.com/jan/fm-rds-tx/internal v0.0.0
 
+require github.com/hajimehoshi/go-mp3 v0.3.4 // indirect
+
 replace github.com/jan/fm-rds-tx/internal => ./internal

go.sum

@@ -0,0 +1,4 @@
+github.com/hajimehoshi/go-mp3 v0.3.4 h1:NUP7pBYH8OguP4diaTZ9wJbUbk3tC0KlfzsEpWmYj68=
+github.com/hajimehoshi/go-mp3 v0.3.4/go.mod h1:fRtZraRFcWb0pu7ok0LqyFhCUrPeMsGRSVop0eemFmo=
+github.com/hajimehoshi/oto/v2 v2.3.1/go.mod h1:seWLbgHH7AyUMYKfKYT9pg7PhUu9/SisyJvNTT+ASQo=
+golang.org/x/sys v0.0.0-20220712014510-0a85c31ab51e/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=

internal/go.mod

@@ -1,3 +1,5 @@
 module github.com/jan/fm-rds-tx/internal
 
 go 1.21
+
+require github.com/hajimehoshi/go-mp3 v0.3.4

internal/go.sum

@@ -0,0 +1,4 @@
+github.com/hajimehoshi/go-mp3 v0.3.4 h1:NUP7pBYH8OguP4diaTZ9wJbUbk3tC0KlfzsEpWmYj68=
+github.com/hajimehoshi/go-mp3 v0.3.4/go.mod h1:fRtZraRFcWb0pu7ok0LqyFhCUrPeMsGRSVop0eemFmo=
+github.com/hajimehoshi/oto/v2 v2.3.1/go.mod h1:seWLbgHH7AyUMYKfKYT9pg7PhUu9/SisyJvNTT+ASQo=
+golang.org/x/sys v0.0.0-20220712014510-0a85c31ab51e/go.mod h1:oPkhp1MJrh7nUepCBck5+mAzfO9JrbApNNgaTdGDITg=

internal/ingest/decoder/mp3/decoder.go

@@ -2,9 +2,12 @@ package mp3
 
 import (
 	"context"
+	"encoding/binary"
 	"fmt"
 	"io"
+	"time"
 
+	gomp3 "github.com/hajimehoshi/go-mp3"
 	"github.com/jan/fm-rds-tx/internal/ingest"
 	"github.com/jan/fm-rds-tx/internal/ingest/decoder"
 )
@@ -15,6 +18,73 @@ func New() *Decoder { return &Decoder{} }
 
 func (d *Decoder) Name() string { return "mp3-native" }
 
-func (d *Decoder) DecodeStream(_ context.Context, _ io.Reader, _ decoder.StreamMeta, _ func(ingest.PCMChunk) error) error {
-	return fmt.Errorf("%w: mp3 native decoder not wired yet", decoder.ErrUnsupported)
+func (d *Decoder) DecodeStream(ctx context.Context, r io.Reader, meta decoder.StreamMeta, emit func(ingest.PCMChunk) error) error {
+	if r == nil {
+		return fmt.Errorf("%w: mp3 decoder stream reader is nil", decoder.ErrUnsupported)
+	}
+	if emit == nil {
+		return fmt.Errorf("%w: mp3 decoder emit callback is nil", decoder.ErrUnsupported)
+	}
+
+	dec, err := gomp3.NewDecoder(r)
+	if err != nil {
+		return fmt.Errorf("%w: mp3 decoder init: %v", decoder.ErrUnsupported, err)
+	}
+
+	const channels = 2 // go-mp3 always decodes to stereo s16le
+	sampleRate := dec.SampleRate()
+	if sampleRate <= 0 {
+		if meta.SampleRateHz > 0 {
+			sampleRate = meta.SampleRateHz
+		} else {
+			sampleRate = 44100
+		}
+	}
+
+	const chunkFrames = 1024
+	const frameBytes = channels * 2
+	buf := make([]byte, chunkFrames*frameBytes)
+	seq := uint64(0)
+
+	for {
+		select {
+		case <-ctx.Done():
+			return nil
+		default:
+		}
+
+		n, readErr := io.ReadAtLeast(dec, buf, frameBytes)
+		if readErr != nil {
+			if readErr == io.EOF || readErr == io.ErrUnexpectedEOF {
+				if n > 0 {
+					if err := emitChunk(buf[:n], seq, sampleRate, meta.SourceID, emit); err != nil {
+						return err
+					}
+				}
+				return nil
+			}
+			return fmt.Errorf("mp3 decoder read pcm: %w", readErr)
+		}
+
+		if err := emitChunk(buf[:n], seq, sampleRate, meta.SourceID, emit); err != nil {
+			return err
+		}
+		seq++
+	}
+}
+
+func emitChunk(data []byte, seq uint64, sampleRate int, sourceID string, emit func(ingest.PCMChunk) error) error {
+	samples := make([]int32, 0, len(data)/2)
+	for i := 0; i+1 < len(data); i += 2 {
+		v := int16(binary.LittleEndian.Uint16(data[i : i+2]))
+		samples = append(samples, int32(v)<<16)
+	}
+	return emit(ingest.PCMChunk{
+		Samples:      samples,
+		Channels:     2,
+		SampleRateHz: sampleRate,
+		Sequence:     seq,
+		Timestamp:    time.Now(),
+		SourceID:     sourceID,
+	})
 }

internal/ingest/decoder/mp3/decoder_test.go

@@ -0,0 +1,60 @@
+package mp3
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/jan/fm-rds-tx/internal/ingest"
+	"github.com/jan/fm-rds-tx/internal/ingest/decoder"
+)
+
+func TestDecodeStream(t *testing.T) {
+	tonePath := filepath.Join("testdata", "tone_44k_stereo.mp3")
+	data, err := os.ReadFile(tonePath)
+	if err != nil {
+		t.Fatalf("read fixture: %v", err)
+	}
+
+	var chunks []ingest.PCMChunk
+	d := New()
+	err = d.DecodeStream(context.Background(), bytes.NewReader(data), decoder.StreamMeta{
+		ContentType: "audio/mpeg",
+		SourceID:    "mp3-test",
+	}, func(c ingest.PCMChunk) error {
+		chunks = append(chunks, c)
+		return nil
+	})
+	if err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(chunks) == 0 {
+		t.Fatal("expected chunks")
+	}
+	if chunks[0].Channels != 2 {
+		t.Fatalf("channels=%d want 2", chunks[0].Channels)
+	}
+	if chunks[0].SampleRateHz != 44100 {
+		t.Fatalf("sampleRate=%d want 44100", chunks[0].SampleRateHz)
+	}
+	if len(chunks[0].Samples) == 0 {
+		t.Fatal("expected samples in first chunk")
+	}
+}
+
+func TestDecodeStreamNilReader(t *testing.T) {
+	err := New().DecodeStream(context.Background(), nil, decoder.StreamMeta{}, func(ingest.PCMChunk) error { return nil })
+	if !errors.Is(err, decoder.ErrUnsupported) {
+		t.Fatalf("expected unsupported, got %v", err)
+	}
+}
+
+func TestDecodeStreamNilEmit(t *testing.T) {
+	err := New().DecodeStream(context.Background(), bytes.NewReader([]byte("not-mp3")), decoder.StreamMeta{}, nil)
+	if !errors.Is(err, decoder.ErrUnsupported) {
+		t.Fatalf("expected unsupported, got %v", err)
+	}
+}