Merge pull request #173 from gopxl/fix-flac-seek

Fix `flac.Seek()`

flac/decode.go

@@ -1,10 +1,10 @@
 package flac
 
 import (
-	"fmt"
 	"io"
 
 	"github.com/mewkiz/flac"
+	"github.com/mewkiz/flac/frame"
 	"github.com/pkg/errors"
 
 	"github.com/gopxl/beep/v2"
@@ -32,10 +32,17 @@ func Decode(r io.Reader) (s beep.StreamSeekCloser, format beep.Format, err error
 	} else {
 		d.stream, err = flac.New(r)
 	}
+	if err != nil {
+		return nil, beep.Format{}, errors.Wrap(err, "flac")
+	}
 
+	// Read the first frame
+	d.frame, err = d.stream.ParseNext()
 	if err != nil {
 		return nil, beep.Format{}, errors.Wrap(err, "flac")
 	}
+	d.hasFixedBlockSize = d.frame.HasFixedBlockSize
+
 	format = beep.Format{
 		SampleRate:  beep.SampleRate(d.stream.Info.SampleRate),
 		NumChannels: int(d.stream.Info.NChannels),
@@ -47,96 +54,70 @@ func Decode(r io.Reader) (s beep.StreamSeekCloser, format beep.Format, err error
 type decoder struct {
 	r           io.Reader
 	stream      *flac.Stream
-	buf         [][2]float64
-	pos         int
+	frame       *frame.Frame
+	posInFrame  int
 	err         error
 	seekEnabled bool
+
+	hasFixedBlockSize bool
 }
 
 func (d *decoder) Stream(samples [][2]float64) (n int, ok bool) {
-	if d.err != nil {
+	if d.err != nil || d.frame == nil {
 		return 0, false
 	}
-	// Copy samples from buffer.
-	j := 0
-	for i := range samples {
-		if j >= len(d.buf) {
-			// refill buffer.
-			if err := d.refill(); err != nil {
-				d.pos += n
+
+	for len(samples) > 0 {
+		samplesLeft := int(d.frame.BlockSize) - d.posInFrame
+		if samplesLeft <= 0 {
+			// Read next frame
+			var err error
+			d.frame, err = d.stream.ParseNext()
+			if err != nil {
+				d.frame = nil
 				if err == io.EOF {
 					return n, n > 0
 				}
-				d.err = err
+				d.err = errors.Wrap(err, "flac")
 				return 0, false
 			}
-			j = 0
+			d.posInFrame = 0
+			continue
 		}
-		samples[i] = d.buf[j]
-		j++
-		n++
+
+		toFill := min(samplesLeft, len(samples))
+		d.decodeFrameRangeInto(d.frame, d.posInFrame, toFill, samples)
+		d.posInFrame += toFill
+		n += toFill
+		samples = samples[toFill:]
 	}
-	d.buf = d.buf[j:]
-	d.pos += n
+
 	return n, true
 }
 
-// refill decodes audio samples to fill the decode buffer.
-func (d *decoder) refill() error {
-	// Empty buffer.
-	d.buf = d.buf[:0]
-	// Parse audio frame.
-	frame, err := d.stream.ParseNext()
-	if err != nil {
-		return err
-	}
-	// Expand buffer size if needed.
-	n := len(frame.Subframes[0].Samples)
-	if cap(d.buf) < n {
-		d.buf = make([][2]float64, n)
-	} else {
-		d.buf = d.buf[:n]
-	}
-	// Decode audio samples.
+// decodeFrameRangeInto decodes the samples frame from the position `start` up to `start + num`
+// and stores them in Beep's format into the provided slice `into`.
+func (d *decoder) decodeFrameRangeInto(frame *frame.Frame, start, num int, into [][2]float64) {
 	bps := d.stream.Info.BitsPerSample
-	nchannels := d.stream.Info.NChannels
+	numChannels := d.stream.Info.NChannels
 	s := 1 << (bps - 1)
 	q := 1 / float64(s)
-	switch {
-	case bps == 8 && nchannels == 1:
-		for i := 0; i < n; i++ {
-			d.buf[i][0] = float64(int8(frame.Subframes[0].Samples[i])) * q
-			d.buf[i][1] = float64(int8(frame.Subframes[0].Samples[i])) * q
-		}
-	case bps == 16 && nchannels == 1:
-		for i := 0; i < n; i++ {
-			d.buf[i][0] = float64(int16(frame.Subframes[0].Samples[i])) * q
-			d.buf[i][1] = float64(int16(frame.Subframes[0].Samples[i])) * q
-		}
-	case bps == 24 && nchannels == 1:
-		for i := 0; i < n; i++ {
-			d.buf[i][0] = float64(int32(frame.Subframes[0].Samples[i])) * q
-			d.buf[i][1] = float64(int32(frame.Subframes[0].Samples[i])) * q
-		}
-	case bps == 8 && nchannels >= 2:
-		for i := 0; i < n; i++ {
-			d.buf[i][0] = float64(int8(frame.Subframes[0].Samples[i])) * q
-			d.buf[i][1] = float64(int8(frame.Subframes[1].Samples[i])) * q
-		}
-	case bps == 16 && nchannels >= 2:
-		for i := 0; i < n; i++ {
-			d.buf[i][0] = float64(int16(frame.Subframes[0].Samples[i])) * q
-			d.buf[i][1] = float64(int16(frame.Subframes[1].Samples[i])) * q
+
+	if numChannels == 1 {
+		samples1 := frame.Subframes[0].Samples[start:]
+		for i := 0; i < num; i++ {
+			v := float64(samples1[i]) * q
+			into[i][0] = v
+			into[i][1] = v
 		}
-	case bps == 24 && nchannels >= 2:
-		for i := 0; i < n; i++ {
-			d.buf[i][0] = float64(frame.Subframes[0].Samples[i]) * q
-			d.buf[i][1] = float64(frame.Subframes[1].Samples[i]) * q
+	} else {
+		samples1 := frame.Subframes[0].Samples[start:]
+		samples2 := frame.Subframes[1].Samples[start:]
+		for i := 0; i < num; i++ {
+			into[i][0] = float64(samples1[i]) * q
+			into[i][1] = float64(samples2[i]) * q
 		}
-	default:
-		panic(fmt.Errorf("support for %d bits-per-sample and %d channels combination not yet implemented", bps, nchannels))
 	}
-	return nil
 }
 
 func (d *decoder) Err() error {
@@ -148,18 +129,68 @@ func (d *decoder) Len() int {
 }
 
 func (d *decoder) Position() int {
-	return d.pos
+	if d.frame == nil {
+		return d.Len()
+	}
+
+	// Temporary workaround until https://github.com/mewkiz/flac/pull/73 is resolved.
+	if d.hasFixedBlockSize {
+		return int(d.frame.Num)*int(d.stream.Info.BlockSizeMax) + d.posInFrame
+	}
+
+	return int(d.frame.SampleNumber()) + d.posInFrame
 }
 
-// p represents flac sample num perhaps?
 func (d *decoder) Seek(p int) error {
 	if !d.seekEnabled {
 		return errors.New("flac.decoder.Seek: not enabled")
 	}
 
+	// Temporary workaround until https://github.com/mewkiz/flac/pull/73 is resolved.
+	// frame.SampleNumber() doesn't work for the last frame of a fixed block size stream
+	// with the result that seeking to that frame doesn't work either. Therefore, if such
+	// a seek is requested, we seek to one of the frames before it and consume until the
+	// desired position is reached.
+	if d.hasFixedBlockSize {
+		lastFrameStartLowerBound := d.Len() - int(d.stream.Info.BlockSizeMax)
+		if p >= lastFrameStartLowerBound {
+			// Seek to & consume an earlier frame.
+			_, err := d.stream.Seek(uint64(lastFrameStartLowerBound - 1))
+			if err != nil {
+				return errors.Wrap(err, "flac")
+			}
+			for {
+				d.frame, err = d.stream.ParseNext()
+				if err != nil {
+					return errors.Wrap(err, "flac")
+				}
+				// Calculate the frame start position manually, because this doesn't
+				// work for the last frame.
+				frameStart := d.frame.Num * uint64(d.stream.Info.BlockSizeMax)
+				if frameStart+uint64(d.frame.BlockSize) >= d.stream.Info.NSamples {
+					// Found the desired frame.
+					d.posInFrame = p - int(frameStart)
+					return nil
+				}
+			}
+		}
+	}
+
+	// d.stream.Seek() doesn't seek to the exact position p, instead
+	// it seeks to the start of the frame p is in. The frame position
+	// is returned and stored in pos.
 	pos, err := d.stream.Seek(uint64(p))
-	d.pos = int(pos)
-	return err
+	if err != nil {
+		return errors.Wrap(err, "flac")
+	}
+	d.posInFrame = p - int(pos)
+
+	d.frame, err = d.stream.ParseNext()
+	if err != nil {
+		return errors.Wrap(err, "flac")
+	}
+
+	return nil
 }
 
 func (d *decoder) Close() error {