flac: add preliminary flac decoder

As noted in zikichombo#3 (comment)
there are some bugs to be ironed out.

flac/decoder.go

@@ -0,0 +1,109 @@
+// TODO: add seek support; only enabled in the dev branch of mewkiz/flac.
+
+package flac
+
+import (
+	"io"
+
+	"github.com/mewkiz/flac"
+	"github.com/mewkiz/flac/frame"
+	"zikichombo.org/sound"
+	"zikichombo.org/sound/freq"
+)
+
+// Decoder encapsulates state for decoding and seeking a FLAC audio stream.
+type Decoder struct {
+	stream *flac.Stream
+	frame  *frame.Frame // current frame.
+	i      int          // index of current sample in subframe(s).
+}
+
+// NewDecoder creates a decoder from a FLAC audio stream (seekable, readable).
+func NewDecoder(r io.Reader) (*Decoder, error) {
+	stream, err := flac.New(r)
+	if err != nil {
+		return nil, err
+	}
+	d := &Decoder{
+		stream: stream,
+	}
+	return d, nil
+}
+
+func (d *Decoder) Receive(dst []float64) (int, error) {
+	if len(dst)%d.Channels() != 0 {
+		return 0, sound.ChannelAlignmentError
+	}
+	n := 0                       // number of frames (samples per channel) buffered.
+	m := len(dst) / d.Channels() // number of frames (samples per channel) to buffer.
+	bps := int(d.stream.Info.BitsPerSample)
+	for n < m {
+		if d.frame == nil {
+			frame, err := d.stream.ParseNext()
+			if err != nil {
+				return n, err
+			}
+			d.frame = frame
+			d.i = 0
+		}
+		samplesLeft := len(d.frame.Subframes[0].Samples[d.i:])
+		j := m
+		if j > samplesLeft {
+			j = samplesLeft
+		}
+		if n+j > m {
+			j = m - n
+		}
+		if j == 0 {
+			panic("cannot retrieve zero samples")
+		}
+		for c := 0; c < d.Channels(); c++ {
+			toFloats(dst[c*m:c*m+j], d.frame.Subframes[c].Samples[d.i:d.i+j], bps)
+		}
+		d.i += j
+		n += j
+		if len(d.frame.Subframes[0].Samples[d.i:]) == 0 {
+			d.frame = nil
+			d.i = 0
+		}
+	}
+	if n < m {
+		// TODO: handle n < m. Move frames in dst to their correct position
+		//
+		//    dst[c*n:(c+1)*n]
+		panic("flac.Decoder.Receive: reached end of stream with n < m; support for this case is not yet implemented;\n\nThe implementation should move frames (in ziki terminology) in dst to their correct position, i.e.\n\n   dst[c*n:(c+1)*n]")
+	}
+	return n, nil
+}
+
+func (d *Decoder) Channels() int {
+	return int(d.stream.Info.NChannels)
+}
+
+func (d *Decoder) SampleRate() freq.T {
+	return freq.T(d.stream.Info.SampleRate) * freq.Hertz
+}
+
+func (d *Decoder) Close() error {
+	return d.stream.Close()
+}
+
+// ### [ Helper functions ] ####################################################
+
+// Copied from sound/sample/fix.go and adjusted from int64 to int32.
+
+func toFloat(d int32, nBits int) float64 {
+	s := float64(int32(1 << uint(nBits-1)))
+	return float64(d) / s
+}
+
+func toFloats(dst []float64, src []int32, nBits int) []float64 {
+	if cap(dst) < len(src) {
+		panic("capacity of dst buffer too small")
+	}
+	dst = dst[:len(src)]
+	for i, v := range src {
+		dst[i] = toFloat(v, nBits)
+	}
+	return dst
+}

flac/doc.go

@@ -0,0 +1,2 @@
+// Package flac provides decoding of FLAC audio streams.
+package flac /* import "zikichombo.org/codec/flac" */