Implement seek

flac.go

@@ -29,6 +29,7 @@ package flac
 import (
 	"bufio"
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -47,6 +48,15 @@ type Stream struct {
 	Info *meta.StreamInfo
 	// Zero or more metadata blocks.
 	Blocks []*meta.Block
+
+	// seekTable contains one or more pre-calculated audio frame seek points of the stream; nil if uninitialized.
+	seekTable *meta.SeekTable
+	// seekTableSize determines how many seek points the seekTable should have if the flac file does not include one
+	// in the metadata.
+	seekTableSize int
+	// dataStart is the offset of the first frame header since SeekPoint.Offset is relative to this position.
+	dataStart int64
+
 	// Underlying io.Reader.
 	r io.Reader
 	// Underlying io.Closer of file if opened with Open and ParseFile, and nil
@@ -64,71 +74,118 @@ func New(r io.Reader) (stream *Stream, err error) {
 	// Verify FLAC signature and parse the StreamInfo metadata block.
 	br := bufio.NewReader(r)
 	stream = &Stream{r: br}
-	isLast, err := stream.parseStreamInfo()
+	block, err := stream.parseStreamInfo()
 	if err != nil {
 		return nil, err
 	}
 
 	// Skip the remaining metadata blocks.
-	for !isLast {
-		block, err := meta.New(br)
+	for !block.IsLast {
+		block, err = meta.New(br)
 		if err != nil && err != meta.ErrReservedType {
 			return stream, err
 		}
 		if err = block.Skip(); err != nil {
 			return stream, err
 		}
-		isLast = block.IsLast
 	}
 
 	return stream, nil
 }
 
-// flacSignature marks the beginning of a FLAC stream.
-var flacSignature = []byte("fLaC")
+// NewSeek returns a Stream that has seeking enabled.  The incoming
+// io.ReadSeeker will not be buffered, which might result in performance issues.
+// Using an in-memory buffer like *bytes.Reader should work well.
+func NewSeek(r io.Reader) (stream *Stream, err error) {
+	rs, ok := r.(io.ReadSeeker)
+	if !ok {
+		return stream, ErrNoSeeker
+	}
+
+	stream = &Stream{r: rs, seekTableSize: defaultSeekTableSize}
+
+	// Verify FLAC signature and parse the StreamInfo metadata block.
+	block, err := stream.parseStreamInfo()
+	if err != nil {
+		return stream, err
+	}
+
+	for !block.IsLast {
+		block, err = meta.Parse(stream.r)
+		if err != nil {
+			if err != meta.ErrReservedType {
+				return stream, err
+			} else {
+				if err = block.Skip(); err != nil {
+					return stream, err
+				}
+			}
+		}
+
+		if block.Header.Type == meta.TypeSeekTable {
+			stream.seekTable = block.Body.(*meta.SeekTable)
+		}
+	}
+
+	// Record file offset of the first frame header.
+	stream.dataStart, err = rs.Seek(0, io.SeekCurrent)
+	return stream, err
+}
+
+var (
+	// flacSignature marks the beginning of a FLAC stream.
+	flacSignature = []byte("fLaC")
+
+	// id3Signature marks the beginning of an ID3 stream, used to skip over ID3 data.
+	id3Signature = []byte("ID3")
+
+	ErrInvalidSeek = errors.New("stream.Seek: out of stream seek")
+	ErrNoSeeker    = errors.New("stream.Seek: not a Seeker")
+)
 
-// id3Signature marks the beginning of an ID3 stream, used to skip over ID3 data.
-var id3Signature = []byte("ID3")
+const (
+	defaultSeekTableSize = 100
+)
 
 // parseStreamInfo verifies the signature which marks the beginning of a FLAC
 // stream, and parses the StreamInfo metadata block. It returns a boolean value
 // which specifies if the StreamInfo block was the last metadata block of the
 // FLAC stream.
-func (stream *Stream) parseStreamInfo() (isLast bool, err error) {
+func (stream *Stream) parseStreamInfo() (block *meta.Block, err error) {
 	// Verify FLAC signature.
 	r := stream.r
 	var buf [4]byte
 	if _, err = io.ReadFull(r, buf[:]); err != nil {
-		return false, err
+		return block, err
 	}
 
 	// Skip prepended ID3v2 data.
 	if bytes.Equal(buf[:3], id3Signature) {
 		if err := stream.skipID3v2(); err != nil {
-			return false, err
+			return block, err
 		}
 
 		// Second attempt at verifying signature.
 		if _, err = io.ReadFull(r, buf[:]); err != nil {
-			return false, err
+			return block, err
 		}
 	}
 
 	if !bytes.Equal(buf[:], flacSignature) {
-		return false, fmt.Errorf("flac.parseStreamInfo: invalid FLAC signature; expected %q, got %q", flacSignature, buf)
+		return block, fmt.Errorf("flac.parseStreamInfo: invalid FLAC signature; expected %q, got %q", flacSignature, buf)
 	}
 
 	// Parse StreamInfo metadata block.
-	block, err := meta.Parse(r)
+	block, err = meta.Parse(r)
 	if err != nil {
-		return false, err
+		return block, err
 	}
 	si, ok := block.Body.(*meta.StreamInfo)
 	if !ok {
-		return false, fmt.Errorf("flac.parseStreamInfo: incorrect type of first metadata block; expected *meta.StreamInfo, got %T", si)
+		return block, fmt.Errorf("flac.parseStreamInfo: incorrect type of first metadata block; expected *meta.StreamInfo, got %T", si)
 	}
 	stream.Info = si
-	return block.IsLast, nil
+	return block, nil
 }
 
 // skipID3v2 skips ID3v2 data prepended to flac files.
@@ -161,14 +218,14 @@ func Parse(r io.Reader) (stream *Stream, err error) {
 	// Verify FLAC signature and parse the StreamInfo metadata block.
 	br := bufio.NewReader(r)
 	stream = &Stream{r: br}
-	isLast, err := stream.parseStreamInfo()
+	block, err := stream.parseStreamInfo()
 	if err != nil {
 		return nil, err
 	}
 
 	// Parse the remaining metadata blocks.
-	for !isLast {
-		block, err := meta.Parse(br)
+	for !block.IsLast {
+		block, err = meta.Parse(br)
 		if err != nil {
 			if err != meta.ErrReservedType {
 				return stream, err
@@ -182,7 +239,6 @@ func Parse(r io.Reader) (stream *Stream, err error) {
 			}
 		}
 		stream.Blocks = append(stream.Blocks, block)
-		isLast = block.IsLast
 	}
 
 	return stream, nil
@@ -201,6 +257,7 @@ func Open(path string) (stream *Stream, err error) {
 	if err != nil {
 		return nil, err
 	}
+
 	stream, err = New(f)
 	if err != nil {
 		return nil, err
@@ -253,7 +310,142 @@ func (stream *Stream) ParseNext() (f *frame.Frame, err error) {
 	return frame.Parse(stream.r)
 }
 
-// Seek is not implement yet.
-func (stream *Stream) Seek(offset int64, whence int) (read int64, err error) {
-	return 0, nil
+// Seek to a specific sample number in the flac stream.
+//
+// sample is valid if:
+// whence == io.SeekEnd and sample is negative
+// whence == io.SeekStart and sample is positive
+// whence == io.SeekCurrent and sample + current sample > 0 and < stream.Info.NSamples
+//
+// If sample does not match one of the above conditions then the result will
+// probably be seeking to the beginning or very end of the data and no error
+// will be returned.
+//
+// The returned value, result, represents the closest match to sampleNum from the seek table.
+// Note that result will always be >= sampleNum
+func (stream *Stream) Seek(sampleNum int64, whence int) (result int64, err error) {
+	if stream.seekTable == nil && stream.seekTableSize > 0 {
+		if err := stream.makeSeekTable(); err != nil {
+			return 0, err
+		}
+	}
+
+	rs := stream.r.(io.ReadSeeker)
+
+	var point meta.SeekPoint
+	switch whence {
+	case io.SeekStart:
+		point = stream.searchFromStart(sampleNum)
+	case io.SeekCurrent:
+		point, err = stream.searchFromCurrent(sampleNum, rs)
+	case io.SeekEnd:
+		point = stream.searchFromEnd(sampleNum)
+	default:
+		return 0, ErrInvalidSeek
+	}
+
+	if err != nil {
+		return 0, err
+	}
+
+	_, err = rs.Seek(stream.dataStart+int64(point.Offset), io.SeekStart)
+	return int64(point.SampleNum), err
+}
+
+func (stream *Stream) searchFromCurrent(sample int64, rs io.ReadSeeker) (p meta.SeekPoint, err error) {
+	o, err := rs.Seek(0, io.SeekCurrent)
+	if err != nil {
+		return p, err
+	}
+
+	offset := o - stream.dataStart
+	for _, p = range stream.seekTable.Points {
+		if int64(p.Offset) >= offset {
+			return stream.searchFromStart(int64(p.SampleNum) + sample), nil
+		}
+	}
+	return p, nil
+}
+
+// searchFromEnd expects sample to be negative.
+// If it is positive, it's ok, the last seek point will be returned.
+func (stream *Stream) searchFromEnd(sample int64) (p meta.SeekPoint) {
+	return stream.searchFromStart(int64(stream.Info.NSamples) + sample)
+}
+
+func (stream *Stream) searchFromStart(sample int64) (p meta.SeekPoint) {
+	var last meta.SeekPoint
+	var i int
+	for i, p = range stream.seekTable.Points {
+		if int64(p.SampleNum) >= sample {
+			if i == 0 {
+				return p
+			}
+			return last
+		}
+		last = p
+	}
+	return p
+}
+
+func (stream *Stream) makeSeekTable() (err error) {
+	rs, ok := stream.r.(io.ReadSeeker)
+	if !ok {
+		return ErrNoSeeker
+	}
+
+	pos, err := rs.Seek(0, io.SeekCurrent)
+	if err != nil {
+		return err
+	}
+
+	_, err = rs.Seek(stream.dataStart, io.SeekStart)
+	if err != nil {
+		return err
+	}
+
+	var i int
+	var sampleNum uint64
+	var tmp []meta.SeekPoint
+	for {
+		f, err := stream.ParseNext()
+		if err == io.EOF {
+			break
+		}
+
+		if err != nil {
+			return err
+		}
+
+		o, err := rs.Seek(0, io.SeekCurrent)
+		if err != nil {
+			return err
+		}
+
+		tmp = append(tmp, meta.SeekPoint{
+			SampleNum: sampleNum,
+			Offset:    uint64(o - stream.dataStart),
+			NSamples:  f.BlockSize,
+		})
+
+		sampleNum += uint64(f.BlockSize)
+		i++
+	}
+
+	// reduce the number of seek points down to the specified resolution
+	m := 1
+	if len(tmp) > stream.seekTableSize {
+		m = len(tmp) / stream.seekTableSize
+	}
+	points := make([]meta.SeekPoint, 0, stream.seekTableSize+1)
+	for i, p := range tmp {
+		if i%m == 0 {
+			points = append(points, p)
+		}
+	}
+
+	stream.seekTable = &meta.SeekTable{Points: points}
+
+	_, err = rs.Seek(pos, io.SeekStart)
+	return err
 }