Shortcut asking Decoder for current position, replace panics with errors (#43)

This change
* adds a shortcut to get the current position by Seek for performance
* replaces panics with errors so that go-mp3 behaves more gracefully

decode.go

@@ -15,7 +15,7 @@
 package mp3
 
 import (
-	"fmt"
+	"errors"
 	"io"
 
 	"github.com/hajimehoshi/go-mp3/internal/consts"
@@ -69,8 +69,17 @@ func (d *Decoder) Read(buf []byte) (int, error) {
 
 // Seek is io.Seeker's Seek.
 //
-// Seek panics when the underlying source is not io.Seeker.
+// Seek returns an error when the underlying source is not io.Seeker.
+//
+// Note that seek uses a byte offset but samples are aligned to 4 bytes (2
+// channels, 2 bytes each). Be careful to seek to an offset that is divisible by
+// 4 if you want to read at full sample boundaries.
 func (d *Decoder) Seek(offset int64, whence int) (int64, error) {
+	if offset == 0 && whence == io.SeekCurrent {
+		// Handle the special case of asking for the current position specially.
+		return d.pos, nil
+	}
+
 	npos := int64(0)
 	switch whence {
 	case io.SeekStart:
@@ -80,7 +89,7 @@ func (d *Decoder) Seek(offset int64, whence int) (int64, error) {
 	case io.SeekEnd:
 		npos = d.Length() + offset
 	default:
-		panic(fmt.Sprintf("mp3: invalid whence: %v", whence))
+		return 0, errors.New("mp3: invalid whence")
 	}
 	d.pos = npos
 	d.buf = nil

internal/frameheader/frameheader.go

@@ -15,6 +15,7 @@
 package frameheader
 
 import (
+	"errors"
 	"fmt"
 	"io"
 
@@ -49,16 +50,16 @@ func (f FrameHeader) SamplingFrequency() consts.SamplingFrequency {
 	return consts.SamplingFrequency(int(f&0x00000c00) >> 10)
 }
 
-func (f FrameHeader) SamplingFrequencyValue() int {
+func (f FrameHeader) SamplingFrequencyValue() (int, error) {
 	switch f.SamplingFrequency() {
 	case 0:
-		return 44100 >> uint(f.LowSamplingFrequency())
+		return 44100 >> uint(f.LowSamplingFrequency()), nil
 	case 1:
-		return 48000 >> uint(f.LowSamplingFrequency())
+		return 48000 >> uint(f.LowSamplingFrequency()), nil
 	case 2:
-		return 32000 >> uint(f.LowSamplingFrequency())
+		return 32000 >> uint(f.LowSamplingFrequency()), nil
 	}
-	panic("not reached")
+	return 0, errors.New("mp3: frame header has invalid sample frequency")
 }
 
 // PaddingBit returns the padding bit stored in position 9
@@ -185,10 +186,14 @@ func (f FrameHeader) Bitrate() int {
 	return bitrates[f.LowSamplingFrequency()][f.Layer()-1][f.BitrateIndex()]
 }
 
-func (f FrameHeader) FrameSize() int {
-	return ((144*f.Bitrate())/
-		f.SamplingFrequencyValue() +
+func (f FrameHeader) FrameSize() (int, error) {
+	freq, err := f.SamplingFrequencyValue()
+	if err != nil {
+		return 0, err
+	}
+	size := ((144*f.Bitrate())/freq +
 		int(f.PaddingBit())) >> uint(f.LowSamplingFrequency())
+	return size, nil
 }
 
 func (f FrameHeader) SideInfoSize() int {

internal/sideinfo/sideinfo.go

@@ -64,7 +64,10 @@ var sideInfoBitsToRead = [2][4]int{
 
 func Read(source FullReader, header frameheader.FrameHeader) (*SideInfo, error) {
 	nch := header.NumberOfChannels()
-	framesize := header.FrameSize()
+	framesize, err := header.FrameSize()
+	if err != nil {
+		return nil, err
+	}
 	if framesize > 2000 {
 		return nil, fmt.Errorf("mp3: framesize = %d\n", framesize)
 	}

source.go

@@ -15,6 +15,7 @@
 package mp3
 
 import (
+	"errors"
 	"io"
 )
 
@@ -27,7 +28,7 @@ type source struct {
 func (s *source) Seek(position int64, whence int) (int64, error) {
 	seeker, ok := s.reader.(io.Seeker)
 	if !ok {
-		panic("mp3: source must be io.Seeker")
+		return 0, errors.New("mp3: source must be io.Seeker")
 	}
 	s.buf = nil
 	n, err := seeker.Seek(position, whence)