232 lines
5.2 KiB
Go
232 lines
5.2 KiB
Go
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// Copyright 2017 Hajime Hoshi
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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package mp3
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import (
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"errors"
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"io"
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"github.com/hajimehoshi/go-mp3/internal/consts"
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"github.com/hajimehoshi/go-mp3/internal/frame"
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"github.com/hajimehoshi/go-mp3/internal/frameheader"
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)
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// A Decoder is a MP3-decoded stream.
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//
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// Decoder decodes its underlying source on the fly.
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type Decoder struct {
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source *source
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sampleRate int
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length int64
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frameStarts []int64
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buf []byte
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frame *frame.Frame
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pos int64
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bytesPerFrame int64
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}
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func (d *Decoder) readFrame() error {
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var err error
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d.frame, _, err = frame.Read(d.source, d.source.pos, d.frame)
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if err != nil {
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if err == io.EOF {
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return io.EOF
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}
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if _, ok := err.(*consts.UnexpectedEOF); ok {
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// TODO: Log here?
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return io.EOF
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}
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return err
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}
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d.buf = append(d.buf, d.frame.Decode()...)
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return nil
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}
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// Read is io.Reader's Read.
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func (d *Decoder) Read(buf []byte) (int, error) {
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for len(d.buf) == 0 {
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if err := d.readFrame(); err != nil {
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return 0, err
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}
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}
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n := copy(buf, d.buf)
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d.buf = d.buf[n:]
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d.pos += int64(n)
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return n, nil
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}
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// Seek is io.Seeker's Seek.
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//
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// Seek returns an error when the underlying source is not io.Seeker.
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//
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// Note that seek uses a byte offset but samples are aligned to 4 bytes (2
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// channels, 2 bytes each). Be careful to seek to an offset that is divisible by
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// 4 if you want to read at full sample boundaries.
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func (d *Decoder) Seek(offset int64, whence int) (int64, error) {
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if offset == 0 && whence == io.SeekCurrent {
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// Handle the special case of asking for the current position specially.
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return d.pos, nil
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}
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npos := int64(0)
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switch whence {
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case io.SeekStart:
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npos = offset
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case io.SeekCurrent:
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npos = d.pos + offset
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case io.SeekEnd:
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npos = d.Length() + offset
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default:
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return 0, errors.New("mp3: invalid whence")
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}
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d.pos = npos
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d.buf = nil
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d.frame = nil
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f := d.pos / d.bytesPerFrame
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// If the frame is not first, read the previous ahead of reading that
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// because the previous frame can affect the targeted frame.
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if f > 0 {
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f--
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if _, err := d.source.Seek(d.frameStarts[f], 0); err != nil {
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return 0, err
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}
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if err := d.readFrame(); err != nil {
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return 0, err
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}
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if err := d.readFrame(); err != nil {
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return 0, err
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}
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d.buf = d.buf[d.bytesPerFrame+(d.pos%d.bytesPerFrame):]
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} else {
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if _, err := d.source.Seek(d.frameStarts[f], 0); err != nil {
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return 0, err
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}
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if err := d.readFrame(); err != nil {
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return 0, err
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}
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d.buf = d.buf[d.pos:]
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}
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return npos, nil
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}
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// SampleRate returns the sample rate like 44100.
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//
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// Note that the sample rate is retrieved from the first frame.
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func (d *Decoder) SampleRate() int {
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return d.sampleRate
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}
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func (d *Decoder) ensureFrameStartsAndLength() error {
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if d.length != invalidLength {
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return nil
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}
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if _, ok := d.source.reader.(io.Seeker); !ok {
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return nil
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}
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// Keep the current position.
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pos, err := d.source.Seek(0, io.SeekCurrent)
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if err != nil {
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return err
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}
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if err := d.source.rewind(); err != nil {
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return err
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}
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if err := d.source.skipTags(); err != nil {
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return err
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}
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l := int64(0)
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for {
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h, pos, err := frameheader.Read(d.source, d.source.pos)
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if err != nil {
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if err == io.EOF {
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break
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}
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if _, ok := err.(*consts.UnexpectedEOF); ok {
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// TODO: Log here?
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break
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}
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return err
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}
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d.frameStarts = append(d.frameStarts, pos)
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d.bytesPerFrame = int64(h.BytesPerFrame())
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l += d.bytesPerFrame
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framesize, err := h.FrameSize()
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if err != nil {
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return err
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}
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buf := make([]byte, framesize-4)
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if _, err := d.source.ReadFull(buf); err != nil {
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if err == io.EOF {
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break
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}
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return err
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}
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}
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d.length = l
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if _, err := d.source.Seek(pos, io.SeekStart); err != nil {
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return err
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}
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return nil
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}
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const invalidLength = -1
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// Length returns the total size in bytes.
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//
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// Length returns -1 when the total size is not available
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// e.g. when the given source is not io.Seeker.
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func (d *Decoder) Length() int64 {
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return d.length
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}
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// NewDecoder decodes the given io.Reader and returns a decoded stream.
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//
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// The stream is always formatted as 16bit (little endian) 2 channels
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// even if the source is single channel MP3.
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// Thus, a sample always consists of 4 bytes.
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func NewDecoder(r io.Reader) (*Decoder, error) {
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s := &source{
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reader: r,
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}
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d := &Decoder{
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source: s,
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length: invalidLength,
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}
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if err := s.skipTags(); err != nil {
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return nil, err
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}
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// TODO: Is readFrame here really needed?
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if err := d.readFrame(); err != nil {
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return nil, err
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}
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freq, err := d.frame.SamplingFrequency()
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if err != nil {
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return nil, err
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}
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d.sampleRate = freq
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if err := d.ensureFrameStartsAndLength(); err != nil {
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return nil, err
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}
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return d, nil
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}
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