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ogg_encoder.go
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ogg_encoder.go
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package audiometa
import (
"bytes"
"io"
)
// Encoder converts raw bytes into an ogg stream.
type oggEncoder struct {
serial uint32
page uint32
dummy [1][]byte
w io.Writer
buf [maxPageSize]byte
}
// NewEncoder initializes an ogg encoder with a given serial ID.
// When using multiple Encoders for multiplexed logical streams, ensure unique IDs.
// Encode streams as per ogg RFC for Grouping and Chaining.
func newOggEncoder(id uint32, w io.Writer) *oggEncoder {
return &oggEncoder{serial: id, w: w}
}
// EncodeBOS writes a beginning-of-stream packet to the ogg stream with a given granule position.
// Large packets are split across multiple pages with continuation-of-packet flag set.
// Packets can be empty or nil, resulting in a single segment of size 0.
func (w *oggEncoder) encodeBOS(granule int64, packets [][]byte) error {
if len(packets) == 0 {
packets = w.dummy[:]
}
return w.writePackets(BOS, granule, packets)
}
// Encode writes a data packet to the ogg stream with a given granule position.
// Large packets are split across multiple pages with continuation-of-packet flag set.
// Packets can be empty or nil, resulting in a single segment of size 0.
func (w *oggEncoder) encode(granule int64, packets [][]byte) error {
if len(packets) == 0 {
packets = w.dummy[:]
}
return w.writePackets(0, granule, packets)
}
// EncodeEOS writes a end-of-stream packet to the ogg stream.
// Packets can be empty or nil, resulting in a single segment of size 0.
func (w *oggEncoder) encodeEOS(granule int64, packets [][]byte) error {
if len(packets) == 0 {
packets = w.dummy[:]
}
return w.writePackets(EOS, granule, packets)
}
func (w *oggEncoder) writePackets(kind byte, granule int64, packets [][]byte) error {
h := oggPageHeader{
Magic: [4]byte{'O', 'g', 'g', 'S'},
Flags: kind,
SerialNumber: w.serial,
GranulePosition: granule,
}
segtbl, car, cdr := w.segmentize(payload{packets[0], packets[1:], nil})
if err := w.writePage(&h, segtbl, car); err != nil {
return err
}
h.Flags |= COP
for len(cdr.leftover) > 0 {
segtbl, car, cdr = w.segmentize(cdr)
if err := w.writePage(&h, segtbl, car); err != nil {
return err
}
}
return nil
}
func (w *oggEncoder) writePage(h *oggPageHeader, segtbl []byte, pay payload) error {
page := &oggPage{}
h.SequenceNumber = w.page
w.page++
h.Segments = byte(len(segtbl))
hb := bytes.NewBuffer(w.buf[0:0:cap(w.buf)])
//_ = binary.Write(hb, byteOrder, h)
hb.Write(segtbl)
hb.Write(pay.leftover)
for _, p := range pay.packets {
hb.Write(p)
}
hb.Write(pay.rightover)
bb := hb.Bytes()
page.Header = h
page.Body = bb
OggPageChecksumSet(page)
buf := page.Header.toBytesBuffer()
if _, err := buf.WriteTo(w.w); err != nil {
return err
}
if _, err := hb.WriteTo(w.w); err != nil {
return err
}
return nil
}
// payload represents a potentially split group of packets.
// ASCII example (each letter run represents one packet):
// Page 1 (left): [aaaabbbbccccd], Page 2 (right): [dddeeeffff]
type payload struct {
leftover []byte
packets [][]byte
rightover []byte
}
// segmentize calculates the lacing values for the segment table based on given packets.
// Returns the segment table, the payload for the current page and any leftover payload.
func (w *oggEncoder) segmentize(pay payload) ([]byte, payload, payload) {
segtbl := w.buf[headerSize : headerSize+maxSegSize]
i := 0
s255s := len(pay.leftover) / maxSegSize
rem := len(pay.leftover) % maxSegSize
for i < len(segtbl) && s255s > 0 {
segtbl[i] = maxSegSize
i++
s255s--
}
if i < maxSegSize {
segtbl[i] = byte(rem)
i++
} else {
leftStart := len(pay.leftover) - (s255s * maxSegSize) - rem
good := payload{pay.leftover[0:leftStart], nil, nil}
bad := payload{pay.leftover[leftStart:], pay.packets, nil}
return segtbl, good, bad
}
// Now loop through the rest and track if we need to split
for p := 0; p < len(pay.packets); p++ {
s255s := len(pay.packets[p]) / maxSegSize
rem := len(pay.packets[p]) % maxSegSize
for i < len(segtbl) && s255s > 0 {
segtbl[i] = maxSegSize
i++
s255s--
}
if i < maxSegSize {
segtbl[i] = byte(rem)
i++
} else {
right := len(pay.packets[p]) - (s255s * maxSegSize) - rem
good := payload{pay.leftover, pay.packets[0:p], pay.packets[p][0:right]}
bad := payload{pay.packets[p][right:], pay.packets[p+1:], nil}
return segtbl, good, bad
}
}
good := pay
bad := payload{}
return segtbl[0:i], good, bad
}