streaming

commit 59f5025d611570198060cb837a0cac5dac6d5b26
parent 68956c4dceeca0d5f3e883c71743841791eca651
Author: Oliver Lowe <o@olowe.co>
Date:   Thu,  6 Jun 2024 15:44:12 +1000

mpegts: initial, basic decoding of packets

Diffstat:
A
mpegts/codec.go
 | 
53
+++++++++++++++++++++++++++++++++++++++++++++++++++++
M
mpegts/mpegts.go
 | 
78
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--------

2 files changed, 123 insertions(+), 8 deletions(-)
diff --git a/mpegts/codec.go b/mpegts/codec.go
@@ -0,0 +1,53 @@
+package mpegts
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+)
+
+func Decode(r io.Reader) (*Packet, error) {
+	buf := make([]byte, PacketSize)
+	n, err := r.Read(buf)
+	if n != PacketSize {
+		if err != nil {
+			return nil, fmt.Errorf("short read (%d bytes): %w", n, err)
+		}
+		return nil, fmt.Errorf("short read (%d bytes)", n)
+	}
+	if err != nil {
+		return nil, err
+	}
+	if buf[0] != Sync {
+		return nil, fmt.Errorf("expected sync byte, got %x", buf[0])
+	}
+	var p Packet
+	if buf[1]&0x80 > 0 {
+		p.Error = true
+	}
+	if buf[1]&0x40 > 0 {
+		p.PayloadStart = true
+	}
+	if buf[1]&0x20 > 0 {
+		p.Priority = true
+	}
+	// Want next 13 bits. 5 from buf[1] and all of buf[2].
+	pid := binary.BigEndian.Uint16([]byte{buf[1] & 0x1f, buf[2]})
+	p.PID = PacketID(pid)
+
+	// next 2 bits
+	p.Scrambling = Scramble(buf[3] & 0xc0)
+	// skip next 2 bits until later when we need to decode adaptation or payload.
+	// now just get last 4 bits
+	p.Continuity = buf[3] & 0x0f
+	afc := buf[3] >> 4
+	switch afc {
+	case 0x01:
+		p.Payload = buf[4:]
+	case 0x03:
+		p.Adaptation = buf[4:]
+	default:
+		return &p, fmt.Errorf("cannot decode both payload and adaptation field")
+	}
+	return &p, nil
+}
diff --git a/mpegts/mpegts.go b/mpegts/mpegts.go
@@ -1,22 +1,27 @@
 // Package mpegts implements encoding and decoding of MPEG-TS packets as specified in ITU-T H.222.0.
 package mpegts
 
+import "fmt"
+
 const PacketSize int = 188
 
+const Sync byte = 'G'
+
 type Packet struct {
 	// If true, the packet should be discarded.
 	Error bool
+	// Indicates whether the payload holds the first byte of a particular payload such as ... TODO(otl)
+	PayloadStart bool
 	// If true, this packet has higher priority than other packets
 	// with the same PID.
 	Priority bool
-	// Indicates whether the payload holds the first byte of a particular payload such as ... TODO(otl)
-	PayloadStart bool
-	// Identifies the type of the payload.
-	PID uint16
+	// Identifies the type of the packet.
+	PID PacketID
 	// Specifies which algorithm, if any, is used to encrypt the payload.
-	ScrambleConrol Scramble
-	Adaptation     *Adaptation
-	Payload        []byte
+	Scrambling Scramble
+	Continuity uint8
+	Adaptation []byte
+	Payload    []byte
 }
 
 type Scramble uint8
@@ -28,4 +33,61 @@ const (
 	ScrambleOdd  Scramble = 0xc0
 )
 
-type Adaptation struct{}
+type PacketID uint16
+
+const (
+	PAT  PacketID = iota // Program association table
+	CAT                  // Conditional access table
+	TSDT                 // Transport stream description table
+	IPMP
+	_          // reserved through to
+	PacketNull = 8191
+)
+
+func (id PacketID) String() string {
+	switch id {
+	case PAT:
+		return "PAT"
+	case CAT:
+		return "CAT"
+	case TSDT:
+		return "TSDT"
+	case IPMP:
+		return "IPMP"
+	case PacketNull:
+		return "null"
+	}
+	return fmt.Sprintf("%d", id)
+}
+
+// Adaptation represents an adaptation field including the header.
+type Adaptation struct {
+	// If true, there is a discontinuity between this packet and
+	// either the continuity conter or PCR.
+	Discontinuous bool
+	// If true, the packet may be used as an index from which to
+	// randomly access other points in the stream.
+	RandomAccess bool
+	// Whether this stream is high priority.
+	Priority bool
+	PCR      *PCR
+	// Original program clock reference.
+	OPCR *PCR
+	// The number of packets remaining until a splicing point.
+	SpliceCountdown uint8
+	// Raw bytes of any application-specific data.
+	Private []byte
+	// Raw bytes of the adaptation extension field.
+	Extension []byte
+	// A slice of bytes all with the value 0xff; enough to ensure
+	// a packet's length is always PacketSize.
+	Stuffing []byte
+}
+
+// PCR represents a Program Clock Reference
+type PCR struct {
+	// Number of ticks of a 27MHz clock as a 33-bit integer.
+	Ticks uint64
+	// 9-bit integer
+	Extension uint16
+}