codec/h265: started writing lexer

Started writing lexer for h265. Wrote type Lexer as we required a data struct we can initialize before starting the lexing process, with for example, information regarding the presense of DONL and
DOND. Wrote handler for aggregation packet. Still need to do fragmentation packet.

codec/h265/lex.go

@@ -0,0 +1,157 @@
+/*
+NAME
+  lex.go
+
+DESCRIPTION
+  lex.go provides a lexer for taking h265 rtp format and lexing into access units.
+
+AUTHORS
+  Saxon A. Nelson-Milton <saxon@ausocean.org>
+
+LICENSE
+  Copyright (C) 2019 the Australian Ocean Lab (AusOcean).
+
+  It is free software: you can redistribute it and/or modify them
+  under the terms of the GNU General Public License as published by the
+  Free Software Foundation, either version 3 of the License, or (at your
+  option) any later version.
+
+  It is distributed in the hope that it will be useful, but WITHOUT
+  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+ for more details.
+
+  You should have received a copy of the GNU General Public License
+  in gpl.txt.  If not, see http://www.gnu.org/licenses.
+*/
+
+package h265
+
+import (
+	"encoding/binary"
+	"fmt"
+	"io"
+	"time"
+
+	"bitbucket.org/ausocean/av/protocol/rtp"
+)
+
+// NALU types.
+const (
+	typeAggregation   = 48
+	typeFragmentation = 49
+	typePACI          = 50
+)
+
+// Buffer sizes.
+const (
+	maxAUSize  = 100000
+	maxRTPSize = 4096
+)
+
+// Lexer is an H265 lexer.
+type Lexer struct {
+	UsingDON   bool            // Indicates whether DONL and DOND will be used for in RTP stream.
+	buf        [maxAUSize]byte // Holds current access unit.
+	off        int             // Holds offset into buf that we're occupying with access unit.
+	fragmented bool            // Indicates if we're currently dealing with a fragmentation unit.
+}
+
+// Lex continually reads RTP packets from the io.Reader src and lexes into
+// access units which are written to the io.Writer dst. Lex expects that for
+// each read from src, a single RTP packet is received.
+func (l *Lexer) Lex(dst io.Writer, src io.Reader, delay time.Duration) error {
+	buf := make([]byte, maxRTPSize)
+	for {
+		n, err := src.Read(buf)
+		switch err {
+		case nil:
+		case io.EOF:
+			continue
+		default:
+			return fmt.Errorf("source read error: %v\n", err)
+		}
+
+		// Get payload from RTP packet.
+		payload, err := rtp.Payload(buf[:n])
+		if err != nil {
+			return fmt.Errorf("could not get rtp payload, failed with err: %v\n", err)
+		}
+
+		nalType := (buf[0] >> 1) & 0x3f
+
+		// If not currently fragmented then we ignore current write
+		if l.fragmented && nalType != typeFragmentation {
+			l.off = 0
+			continue
+		}
+
+		switch nalType {
+		case typeAggregation:
+			l.handleAggregation(payload)
+		case typeFragmentation:
+			l.handleFragmentation(payload)
+		case typePACI:
+			l.handlePACI(payload)
+		default:
+			l.write(payload)
+		}
+
+		m, err := rtp.Marker(buf[:n])
+		if err != nil {
+			return fmt.Errorf("could not get marker bit, failed with err: %v\n", err)
+		}
+
+		if m {
+			_, err := dst.Write(l.buf[:l.off])
+			if err != nil {
+				// TODO: work out what to do here.
+			}
+			l.off = 0
+		}
+	}
+	return nil
+}
+
+// handleAggregation parses NAL units from an aggregation packet and writing
+// them to the Lexers buffer buf.
+func (l *Lexer) handleAggregation(d []byte) {
+	idx := 2
+	for idx < len(d) {
+		if l.UsingDON {
+			switch idx {
+			case 2:
+				idx += 2
+			default:
+				idx += 1
+			}
+		}
+		size := int(binary.BigEndian.Uint16(d[idx:]))
+		idx += 2
+		nalu := d[idx : idx+size]
+		idx += size
+		l.write(nalu)
+	}
+}
+
+// handleFragmentation parses NAL units from fragmentation packets and writes
+// them to the Lexer's buf.
+func (l *Lexer) handleFragmentation(d []byte) {
+
+}
+
+// handlePACI will handl PACI packets
+//
+// TODO: complete this
+func (l *Lexer) handlePACI(d []byte) {
+	panic("unsupported nal type")
+}
+
+// write writes a NAL unit to the Lexers buf in byte stream format using the
+// start code.
+func (l *Lexer) write(d []byte) {
+	const startCode = "\x00\x00\x00\x01"
+	copy(l.buf[l.off:], []byte(startCode))
+	copy(l.buf[l.off+4:], d)
+	l.off += len(d) + 4
+}

protocol/rtp/parse.go

@@ -34,6 +34,19 @@ import (
 
 const badVer = "incompatible RTP version"
 
+// Marker returns the state of the RTP marker bit, and an error if parsing fails.
+func Marker(d []byte) (bool, error) {
+	if len(d) < defaultHeadSize {
+		panic("invalid RTP packet length")
+	}
+
+	if version(d) != rtpVer {
+		return false, errors.New(badVer)
+	}
+
+	return d[1]&0x80 == 1, nil
+}
+
 // Payload returns the payload from an RTP packet provided the version is
 // compatible, otherwise an error is returned.
 func Payload(d []byte) ([]byte, error) {