Detect partition head in SampleBuilder

Add functional option to set PartitionHeadChecker to detect the
beginning of the frame partition and avoid dropping the first
packet.

pkg/media/samplebuilder/samplebuilder.go

@@ -25,11 +25,18 @@ type SampleBuilder struct {
 	isContiguous     bool
 	lastPopSeq       uint16
 	lastPopTimestamp uint32
+
+	// Interface that checks whether the packet is the first fragment of the frame or not
+	partitionHeadChecker rtp.PartitionHeadChecker
 }
 
 // New constructs a new SampleBuilder
-func New(maxLate uint16, depacketizer rtp.Depacketizer) *SampleBuilder {
-	return &SampleBuilder{maxLate: maxLate, depacketizer: depacketizer}
+func New(maxLate uint16, depacketizer rtp.Depacketizer, opts ...Option) *SampleBuilder {
+	s := &SampleBuilder{maxLate: maxLate, depacketizer: depacketizer}
+	for _, o := range opts {
+		o(s)
+	}
+	return s
 }
 
 // Push adds a RTP Packet to the sample builder
@@ -47,9 +54,14 @@ func (s *SampleBuilder) buildSample(firstBuffer uint16) (*media.Sample, uint32)
 	for i := firstBuffer; s.buffer[i] != nil; i++ {
 		if s.buffer[i].Timestamp != s.buffer[firstBuffer].Timestamp {
 			lastTimeStamp := s.lastPopTimestamp
-			if !s.isContiguous && s.buffer[firstBuffer-1] != nil {
-				// firstBuffer-1 should always pass, but just to be safe if there is a bug in Pop()
-				lastTimeStamp = s.buffer[firstBuffer-1].Timestamp
+			if !s.isContiguous {
+				if s.buffer[firstBuffer-1] != nil {
+					lastTimeStamp = s.buffer[firstBuffer-1].Timestamp
+				} else {
+					// If PartitionHeadChecker detects that the first packet is a head,
+					// the duration of the packet is not guessable
+					lastTimeStamp = s.buffer[firstBuffer].Timestamp
+				}
 			}
 
 			samples := s.buffer[i-1].Timestamp - lastTimeStamp
@@ -106,18 +118,21 @@ func (s *SampleBuilder) PopWithTimestamp() (*media.Sample, uint32) {
 	for ; i != s.lastPush; i++ {
 		curr := s.buffer[i]
 		if curr == nil {
-			if s.buffer[i-1] != nil {
-				break // there is a gap, we can't proceed
-			}
-
 			continue // we haven't hit a buffer yet, keep moving
 		}
 
 		if !s.isContiguous {
-			if s.buffer[i-1] == nil {
-				continue // We have never popped a buffer, so we can't assert that the first RTP packet we encounter is valid
-			} else if s.buffer[i-1].Timestamp == curr.Timestamp {
-				continue // We have the same timestamps, so it is data that spans multiple RTP packets
+			if s.partitionHeadChecker == nil {
+				if s.buffer[i-1] == nil {
+					continue // We have never popped a buffer, so we can't assert that the first RTP packet we encounter is valid
+				} else if s.buffer[i-1].Timestamp == curr.Timestamp {
+					continue // We have the same timestamps, so it is data that spans multiple RTP packets
+				}
+			} else {
+				if !s.partitionHeadChecker.IsPartitionHead(curr.Payload) {
+					continue
+				}
+				// We can start using this frame as it is a head of frame partition
 			}
 		}
 
@@ -126,3 +141,13 @@ func (s *SampleBuilder) PopWithTimestamp() (*media.Sample, uint32) {
 	}
 	return nil, 0
 }
+
+// Option configures SampleBuilder
+type Option func(o *SampleBuilder)
+
+// WithPartitionHeadChecker assigns codec specific PartitionHeadChecker to SampleBuilder
+func WithPartitionHeadChecker(checker rtp.PartitionHeadChecker) Option {
+	return func(o *SampleBuilder) {
+		o.partitionHeadChecker = checker
+	}
+}