Add PacketsHeld to TWCC recorder

When using `twcc.Recorder` outside the context of
`twcc.SenderInterceptor`, it is useful to have access to
the number of packets currently held. This enables writing
logic other than an interval to determine when to build
feedback packets.

AUTHORS.txt

@@ -3,6 +3,8 @@
 #
 # This file is auto generated, using git to list all individuals contributors.
 # see https://github.com/pion/.goassets/blob/master/scripts/generate-authors.sh for the scripting
+ Sean DuBois <[email protected]>
+ ziminghua <[email protected]>
 Aaron Boushley <[email protected]>
 Adam Kiss <[email protected]>
 adamroach <[email protected]>
@@ -23,8 +25,8 @@ Quentin Renard <[email protected]>
 Rayleigh Li <[email protected]>
 Sean DuBois <[email protected]>
 Steffen Vogel <[email protected]>
+treyhakanson <[email protected]>
 XLPolar <[email protected]>
-ziminghua <[email protected]>
 
 # List of contributors not appearing in Git history
 

pkg/twcc/twcc.go

@@ -51,6 +51,11 @@ func (r *Recorder) Record(mediaSSRC uint32, sequenceNumber uint16, arrivalTime i
 	r.lastSequenceNumber = sequenceNumber
 }
 
+// PacketsHeld returns the number of received packets currently held by the recorder
+func (r *Recorder) PacketsHeld() int {
+	return len(r.receivedPackets)
+}
+
 func insertSorted(list []pktInfo, element pktInfo) []pktInfo {
 	if len(list) == 0 {
 		return append(list, element)

pkg/twcc/twcc_test.go

@@ -940,3 +940,25 @@ func TestInsertSorted(t *testing.T) {
 		})
 	}
 }
+
+func TestPacketsHheld(t *testing.T) {
+	r := NewRecorder(5000)
+	assert.Zero(t, r.PacketsHeld())
+
+	arrivalTime := int64(scaleFactorReferenceTime)
+	addRun(t, r, []uint16{0, 1, 2}, []int64{
+		arrivalTime,
+		increaseTime(&arrivalTime, rtcp.TypeTCCDeltaScaleFactor),
+		increaseTime(&arrivalTime, rtcp.TypeTCCDeltaScaleFactor),
+	})
+	assert.Equal(t, r.PacketsHeld(), 3)
+
+	addRun(t, r, []uint16{3, 4}, []int64{
+		increaseTime(&arrivalTime, rtcp.TypeTCCDeltaScaleFactor),
+		increaseTime(&arrivalTime, rtcp.TypeTCCDeltaScaleFactor),
+	})
+	assert.Equal(t, r.PacketsHeld(), 5)
+
+	r.BuildFeedbackPacket()
+	assert.Zero(t, r.PacketsHeld())
+}