[CODEC] Add encoder bitrate tracker (#646)

add encoder bitrate tracker

pkg/codec/bitrate_tracker.go

@@ -0,0 +1,48 @@
+package codec
+
+import (
+	"time"
+)
+
+type BitrateTracker struct {
+	windowSize time.Duration
+	buffer     []int
+	times      []time.Time
+}
+
+func NewBitrateTracker(windowSize time.Duration) *BitrateTracker {
+	return &BitrateTracker{
+		windowSize: windowSize,
+	}
+}
+
+func (bt *BitrateTracker) AddFrame(sizeBytes int, timestamp time.Time) {
+	bt.buffer = append(bt.buffer, sizeBytes)
+	bt.times = append(bt.times, timestamp)
+
+	// Remove old entries outside the window
+	cutoff := timestamp.Add(-bt.windowSize)
+	i := 0
+	for ; i < len(bt.times); i++ {
+		if bt.times[i].After(cutoff) {
+			break
+		}
+	}
+	bt.buffer = bt.buffer[i:]
+	bt.times = bt.times[i:]
+}
+
+func (bt *BitrateTracker) GetBitrate() float64 {
+	if len(bt.times) < 2 {
+		return 0
+	}
+	totalBytes := 0
+	for _, b := range bt.buffer {
+		totalBytes += b
+	}
+	duration := bt.times[len(bt.times)-1].Sub(bt.times[0]).Seconds()
+	if duration <= 0 {
+		return 0
+	}
+	return float64(totalBytes*8) / duration // bits per second
+}

pkg/codec/bitrate_tracker_test.go

@@ -0,0 +1,19 @@
+package codec
+
+import (
+	"math"
+	"testing"
+	"time"
+)
+
+func TestBitrateTracker(t *testing.T) {
+	packetSize := 1000
+	bt := NewBitrateTracker(time.Second)
+	bt.AddFrame(packetSize, time.Now())
+	bt.AddFrame(packetSize, time.Now().Add(time.Millisecond*100))
+	bt.AddFrame(packetSize, time.Now().Add(time.Millisecond*999))
+	eps := float64(packetSize*8) / 10
+	if got, want := bt.GetBitrate(), float64(packetSize*8)*3; math.Abs(got-want) > eps {
+		t.Fatalf("GetBitrate() = %v, want %v (|diff| <= %v)", got, want, eps)
+	}
+}