If the array size is even, the median should be average of its two middlemost elements.

NADA unittests updated accordingly.

BUG=webrtc:4550
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1246083006 .

Cr-Commit-Position: refs/heads/master@{#9607}

webrtc/modules/remote_bitrate_estimator/test/estimators/nada.cc

@@ -124,12 +124,20 @@ FeedbackPacket* NadaBweReceiver::GetFeedback(int64_t now_ms) {
                           derivative, RecentKbps(), corrected_send_time_ms);
 }
 
+// If size is even, the median is the average of the two middlemost numbers.
 int64_t NadaBweReceiver::MedianFilter(int64_t* last_delays_ms, int size) {
-  // Typically, size = 5.
   std::vector<int64_t> array_copy(last_delays_ms, last_delays_ms + size);
   std::nth_element(array_copy.begin(), array_copy.begin() + size / 2,
                    array_copy.end());
-  return array_copy.at(size / 2);
+  if (size % 2 == 1) {
+    // Typically, size = 5. For odd size values, right and left are equal.
+    return array_copy.at(size / 2);
+  }
+  int64_t right = array_copy.at(size / 2);
+  std::nth_element(array_copy.begin(), array_copy.begin() + (size - 1) / 2,
+                   array_copy.end());
+  int64_t left = array_copy.at((size - 1) / 2);
+  return (left + right + 1) / 2;
 }
 
 int64_t NadaBweReceiver::ExponentialSmoothingFilter(int64_t new_value,

webrtc/modules/remote_bitrate_estimator/test/estimators/nada_unittest.cc

@@ -356,7 +356,7 @@ TEST_F(NadaReceiverSideTest, FeedbackIncreasingDelay) {
   // Raw delays are = [10 20 30 40 50 60 70 80] ms.
   // Baseline delay will be 50 ms.
   // Delay signals should be: [0 10 20 30 40 50 60 70] ms.
-  const int64_t kMedianFilteredDelaysMs[] = {0, 10, 10, 20, 20, 30, 40, 50};
+  const int64_t kMedianFilteredDelaysMs[] = {0, 5, 10, 15, 20, 30, 40, 50};
   const int kNumPackets = ARRAY_SIZE(kMedianFilteredDelaysMs);
   const float kAlpha = 0.1f;  // Used for exponential smoothing.
 
@@ -425,7 +425,7 @@ TEST_F(NadaReceiverSideTest, FeedbackWarpedDelay) {
   // Baseline delay will be 50 ms.
   // Delay signals should be: [0 200 400 600 800 1000 1200 1400] ms.
   const int64_t kMedianFilteredDelaysMs[] = {
-      0, 200, 200, 400, 400, 600, 800, 1000};
+      0, 100, 200, 300, 400, 600, 800, 1000};
   const int kNumPackets = ARRAY_SIZE(kMedianFilteredDelaysMs);
   const float kAlpha = 0.1f;  // Used for exponential smoothing.