Unit test for media packet reordering in ForwardErrorCorrection.

webrtc/modules/rtp_rtcp/source/forward_error_correction.cc

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 505 matching lines @@
   for (uint16_t byte_idx = 0; byte_idx < mask_size_bytes; ++byte_idx) {
     uint8_t packet_mask = fec_packet->pkt->data[12 + byte_idx];
     for (uint16_t bit_idx = 0; bit_idx < 8; ++bit_idx) {
       if (packet_mask & (1 << (7 - bit_idx))) {
         std::unique_ptr<ProtectedPacket> protected_packet(
             new ProtectedPacket());
         // This wraps naturally with the sequence number.
         protected_packet->seq_num =
             static_cast<uint16_t>(seq_num_base + (byte_idx << 3) + bit_idx);
         protected_packet->pkt = nullptr;
+        // Note that |protected_pkt_list| is sorted (according to sequence
+        // number) by construction.
         fec_packet->protected_packets.push_back(std::move(protected_packet));
       }
     }
   }
   if (fec_packet->protected_packets.empty()) {
     // All-zero packet mask; we can discard this FEC packet.
     LOG(LS_WARNING) << "Received FEC packet has an all-zero packet mask.";
   } else {
     AssignRecoveredPackets(fec_packet.get(), recovered_packets);
     // TODO(holmer): Consider replacing this with a binary search for the right
     // position, and then just insert the new packet. Would get rid of the sort.
+    //
+    // For correct decoding, |fec_packet_list_| does not necessarily
+    // need to be sorted by sequence number (see decoding algorithm in
+    // AttemptRecover()), but by keeping it sorted we try to recover the
+    // oldest lost packets first.
     received_fec_packets_.push_back(std::move(fec_packet));
     received_fec_packets_.sort(SortablePacket::LessThan());
     if (received_fec_packets_.size() > kMaxFecPackets) {
       received_fec_packets_.pop_front();
     }
     RTC_DCHECK_LE(received_fec_packets_.size(), kMaxFecPackets);
   }
 }
 
 void ForwardErrorCorrection::AssignRecoveredPackets(
@@ skipping 249 matching lines @@
   }
   InsertPackets(received_packets, recovered_packets);
   AttemptRecover(recovered_packets);
   return 0;
 }
 
 size_t ForwardErrorCorrection::PacketOverhead() {
   return kFecHeaderSize + kUlpHeaderSizeLBitSet;
 }
 }  // namespace webrtc