Reland of Only compare sequence numbers from the same SSRC in ForwardErrorCorrection.

webrtc/modules/rtp_rtcp/source/forward_error_correction.h

Index: webrtc/modules/rtp_rtcp/source/forward_error_correction.h
diff --git a/webrtc/modules/rtp_rtcp/source/forward_error_correction.h b/webrtc/modules/rtp_rtcp/source/forward_error_correction.h
index f45473cb1f417cbb36dcdabb0f2c99542af9ccca..e70d7db493192cb32b19c17588fda1497c0445d9 100644
--- a/webrtc/modules/rtp_rtcp/source/forward_error_correction.h
+++ b/webrtc/modules/rtp_rtcp/source/forward_error_correction.h
@@ -33,6 +33,8 @@ class FecHeaderWriter;
 // Option exists to enable unequal protection (UEP) across packets.
 // This is not to be confused with protection within packets
 // (referred to as uneven level protection (ULP) in RFC 5109).
+// TODO(brandtr): Split this class into a separate encoder
+// and a separate decoder.
 class ForwardErrorCorrection {
  public:
   // TODO(holmer): As a next step all these struct-like packet classes should be
@@ -62,29 +64,25 @@ class ForwardErrorCorrection {
   class SortablePacket {
    public:
     // Functor which returns true if the sequence number of |first|
-    // is < the sequence number of |second|.
+    // is < the sequence number of |second|. Should only ever be called for
+    // packets belonging to the same SSRC.
     struct LessThan {
       template <typename S, typename T>
       bool operator() (const S& first, const T& second);
     };
 
+    uint32_t ssrc;
     uint16_t seq_num;
   };
 
   // The received list parameter of DecodeFec() references structs of this type.
   //
-  // The ssrc member is needed to ensure that we can restore the SSRC field of
-  // recovered packets. In most situations this could be retrieved from other
-  // media packets, but in the case of an FEC packet protecting a single
-  // missing media packet, we have no other means of obtaining it.
   // TODO(holmer): Refactor into a proper class.
   class ReceivedPacket : public SortablePacket {
    public:
     ReceivedPacket();
     ~ReceivedPacket();
 
-    uint32_t ssrc;  // SSRC of the current frame. Must be set for FEC
-                    // packets, but not required for media packets.
     bool is_fec;    // Set to true if this is an FEC packet and false
                     // otherwise.
     rtc::scoped_refptr<Packet> pkt;  // Pointer to the packet storage.
@@ -109,7 +107,7 @@ class ForwardErrorCorrection {
   // Used to link media packets to their protecting FEC packets.
   //
   // TODO(holmer): Refactor into a proper class.
-  class ProtectedPacket : public ForwardErrorCorrection::SortablePacket {
+  class ProtectedPacket : public SortablePacket {
    public:
     ProtectedPacket();
     ~ProtectedPacket();
@@ -122,7 +120,7 @@ class ForwardErrorCorrection {
   // Used for internal storage of received FEC packets in a list.
   //
   // TODO(holmer): Refactor into a proper class.
-  class ReceivedFecPacket : public ForwardErrorCorrection::SortablePacket {
+  class ReceivedFecPacket : public SortablePacket {
    public:
     ReceivedFecPacket();
     ~ReceivedFecPacket();
@@ -150,8 +148,10 @@ class ForwardErrorCorrection {
   ~ForwardErrorCorrection();
 
   // Creates a ForwardErrorCorrection tailored for a specific FEC scheme.
-  static std::unique_ptr<ForwardErrorCorrection> CreateUlpfec();
-  static std::unique_ptr<ForwardErrorCorrection> CreateFlexfec();
+  static std::unique_ptr<ForwardErrorCorrection> CreateUlpfec(uint32_t ssrc);
+  static std::unique_ptr<ForwardErrorCorrection> CreateFlexfec(
+      uint32_t ssrc,
+      uint32_t protected_media_ssrc);
 
   // Generates a list of FEC packets from supplied media packets.
   //
@@ -242,7 +242,9 @@ class ForwardErrorCorrection {
 
  protected:
   ForwardErrorCorrection(std::unique_ptr<FecHeaderReader> fec_header_reader,
-                         std::unique_ptr<FecHeaderWriter> fec_header_writer);
+                         std::unique_ptr<FecHeaderWriter> fec_header_writer,
+                         uint32_t ssrc,
+                         uint32_t protected_media_ssrc);
 
  private:
   // Analyzes |media_packets| for holes in the sequence and inserts zero columns
@@ -330,6 +332,10 @@ class ForwardErrorCorrection {
   // for recovering lost packets.
   void DiscardOldRecoveredPackets(RecoveredPacketList* recovered_packets);
 
+  // These SSRCs are only used by the decoder.
+  const uint32_t ssrc_;
+  const uint32_t protected_media_ssrc_;
+
   std::unique_ptr<FecHeaderReader> fec_header_reader_;
   std::unique_ptr<FecHeaderWriter> fec_header_writer_;