Removing `preference` field from `cricket::Codec`.

webrtc/pc/mediasession.cc

Index: webrtc/pc/mediasession.cc
diff --git a/webrtc/pc/mediasession.cc b/webrtc/pc/mediasession.cc
index a9d1b95c22aee6aae9299315a9ecc05586242336..6b05736f0a3f3e74d662bb305e2458b7b7eba8ed 100644
--- a/webrtc/pc/mediasession.cc
+++ b/webrtc/pc/mediasession.cc
@@ -10,11 +10,12 @@
 
 #include "webrtc/pc/mediasession.h"
 
-#include <algorithm>  // For std::find_if.
+#include <algorithm>  // For std::find_if, std::sort.
 #include <functional>
 #include <map>
 #include <memory>
 #include <set>
+#include <unordered_map>
 #include <utility>
 
 #include "webrtc/base/helpers.h"
@@ -749,7 +750,6 @@ static bool CreateMediaContentOffer(
     StreamParamsVec* current_streams,
     MediaContentDescriptionImpl<C>* offer) {
   offer->AddCodecs(codecs);
-  offer->SortCodecs();
 
   if (secure_policy == SEC_REQUIRED) {
     offer->set_crypto_required(CT_SDES);
@@ -810,6 +810,8 @@ static void NegotiateCodecs(const std::vector<C>& local_codecs,
                             std::vector<C>* negotiated_codecs) {
   for (const C& ours : local_codecs) {
     C theirs;
+    // Note that we intentionally only find one matching codec for each of our
+    // local codecs, in case the remote offer contains duplicate codecs.
     if (FindMatchingCodec(local_codecs, offered_codecs, ours, &theirs)) {
       C negotiated = ours;
       negotiated.IntersectFeedbackParams(theirs);
@@ -822,14 +824,23 @@ static void NegotiateCodecs(const std::vector<C>& local_codecs,
                             offered_apt_value);
       }
       negotiated.id = theirs.id;
-      // RFC3264: Although the answerer MAY list the formats in their desired
-      // order of preference, it is RECOMMENDED that unless there is a
-      // specific reason, the answerer list formats in the same relative order
-      // they were present in the offer.
-      negotiated.preference = theirs.preference;
       negotiated_codecs->push_back(negotiated);
     }
   }
+  // RFC3264: Although the answerer MAY list the formats in their desired
+  // order of preference, it is RECOMMENDED that unless there is a
+  // specific reason, the answerer list formats in the same relative order
+  // they were present in the offer.
+  std::unordered_map<int, int> payload_type_preferences;
+  int preference = static_cast<int>(offered_codecs.size() + 1);
+  for (const C& codec : offered_codecs) {
+    payload_type_preferences[codec.id] = preference--;
+  }
+  std::sort(negotiated_codecs->begin(), negotiated_codecs->end(),
+            [&payload_type_preferences](const C& a, const C& b) {
+              return payload_type_preferences[a.id] >
+                     payload_type_preferences[b.id];
+            });
 }
 
 // Finds a codec in |codecs2| that matches |codec_to_match|, which is
@@ -1042,7 +1053,6 @@ static bool CreateMediaContentAnswer(
   std::vector<C> negotiated_codecs;
   NegotiateCodecs(local_codecs, offer->codecs(), &negotiated_codecs);
   answer->AddCodecs(negotiated_codecs);
-  answer->SortCodecs();
   answer->set_protocol(offer->protocol());
   RtpHeaderExtensions negotiated_rtp_extensions;
   NegotiateRtpHeaderExtensions(local_rtp_extenstions,
@@ -1365,8 +1375,8 @@ SessionDescription* MediaSessionDescriptionFactory::CreateAnswer(
     const SessionDescription* offer, const MediaSessionOptions& options,
     const SessionDescription* current_description) const {
   // The answer contains the intersection of the codecs in the offer with the
-  // codecs we support, ordered by our local preference. As indicated by
-  // XEP-0167, we retain the same payload ids from the offer in the answer.
+  // codecs we support. As indicated by XEP-0167, we retain the same payload ids
+  // from the offer in the answer.
   std::unique_ptr<SessionDescription> answer(new SessionDescription());
 
   StreamParamsVec current_streams;