Add helper functions for negotiating H264 profile level id

webrtc/common_video/h264/profile_level_id.h

Index: webrtc/common_video/h264/profile_level_id.h
diff --git a/webrtc/common_video/h264/profile_level_id.h b/webrtc/common_video/h264/profile_level_id.h
index f69b7f6cdcd27dee0cd409f5a79b7c0769ca9c76..bb1c435926bcf4311dbe0c628644efc2acbd1262 100644
--- a/webrtc/common_video/h264/profile_level_id.h
+++ b/webrtc/common_video/h264/profile_level_id.h
@@ -11,6 +11,7 @@
 #ifndef WEBRTC_COMMON_VIDEO_H264_PROFILE_LEVEL_ID_H_
 #define WEBRTC_COMMON_VIDEO_H264_PROFILE_LEVEL_ID_H_
 
+#include <map>
 #include <string>
 
 #include "webrtc/base/optional.h"
@@ -18,6 +19,9 @@
 namespace webrtc {
 namespace H264 {
 
+// Map containting SDP codec parameters.
+typedef std::map<std::string, std::string> CodecParameterMap;
+
 enum Profile {
   kProfileConstrainedBaseline,
   kProfileBaseline,
@@ -60,6 +64,13 @@ struct ProfileLevelId {
 // profile level id.
 rtc::Optional<ProfileLevelId> ParseProfileLevelId(const char* str);
 
+// Parse profile level id that is represented as a string of 3 hex bytes
+// contained in an SDP key-value map. A default profile level id will be
+// returned if the profile-level-id key is missing. Nothing will be returned if
+// the key is present but the string is invalid.
+rtc::Optional<ProfileLevelId> ParseSdpProfileLevelId(
+    const CodecParameterMap& params);
+
 // Given that a decoder supports up to a given frame size (in pixels) at up to a
 // given number of frames per second, return the highest H.264 level where it
 // can guarantee that it will be able to support all valid encoded streams that
@@ -71,6 +82,27 @@ rtc::Optional<Level> SupportedLevel(int max_frame_pixel_count, float max_fps);
 rtc::Optional<std::string> ProfileLevelIdToString(
     const ProfileLevelId& profile_level_id);
 
+// Generate codec parameters that will be used as answer in an SDP negotiation
+// based on local supported parameters and remote offered parameters. Both
+// |local_supported_params|, |remote_offered_params|, and |answer_params|
+// represent sendrecv media descriptions, i.e they are a mix of both encode and
+// decode capabilities. In theory, when the profile in |local_supported_params|
+// represent a strict superset of the profile in |remote_offered_params|, we
+// could limit the profile in |answer_params| to the profile in
+// |remote_offered_params|. However, to simplify the code, each supported H264
+// profile should be listed explicitly in the list of local supported codecs,
+// even if they are redundant. Then each local codec in the list should be
+// tested one at a time against the remote codec, and only when the profiles are
+// equal should this function be called. Therefore, this function does not need
+// to handle profile intersection, and the profile of |local_supported_params|
+// and |remote_offered_params| must be equal before calling this function. The
+// parameters that are used when negotiating are the level part of
+// profile-level-id and level-asymmetry-allowed.
+void GenerateProfileLevelIdForAnswer(
+    const CodecParameterMap& local_supported_params,
+    const CodecParameterMap& remote_offered_params,
+    CodecParameterMap* answer_params);
+
 }  // namespace H264
 }  // namespace webrtc