Piggybacking simulcast id and ALR experiment id into video content type extension.

webrtc/api/video/video_content_type.cc

Index: webrtc/api/video/video_content_type.cc
diff --git a/webrtc/api/video/video_content_type.cc b/webrtc/api/video/video_content_type.cc
new file mode 100644
index 0000000000000000000000000000000000000000..442ebcd9fa5d9d85abe51505365b8cea4fa372d7
--- /dev/null
+++ b/webrtc/api/video/video_content_type.cc
@@ -0,0 +1,93 @@
+/*
+ *  Copyright (c) 2017 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.
+ */
+
+#include "webrtc/api/video/video_content_type.h"
+
+// VideoContentType stores a single byte, which is sent other the network.
+// Structure:
+//
+//  0 1 2 3 4 5 6 7
+// +---------------+
+// |r r e e e s s c|
+//
+// where:
+// r - reserved bits.
+// e - 3-bit number of an experiment group counted from 1. 0 means there's no
+// experiment ongoing.
+// s - 2-bit simulcast stream id or spatial layer, counted from 1. 0 means that
+// no simulcast information is set.
+// c - content type. 0 means real-time video, 1 means screenshare.
+//
+static const uint8_t kScreenshareBitsMask = 0x01;
+
+static const uint8_t kSimulcastShift = 1;
+static const uint8_t kSimulcastBitsMask = 0x06;  // 0b00000110
+static const uint8_t kSimulcastBitsSize = 2;
+
+static const uint8_t kExperimentShift = 3;
+static const uint8_t kExperimentBitsMask = 0x38;  // 0b00111000
+static const uint8_t kExperimentBitsSize = 3;
+
+namespace webrtc {
+const VideoContentType VideoContentType::UNSPECIFIED = VideoContentType({0});

[tommi, 2017/08/29 09:25:45]

if for some reason we can't use enum, can we use constexpr here? (global
construction will cause the cl to be reverted)

[ilnik, 2017/08/29 10:26:14]

Done. However, it's not constexpr in the .h file, is it fine? It can't be set
there as constexpr because it will need initializer there, yet at that place the
class is still incomplete and can't be used.

One solution would be to use uint8_t constants, but the problem is that we can't
use them for initializing VideoContentType because we can't define constructor
or this struct will become complex again.

+const VideoContentType VideoContentType::SCREENSHARE = VideoContentType({1});
+
+VideoContentType::operator uint8_t() const {
+  return content_type;
+}
+
+uint8_t VideoContentType::operator=(uint8_t value) {
+  content_type = value;
+  return value;
+}
+
+bool VideoContentType::operator==(const VideoContentType& other) {
+  return content_type == other.content_type;
+}
+
+bool VideoContentType::operator==(uint8_t value) {
+  return content_type == value;
+}
+
+bool VideoContentType::SetExperimentId(uint8_t experiment_id) {
+  // Store in bits 3-5.
+  if (experiment_id >= (1 << kExperimentBitsSize))
+    return false;
+  content_type = (content_type & ~kExperimentBitsMask) |
+                 ((experiment_id << kExperimentShift) & kExperimentBitsMask);
+  return true;
+}
+
+bool VideoContentType::SetSimulcastId(uint8_t simulcast_id) {
+  // Store in bits 1-2.
+  if (simulcast_id >= (1 << kSimulcastBitsSize))
+    return false;
+  content_type = (content_type & ~kSimulcastBitsMask) |
+                 ((simulcast_id << kSimulcastShift) & kSimulcastBitsMask);
+  return true;
+}
+
+uint8_t VideoContentType::GetExperimentId() const {
+  return (content_type & kExperimentBitsMask) >> kExperimentShift;
+}
+
+uint8_t VideoContentType::GetSimulcastId() const {
+  return (content_type & kSimulcastBitsMask) >> kSimulcastShift;
+}
+
+bool VideoContentType::IsValidContentType(uint8_t value) {
+  // Any 6-bit value is allowed.
+  return value < (1 << 6);
+}
+
+bool VideoContentType::IsScreenshare() const {
+  return (content_type & kScreenshareBitsMask) > 0;
+}
+}  // namespace webrtc