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

webrtc/api/video/video_content_type.cc

+/*
+ *  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 stored as a single byte, which is sent over 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.
+//
+
+namespace webrtc {
+namespace videocontenttypehelpers {
+
+namespace {
+static constexpr uint8_t kScreenshareBitsSize = 1;
+static constexpr uint8_t kScreenshareBitsMask =
+    (1u << kScreenshareBitsSize) - 1;
+
+static constexpr uint8_t kSimulcastShift = 1;
+static constexpr uint8_t kSimulcastBitsSize = 2;
+static constexpr uint8_t kSimulcastBitsMask = ((1u << kSimulcastBitsSize) - 1)
+                                              << kSimulcastShift;  // 0b00000110
+
+static constexpr uint8_t kExperimentShift = 3;
+static constexpr uint8_t kExperimentBitsSize = 3;
+static constexpr uint8_t kExperimentBitsMask =
+    ((1u << kExperimentBitsSize) - 1) << kExperimentShift;  // 0b00111000
+
+static constexpr uint8_t kTotalBitsSize =
+    kScreenshareBitsSize + kSimulcastBitsSize + kExperimentBitsSize;
+}  // namespace
+
+bool SetExperimentId(VideoContentType* content_type,
+                                              uint8_t experiment_id) {
+  // Store in bits 2-4.
+  if (experiment_id >= (1 << kExperimentBitsSize))
+    return false;
+  *content_type = static_cast<VideoContentType>(
+      (static_cast<uint8_t>(*content_type) & ~kExperimentBitsMask) |
+      ((experiment_id << kExperimentShift) & kExperimentBitsMask));
+  return true;
+}
+
+bool SetSimulcastId(VideoContentType* content_type,
+                                             uint8_t simulcast_id) {
+  // Store in bits 5-6.
+  if (simulcast_id >= (1 << kSimulcastBitsSize))
+    return false;
+  *content_type = static_cast<VideoContentType>(
+      (static_cast<uint8_t>(*content_type) & ~kSimulcastBitsMask) |
+      ((simulcast_id << kSimulcastShift) & kSimulcastBitsMask));
+  return true;
+}
+
+uint8_t GetExperimentId(
+    const VideoContentType& content_type) {
+  return (static_cast<uint8_t>(content_type) & kExperimentBitsMask) >>
+         kExperimentShift;
+}
+uint8_t GetSimulcastId(
+    const VideoContentType& content_type) {
+  return (static_cast<uint8_t>(content_type) & kSimulcastBitsMask) >>
+         kSimulcastShift;
+}
+
+bool IsScreenshare(
+    const VideoContentType& content_type) {
+  return (static_cast<uint8_t>(content_type) & kScreenshareBitsMask) > 0;
+}
+
+bool IsValidContentType(uint8_t value) {
+  // Any 6-bit value is allowed.
+  return value < (1 << kTotalBitsSize);
+}
+}  // namespace videocontenttypehelpers
+}  // namespace webrtc