Implement WiFi Display media packetizer.

extensions/renderer/api/display_source/wifi_display/wifi_display_media_packetizer.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "extensions/renderer/api/display_source/wifi_display/wifi_display_media_packetizer.h"
+
+#include <utility>
+
+#include "base/logging.h"
+#include "base/rand_util.h"
+#include "crypto/random.h"
+#include "extensions/renderer/api/display_source/wifi_display/wifi_display_elementary_stream_info.h"
+
+namespace extensions {
+namespace {
+const size_t kMaxTransportStreamPacketCount = 7u;
+const uint8_t kProtocolPayloadTypeMP2T = 33u;
+const uint8_t kProtocolVersion = 2u;
+}  // namespace
+
+WiFiDisplayMediaDatagramPacket::WiFiDisplayMediaDatagramPacket() = default;
+
+WiFiDisplayMediaDatagramPacket::WiFiDisplayMediaDatagramPacket(
+    WiFiDisplayMediaDatagramPacket&&) = default;
+
+WiFiDisplayMediaPacketizer::WiFiDisplayMediaPacketizer(
+    const base::TimeDelta& delay_for_unit_time_stamps,
+    std::vector<WiFiDisplayElementaryStreamInfo> stream_infos,
+    const PacketizedCallback& on_packetized)
+    : WiFiDisplayTransportStreamPacketizer(delay_for_unit_time_stamps,
+                                           std::move(stream_infos)),
+      on_packetized_media_datagram_packet_(on_packetized) {
+  crypto::RandBytes(&sequence_number_, sizeof(sequence_number_));

[asargent_no_longer_on_chrome, 2016/03/22 19:15:36]

My immediate reaction reading this line is to be a little worried about whether
uint16_t has a wide enough range to effectively prevent collisions. Are there
going to a be a lot of these objects alive at the same time (in the "objects in
memory" or "packets on the local network" sense)? Are there bad consequences if
you did get a collision? Is it only the combination of
synchronization_source_identifier_ and sequence_number_ that need to be unique?

If you've already thought about all this and concluded it's ok, consider adding
a comment explaining why. 

+  base::RandBytes(&synchronization_source_identifier_,
+                  sizeof(synchronization_source_identifier_));
+}
+
+WiFiDisplayMediaPacketizer::~WiFiDisplayMediaPacketizer() {}
+
+bool WiFiDisplayMediaPacketizer::OnPacketizedTransportStreamPacket(
+    const WiFiDisplayTransportStreamPacket& transport_stream_packet,
+    bool flush) {
+  DCHECK(CalledOnValidThread());
+
+  if (media_datagram_packet_.empty()) {
+    // Convert time to the number of 90 kHz ticks since some epoch.
+    const uint64_t us = static_cast<uint64_t>(
+        (base::TimeTicks::Now() - base::TimeTicks()).InMicroseconds());
+    const uint64_t time_stamp = (us * 90u + 500u) / 1000u;
+    const uint8_t header_without_identifiers[] = {
+        (kProtocolVersion << 6 |          // Version (2 bits)
+         0x0u << 5 |                      // Padding (no)
+         0x0u << 4 |                      // Extension (no)
+         0u << 0),                        // CSRC count (4 bits)
+        (0x0u << 7 |                      // Marker (no)
+         kProtocolPayloadTypeMP2T << 0),  // Payload type (7 bits)
+        sequence_number_ >> 8,            // Sequence number (16 bits)
+        sequence_number_ & 0xFFu,         //
+        (time_stamp >> 24) & 0xFFu,       // Time stamp (32 bits)
+        (time_stamp >> 16) & 0xFFu,       //
+        (time_stamp >> 8) & 0xFFu,        //
+        time_stamp & 0xFFu};              //
+    ++sequence_number_;
+    media_datagram_packet_.reserve(
+        sizeof(header_without_identifiers) +
+        sizeof(synchronization_source_identifier_) +
+        kMaxTransportStreamPacketCount *
+            WiFiDisplayTransportStreamPacket::kPacketSize);
+    media_datagram_packet_.insert(media_datagram_packet_.end(),
+                                  std::begin(header_without_identifiers),
+                                  std::end(header_without_identifiers));
+    media_datagram_packet_.insert(
+        media_datagram_packet_.end(),
+        std::begin(synchronization_source_identifier_),
+        std::end(synchronization_source_identifier_));
+    DCHECK_EQ(0u, media_datagram_packet_.size() /
+                      WiFiDisplayTransportStreamPacket::kPacketSize);
+  }
+
+  // Append header and payload data.
+  media_datagram_packet_.insert(media_datagram_packet_.end(),
+                                transport_stream_packet.header().begin(),
+                                transport_stream_packet.header().end());
+  media_datagram_packet_.insert(media_datagram_packet_.end(),
+                                transport_stream_packet.payload().begin(),
+                                transport_stream_packet.payload().end());
+  media_datagram_packet_.insert(media_datagram_packet_.end(),
+                                transport_stream_packet.filler().size(),
+                                transport_stream_packet.filler().value());
+
+  // Combine multiple transport stream packets into one datagram packet
+  // by delaying delegation whenever possible.
+  if (!flush) {
+    const size_t transport_stream_packet_count =
+        media_datagram_packet_.size() /
+        WiFiDisplayTransportStreamPacket::kPacketSize;
+    if (transport_stream_packet_count < kMaxTransportStreamPacketCount)
+      return true;
+  }
+
+  WiFiDisplayMediaDatagramPacket packet;
+  packet.swap(media_datagram_packet_);
+  return on_packetized_media_datagram_packet_.Run(std::move(packet));
+}
+
+}  // namespace extensions