factor out path id + packet number packing code from the QUIC aead code

net/quic/quic_utils.cc

 // Copyright (c) 2012 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 "net/quic/quic_utils.h"
 
 #include <ctype.h>
 #include <stdint.h>
 
 #include <algorithm>
@@ skipping 446 matching lines @@
 void QuicUtils::ClearSerializedPacket(SerializedPacket* serialized_packet) {
   if (serialized_packet->retransmittable_frames != nullptr) {
     DeleteFrames(serialized_packet->retransmittable_frames);
   }
   delete serialized_packet->retransmittable_frames;
   delete serialized_packet->packet;
   serialized_packet->retransmittable_frames = nullptr;
   serialized_packet->packet = nullptr;
 }
 
+// static
+uint64_t QuicUtils::PackPathIdAndPacketNumber(QuicPathId path_id,
+                                              QuicPacketNumber packet_number) {
+  // Setting the nonce below relies on QuicPathId and QuicPacketNumber being
+  // specific sizes.
+  static_assert(sizeof(path_id) == 1, "Size of QuicPathId changed.");
+  static_assert(sizeof(packet_number) == 8,
+                "Size of QuicPacketNumber changed.");
+  // Use path_id and lower 7 bytes of packet_number as lower 8 bytes of nonce.
+  uint64_t path_id_packet_number =
+      (static_cast<uint64_t>(path_id) << 56) | packet_number;
+  DCHECK(path_id != kDefaultPathId || path_id_packet_number == packet_number);
+  return path_id_packet_number;
+}
+
 }  // namespace net