quiche/quic/moqt/moqt_priority.cc

// Copyright 2024 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 "quiche/quic/moqt/moqt_priority.h"

#include <cstdint>
#include <limits>

#include "quiche/web_transport/web_transport.h"

namespace moqt {

namespace {
template <uint64_t NumBits>
constexpr uint64_t Flip(uint64_t number) {
  static_assert(NumBits <= 63);
  return (1ull << NumBits) - 1 - number;
}
template <uint64_t N>
constexpr uint64_t OnlyLowestNBits(uint64_t value) {
  static_assert(N <= 62);
  return value & ((1ull << (N + 1)) - 1);
}
}  // namespace

// The send order is packed into a signed 64-bit integer as follows:
//   63: always zero to indicate a positive number
//   62: 0 for data streams, 1 for control streams
//   54-61: subscriber priority
//   46-53: publisher priority
//     (if stream-per-group)
//   0-45: group ID
//     (if stream-per-object)
//   20-45: group ID
//   0-19: object ID

webtransport::SendOrder SendOrderForStream(MoqtPriority subscriber_priority,
                                           MoqtPriority publisher_priority,
                                           uint64_t group_id,
                                           MoqtDeliveryOrder delivery_order) {
  const int64_t track_bits = (Flip<8>(subscriber_priority) << 54) |
                             (Flip<8>(publisher_priority) << 46);
  group_id = OnlyLowestNBits<46>(group_id);
  if (delivery_order == MoqtDeliveryOrder::kAscending) {
    group_id = Flip<46>(group_id);
  }
  return track_bits | group_id;
}

webtransport::SendOrder SendOrderForStream(MoqtPriority subscriber_priority,
                                           MoqtPriority publisher_priority,
                                           uint64_t group_id,
                                           uint64_t object_id,
                                           MoqtDeliveryOrder delivery_order) {
  const int64_t track_bits = (Flip<8>(subscriber_priority) << 54) |
                             (Flip<8>(publisher_priority) << 46);
  group_id = OnlyLowestNBits<26>(group_id);
  object_id = OnlyLowestNBits<20>(object_id);
  if (delivery_order == MoqtDeliveryOrder::kAscending) {
    group_id = Flip<26>(group_id);
  }
  object_id = Flip<20>(object_id);  // Object ID is always ascending.
  return track_bits | (group_id << 20) | object_id;
}

const webtransport::SendOrder kMoqtControlStreamSendOrder =
    std::numeric_limits<webtransport::SendOrder>::max();

}  // namespace moqt