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* Copyright (c) 2021 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 <algorithm>
#include <cstddef>
#include <cstdint>
#include <memory>
#include "api/units/timestamp.h"
#include "rtc_base/checks.h"
namespace webrtc {
// PacketArrivalTimeMap is an optimized map of packet sequence number to arrival
// time, limited in size to never exceed `kMaxNumberOfPackets`. It will grow as
// needed, and remove old packets, and will expand to allow earlier packets to
// be added (out-of-order).
// Not yet received packets have the arrival time zero. The queue will not span
// larger than necessary and the last packet should always be received. The
// first packet in the queue doesn't have to be received in case of receiving
// packets out-of-order.
class PacketArrivalTimeMap {
struct PacketArrivalTime {
Timestamp arrival_time;
int64_t sequence_number;
// Impossible to request feedback older than what can be represented by 15
// bits.
static constexpr int kMaxNumberOfPackets = (1 << 15);
PacketArrivalTimeMap() = default;
PacketArrivalTimeMap(const PacketArrivalTimeMap&) = delete;
PacketArrivalTimeMap& operator=(const PacketArrivalTimeMap&) = delete;
~PacketArrivalTimeMap() = default;
// Indicates if the packet with `sequence_number` has already been received.
bool has_received(int64_t sequence_number) const {
return sequence_number >= begin_sequence_number() &&
sequence_number < end_sequence_number() &&
arrival_times_[Index(sequence_number)] >= Timestamp::Zero();
// Returns the sequence number of the first entry in the map, i.e. the
// sequence number that a `begin()` iterator would represent.
int64_t begin_sequence_number() const { return begin_sequence_number_; }
// Returns the sequence number of the element just after the map, i.e. the
// sequence number that an `end()` iterator would represent.
int64_t end_sequence_number() const { return end_sequence_number_; }
// Returns an element by `sequence_number`, which must be valid, i.e.
// between [begin_sequence_number, end_sequence_number).
Timestamp get(int64_t sequence_number) {
RTC_DCHECK_GE(sequence_number, begin_sequence_number());
RTC_DCHECK_LT(sequence_number, end_sequence_number());
return arrival_times_[Index(sequence_number)];
// Returns timestamp and sequence number of the received packet with sequence
// number equal or larger than `sequence_number`. `sequence_number` must be in
// range [begin_sequence_number, end_sequence_number).
PacketArrivalTime FindNextAtOrAfter(int64_t sequence_number) const {
RTC_DCHECK_GE(sequence_number, begin_sequence_number());
RTC_DCHECK_LT(sequence_number, end_sequence_number());
while (true) {
Timestamp t = arrival_times_[Index(sequence_number)];
if (t >= Timestamp::Zero()) {
return {.arrival_time = t, .sequence_number = sequence_number};
// Clamps `sequence_number` between [begin_sequence_number,
// end_sequence_number].
int64_t clamp(int64_t sequence_number) const {
return std::clamp(sequence_number, begin_sequence_number(),
// Erases all elements from the beginning of the map until `sequence_number`.
void EraseTo(int64_t sequence_number);
// Records the fact that a packet with `sequence_number` arrived at
// `arrival_time_ms`.
void AddPacket(int64_t sequence_number, Timestamp arrival_time);
// Removes packets from the beginning of the map as long as they are received
// before `sequence_number` and with an age older than `arrival_time_limit`
void RemoveOldPackets(int64_t sequence_number, Timestamp arrival_time_limit);
static constexpr int kMinCapacity = 128;
// Returns index in the `arrival_times_` for value for `sequence_number`.
int Index(int64_t sequence_number) const {
// Note that sequence_number might be negative, thus taking '%' requires
// extra handling and can be slow. Because capacity is a power of two, it
// is much faster to use '&' operator.
return sequence_number & capacity_minus_1_;
void SetNotReceived(int64_t begin_sequence_number_inclusive,
int64_t end_sequence_number_exclusive);
// Adjust capacity to match new_size, may reduce capacity.
// On return guarantees capacity >= new_size.
void AdjustToSize(int new_size);
void Reallocate(int new_capacity);
int capacity() const { return capacity_minus_1_ + 1; }
bool has_seen_packet() const { return arrival_times_ != nullptr; }
// Circular buffer. Packet with sequence number `sequence_number`
// is stored in the slot `sequence_number % capacity_`
std::unique_ptr<Timestamp[]> arrival_times_ = nullptr;
// Allocated size of the `arrival_times_`
// capacity_ is a power of 2 in range [kMinCapacity, kMaxNumberOfPackets]
// `capacity - 1` is used much more often than `capacity`, thus that value is
// stored.
int capacity_minus_1_ = -1;
// The unwrapped sequence number for valid range of sequence numbers.
// arrival_times_ entries only valid for sequence numbers in range
// `begin_sequence_number_ <= sequence_number < end_sequence_number_`
int64_t begin_sequence_number_ = 0;
int64_t end_sequence_number_ = 0;
} // namespace webrtc