Refactor delay manager.

Split out `RelativeArrivalDelayTracker` and `DelayOptimizer` logic.

This is in preparation for adding another `DelayOptimizer` specialized in handling reordered packets.

Bug: webrtc:10178
Change-Id: Id3c1746d91980b171fa524f9b2b71cf11fc75f64
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/231224
Commit-Queue: Jakob Ivarsson <jakobi@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34938}
diff --git a/modules/audio_coding/BUILD.gn b/modules/audio_coding/BUILD.gn
index 7914806..d5fb1a4 100644
--- a/modules/audio_coding/BUILD.gn
+++ b/modules/audio_coding/BUILD.gn
@@ -974,6 +974,8 @@
     "neteq/random_vector.h",
     "neteq/red_payload_splitter.cc",
     "neteq/red_payload_splitter.h",
+    "neteq/relative_arrival_delay_tracker.cc",
+    "neteq/relative_arrival_delay_tracker.h",
     "neteq/statistics_calculator.cc",
     "neteq/statistics_calculator.h",
     "neteq/sync_buffer.cc",
@@ -982,6 +984,8 @@
     "neteq/time_stretch.h",
     "neteq/timestamp_scaler.cc",
     "neteq/timestamp_scaler.h",
+    "neteq/underrun_optimizer.cc",
+    "neteq/underrun_optimizer.h",
   ]
 
   deps = [
@@ -2011,12 +2015,14 @@
         "neteq/post_decode_vad_unittest.cc",
         "neteq/random_vector_unittest.cc",
         "neteq/red_payload_splitter_unittest.cc",
+        "neteq/relative_arrival_delay_tracker_unittest.cc",
         "neteq/statistics_calculator_unittest.cc",
         "neteq/sync_buffer_unittest.cc",
         "neteq/time_stretch_unittest.cc",
         "neteq/timestamp_scaler_unittest.cc",
         "neteq/tools/input_audio_file_unittest.cc",
         "neteq/tools/packet_unittest.cc",
+        "neteq/underrun_optimizer_unittest.cc",
       ]
 
       deps = [
diff --git a/modules/audio_coding/neteq/decision_logic.cc b/modules/audio_coding/neteq/decision_logic.cc
index ceefe50..30463fc 100644
--- a/modules/audio_coding/neteq/decision_logic.cc
+++ b/modules/audio_coding/neteq/decision_logic.cc
@@ -22,21 +22,28 @@
 #include "rtc_base/numerics/safe_conversions.h"
 #include "system_wrappers/include/field_trial.h"
 
+namespace webrtc {
+
 namespace {
 
 constexpr int kPostponeDecodingLevel = 50;
 constexpr int kDefaultTargetLevelWindowMs = 100;
 constexpr int kDecelerationTargetLevelOffsetMs = 85;
 
-}  // namespace
+std::unique_ptr<DelayManager> CreateDelayManager(
+    const NetEqController::Config& neteq_config) {
+  DelayManager::Config config;
+  config.max_packets_in_buffer = neteq_config.max_packets_in_buffer;
+  config.base_minimum_delay_ms = neteq_config.base_min_delay_ms;
+  config.Log();
+  return std::make_unique<DelayManager>(config, neteq_config.tick_timer);
+}
 
-namespace webrtc {
+}  // namespace
 
 DecisionLogic::DecisionLogic(NetEqController::Config config)
     : DecisionLogic(config,
-                    DelayManager::Create(config.max_packets_in_buffer,
-                                         config.base_min_delay_ms,
-                                         config.tick_timer),
+                    CreateDelayManager(config),
                     std::make_unique<BufferLevelFilter>()) {}
 
 DecisionLogic::DecisionLogic(
diff --git a/modules/audio_coding/neteq/decision_logic_unittest.cc b/modules/audio_coding/neteq/decision_logic_unittest.cc
index fc58035..b821653 100644
--- a/modules/audio_coding/neteq/decision_logic_unittest.cc
+++ b/modules/audio_coding/neteq/decision_logic_unittest.cc
@@ -61,11 +61,8 @@
     NetEqController::Config config;
     config.tick_timer = &tick_timer_;
     config.allow_time_stretching = true;
-    std::unique_ptr<Histogram> histogram =
-        std::make_unique<Histogram>(200, 12345, 2);
     auto delay_manager = std::make_unique<MockDelayManager>(
-        200, 0, 12300, absl::nullopt, 2000, config.tick_timer,
-        std::move(histogram));
+        DelayManager::Config(), config.tick_timer);
     mock_delay_manager_ = delay_manager.get();
     auto buffer_level_filter = std::make_unique<MockBufferLevelFilter>();
     mock_buffer_level_filter_ = buffer_level_filter.get();
diff --git a/modules/audio_coding/neteq/delay_manager.cc b/modules/audio_coding/neteq/delay_manager.cc
index c250977..9f7eebd 100644
--- a/modules/audio_coding/neteq/delay_manager.cc
+++ b/modules/audio_coding/neteq/delay_manager.cc
@@ -18,10 +18,8 @@
 #include <numeric>
 #include <string>
 
-#include "modules/audio_coding/neteq/histogram.h"
 #include "modules/include/module_common_types_public.h"
 #include "rtc_base/checks.h"
-#include "rtc_base/experiments/struct_parameters_parser.h"
 #include "rtc_base/logging.h"
 #include "rtc_base/numerics/safe_conversions.h"
 #include "rtc_base/numerics/safe_minmax.h"
@@ -32,157 +30,94 @@
 
 constexpr int kMinBaseMinimumDelayMs = 0;
 constexpr int kMaxBaseMinimumDelayMs = 10000;
-constexpr int kDelayBuckets = 100;
-constexpr int kBucketSizeMs = 20;
 constexpr int kStartDelayMs = 80;
 
-struct DelayManagerConfig {
-  double quantile = 0.97;
-  double forget_factor = 0.9993;
-  absl::optional<double> start_forget_weight = 2;
-  absl::optional<int> resample_interval_ms;
-  int max_history_ms = 2000;
-
-  std::unique_ptr<webrtc::StructParametersParser> Parser() {
-    return webrtc::StructParametersParser::Create(      //
-        "quantile", &quantile,                          //
-        "forget_factor", &forget_factor,                //
-        "start_forget_weight", &start_forget_weight,    //
-        "resample_interval_ms", &resample_interval_ms,  //
-        "max_history_ms", &max_history_ms);
-  }
-
-  // TODO(jakobi): remove legacy field trial.
-  void MaybeUpdateFromLegacyFieldTrial() {
-    constexpr char kDelayHistogramFieldTrial[] =
-        "WebRTC-Audio-NetEqDelayHistogram";
-    if (!webrtc::field_trial::IsEnabled(kDelayHistogramFieldTrial)) {
-      return;
-    }
-    const auto field_trial_string =
-        webrtc::field_trial::FindFullName(kDelayHistogramFieldTrial);
-    double percentile = -1.0;
-    double forget_factor = -1.0;
-    double start_forget_weight = -1.0;
-    if (sscanf(field_trial_string.c_str(), "Enabled-%lf-%lf-%lf", &percentile,
-               &forget_factor, &start_forget_weight) >= 2 &&
-        percentile >= 0.0 && percentile <= 100.0 && forget_factor >= 0.0 &&
-        forget_factor <= 1.0) {
-      this->quantile = percentile / 100;
-      this->forget_factor = forget_factor;
-      this->start_forget_weight = start_forget_weight >= 1
-                                      ? absl::make_optional(start_forget_weight)
-                                      : absl::nullopt;
-    }
-  }
-
-  explicit DelayManagerConfig() {
-    Parser()->Parse(webrtc::field_trial::FindFullName(
-        "WebRTC-Audio-NetEqDelayManagerConfig"));
-    MaybeUpdateFromLegacyFieldTrial();
-    RTC_LOG(LS_INFO) << "Delay manager config:"
-                        " quantile="
-                     << quantile << " forget_factor=" << forget_factor
-                     << " start_forget_weight="
-                     << start_forget_weight.value_or(0)
-                     << " resample_interval_ms="
-                     << resample_interval_ms.value_or(0)
-                     << " max_history_ms=" << max_history_ms;
-  }
-};
-
 }  // namespace
 
-DelayManager::DelayManager(int max_packets_in_buffer,
-                           int base_minimum_delay_ms,
-                           int histogram_quantile,
-                           absl::optional<int> resample_interval_ms,
-                           int max_history_ms,
-                           const TickTimer* tick_timer,
-                           std::unique_ptr<Histogram> histogram)
-    : max_packets_in_buffer_(max_packets_in_buffer),
-      histogram_(std::move(histogram)),
-      histogram_quantile_(histogram_quantile),
-      tick_timer_(tick_timer),
-      resample_interval_ms_(resample_interval_ms),
-      max_history_ms_(max_history_ms),
-      base_minimum_delay_ms_(base_minimum_delay_ms),
-      effective_minimum_delay_ms_(base_minimum_delay_ms),
+DelayManager::Config::Config() {
+  Parser()->Parse(webrtc::field_trial::FindFullName(
+      "WebRTC-Audio-NetEqDelayManagerConfig"));
+  MaybeUpdateFromLegacyFieldTrial();
+}
+
+void DelayManager::Config::Log() {
+  RTC_LOG(LS_INFO) << "Delay manager config:"
+                      " quantile="
+                   << quantile << " forget_factor=" << forget_factor
+                   << " start_forget_weight=" << start_forget_weight.value_or(0)
+                   << " resample_interval_ms="
+                   << resample_interval_ms.value_or(0)
+                   << " max_history_ms=" << max_history_ms;
+}
+
+std::unique_ptr<StructParametersParser> DelayManager::Config::Parser() {
+  return StructParametersParser::Create(              //
+      "quantile", &quantile,                          //
+      "forget_factor", &forget_factor,                //
+      "start_forget_weight", &start_forget_weight,    //
+      "resample_interval_ms", &resample_interval_ms,  //
+      "max_history_ms", &max_history_ms);
+}
+
+// TODO(jakobi): remove legacy field trial.
+void DelayManager::Config::MaybeUpdateFromLegacyFieldTrial() {
+  constexpr char kDelayHistogramFieldTrial[] =
+      "WebRTC-Audio-NetEqDelayHistogram";
+  if (!webrtc::field_trial::IsEnabled(kDelayHistogramFieldTrial)) {
+    return;
+  }
+  const auto field_trial_string =
+      webrtc::field_trial::FindFullName(kDelayHistogramFieldTrial);
+  double percentile = -1.0;
+  double forget_factor = -1.0;
+  double start_forget_weight = -1.0;
+  if (sscanf(field_trial_string.c_str(), "Enabled-%lf-%lf-%lf", &percentile,
+             &forget_factor, &start_forget_weight) >= 2 &&
+      percentile >= 0.0 && percentile <= 100.0 && forget_factor >= 0.0 &&
+      forget_factor <= 1.0) {
+    this->quantile = percentile / 100;
+    this->forget_factor = forget_factor;
+    this->start_forget_weight = start_forget_weight >= 1
+                                    ? absl::make_optional(start_forget_weight)
+                                    : absl::nullopt;
+  }
+}
+
+DelayManager::DelayManager(const Config& config, const TickTimer* tick_timer)
+    : max_packets_in_buffer_(config.max_packets_in_buffer),
+      underrun_optimizer_(tick_timer,
+                          (1 << 30) * config.quantile,
+                          (1 << 15) * config.forget_factor,
+                          config.start_forget_weight,
+                          config.resample_interval_ms),
+      relative_arrival_delay_tracker_(tick_timer, config.max_history_ms),
+      base_minimum_delay_ms_(config.base_minimum_delay_ms),
+      effective_minimum_delay_ms_(config.base_minimum_delay_ms),
       minimum_delay_ms_(0),
       maximum_delay_ms_(0),
       target_level_ms_(kStartDelayMs) {
-  RTC_CHECK(histogram_);
   RTC_DCHECK_GE(base_minimum_delay_ms_, 0);
 
   Reset();
 }
 
-std::unique_ptr<DelayManager> DelayManager::Create(
-    int max_packets_in_buffer,
-    int base_minimum_delay_ms,
-    const TickTimer* tick_timer) {
-  DelayManagerConfig config;
-  int forget_factor_q15 = (1 << 15) * config.forget_factor;
-  int quantile_q30 = (1 << 30) * config.quantile;
-  std::unique_ptr<Histogram> histogram = std::make_unique<Histogram>(
-      kDelayBuckets, forget_factor_q15, config.start_forget_weight);
-  return std::make_unique<DelayManager>(
-      max_packets_in_buffer, base_minimum_delay_ms, quantile_q30,
-      config.resample_interval_ms, config.max_history_ms, tick_timer,
-      std::move(histogram));
-}
-
 DelayManager::~DelayManager() {}
 
 absl::optional<int> DelayManager::Update(uint32_t timestamp,
                                          int sample_rate_hz,
                                          bool reset) {
-  if (sample_rate_hz <= 0) {
+  if (reset) {
+    relative_arrival_delay_tracker_.Reset();
+  }
+  absl::optional<int> relative_delay =
+      relative_arrival_delay_tracker_.Update(timestamp, sample_rate_hz);
+  if (!relative_delay) {
     return absl::nullopt;
   }
 
-  if (!last_timestamp_ || reset) {
-    // Restart relative delay esimation from this packet.
-    delay_history_.clear();
-    packet_iat_stopwatch_ = tick_timer_->GetNewStopwatch();
-    last_timestamp_ = timestamp;
-    resample_stopwatch_ = tick_timer_->GetNewStopwatch();
-    max_delay_in_interval_ms_ = 0;
-    return absl::nullopt;
-  }
-
-  const int expected_iat_ms =
-      1000ll * static_cast<int32_t>(timestamp - *last_timestamp_) /
-      sample_rate_hz;
-  const int iat_ms = packet_iat_stopwatch_->ElapsedMs();
-  const int iat_delay_ms = iat_ms - expected_iat_ms;
-  UpdateDelayHistory(iat_delay_ms, timestamp, sample_rate_hz);
-  int relative_delay = CalculateRelativePacketArrivalDelay();
-
-  absl::optional<int> histogram_update;
-  if (resample_interval_ms_) {
-    if (static_cast<int>(resample_stopwatch_->ElapsedMs()) >
-        *resample_interval_ms_) {
-      histogram_update = max_delay_in_interval_ms_;
-      resample_stopwatch_ = tick_timer_->GetNewStopwatch();
-      max_delay_in_interval_ms_ = 0;
-    }
-    max_delay_in_interval_ms_ =
-        std::max(max_delay_in_interval_ms_, relative_delay);
-  } else {
-    histogram_update = relative_delay;
-  }
-  if (histogram_update) {
-    const int index = *histogram_update / kBucketSizeMs;
-    if (index < histogram_->NumBuckets()) {
-      // Maximum delay to register is 2000 ms.
-      histogram_->Add(index);
-    }
-  }
-
-  // Calculate new `target_level_ms_` based on updated statistics.
-  int bucket_index = histogram_->Quantile(histogram_quantile_);
-  target_level_ms_ = (1 + bucket_index) * kBucketSizeMs;
+  underrun_optimizer_.Update(*relative_delay);
+  target_level_ms_ =
+      underrun_optimizer_.GetOptimalDelayMs().value_or(kStartDelayMs);
   target_level_ms_ = std::max(target_level_ms_, effective_minimum_delay_ms_);
   if (maximum_delay_ms_ > 0) {
     target_level_ms_ = std::min(target_level_ms_, maximum_delay_ms_);
@@ -195,37 +130,9 @@
         target_level_ms_, 3 * max_packets_in_buffer_ * packet_len_ms_ / 4);
   }
 
-  // Prepare for next packet arrival.
-  packet_iat_stopwatch_ = tick_timer_->GetNewStopwatch();
-  last_timestamp_ = timestamp;
   return relative_delay;
 }
 
-void DelayManager::UpdateDelayHistory(int iat_delay_ms,
-                                      uint32_t timestamp,
-                                      int sample_rate_hz) {
-  PacketDelay delay;
-  delay.iat_delay_ms = iat_delay_ms;
-  delay.timestamp = timestamp;
-  delay_history_.push_back(delay);
-  while (static_cast<int32_t>(timestamp - delay_history_.front().timestamp) >
-         max_history_ms_ * sample_rate_hz / 1000) {
-    delay_history_.pop_front();
-  }
-}
-
-int DelayManager::CalculateRelativePacketArrivalDelay() const {
-  // This effectively calculates arrival delay of a packet relative to the
-  // packet preceding the history window. If the arrival delay ever becomes
-  // smaller than zero, it means the reference packet is invalid, and we
-  // move the reference.
-  int relative_delay = 0;
-  for (const PacketDelay& delay : delay_history_) {
-    relative_delay += delay.iat_delay_ms;
-    relative_delay = std::max(relative_delay, 0);
-  }
-  return relative_delay;
-}
 
 int DelayManager::SetPacketAudioLength(int length_ms) {
   if (length_ms <= 0) {
@@ -238,13 +145,9 @@
 
 void DelayManager::Reset() {
   packet_len_ms_ = 0;
-  histogram_->Reset();
-  delay_history_.clear();
+  underrun_optimizer_.Reset();
+  relative_arrival_delay_tracker_.Reset();
   target_level_ms_ = kStartDelayMs;
-  packet_iat_stopwatch_ = tick_timer_->GetNewStopwatch();
-  last_timestamp_ = absl::nullopt;
-  resample_stopwatch_ = tick_timer_->GetNewStopwatch();
-  max_delay_in_interval_ms_ = 0;
 }
 
 int DelayManager::TargetDelayMs() const {
diff --git a/modules/audio_coding/neteq/delay_manager.h b/modules/audio_coding/neteq/delay_manager.h
index cc98203..c751836 100644
--- a/modules/audio_coding/neteq/delay_manager.h
+++ b/modules/audio_coding/neteq/delay_manager.h
@@ -19,28 +19,38 @@
 #include "absl/types/optional.h"
 #include "api/neteq/tick_timer.h"
 #include "modules/audio_coding/neteq/histogram.h"
+#include "modules/audio_coding/neteq/relative_arrival_delay_tracker.h"
+#include "modules/audio_coding/neteq/underrun_optimizer.h"
 #include "rtc_base/constructor_magic.h"
+#include "rtc_base/experiments/struct_parameters_parser.h"
 
 namespace webrtc {
 
 class DelayManager {
  public:
-  DelayManager(int max_packets_in_buffer,
-               int base_minimum_delay_ms,
-               int histogram_quantile,
-               absl::optional<int> resample_interval_ms,
-               int max_history_ms,
-               const TickTimer* tick_timer,
-               std::unique_ptr<Histogram> histogram);
+  struct Config {
+    Config();
+    void Log();
 
-  // Create a DelayManager object. Notify the delay manager that the packet
-  // buffer can hold no more than `max_packets_in_buffer` packets (i.e., this
-  // is the number of packet slots in the buffer) and that the target delay
-  // should be greater than or equal to `base_minimum_delay_ms`. Supply a
-  // PeakDetector object to the DelayManager.
-  static std::unique_ptr<DelayManager> Create(int max_packets_in_buffer,
-                                              int base_minimum_delay_ms,
-                                              const TickTimer* tick_timer);
+    // Options that can be configured via field trial.
+    double quantile = 0.97;
+    double forget_factor = 0.9993;
+    absl::optional<double> start_forget_weight = 2;
+    absl::optional<int> resample_interval_ms;
+    int max_history_ms = 2000;
+
+    // Options that are externally populated.
+    int max_packets_in_buffer = 200;
+    int base_minimum_delay_ms = 0;
+
+   private:
+    std::unique_ptr<StructParametersParser> Parser();
+
+    // TODO(jakobi): remove legacy field trial.
+    void MaybeUpdateFromLegacyFieldTrial();
+  };
+
+  DelayManager(const Config& config, const TickTimer* tick_timer);
 
   virtual ~DelayManager();
 
@@ -73,22 +83,12 @@
   int effective_minimum_delay_ms_for_test() const {
     return effective_minimum_delay_ms_;
   }
-  int histogram_quantile() const { return histogram_quantile_; }
-  Histogram* histogram() const { return histogram_.get(); }
 
  private:
   // Provides value which minimum delay can't exceed based on current buffer
   // size and given `maximum_delay_ms_`. Lower bound is a constant 0.
   int MinimumDelayUpperBound() const;
 
-  // Updates `delay_history_`.
-  void UpdateDelayHistory(int iat_delay_ms,
-                          uint32_t timestamp,
-                          int sample_rate_hz);
-
-  // Calculate relative packet arrival delay from `delay_history_`.
-  int CalculateRelativePacketArrivalDelay() const;
-
   // Updates `effective_minimum_delay_ms_` delay based on current
   // `minimum_delay_ms_`, `base_minimum_delay_ms_` and `maximum_delay_ms_`
   // and buffer size.
@@ -103,11 +103,8 @@
 
   // TODO(jakobi): set maximum buffer delay instead of number of packets.
   const int max_packets_in_buffer_;
-  std::unique_ptr<Histogram> histogram_;
-  const int histogram_quantile_;
-  const TickTimer* tick_timer_;
-  const absl::optional<int> resample_interval_ms_;
-  const int max_history_ms_;
+  UnderrunOptimizer underrun_optimizer_;
+  RelativeArrivalDelayTracker relative_arrival_delay_tracker_;
 
   int base_minimum_delay_ms_;
   int effective_minimum_delay_ms_;  // Used as lower bound for target delay.
@@ -115,19 +112,7 @@
   int maximum_delay_ms_;            // Externally set maximum allowed delay.
 
   int packet_len_ms_ = 0;
-  std::unique_ptr<TickTimer::Stopwatch>
-      packet_iat_stopwatch_;  // Time elapsed since last packet.
   int target_level_ms_;       // Currently preferred buffer level.
-  absl::optional<uint32_t>
-      last_timestamp_;  // Timestamp for the last received packet.
-  int max_delay_in_interval_ms_ = 0;
-  std::unique_ptr<TickTimer::Stopwatch> resample_stopwatch_;
-
-  struct PacketDelay {
-    int iat_delay_ms;
-    uint32_t timestamp;
-  };
-  std::deque<PacketDelay> delay_history_;
 
   RTC_DISALLOW_COPY_AND_ASSIGN(DelayManager);
 };
diff --git a/modules/audio_coding/neteq/delay_manager_unittest.cc b/modules/audio_coding/neteq/delay_manager_unittest.cc
index 427386e..fc4e0cb 100644
--- a/modules/audio_coding/neteq/delay_manager_unittest.cc
+++ b/modules/audio_coding/neteq/delay_manager_unittest.cc
@@ -28,60 +28,36 @@
 namespace webrtc {
 
 namespace {
-constexpr int kMaxNumberOfPackets = 240;
-constexpr int kMinDelayMs = 0;
-constexpr int kMaxHistoryMs = 2000;
+constexpr int kMaxNumberOfPackets = 200;
 constexpr int kTimeStepMs = 10;
 constexpr int kFs = 8000;
 constexpr int kFrameSizeMs = 20;
 constexpr int kTsIncrement = kFrameSizeMs * kFs / 1000;
 constexpr int kMaxBufferSizeMs = kMaxNumberOfPackets * kFrameSizeMs;
-constexpr int kDefaultHistogramQuantile = 1020054733;
-constexpr int kNumBuckets = 100;
-constexpr int kForgetFactor = 32745;
+
 }  // namespace
 
 class DelayManagerTest : public ::testing::Test {
  protected:
   DelayManagerTest();
   virtual void SetUp();
-  void RecreateDelayManager();
   absl::optional<int> InsertNextPacket();
   void IncreaseTime(int inc_ms);
 
-  std::unique_ptr<DelayManager> dm_;
+  DelayManager dm_;
   TickTimer tick_timer_;
-  MockStatisticsCalculator stats_;
-  MockHistogram* mock_histogram_;
   uint32_t ts_;
-  bool use_mock_histogram_ = false;
-  absl::optional<int> resample_interval_ms_;
 };
 
 DelayManagerTest::DelayManagerTest()
-    : dm_(nullptr),
-      ts_(0x12345678) {}
+    : dm_(DelayManager::Config(), &tick_timer_), ts_(0x12345678) {}
 
 void DelayManagerTest::SetUp() {
-  RecreateDelayManager();
-}
-
-void DelayManagerTest::RecreateDelayManager() {
-  if (use_mock_histogram_) {
-    mock_histogram_ = new MockHistogram(kNumBuckets, kForgetFactor);
-    std::unique_ptr<Histogram> histogram(mock_histogram_);
-    dm_ = std::make_unique<DelayManager>(kMaxNumberOfPackets, kMinDelayMs,
-                                         kDefaultHistogramQuantile,
-                                         resample_interval_ms_, kMaxHistoryMs,
-                                         &tick_timer_, std::move(histogram));
-  } else {
-    dm_ = DelayManager::Create(kMaxNumberOfPackets, kMinDelayMs, &tick_timer_);
-  }
-  dm_->SetPacketAudioLength(kFrameSizeMs);
+  dm_.SetPacketAudioLength(kFrameSizeMs);
 }
 
 absl::optional<int> DelayManagerTest::InsertNextPacket() {
-  auto relative_delay = dm_->Update(ts_, kFs);
+  auto relative_delay = dm_.Update(ts_, kFs);
   ts_ += kTsIncrement;
   return relative_delay;
 }
@@ -104,7 +80,7 @@
   IncreaseTime(kFrameSizeMs);
   // Second packet arrival.
   InsertNextPacket();
-  EXPECT_EQ(20, dm_->TargetDelayMs());
+  EXPECT_EQ(20, dm_.TargetDelayMs());
 }
 
 TEST_F(DelayManagerTest, UpdateLongInterArrivalTime) {
@@ -114,7 +90,7 @@
   IncreaseTime(2 * kFrameSizeMs);
   // Second packet arrival.
   InsertNextPacket();
-  EXPECT_EQ(40, dm_->TargetDelayMs());
+  EXPECT_EQ(40, dm_.TargetDelayMs());
 }
 
 TEST_F(DelayManagerTest, MaxDelay) {
@@ -126,16 +102,16 @@
   InsertNextPacket();
 
   // No limit is set.
-  EXPECT_EQ(kExpectedTarget, dm_->TargetDelayMs());
+  EXPECT_EQ(kExpectedTarget, dm_.TargetDelayMs());
 
   const int kMaxDelayMs = 3 * kFrameSizeMs;
-  EXPECT_TRUE(dm_->SetMaximumDelay(kMaxDelayMs));
+  EXPECT_TRUE(dm_.SetMaximumDelay(kMaxDelayMs));
   IncreaseTime(kFrameSizeMs);
   InsertNextPacket();
-  EXPECT_EQ(kMaxDelayMs, dm_->TargetDelayMs());
+  EXPECT_EQ(kMaxDelayMs, dm_.TargetDelayMs());
 
   // Target level at least should be one packet.
-  EXPECT_FALSE(dm_->SetMaximumDelay(kFrameSizeMs - 1));
+  EXPECT_FALSE(dm_.SetMaximumDelay(kFrameSizeMs - 1));
 }
 
 TEST_F(DelayManagerTest, MinDelay) {
@@ -147,23 +123,23 @@
   InsertNextPacket();
 
   // No limit is applied.
-  EXPECT_EQ(kExpectedTarget, dm_->TargetDelayMs());
+  EXPECT_EQ(kExpectedTarget, dm_.TargetDelayMs());
 
   int kMinDelayMs = 7 * kFrameSizeMs;
-  dm_->SetMinimumDelay(kMinDelayMs);
+  dm_.SetMinimumDelay(kMinDelayMs);
   IncreaseTime(kFrameSizeMs);
   InsertNextPacket();
-  EXPECT_EQ(kMinDelayMs, dm_->TargetDelayMs());
+  EXPECT_EQ(kMinDelayMs, dm_.TargetDelayMs());
 }
 
 TEST_F(DelayManagerTest, BaseMinimumDelayCheckValidRange) {
   // Base minimum delay should be between [0, 10000] milliseconds.
-  EXPECT_FALSE(dm_->SetBaseMinimumDelay(-1));
-  EXPECT_FALSE(dm_->SetBaseMinimumDelay(10001));
-  EXPECT_EQ(dm_->GetBaseMinimumDelay(), 0);
+  EXPECT_FALSE(dm_.SetBaseMinimumDelay(-1));
+  EXPECT_FALSE(dm_.SetBaseMinimumDelay(10001));
+  EXPECT_EQ(dm_.GetBaseMinimumDelay(), 0);
 
-  EXPECT_TRUE(dm_->SetBaseMinimumDelay(7999));
-  EXPECT_EQ(dm_->GetBaseMinimumDelay(), 7999);
+  EXPECT_TRUE(dm_.SetBaseMinimumDelay(7999));
+  EXPECT_EQ(dm_.GetBaseMinimumDelay(), 7999);
 }
 
 TEST_F(DelayManagerTest, BaseMinimumDelayLowerThanMinimumDelay) {
@@ -174,9 +150,9 @@
   // minimum delay is lower than minimum delay we use minimum delay.
   RTC_DCHECK_LT(kBaseMinimumDelayMs, kMinimumDelayMs);
 
-  EXPECT_TRUE(dm_->SetBaseMinimumDelay(kBaseMinimumDelayMs));
-  EXPECT_TRUE(dm_->SetMinimumDelay(kMinimumDelayMs));
-  EXPECT_EQ(dm_->effective_minimum_delay_ms_for_test(), kMinimumDelayMs);
+  EXPECT_TRUE(dm_.SetBaseMinimumDelay(kBaseMinimumDelayMs));
+  EXPECT_TRUE(dm_.SetMinimumDelay(kMinimumDelayMs));
+  EXPECT_EQ(dm_.effective_minimum_delay_ms_for_test(), kMinimumDelayMs);
 }
 
 TEST_F(DelayManagerTest, BaseMinimumDelayGreaterThanMinimumDelay) {
@@ -187,9 +163,9 @@
   // minimum delay is greater than minimum delay we use base minimum delay.
   RTC_DCHECK_GT(kBaseMinimumDelayMs, kMinimumDelayMs);
 
-  EXPECT_TRUE(dm_->SetBaseMinimumDelay(kBaseMinimumDelayMs));
-  EXPECT_TRUE(dm_->SetMinimumDelay(kMinimumDelayMs));
-  EXPECT_EQ(dm_->effective_minimum_delay_ms_for_test(), kBaseMinimumDelayMs);
+  EXPECT_TRUE(dm_.SetBaseMinimumDelay(kBaseMinimumDelayMs));
+  EXPECT_TRUE(dm_.SetMinimumDelay(kMinimumDelayMs));
+  EXPECT_EQ(dm_.effective_minimum_delay_ms_for_test(), kBaseMinimumDelayMs);
 }
 
 TEST_F(DelayManagerTest, BaseMinimumDelayGreaterThanBufferSize) {
@@ -198,7 +174,7 @@
   constexpr int kMaximumDelayMs = 20;
   constexpr int kMaxBufferSizeMsQ75 = 3 * kMaxBufferSizeMs / 4;
 
-  EXPECT_TRUE(dm_->SetMaximumDelay(kMaximumDelayMs));
+  EXPECT_TRUE(dm_.SetMaximumDelay(kMaximumDelayMs));
 
   // Base minimum delay is greater than minimum delay, that is why we clamp
   // it to current the highest possible value which is maximum delay.
@@ -207,15 +183,15 @@
   RTC_DCHECK_GT(kBaseMinimumDelayMs, kMaximumDelayMs);
   RTC_DCHECK_LT(kMaximumDelayMs, kMaxBufferSizeMsQ75);
 
-  EXPECT_TRUE(dm_->SetMinimumDelay(kMinimumDelayMs));
-  EXPECT_TRUE(dm_->SetBaseMinimumDelay(kBaseMinimumDelayMs));
+  EXPECT_TRUE(dm_.SetMinimumDelay(kMinimumDelayMs));
+  EXPECT_TRUE(dm_.SetBaseMinimumDelay(kBaseMinimumDelayMs));
 
   // Unset maximum value.
-  EXPECT_TRUE(dm_->SetMaximumDelay(0));
+  EXPECT_TRUE(dm_.SetMaximumDelay(0));
 
   // With maximum value unset, the highest possible value now is 75% of
   // currently possible maximum buffer size.
-  EXPECT_EQ(dm_->effective_minimum_delay_ms_for_test(), kMaxBufferSizeMsQ75);
+  EXPECT_EQ(dm_.effective_minimum_delay_ms_for_test(), kMaxBufferSizeMsQ75);
 }
 
 TEST_F(DelayManagerTest, BaseMinimumDelayGreaterThanMaximumDelay) {
@@ -229,10 +205,10 @@
   RTC_DCHECK_GT(kBaseMinimumDelayMs, kMaximumDelayMs);
   RTC_DCHECK_LT(kMaximumDelayMs, kMaxBufferSizeMs);
 
-  EXPECT_TRUE(dm_->SetMaximumDelay(kMaximumDelayMs));
-  EXPECT_TRUE(dm_->SetMinimumDelay(kMinimumDelayMs));
-  EXPECT_TRUE(dm_->SetBaseMinimumDelay(kBaseMinimumDelayMs));
-  EXPECT_EQ(dm_->effective_minimum_delay_ms_for_test(), kMaximumDelayMs);
+  EXPECT_TRUE(dm_.SetMaximumDelay(kMaximumDelayMs));
+  EXPECT_TRUE(dm_.SetMinimumDelay(kMinimumDelayMs));
+  EXPECT_TRUE(dm_.SetBaseMinimumDelay(kBaseMinimumDelayMs));
+  EXPECT_EQ(dm_.effective_minimum_delay_ms_for_test(), kMaximumDelayMs);
 }
 
 TEST_F(DelayManagerTest, BaseMinimumDelayLowerThanMaxSize) {
@@ -245,10 +221,10 @@
   RTC_DCHECK_GT(kBaseMinimumDelayMs, kMinimumDelayMs);
   RTC_DCHECK_LT(kBaseMinimumDelayMs, kMaximumDelayMs);
 
-  EXPECT_TRUE(dm_->SetMaximumDelay(kMaximumDelayMs));
-  EXPECT_TRUE(dm_->SetMinimumDelay(kMinimumDelayMs));
-  EXPECT_TRUE(dm_->SetBaseMinimumDelay(kBaseMinimumDelayMs));
-  EXPECT_EQ(dm_->effective_minimum_delay_ms_for_test(), kBaseMinimumDelayMs);
+  EXPECT_TRUE(dm_.SetMaximumDelay(kMaximumDelayMs));
+  EXPECT_TRUE(dm_.SetMinimumDelay(kMinimumDelayMs));
+  EXPECT_TRUE(dm_.SetBaseMinimumDelay(kBaseMinimumDelayMs));
+  EXPECT_EQ(dm_.effective_minimum_delay_ms_for_test(), kBaseMinimumDelayMs);
 }
 
 TEST_F(DelayManagerTest, MinimumDelayMemorization) {
@@ -260,19 +236,18 @@
   constexpr int kMinimumDelayMs = 20;
   constexpr int kBaseMinimumDelayMsHigh = 30;
 
-  EXPECT_TRUE(dm_->SetBaseMinimumDelay(kBaseMinimumDelayMsLow));
-  EXPECT_TRUE(dm_->SetMinimumDelay(kMinimumDelayMs));
+  EXPECT_TRUE(dm_.SetBaseMinimumDelay(kBaseMinimumDelayMsLow));
+  EXPECT_TRUE(dm_.SetMinimumDelay(kMinimumDelayMs));
   // Minimum delay is used as it is higher than base minimum delay.
-  EXPECT_EQ(dm_->effective_minimum_delay_ms_for_test(), kMinimumDelayMs);
+  EXPECT_EQ(dm_.effective_minimum_delay_ms_for_test(), kMinimumDelayMs);
 
-  EXPECT_TRUE(dm_->SetBaseMinimumDelay(kBaseMinimumDelayMsHigh));
+  EXPECT_TRUE(dm_.SetBaseMinimumDelay(kBaseMinimumDelayMsHigh));
   // Base minimum delay is used as it is now higher than minimum delay.
-  EXPECT_EQ(dm_->effective_minimum_delay_ms_for_test(),
-            kBaseMinimumDelayMsHigh);
+  EXPECT_EQ(dm_.effective_minimum_delay_ms_for_test(), kBaseMinimumDelayMsHigh);
 
-  EXPECT_TRUE(dm_->SetBaseMinimumDelay(kBaseMinimumDelayMsLow));
+  EXPECT_TRUE(dm_.SetBaseMinimumDelay(kBaseMinimumDelayMsLow));
   // Check that minimum delay is memorized and is used again.
-  EXPECT_EQ(dm_->effective_minimum_delay_ms_for_test(), kMinimumDelayMs);
+  EXPECT_EQ(dm_.effective_minimum_delay_ms_for_test(), kMinimumDelayMs);
 }
 
 TEST_F(DelayManagerTest, BaseMinimumDelay) {
@@ -284,16 +259,16 @@
   InsertNextPacket();
 
   // No limit is applied.
-  EXPECT_EQ(kExpectedTarget, dm_->TargetDelayMs());
+  EXPECT_EQ(kExpectedTarget, dm_.TargetDelayMs());
 
   constexpr int kBaseMinimumDelayMs = 7 * kFrameSizeMs;
-  EXPECT_TRUE(dm_->SetBaseMinimumDelay(kBaseMinimumDelayMs));
-  EXPECT_EQ(dm_->GetBaseMinimumDelay(), kBaseMinimumDelayMs);
+  EXPECT_TRUE(dm_.SetBaseMinimumDelay(kBaseMinimumDelayMs));
+  EXPECT_EQ(dm_.GetBaseMinimumDelay(), kBaseMinimumDelayMs);
 
   IncreaseTime(kFrameSizeMs);
   InsertNextPacket();
-  EXPECT_EQ(dm_->GetBaseMinimumDelay(), kBaseMinimumDelayMs);
-  EXPECT_EQ(kBaseMinimumDelayMs, dm_->TargetDelayMs());
+  EXPECT_EQ(dm_.GetBaseMinimumDelay(), kBaseMinimumDelayMs);
+  EXPECT_EQ(kBaseMinimumDelayMs, dm_.TargetDelayMs());
 }
 
 TEST_F(DelayManagerTest, BaseMinimumDelayAffectsTargetDelay) {
@@ -306,7 +281,7 @@
   InsertNextPacket();
 
   // No limit is applied.
-  EXPECT_EQ(kTimeIncrement, dm_->TargetDelayMs());
+  EXPECT_EQ(kTimeIncrement, dm_.TargetDelayMs());
 
   // Minimum delay is lower than base minimum delay, that is why base minimum
   // delay is used to calculate target level.
@@ -317,137 +292,31 @@
   constexpr int kBaseMinimumDelayMs = kBaseMinimumDelayPackets * kFrameSizeMs;
 
   EXPECT_TRUE(kMinimumDelayMs < kBaseMinimumDelayMs);
-  EXPECT_TRUE(dm_->SetMinimumDelay(kMinimumDelayMs));
-  EXPECT_TRUE(dm_->SetBaseMinimumDelay(kBaseMinimumDelayMs));
-  EXPECT_EQ(dm_->GetBaseMinimumDelay(), kBaseMinimumDelayMs);
+  EXPECT_TRUE(dm_.SetMinimumDelay(kMinimumDelayMs));
+  EXPECT_TRUE(dm_.SetBaseMinimumDelay(kBaseMinimumDelayMs));
+  EXPECT_EQ(dm_.GetBaseMinimumDelay(), kBaseMinimumDelayMs);
 
   IncreaseTime(kFrameSizeMs);
   InsertNextPacket();
-  EXPECT_EQ(dm_->GetBaseMinimumDelay(), kBaseMinimumDelayMs);
-  EXPECT_EQ(kBaseMinimumDelayMs, dm_->TargetDelayMs());
+  EXPECT_EQ(dm_.GetBaseMinimumDelay(), kBaseMinimumDelayMs);
+  EXPECT_EQ(kBaseMinimumDelayMs, dm_.TargetDelayMs());
 }
 
 TEST_F(DelayManagerTest, Failures) {
   // Wrong sample rate.
-  EXPECT_EQ(absl::nullopt, dm_->Update(0, -1));
+  EXPECT_EQ(absl::nullopt, dm_.Update(0, -1));
   // Wrong packet size.
-  EXPECT_EQ(-1, dm_->SetPacketAudioLength(0));
-  EXPECT_EQ(-1, dm_->SetPacketAudioLength(-1));
+  EXPECT_EQ(-1, dm_.SetPacketAudioLength(0));
+  EXPECT_EQ(-1, dm_.SetPacketAudioLength(-1));
 
   // Minimum delay higher than a maximum delay is not accepted.
-  EXPECT_TRUE(dm_->SetMaximumDelay(20));
-  EXPECT_FALSE(dm_->SetMinimumDelay(40));
+  EXPECT_TRUE(dm_.SetMaximumDelay(20));
+  EXPECT_FALSE(dm_.SetMinimumDelay(40));
 
   // Maximum delay less than minimum delay is not accepted.
-  EXPECT_TRUE(dm_->SetMaximumDelay(100));
-  EXPECT_TRUE(dm_->SetMinimumDelay(80));
-  EXPECT_FALSE(dm_->SetMaximumDelay(60));
-}
-
-TEST_F(DelayManagerTest, DelayHistogramFieldTrial) {
-  {
-    test::ScopedFieldTrials field_trial(
-        "WebRTC-Audio-NetEqDelayHistogram/Enabled-96-0.998/");
-    RecreateDelayManager();
-    EXPECT_EQ(1030792151, dm_->histogram_quantile());  // 0.96 in Q30.
-    EXPECT_EQ(
-        32702,
-        dm_->histogram()->base_forget_factor_for_testing());  // 0.998 in Q15.
-    EXPECT_FALSE(dm_->histogram()->start_forget_weight_for_testing());
-  }
-  {
-    test::ScopedFieldTrials field_trial(
-        "WebRTC-Audio-NetEqDelayHistogram/Enabled-97.5-0.998/");
-    RecreateDelayManager();
-    EXPECT_EQ(1046898278, dm_->histogram_quantile());  // 0.975 in Q30.
-    EXPECT_EQ(
-        32702,
-        dm_->histogram()->base_forget_factor_for_testing());  // 0.998 in Q15.
-    EXPECT_FALSE(dm_->histogram()->start_forget_weight_for_testing());
-  }
-  // Test parameter for new call start adaptation.
-  {
-    test::ScopedFieldTrials field_trial(
-        "WebRTC-Audio-NetEqDelayHistogram/Enabled-96-0.998-1/");
-    RecreateDelayManager();
-    EXPECT_EQ(dm_->histogram()->start_forget_weight_for_testing().value(), 1.0);
-  }
-  {
-    test::ScopedFieldTrials field_trial(
-        "WebRTC-Audio-NetEqDelayHistogram/Enabled-96-0.998-1.5/");
-    RecreateDelayManager();
-    EXPECT_EQ(dm_->histogram()->start_forget_weight_for_testing().value(), 1.5);
-  }
-  {
-    test::ScopedFieldTrials field_trial(
-        "WebRTC-Audio-NetEqDelayHistogram/Enabled-96-0.998-0.5/");
-    RecreateDelayManager();
-    EXPECT_FALSE(dm_->histogram()->start_forget_weight_for_testing());
-  }
-}
-
-TEST_F(DelayManagerTest, RelativeArrivalDelay) {
-  use_mock_histogram_ = true;
-  RecreateDelayManager();
-
-  InsertNextPacket();
-
-  IncreaseTime(kFrameSizeMs);
-  EXPECT_CALL(*mock_histogram_, Add(0));  // Not delayed.
-  InsertNextPacket();
-
-  IncreaseTime(2 * kFrameSizeMs);
-  EXPECT_CALL(*mock_histogram_, Add(1));  // 20ms delayed.
-  dm_->Update(ts_, kFs);
-
-  EXPECT_CALL(*mock_histogram_, Add(3));  // Reordered, 60ms delayed.
-  dm_->Update(ts_ - 2 * kTsIncrement, kFs);
-
-  IncreaseTime(2 * kFrameSizeMs);
-  EXPECT_CALL(*mock_histogram_, Add(2));  // 40ms delayed.
-  dm_->Update(ts_ + kTsIncrement, kFs);
-}
-
-TEST_F(DelayManagerTest, ReorderedPackets) {
-  use_mock_histogram_ = true;
-  RecreateDelayManager();
-
-  // Insert first packet.
-  InsertNextPacket();
-
-  // Insert reordered packet.
-  EXPECT_CALL(*mock_histogram_, Add(4));
-  dm_->Update(ts_ - 5 * kTsIncrement, kFs);
-
-  // Insert another reordered packet.
-  EXPECT_CALL(*mock_histogram_, Add(1));
-  dm_->Update(ts_ - 2 * kTsIncrement, kFs);
-
-  // Insert the next packet in order and verify that the relative delay is
-  // estimated based on the first inserted packet.
-  IncreaseTime(4 * kFrameSizeMs);
-  EXPECT_CALL(*mock_histogram_, Add(3));
-  InsertNextPacket();
-}
-
-TEST_F(DelayManagerTest, MaxDelayHistory) {
-  use_mock_histogram_ = true;
-  RecreateDelayManager();
-
-  InsertNextPacket();
-
-  // Insert 20 ms iat delay in the delay history.
-  IncreaseTime(2 * kFrameSizeMs);
-  EXPECT_CALL(*mock_histogram_, Add(1));  // 20ms delayed.
-  InsertNextPacket();
-
-  // Insert next packet with a timestamp difference larger than maximum history
-  // size. This removes the previously inserted iat delay from the history.
-  constexpr int kMaxHistoryMs = 2000;
-  IncreaseTime(kMaxHistoryMs + kFrameSizeMs);
-  ts_ += kFs * kMaxHistoryMs / 1000;
-  EXPECT_CALL(*mock_histogram_, Add(0));  // Not delayed.
-  dm_->Update(ts_, kFs);
+  EXPECT_TRUE(dm_.SetMaximumDelay(100));
+  EXPECT_TRUE(dm_.SetMinimumDelay(80));
+  EXPECT_FALSE(dm_.SetMaximumDelay(60));
 }
 
 TEST_F(DelayManagerTest, RelativeArrivalDelayStatistic) {
@@ -459,31 +328,4 @@
   EXPECT_EQ(20, InsertNextPacket());
 }
 
-TEST_F(DelayManagerTest, ResamplePacketDelays) {
-  use_mock_histogram_ = true;
-  resample_interval_ms_ = 500;
-  RecreateDelayManager();
-
-  // The histogram should be updated once with the maximum delay observed for
-  // the following sequence of packets.
-  EXPECT_CALL(*mock_histogram_, Add(5)).Times(1);
-
-  EXPECT_EQ(absl::nullopt, InsertNextPacket());
-
-  IncreaseTime(kFrameSizeMs);
-  EXPECT_EQ(0, InsertNextPacket());
-  IncreaseTime(3 * kFrameSizeMs);
-  EXPECT_EQ(2 * kFrameSizeMs, InsertNextPacket());
-  IncreaseTime(4 * kFrameSizeMs);
-  EXPECT_EQ(5 * kFrameSizeMs, InsertNextPacket());
-
-  for (int i = 4; i >= 0; --i) {
-    EXPECT_EQ(i * kFrameSizeMs, InsertNextPacket());
-  }
-  for (int i = 0; i < *resample_interval_ms_ / kFrameSizeMs; ++i) {
-    IncreaseTime(kFrameSizeMs);
-    EXPECT_EQ(0, InsertNextPacket());
-  }
-}
-
 }  // namespace webrtc
diff --git a/modules/audio_coding/neteq/mock/mock_delay_manager.h b/modules/audio_coding/neteq/mock/mock_delay_manager.h
index 5b5133e..d783f87 100644
--- a/modules/audio_coding/neteq/mock/mock_delay_manager.h
+++ b/modules/audio_coding/neteq/mock/mock_delay_manager.h
@@ -11,9 +11,6 @@
 #ifndef MODULES_AUDIO_CODING_NETEQ_MOCK_MOCK_DELAY_MANAGER_H_
 #define MODULES_AUDIO_CODING_NETEQ_MOCK_MOCK_DELAY_MANAGER_H_
 
-#include <memory>
-#include <utility>
-
 #include "api/neteq/tick_timer.h"
 #include "modules/audio_coding/neteq/delay_manager.h"
 #include "test/gmock.h"
@@ -22,20 +19,9 @@
 
 class MockDelayManager : public DelayManager {
  public:
-  MockDelayManager(size_t max_packets_in_buffer,
-                   int base_minimum_delay_ms,
-                   int histogram_quantile,
-                   absl::optional<int> resample_interval_ms,
-                   int max_history_ms,
-                   const TickTimer* tick_timer,
-                   std::unique_ptr<Histogram> histogram)
-      : DelayManager(max_packets_in_buffer,
-                     base_minimum_delay_ms,
-                     histogram_quantile,
-                     resample_interval_ms,
-                     max_history_ms,
-                     tick_timer,
-                     std::move(histogram)) {}
+  MockDelayManager(const MockDelayManager::Config& config,
+                   const TickTimer* tick_timer)
+      : DelayManager(config, tick_timer) {}
   MOCK_METHOD(int, TargetDelayMs, (), (const));
 };
 
diff --git a/modules/audio_coding/neteq/relative_arrival_delay_tracker.cc b/modules/audio_coding/neteq/relative_arrival_delay_tracker.cc
new file mode 100644
index 0000000..5b9580c
--- /dev/null
+++ b/modules/audio_coding/neteq/relative_arrival_delay_tracker.cc
@@ -0,0 +1,76 @@
+/*
+ *  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 "modules/audio_coding/neteq/relative_arrival_delay_tracker.h"
+
+#include <algorithm>
+
+namespace webrtc {
+
+absl::optional<int> RelativeArrivalDelayTracker::Update(uint32_t timestamp,
+                                                        int sample_rate_hz) {
+  if (sample_rate_hz <= 0) {
+    return absl::nullopt;
+  }
+  if (!last_timestamp_) {
+    // Restart relative delay esimation from this packet.
+    delay_history_.clear();
+    packet_iat_stopwatch_ = tick_timer_->GetNewStopwatch();
+    last_timestamp_ = timestamp;
+    return absl::nullopt;
+  }
+
+  const int expected_iat_ms =
+      1000ll * static_cast<int32_t>(timestamp - *last_timestamp_) /
+      sample_rate_hz;
+  const int iat_ms = packet_iat_stopwatch_->ElapsedMs();
+  const int iat_delay_ms = iat_ms - expected_iat_ms;
+  UpdateDelayHistory(iat_delay_ms, timestamp, sample_rate_hz);
+  int relative_delay = CalculateRelativePacketArrivalDelay();
+
+  packet_iat_stopwatch_ = tick_timer_->GetNewStopwatch();
+  last_timestamp_ = timestamp;
+
+  return relative_delay;
+}
+
+void RelativeArrivalDelayTracker::Reset() {
+  delay_history_.clear();
+  packet_iat_stopwatch_ = tick_timer_->GetNewStopwatch();
+  last_timestamp_ = absl::nullopt;
+}
+
+void RelativeArrivalDelayTracker::UpdateDelayHistory(int iat_delay_ms,
+                                                     uint32_t timestamp,
+                                                     int sample_rate_hz) {
+  PacketDelay delay;
+  delay.iat_delay_ms = iat_delay_ms;
+  delay.timestamp = timestamp;
+  delay_history_.push_back(delay);
+  while (static_cast<int32_t>(timestamp - delay_history_.front().timestamp) >
+         max_history_ms_ * sample_rate_hz / 1000) {
+    delay_history_.pop_front();
+  }
+}
+
+int RelativeArrivalDelayTracker::CalculateRelativePacketArrivalDelay() const {
+  // This effectively calculates arrival delay of a packet relative to the
+  // packet preceding the history window. If the arrival delay ever becomes
+  // smaller than zero, it means the reference packet is invalid, and we
+  // move the reference.
+  int relative_delay = 0;
+  for (const PacketDelay& delay : delay_history_) {
+    relative_delay += delay.iat_delay_ms;
+    relative_delay = std::max(relative_delay, 0);
+  }
+  return relative_delay;
+}
+
+}  // namespace webrtc
diff --git a/modules/audio_coding/neteq/relative_arrival_delay_tracker.h b/modules/audio_coding/neteq/relative_arrival_delay_tracker.h
new file mode 100644
index 0000000..da7f121
--- /dev/null
+++ b/modules/audio_coding/neteq/relative_arrival_delay_tracker.h
@@ -0,0 +1,57 @@
+/*
+ *  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.
+ */
+
+#ifndef MODULES_AUDIO_CODING_NETEQ_RELATIVE_ARRIVAL_DELAY_TRACKER_H_
+#define MODULES_AUDIO_CODING_NETEQ_RELATIVE_ARRIVAL_DELAY_TRACKER_H_
+
+#include <deque>
+#include <memory>
+
+#include "absl/types/optional.h"
+#include "api/neteq/tick_timer.h"
+
+namespace webrtc {
+
+class RelativeArrivalDelayTracker {
+ public:
+  RelativeArrivalDelayTracker(const TickTimer* tick_timer, int max_history_ms)
+      : tick_timer_(tick_timer), max_history_ms_(max_history_ms) {}
+
+  absl::optional<int> Update(uint32_t timestamp, int sample_rate_hz);
+
+  void Reset();
+
+ private:
+  // Updates `delay_history_`.
+  void UpdateDelayHistory(int iat_delay_ms,
+                          uint32_t timestamp,
+                          int sample_rate_hz);
+
+  // Calculate relative packet arrival delay from `delay_history_`.
+  int CalculateRelativePacketArrivalDelay() const;
+
+  const TickTimer* tick_timer_;
+  const int max_history_ms_;
+
+  struct PacketDelay {
+    int iat_delay_ms;
+    uint32_t timestamp;
+  };
+  std::deque<PacketDelay> delay_history_;
+
+  absl::optional<uint32_t>
+      last_timestamp_;  // Timestamp for the last received packet.
+
+  std::unique_ptr<TickTimer::Stopwatch>
+      packet_iat_stopwatch_;  // Time elapsed since last packet.
+};
+
+}  // namespace webrtc
+#endif  // MODULES_AUDIO_CODING_NETEQ_RELATIVE_ARRIVAL_DELAY_TRACKER_H_
diff --git a/modules/audio_coding/neteq/relative_arrival_delay_tracker_unittest.cc b/modules/audio_coding/neteq/relative_arrival_delay_tracker_unittest.cc
new file mode 100644
index 0000000..f764b85
--- /dev/null
+++ b/modules/audio_coding/neteq/relative_arrival_delay_tracker_unittest.cc
@@ -0,0 +1,81 @@
+/*
+ *  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 "modules/audio_coding/neteq/relative_arrival_delay_tracker.h"
+
+#include "test/gtest.h"
+
+namespace webrtc {
+
+namespace {
+constexpr int kMaxHistoryMs = 2000;
+constexpr int kFs = 8000;
+constexpr int kFrameSizeMs = 20;
+constexpr int kTsIncrement = kFrameSizeMs * kFs / 1000;
+constexpr uint32_t kTs = 0x12345678;
+}  // namespace
+
+TEST(RelativeArrivalDelayTrackerTest, RelativeArrivalDelay) {
+  TickTimer tick_timer;
+  RelativeArrivalDelayTracker tracker(&tick_timer, kMaxHistoryMs);
+
+  EXPECT_FALSE(tracker.Update(kTs, kFs));
+
+  tick_timer.Increment(kFrameSizeMs / tick_timer.ms_per_tick());
+  EXPECT_EQ(tracker.Update(kTs + kTsIncrement, kFs), 0);
+
+  tick_timer.Increment(2 * kFrameSizeMs / tick_timer.ms_per_tick());
+  EXPECT_EQ(tracker.Update(kTs + 2 * kTsIncrement, kFs), 20);
+
+  EXPECT_EQ(tracker.Update(kTs, kFs), 60);  // Reordered, 60ms delayed.
+
+  tick_timer.Increment(2 * kFrameSizeMs / tick_timer.ms_per_tick());
+  EXPECT_EQ(tracker.Update(kTs + 3 * kTsIncrement, kFs), 40);
+}
+
+TEST(RelativeArrivalDelayTrackerTest, ReorderedPackets) {
+  TickTimer tick_timer;
+  RelativeArrivalDelayTracker tracker(&tick_timer, kMaxHistoryMs);
+
+  // Insert first packet.
+  EXPECT_FALSE(tracker.Update(kTs, kFs));
+
+  // Insert reordered packet.
+  EXPECT_EQ(tracker.Update(kTs - 4 * kTsIncrement, kFs), 80);
+
+  // Insert another reordered packet.
+  EXPECT_EQ(tracker.Update(kTs - kTsIncrement, kFs), 20);
+
+  // Insert the next packet in order and verify that the relative delay is
+  // estimated based on the first inserted packet.
+  tick_timer.Increment(4 * kFrameSizeMs / tick_timer.ms_per_tick());
+  EXPECT_EQ(tracker.Update(kTs + kTsIncrement, kFs), 60);
+}
+
+TEST(RelativeArrivalDelayTrackerTest, MaxDelayHistory) {
+  TickTimer tick_timer;
+  RelativeArrivalDelayTracker tracker(&tick_timer, kMaxHistoryMs);
+
+  EXPECT_FALSE(tracker.Update(kTs, kFs));
+
+  // Insert 20 ms iat delay in the delay history.
+  tick_timer.Increment(2 * kFrameSizeMs / tick_timer.ms_per_tick());
+  EXPECT_EQ(tracker.Update(kTs + kTsIncrement, kFs), 20);
+
+  // Insert next packet with a timestamp difference larger than maximum history
+  // size. This removes the previously inserted iat delay from the history.
+  tick_timer.Increment((kMaxHistoryMs + kFrameSizeMs) /
+                       tick_timer.ms_per_tick());
+  EXPECT_EQ(
+      tracker.Update(kTs + 2 * kTsIncrement + kFs * kMaxHistoryMs / 1000, kFs),
+      0);
+}
+
+}  // namespace webrtc
diff --git a/modules/audio_coding/neteq/underrun_optimizer.cc b/modules/audio_coding/neteq/underrun_optimizer.cc
new file mode 100644
index 0000000..dad0424
--- /dev/null
+++ b/modules/audio_coding/neteq/underrun_optimizer.cc
@@ -0,0 +1,71 @@
+/*
+ *  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 "modules/audio_coding/neteq/underrun_optimizer.h"
+
+#include <algorithm>
+
+namespace webrtc {
+
+namespace {
+
+constexpr int kDelayBuckets = 100;
+constexpr int kBucketSizeMs = 20;
+
+}  // namespace
+
+UnderrunOptimizer::UnderrunOptimizer(const TickTimer* tick_timer,
+                                     int histogram_quantile,
+                                     int forget_factor,
+                                     absl::optional<int> start_forget_weight,
+                                     absl::optional<int> resample_interval_ms)
+    : tick_timer_(tick_timer),
+      histogram_(kDelayBuckets, forget_factor, start_forget_weight),
+      histogram_quantile_(histogram_quantile),
+      resample_interval_ms_(resample_interval_ms) {}
+
+void UnderrunOptimizer::Update(int relative_delay_ms) {
+  absl::optional<int> histogram_update;
+  if (resample_interval_ms_) {
+    if (!resample_stopwatch_) {
+      resample_stopwatch_ = tick_timer_->GetNewStopwatch();
+    }
+    if (static_cast<int>(resample_stopwatch_->ElapsedMs()) >
+        *resample_interval_ms_) {
+      histogram_update = max_delay_in_interval_ms_;
+      resample_stopwatch_ = tick_timer_->GetNewStopwatch();
+      max_delay_in_interval_ms_ = 0;
+    }
+    max_delay_in_interval_ms_ =
+        std::max(max_delay_in_interval_ms_, relative_delay_ms);
+  } else {
+    histogram_update = relative_delay_ms;
+  }
+  if (!histogram_update) {
+    return;
+  }
+
+  const int index = *histogram_update / kBucketSizeMs;
+  if (index < histogram_.NumBuckets()) {
+    // Maximum delay to register is 2000 ms.
+    histogram_.Add(index);
+  }
+  int bucket_index = histogram_.Quantile(histogram_quantile_);
+  optimal_delay_ms_ = (1 + bucket_index) * kBucketSizeMs;
+}
+
+void UnderrunOptimizer::Reset() {
+  histogram_.Reset();
+  resample_stopwatch_ = tick_timer_->GetNewStopwatch();
+  max_delay_in_interval_ms_ = 0;
+  optimal_delay_ms_.reset();
+}
+
+}  // namespace webrtc
diff --git a/modules/audio_coding/neteq/underrun_optimizer.h b/modules/audio_coding/neteq/underrun_optimizer.h
new file mode 100644
index 0000000..b37ce18
--- /dev/null
+++ b/modules/audio_coding/neteq/underrun_optimizer.h
@@ -0,0 +1,50 @@
+/*
+ *  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.
+ */
+
+#ifndef MODULES_AUDIO_CODING_NETEQ_UNDERRUN_OPTIMIZER_H_
+#define MODULES_AUDIO_CODING_NETEQ_UNDERRUN_OPTIMIZER_H_
+
+#include <memory>
+
+#include "absl/types/optional.h"
+#include "api/neteq/tick_timer.h"
+#include "modules/audio_coding/neteq/histogram.h"
+
+namespace webrtc {
+
+// Estimates probability of buffer underrun due to late packet arrival.
+// The optimal delay is decided such that the probability of underrun is lower
+// than 1 - `histogram_quantile`.
+class UnderrunOptimizer {
+ public:
+  UnderrunOptimizer(const TickTimer* tick_timer,
+                    int histogram_quantile,
+                    int forget_factor,
+                    absl::optional<int> start_forget_weight,
+                    absl::optional<int> resample_interval_ms);
+
+  void Update(int relative_delay_ms);
+
+  absl::optional<int> GetOptimalDelayMs() const { return optimal_delay_ms_; }
+
+  void Reset();
+
+ private:
+  const TickTimer* tick_timer_;
+  Histogram histogram_;
+  const int histogram_quantile_;  // In Q30.
+  const absl::optional<int> resample_interval_ms_;
+  std::unique_ptr<TickTimer::Stopwatch> resample_stopwatch_;
+  int max_delay_in_interval_ms_ = 0;
+  absl::optional<int> optimal_delay_ms_;
+};
+
+}  // namespace webrtc
+#endif  // MODULES_AUDIO_CODING_NETEQ_UNDERRUN_OPTIMIZER_H_
diff --git a/modules/audio_coding/neteq/underrun_optimizer_unittest.cc b/modules/audio_coding/neteq/underrun_optimizer_unittest.cc
new file mode 100644
index 0000000..a86e9cf
--- /dev/null
+++ b/modules/audio_coding/neteq/underrun_optimizer_unittest.cc
@@ -0,0 +1,42 @@
+/*
+ *  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 "modules/audio_coding/neteq/underrun_optimizer.h"
+
+#include "test/gtest.h"
+
+namespace webrtc {
+
+namespace {
+
+constexpr int kDefaultHistogramQuantile = 1020054733;  // 0.95 in Q30.
+constexpr int kForgetFactor = 32745;                   // 0.9993 in Q15.
+
+}  // namespace
+
+TEST(UnderrunOptimizerTest, ResamplePacketDelays) {
+  TickTimer tick_timer;
+  constexpr int kResampleIntervalMs = 500;
+  UnderrunOptimizer underrun_optimizer(&tick_timer, kDefaultHistogramQuantile,
+                                       kForgetFactor, absl::nullopt,
+                                       kResampleIntervalMs);
+
+  // The histogram should be updated once with the maximum delay observed for
+  // the following sequence of updates.
+  for (int i = 0; i < 500; i += 20) {
+    underrun_optimizer.Update(i);
+    EXPECT_FALSE(underrun_optimizer.GetOptimalDelayMs());
+  }
+  tick_timer.Increment(kResampleIntervalMs / tick_timer.ms_per_tick() + 1);
+  underrun_optimizer.Update(0);
+  EXPECT_EQ(underrun_optimizer.GetOptimalDelayMs(), 500);
+}
+
+}  // namespace webrtc