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}

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_