Implement THROTTLED priority semantics.

net/base/percentile_estimator.cc

Index: net/base/percentile_estimator.cc
diff --git a/net/base/percentile_estimator.cc b/net/base/percentile_estimator.cc
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+++ b/net/base/percentile_estimator.cc
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+// Copyright 2016 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 "percentile_estimator.h"
+
+#include "base/bind.h"
+#include "base/callback.h"
+#include "base/rand_util.h"
+
+namespace {
+
+// Random number wrapper to allow substitutions for testing.
+int GenerateRand0To99() {
+  return base::RandInt(0, 99);
+}
+
+}  // namespace
+
+namespace net {
+
+// The algorithm used for percentile estimation is "Algorithm 3" from
+// https://arxiv.org/pdf/1407.1121v1.pdf.  There are several parts to the
+// algorithm:
+// * The estimate is conditionally moved towards the sample by a step amount.
+//   This means that if the samples are clustered around a value the estimates
+//   will converge to that sample.
+// * The percentile requested (e.g. 90%l) is handled by the conditional move.
+//   If the estimate is accurate, there is a chance equal to the percentile
+//   value that a sample will be lower than it, and a chance equal to
+//   1-percentile that it will be higher.  So the code balances those
+//   probabilities by increasing the estimate in the percentile fraction
+//   of the cases where the sample is over the estimate, and decreases the
+//   estimate in (1-percentile) fraction of the cases where the sample is under
+//   the estimate.
+//   E.g. in the case of the 90%l estimation, the estimate would
+//   move up in 90% of the cases in which the sample was above the
+//   estimate (which would be 10% of the total samples, presuming the
+//   estimate was accurate), and it would move down in 10% of the cases
+//   in which the sample was below the estimate.
+// * Every time the estimate moves in the same direction, the step
+//   amount is increased by one, and every time the estimate reverses
+//   direction, the step amount is decreased (to 1, if greater than 1,
+//   by one, if zero or negative).  The effective step amount is
+//   Max(step, 1).
+// * If the estimate
+//   would be moved beyond the sample causing its move, it is moved to
+//   be equal to the same (and the step amount set to the distance to
+//   the sample).  See the paper for further details.
+
+PercentileEstimator::PercentileEstimator(int percentile, int initial_estimate)
+    : percentile_(percentile),
+      sign_positive_(true),
+      current_estimate_(initial_estimate),
+      current_step_(1),
+      generator_callback_(base::Bind(&GenerateRand0To99)) {}
+
+PercentileEstimator::~PercentileEstimator() {}
+
+void PercentileEstimator::AddSample(int sample) {
+  int rand100 = generator_callback_.Run();
+  if (sample > current_estimate_ && rand100 > 1 - percentile_) {
+    current_step_ += sign_positive_ ? 1 : -1;
+    current_estimate_ += (current_step_ > 0) ? current_step_ : 1;
+
+    // Clamp movement to distance to sample.
+    if (current_estimate_ > sample) {
+      current_step_ -= current_estimate_ - sample;
+      current_estimate_ = sample;
+    }
+
+    // If we've reversed direction, reset the step down.
+    if (!sign_positive_ && current_step_ > 1)
+      current_step_ = 1;
+
+    sign_positive_ = true;
+  } else if (sample < current_estimate_ && rand100 > percentile_) {
+    current_step_ += !sign_positive_ ? 1 : -1;
+    current_estimate_ -= (current_step_ > 0) ? current_step_ : 1;
+
+    // Clamp movement to distance to sample.
+    if (current_estimate_ < sample) {
+      current_step_ -= sample - current_estimate_;
+      current_estimate_ = sample;
+    }
+
+    // If we've reversed direction, reset the step down.
+    if (sign_positive_ && current_step_ > 1)
+      current_step_ = 1;
+
+    sign_positive_ = false;
+  }
+}
+
+void PercentileEstimator::SetRandomNumberGeneratorForTesting(
+    RandomNumberCallback generator_callback) {
+  generator_callback_ = generator_callback;
+}
+
+}  // namespace net