Implement THROTTLED priority semantics.

net/base/percentile_estimator.h

+// 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.
+
+#ifndef NET_BASE_PERCENTILE_ESTIMATOR_H_
+#define NET_BASE_PERCENTILE_ESTIMATOR_H_
+
+#include "base/callback.h"
+#include "base/macros.h"
+#include "net/base/net_export.h"
+
+namespace net {
+
+// This class estimates statistical percentiles (e.g. 10%l, 50%l) for
+// integer distributions presented in stream form.  These estimates
+// adjust automatically when the stream distribution changes.
+// TODO(rdsmith): Expand the class to maintain floating point
+// estimates rather than integer estimates, when there's a use case
+// for that that deserves the extra complexity and pitfalls of
+// floating point arithmetic.
+class NET_EXPORT PercentileEstimator {
+ public:
+  using RandomNumberCallback = base::Callback<int(void)>;
+
+  static const int kMedianPercentile = 50;
+
+  // |percentile| is a number between 0 and 100 indicating what percentile
+  // should be estimated (e.g. 50 would be a median estimate).
+  // |initial_estimate| is the value the class is seeded with; in other
+  // words, if AddSample() is never called,
+  // |CurrentEstimate() == initial_estimate|.
+  PercentileEstimator(int percentile, int initial_estimate);
+
+  ~PercentileEstimator();
+
+  int current_estimate() const { return current_estimate_; }
+  void AddSample(int sample);
+
+  // Specify a callback that will generate a "random" number
+  // in the range [0,99] on each call.  Used so that tests can
+  // rely on reproducible behavior.
+  void SetRandomNumberGeneratorForTesting(
+      RandomNumberCallback generator_callback);
+
+ private:
+  const int percentile_;
+
+  bool sign_positive_;
+  int current_estimate_;
+  int current_step_;
+
+  RandomNumberCallback generator_callback_;
+
+  DISALLOW_COPY_AND_ASSIGN(PercentileEstimator);
+};
+
+}  // namespace net
+
+#endif  // NET_BASE_PERCENTILE_ESTIMATOR_H_