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+/*
+ *
+ * Copyright 2015, Google Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ */
+
+#ifndef TEST_QPS_INTERARRIVAL_H
+#define TEST_QPS_INTERARRIVAL_H
+
+#include <chrono>
+#include <cmath>
+#include <random>
+
+#include <grpc++/config.h>
+
+namespace grpc {
+namespace testing {
+
+// First create classes that define a random distribution
+// Note that this code does not include C++-specific random distribution
+// features supported in std::random. Although this would make this code easier,
+// this code is required to serve as the template code for other language
+// stacks. Thus, this code only uses a uniform distribution of doubles [0,1)
+// and then provides the distribution functions itself.
+
+class RandomDist {
+ public:
+  RandomDist() {}
+  virtual ~RandomDist() = 0;
+  // Argument to operator() is a uniform double in the range [0,1)
+  virtual double operator()(double uni) const = 0;
+};
+
+inline RandomDist::~RandomDist() {}
+
+// ExpDist implements an exponential distribution, which is the
+// interarrival distribution for a Poisson process. The parameter
+// lambda is the mean rate of arrivals. This is the
+// most useful distribution since it is actually additive and
+// memoryless. It is a good representation of activity coming in from
+// independent identical stationary sources. For more information,
+// see http://en.wikipedia.org/wiki/Exponential_distribution
+
+class ExpDist GRPC_FINAL : public RandomDist {
+ public:
+  explicit ExpDist(double lambda) : lambda_recip_(1.0 / lambda) {}
+  ~ExpDist() GRPC_OVERRIDE {}
+  double operator()(double uni) const GRPC_OVERRIDE {
+    // Note: Use 1.0-uni above to avoid NaN if uni is 0
+    return lambda_recip_ * (-log(1.0 - uni));
+  }
+
+ private:
+  double lambda_recip_;
+};
+
+// UniformDist implements a random distribution that has
+// interarrival time uniformly spread between [lo,hi). The
+// mean interarrival time is (lo+hi)/2. For more information,
+// see http://en.wikipedia.org/wiki/Uniform_distribution_%28continuous%29
+
+class UniformDist GRPC_FINAL : public RandomDist {
+ public:
+  UniformDist(double lo, double hi) : lo_(lo), range_(hi - lo) {}
+  ~UniformDist() GRPC_OVERRIDE {}
+  double operator()(double uni) const GRPC_OVERRIDE {
+    return uni * range_ + lo_;
+  }
+
+ private:
+  double lo_;
+  double range_;
+};
+
+// DetDist provides a random distribution with interarrival time
+// of val. Note that this is not additive, so using this on multiple
+// flows of control (threads within the same client or separate
+// clients) will not preserve any deterministic interarrival gap across
+// requests.
+
+class DetDist GRPC_FINAL : public RandomDist {
+ public:
+  explicit DetDist(double val) : val_(val) {}
+  ~DetDist() GRPC_OVERRIDE {}
+  double operator()(double uni) const GRPC_OVERRIDE { return val_; }
+
+ private:
+  double val_;
+};
+
+// ParetoDist provides a random distribution with interarrival time
+// spread according to a Pareto (heavy-tailed) distribution. In this
+// model, many interarrival times are close to the base, but a sufficient
+// number will be high (up to infinity) as to disturb the mean. It is a
+// good representation of the response times of data center jobs. See
+// http://en.wikipedia.org/wiki/Pareto_distribution
+
+class ParetoDist GRPC_FINAL : public RandomDist {
+ public:
+  ParetoDist(double base, double alpha)
+      : base_(base), alpha_recip_(1.0 / alpha) {}
+  ~ParetoDist() GRPC_OVERRIDE {}
+  double operator()(double uni) const GRPC_OVERRIDE {
+    // Note: Use 1.0-uni above to avoid div by zero if uni is 0
+    return base_ / pow(1.0 - uni, alpha_recip_);
+  }
+
+ private:
+  double base_;
+  double alpha_recip_;
+};
+
+// A class library for generating pseudo-random interarrival times
+// in an efficient re-entrant way. The random table is built at construction
+// time, and each call must include the thread id of the invoker
+
+typedef std::default_random_engine qps_random_engine;
+
+class InterarrivalTimer {
+ public:
+  InterarrivalTimer() {}
+  void init(const RandomDist& r, int threads, int entries = 1000000) {
+    qps_random_engine gen;
+    std::uniform_real_distribution<double> uniform(0.0, 1.0);
+    for (int i = 0; i < entries; i++) {
+      random_table_.push_back(std::chrono::nanoseconds(
+          static_cast<int64_t>(1e9 * r(uniform(gen)))));
+    }
+    // Now set up the thread positions
+    for (int i = 0; i < threads; i++) {
+      thread_posns_.push_back(random_table_.begin() + (entries * i) / threads);
+    }
+  }
+  virtual ~InterarrivalTimer(){};
+
+  std::chrono::nanoseconds operator()(int thread_num) {
+    auto ret = *(thread_posns_[thread_num]++);
+    if (thread_posns_[thread_num] == random_table_.end())
+      thread_posns_[thread_num] = random_table_.begin();
+    return ret;
+  }
+
+ private:
+  typedef std::vector<std::chrono::nanoseconds> time_table;
+  std::vector<time_table::const_iterator> thread_posns_;
+  time_table random_table_;
+};
+}
+}
+
+#endif