metrics: Add RandomSelector, a class that randomly selects items with odds

chrome/browser/metrics/perf/random_selector.h

+// Copyright 2015 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 CHROME_BROWSER_METRICS_PERF_RANDOM_SELECTOR_H_
+#define CHROME_BROWSER_METRICS_PERF_RANDOM_SELECTOR_H_
+
+#include <string>
+#include <vector>
+
+#include "base/macros.h"
+
+// RandomSelector can be used to pick vectors of strings according to certain
+// probabilities. The probabilities are set using SetOdds(). A randomly picked
+// vector can be obtained by calling Select().
+//
+// Sample usage:
+//
+// RandomSelector random_selector;
+// std::vector<RandomSelector::WeightAndValue> odds {
+//   {50, "a"},
+//   {40, "b"},
+//   {10, "c"}
+// };
+// random_selector.SetOdds(odds);
+//
+// std::vector<std::string>& selection = random_selector.Select();
+//
+// The above line should return "a" with a probability of 50%,
+// "b" with a probability of 40%, and "c" with a probability of 10%:
+class RandomSelector {
+ public:
+  struct WeightAndValue {
+    WeightAndValue(double weight, const std::string& value)
+      : weight(weight), value(value) {
+    }
+
+    // Probability weight for selecting this value.
+    double weight;
+    // Value to be returned by Select(), if selected.
+    std::string value;
+  };
+
+  RandomSelector();
+  virtual ~RandomSelector();
+
+  // Set the probabilities for various strings.
+  void SetOdds(const std::vector<WeightAndValue>& odds);
+
+  // Randomly select one of the values from the set.
+  const std::string& Select();
+
+  // Returns the number of string entries.
+  size_t num_values() const {
+    return odds_.size();
+  }
+
+  // Sum of the |weight| fields in the vector.
+  static double SumWeights(const std::vector<WeightAndValue>& odds);
+
+ private:
+  // Get a floating point number between 0.0 and |max|.
+  virtual double RandDoubleUpTo(double max);
+
+  // Get a string corresponding to |random| that is in the odds vector.
+  // |random| must be a number between zero and the sum of the probability
+  // weights.
+  const std::string& GetValueFor(double random);
+
+  // A dictionary representing the strings to choose from and associated odds.
+  std::vector<WeightAndValue> odds_;
+
+  // Sum of the probability weights.
+  double sum_of_weights_;
+
+  DISALLOW_COPY_AND_ASSIGN(RandomSelector);
+};
+
+#endif  // CHROME_BROWSER_METRICS_PERF_RANDOM_SELECTOR_H_