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);
+//
+// // The following should return "a" with a probability of 50%,
+// // "b" with a probability of 40%, and "c" with a probability of 10%:
+//
+// std::vector<std::string>& selection = random_selector.Select();

[Alexei Svitkine (slow), 2015/09/15 18:36:12]

Nit: I find the nested comment above a bit messy. How about moving this code
line right below line 25 and moving the text on line 27 to below it, unnesting
the comment and changing it to say "The above should return ...".

[dhsharp, 2015/09/16 01:01:40]

Done.

+class RandomSelector {
+ public:
+  struct WeightAndValue {
+    WeightAndValue(double weight, const std::string& value)
+      : weight(weight), value(value) {
+    }
+
+    double weight;

[Alexei Svitkine (slow), 2015/09/15 18:36:12]

Nit: Add a comment mention it's the probability weight.

Also add a comment about the value field below.

[dhsharp, 2015/09/16 01:01:40]

Done.

+    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_