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"
+
+namespace metrics {

[Alexei Svitkine (slow), 2015/09/14 19:12:33]

I think 'metrics' namespace is for things in components/metrics. I think the
code in c/b/metrics generally doesn't use a namespace, so I'd drop it.

[dhsharp, 2015/09/15 00:00:21]

You just told me to put it in 'metrics'... Done.

[Alexei Svitkine (slow), 2015/09/15 15:31:18]

Sorry, my previous comment was about your "using" statement in the test - the
point was to make the test be in the same namespace as the code it's testing so
it didn't need a using statement.

+
+// 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 GetNext().
+//
+// Sample usage:
+//
+// RandomSelector random_selector;
+// std::vector<RandomSelector::OddsAndValue> 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.GetNext();
+class RandomSelector {
+ public:
+  struct OddsAndValue {
+    OddsAndValue(double weight, const std::string& value)
+      : weight(weight), value(value) {
+    }
+
+    double weight;
+    std::string value;
+  };
+
+  RandomSelector();
+  virtual ~RandomSelector();
+
+  // Set the probabilities for various strings.
+  void SetOdds(const std::vector<OddsAndValue>& odds);
+
+  // Get the next randomly picked string in |next|.

[Alexei Svitkine (slow), 2015/09/14 19:12:33]

Update comment. No |next|.

Also, document semantics of GetNext() - i.e. mention that each pick is
independent. Or maybe even rename this to not use the "next" term, since that
implies the order is relevant.

[dhsharp, 2015/09/15 00:00:21]

Renamed "Select".

+  const std::string& GetNext();
+
+  // Returns the number of string entries.
+  size_t GetNumValues() const {

[Alexei Svitkine (slow), 2015/09/14 19:12:33]

Nit: User hacker_style for inline trivial functions.

[dhsharp, 2015/09/15 00:00:21]

Done.

+    return odds_.size();
+  }
+
+  // Sum of the |weight| fields in the vector.
+  static double SumOdds(const std::vector<OddsAndValue>& odds);
+
+ private:
+  // Get a floating point number between 0.0 and |max|.
+  virtual double RandDoubleUpTo(double max);
+
+  // Get a string corresponding to a random double |value| that is in our odds
+  // vector. Stores the result in |key|.

[Alexei Svitkine (slow), 2015/09/14 19:12:33]

Comment seems incorrect - there's no |key|.

[dhsharp, 2015/09/15 00:00:21]

Done. Sorry, old comments from debugd.

+  const std::string& GetKeyOf(double value);
+
+  // A dictionary representing the strings to choose from and associated odds.
+  std::vector<OddsAndValue> odds_;
+
+  double sum_of_odds_;
+
+  DISALLOW_COPY_AND_ASSIGN(RandomSelector);
+};
+
+}  // namespace metrics
+
+#endif  // CHROME_BROWSER_METRICS_PERF_RANDOM_SELECTOR_H_