Renovate FieldTrial class to better fit with histogram usage.

base/field_trial.h

Index: base/field_trial.h
===================================================================
--- base/field_trial.h	(revision 10679)
+++ base/field_trial.h	(working copy)
@@ -6,16 +6,65 @@
 // performed by actual users in the field (i.e., in a shipped or beta product).
 // All code is called exclusively on the UI thread currently.
 //
-// The simplest example is a test to see whether one of two options produces
-// "better" results across our user population.  In that scenario, UMA data
-// is uploaded to show the test results, and this class manages the state of
-// each such test (state == which option was pseudo-randomly selected).
+// The simplest example is an experiment to see whether one of two options
+// produces "better" results across our user population.  In that scenario, UMA
+// data is uploaded to aggregate the test results, and this FieldTrial class
+// manages the state of each such experiment (state == which option was
+// pseudo-randomly selected).
+//
 // States are typically generated randomly, either based on a one time
-// randomization (reused during each run of the program), or by a startup
-// randomization (keeping that tests state constant across a run), or by
-// continuous randomization across a run.
-// Only startup randomization is implemented (thus far).
+// randomization (generated randomly once, and then persitently reused in the
+// client during each future run of the program), or by a startup randomization
+// (generated each time the application starts up, but held constant during the
+// duration of the process), or by continuous randomization across a run (where
+// the state can be recalculated again and again, many times during a process).
+// Only startup randomization is implemented thus far.
 
+//------------------------------------------------------------------------------
+// Example:  Suppose we have an experiment involving memory, such as determining
+// the impact of memory model command line flags actual memory use.
+// We assume that we already have a histogram of memory usage, such as:
+
+//   HISTOGRAM_COUNTS("Memory.RendererTotal", count);
+
+// Somewhere in main thread initialization code, we'd probably define an
+// instance of a FieldTrial, with code such as:
+
+// // Note, FieldTrials are reference counted, and persist automagically until
+// // process teardown, courtesy of their automatic registration in
+// // FieldTrialList.
+// trial = new FieldTrial("MemoryExperiment", 1000);
+// group1 = trial->AppendGroup("_high_mem", 20);  // 2% this _high_mem group.
+// group2 = trial->AppendGroup("_low_mem", 20);   // 2% this _low_mem group.
+// // Take action depending of which group we randomly land in.
+// switch (trial->group()) {
+//   case group1:
+//          ... do something
+//          break;
+//   case group2:
+//          ....
+//          break;
+//   default:
+//          ...
+// }
+
+// We then modify any histograms we wish to correlate with our experiment to
+// have slighly different names, depending on what group the trial instance
+// happened (randomly) to be assigned to:
+
+// HISTOGRAM_COUNTS(FieldTrial::MakeName("Memory.RendererTotal",
+//                                       "MemoryExperiment"), count);
+
+// The above code will create 3 distinct histograms, with each run of the
+// application being assigned to of of teh three groups, and for each group, the
+// correspondingly named histogram will be populated:
+
+// Memory.RendererTotal            // 96% of users still fill this histogram.
+// Memory.RendererTotal_high_mem   // 2% of users will fill this histogram.
+// Memory.RendererTotal_low_mem    // 2% of users will fill this histogram.
+
+//------------------------------------------------------------------------------
+
 #ifndef BASE_FIELD_TRIAL_H_
 #define BASE_FIELD_TRIAL_H_
 
@@ -26,27 +75,74 @@
 #include "base/ref_counted.h"
 #include "base/time.h"
 
+
 class FieldTrial : public base::RefCounted<FieldTrial> {
  public:
-  // Constructor for a 2-state (boolean) trial.
+  static const int kNotParticipating = -1;
+
+  typedef int Probability;  // Use scaled up probability.
+
   // The name is used to register the instance with the FieldTrialList class,
   // and can be used to find the trial (only one trial can be present for each
-  // name) using the Find() method.
-  // The probability is a number in the range [0, 1], and is the likliehood that
-  // the assigned boolean value will be true.
-  FieldTrial(const std::wstring& name, double probability);
+  // name).
+  // Group probabilities that are later supplied must sum to less than or equal
+  // to the total_probability.
+  FieldTrial(const std::string& name, Probability total_probability);
 
-  // Return the selected boolean value.
-  bool boolean_value() const { return boolean_value_; }
-  std::wstring name() const { return name_; }
+  // Establish the name and probability of the next group in this trial.
+  // Sometimes, based on construction randomization, this call may causes the
+  // provided group to be *THE* group selected for use in this instance.
+  int AppendGroup(const std::string& name, Probability group_probability);
 
+  // Return the name of the FieldTrial (excluding the group name).
+  std::string name() const { return name_; }
+
+  // Return the randomly selected group number that was assigned.
+  // Return kNotParticipating if the instance is not participating in the
+  // experiment.
+  int group() const { return group_; }
+
+  // If the field trial is not in an experiment, this returns the empty string.
+  // if the group's name is empty, a name of "_" concatenated with the group
+  // number is used as the group name.
+  std::string group_name() const { return group_name_; }
+
+  // Helper function for the most common use: as an argument to specifiy the
+  // name of a HISTOGRAM.  Use the original histogram name as the name_prefix.
+  static std::string MakeName(const std::string& name_prefix,
+                              const std::string& trial_name);
+
  private:
-  const std::wstring name_;
-  bool boolean_value_;
+  // The name of the field trial, as can be found via the FieldTrialList.
+  // This is empty of the trial is not in the experiment.
+  const std::string name_;
 
+  // The maximu sum of all probabilities supplied, which corresponds to 100%.
+  // This is the scaling factor used to adjust supplied probabilities.
+  Probability divisor_;
+
+  // The randomly selected probability that is used to select a group (or have
+  // the instance not participate).  It is the product of divisor_ and a random
+  // number between [0, 1).
+  Probability random_;
+
+  // Sum of the probabilities of all appended groups.
+  Probability accumulated_group_probability_;
+
+  int next_group_number_;
+
+  // The pseudo-randomly assigned group number.
+  // This is kNotParticipating if no group has been assigned.
+  int group_;
+
+  // A textual name for the randomly selected group, including the Trial name.
+  // If this Trial is not a member of an group, this string is empty.
+  std::string group_name_;
+
   DISALLOW_COPY_AND_ASSIGN(FieldTrial);
 };
 
+//------------------------------------------------------------------------------
 // Class with a list of all active field trials.  A trial is active if it has
 // been registered, which includes evaluating its state based on its probaility.
 // Only one instance of this class exists.
@@ -63,12 +159,16 @@
 
   // The Find() method can be used to test to see if a named Trial was already
   // registered, or to retrieve a pointer to it from the global map.
-  static FieldTrial* Find(const std::wstring& name);
+  static FieldTrial* Find(const std::string& name);
 
+  static int FindValue(const std::string& name);
+
+  static std::string FindFullName(const std::string& name);
+
   // The time of construction of the global map is recorded in a static variable
   // and is commonly used by experiments to identify the time since the start
   // of the application.  In some experiments it may be useful to discount
-  // data that is gathered before the application has reach sufficient
+  // data that is gathered before the application has reached sufficient
   // stability (example: most DLL have loaded, etc.)
   static base::Time application_start_time() {
     if (global_)
@@ -78,7 +178,7 @@
   }
 
  private:
-  typedef std::map<std::wstring, FieldTrial*> RegistrationList;
+  typedef std::map<std::string, FieldTrial*> RegistrationList;
 
   static FieldTrialList* global_;  // The singleton of this class.