Added a Unique ID for a Field Trial containing it's hashed name and the selected group ID.

base/metrics/field_trial.h

Index: base/metrics/field_trial.h
===================================================================
--- base/metrics/field_trial.h	(revision 118842)
+++ base/metrics/field_trial.h	(working copy)
@@ -92,6 +92,9 @@
 class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
  public:
   typedef int Probability;  // Probability type for being selected in a trial.
+  // The Unique ID of a trial, first element is a hashed value of the name
+  // and the second one is the group identifier.
+  typedef std::pair<size_t, int> UID;

[jar (doing other things), 2012/01/24 19:19:45]

You might want to send two hashes, rather than one.  The integer will vary
depending on the experiments the programmer includes.  It is common to see the
names of the experiments changed, such as when folks are trying various window
sizes.... decide that some window sizes are very bad... and decide to try some
additional sizes.  Folks *could* learn to give 0% chance to such obsolete cases,
but this is not currently enforceable.

Also, size_t should only be used when required, such as for stl:: array indices,
or malloc sizes.  Better is to use uint32 or such.

The typename UID appears to me to be pretty cryptic, and is in fact an
abbreviation (which the style guide would not endorse).  Given that the type is
in the FieldTrial class, we can assume that much is known, and might refine it
with something like:
NameGroupPair
or
NameAndChoice
other??

Lastly, it is probably better to use a struct than a pair, as readability is
improved (and it is not necessary to "remember" which field comes first).

[MAD, 2012/01/24 20:30:24]

OK! Done...
Thanks! :-)

 
   // A return value to indicate that a given instance has not yet had a group
   // assignment (and hence is not yet participating in the trial).
@@ -148,6 +151,10 @@
   // number is used as the group name.
   std::string group_name();
 
+  // Gets the unique identifier of the Field Trial, but only if a winner was
+  // elected. Returns true if a winner is returned, false otherwise.
+  bool GetUID(UID* uid);
+
   // Return the default group name of the FieldTrial.
   std::string default_group_name() const { return default_group_name_; }
 
@@ -285,11 +292,14 @@
 
   // Create a persistent representation of all FieldTrial instances and the
   // |client_id()| state for resurrection in another process.  This allows
-  // randomization to be done in one process, and secondary processes can by
+  // randomization to be done in one process, and secondary processes can be
   // synchronized on the result. The resulting string contains the
   // |client_id()|, the names, the trial name, and a "/" separator.
   static void StatesToString(std::string* output);
 
+  // Returns an array of Unique IDs for each currently running Field Trials.
+  static void GetFieldTrialUIDs(std::vector<FieldTrial::UID>* uids);
+
   // Use a previously generated state string (re: StatesToString()) augment the
   // current list of field tests to include the supplied tests, and using a 100%
   // probability for each test, force them to have the same group string.  This