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

base/metrics/field_trial.cc

Index: base/metrics/field_trial.cc
===================================================================
--- base/metrics/field_trial.cc	(revision 118842)
+++ base/metrics/field_trial.cc	(working copy)
@@ -42,6 +42,7 @@
                        const int month,
                        const int day_of_month)
   : name_(name),
+    name_hash_(HashName(name)),
     divisor_(total_probability),
     default_group_name_(default_group_name),
     random_(static_cast<Probability>(divisor_ * RandDouble())),
@@ -96,6 +97,7 @@
   if (group_ != kNotFinalized) {
     group_ = kDefaultGroupNumber;
     group_name_ = default_group_name_;
+    group_name_hash_ = HashName(group_name_);
   }
 }
 
@@ -117,6 +119,7 @@
       StringAppendF(&group_name_, "%d", group_);
     else
       group_name_ = name;
+    group_name_hash_ = HashName(group_name_);
     FieldTrialList::NotifyFieldTrialGroupSelection(name_, group_name_);
   }
   return next_group_number_++;
@@ -127,6 +130,7 @@
     accumulated_group_probability_ = divisor_;
     group_ = kDefaultGroupNumber;
     group_name_ = default_group_name_;
+    group_name_hash_ = HashName(group_name_);
     FieldTrialList::NotifyFieldTrialGroupSelection(name_, group_name_);
   }
   return group_;
@@ -138,6 +142,14 @@
   return group_name_;
 }
 
+bool FieldTrial::GetNameGroupId(NameGroupId* name_group_id) {
+  if (group_ == kNotFinalized)

[jar (doing other things), 2012/01/24 21:51:23]

You need to check for the (reserved) initial value of group_name_hash_.

[MAD, 2012/01/25 00:14:48]

Done.

+    return false;
+  name_group_id->name = name_hash_;
+  name_group_id->group = group_name_hash_;
+  return true;
+}
+
 // static
 std::string FieldTrial::MakeName(const std::string& name_prefix,
                                  const std::string& trial_name) {
@@ -173,6 +185,21 @@
   return BitsToOpenEndedUnitInterval(*bits);
 }
 
+// static
+uint32 FieldTrial::HashName(const std::string& name) {
+  // SHA-1 is designed to produce a uniformly random spread in its output space,
+  // even for nearly-identical name.
+  unsigned char sha1_hash[kSHA1Length];
+  SHA1HashBytes(reinterpret_cast<const unsigned char*>(name.c_str()),
+                name.size(),
+                sha1_hash);
+
+  COMPILE_ASSERT(sizeof(uint32) < sizeof(sha1_hash), need_more_data);
+  uint32* bits = reinterpret_cast<uint32*>(&sha1_hash[0]);
+
+  return *bits;
+}
+
 //------------------------------------------------------------------------------
 // FieldTrialList methods and members.
 
@@ -272,6 +299,22 @@
 }
 
 // static
+void FieldTrialList::GetFieldTrialNameGroupIds(
+    std::vector<FieldTrial::NameGroupId>* name_group_ids) {
+  if (!global_)
+    return;
+  DCHECK(name_group_ids->empty());
+  AutoLock auto_lock(global_->lock_);
+
+  for (RegistrationList::iterator it = global_->registered_.begin();
+       it != global_->registered_.end(); ++it) {
+    FieldTrial::NameGroupId name_group_id;
+    if (it->second->GetNameGroupId(&name_group_id))
+      name_group_ids->push_back(name_group_id);
+  }
+}
+
+// static
 bool FieldTrialList::CreateTrialsInChildProcess(
     const std::string& parent_trials) {
   DCHECK(global_);