Add and implement API to associate Google experiment IDs with FieldTrials.

chrome/common/metrics/experiments_helper.cc

Index: chrome/common/metrics/experiments_helper.cc
diff --git a/chrome/common/metrics/experiments_helper.cc b/chrome/common/metrics/experiments_helper.cc
index 09e41cc362e3375dd1f2e8ad03fae64c09e4a095..11c7c1389fca180304be1513a6498446e06d441e 100644
--- a/chrome/common/metrics/experiments_helper.cc
+++ b/chrome/common/metrics/experiments_helper.cc
@@ -6,13 +6,79 @@
 
 #include <vector>
 
-#include "base/metrics/field_trial.h"
+#include "base/memory/singleton.h"
 #include "base/string16.h"
 #include "base/stringprintf.h"
 #include "base/utf_string_conversions.h"
 #include "chrome/common/child_process_logging.h"
 
-namespace ExperimentsHelper {
+namespace {
+
+// We need to pass a Compare class to the std::map template since NameGroupId
+// is a user-defined type.
+struct NameGroupIdCompare {
+  bool operator() (const base::FieldTrial::NameGroupId& lhs,
+                   const base::FieldTrial::NameGroupId& rhs) const {
+    // The group and name fields are just SHA-1 Hashes, so we just need to treat
+    // them as IDs and do a less-than comparison. We test group first, since
+    // name is more likely to collide.
+    return lhs.group == rhs.group ? lhs.name < rhs.name :
+        lhs.group < rhs.group;
+  }
+};
+
+// The internal singleton accessor for the map, used to keep it thread-safe.
+class GroupMapAccessor {
+ public:
+  // Retrieve the singleton.
+  static GroupMapAccessor* GetInstance() {
+    return Singleton<GroupMapAccessor>::get();
+  }
+
+  GroupMapAccessor() {}
+  ~GroupMapAccessor() {}
+
+  void AssociateID(const base::FieldTrial::NameGroupId& group_identifier,
+                   experiments_helper::GoogleExperimentID id) {
+    base::AutoLock scoped_lock(lock_);
+    DCHECK(group_to_id_map_.find(group_identifier) == group_to_id_map_.end()) <<
+        "You can associate a group with a GoogleExperimentID only once.";
+    group_to_id_map_[group_identifier] = id;
+  }
+
+  experiments_helper::GoogleExperimentID GetID(
+      const base::FieldTrial::NameGroupId& group_identifier) {
+    base::AutoLock scoped_lock(lock_);
+    GroupToIDMap::const_iterator it = group_to_id_map_.find(group_identifier);
+    if (it == group_to_id_map_.end())
+      return experiments_helper::kEmptyGoogleExperimentID;
+    return it->second;
+  }
+
+ private:
+  typedef std::map<base::FieldTrial::NameGroupId,
+      experiments_helper::GoogleExperimentID, NameGroupIdCompare> GroupToIDMap;
+
+  base::Lock lock_;
+  GroupToIDMap group_to_id_map_;
+};
+
+}  // namespace
+
+namespace experiments_helper {
+
+const GoogleExperimentID kEmptyGoogleExperimentID = 0;
+
+void AssociateGoogleExperimentID(
+    const base::FieldTrial::NameGroupId& group_identifier,
+    GoogleExperimentID id) {
+  GroupMapAccessor::GetInstance()->AssociateID(group_identifier, id);
+}
+
+GoogleExperimentID GetGoogleExperimentID(
+    const base::FieldTrial::NameGroupId& group_identifier) {
+  return GroupMapAccessor::GetInstance()->GetID(group_identifier);
+}
 
 void SetChildProcessLoggingExperimentList() {
   std::vector<base::FieldTrial::NameGroupId> name_group_ids;
@@ -26,4 +92,4 @@ void SetChildProcessLoggingExperimentList() {
   child_process_logging::SetExperimentList(experiment_strings);
 }
 
-}  // namespace ExperimentsHelper
+}  // namespace experiments_helper