Multi-strategy based tracking.

chrome/browser/prefs/pref_hash_filter.h

Index: chrome/browser/prefs/pref_hash_filter.h
diff --git a/chrome/browser/prefs/pref_hash_filter.h b/chrome/browser/prefs/pref_hash_filter.h
index baa7811a05aca3fd5ed75039ac5f2f8f626e780a..ead8dcebbb1134fc95cebc2057666919811fea68 100644
--- a/chrome/browser/prefs/pref_hash_filter.h
+++ b/chrome/browser/prefs/pref_hash_filter.h
@@ -12,8 +12,7 @@
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/prefs/pref_filter.h"
-
-class PrefHashStore;
+#include "chrome/browser/prefs/pref_hash_store.h"
 
 namespace base {
 class DictionaryValue;
@@ -25,7 +24,29 @@ class Value;
 // are changed.
 class PrefHashFilter : public PrefFilter {
  public:
+  enum PrefTrackingStrategy {
+    // Atomic preferences are tracked as a whole.
+    TRACKING_STRATEGY_ATOMIC,
+    // Split preferences are dictionaries for which each top-level entry is
+    // tracked independently.

[Alexei Svitkine (slow), 2013/12/18 18:28:02]

Can you document that adding new split strategy prefs requires modifying the
histograms.xml file to list them?

[gab, 2013/12/18 18:48:03]

Done.

[Alexei Svitkine (slow), 2013/12/18 20:02:09]

Nit: I think there's value documenting it in this comment. So that someone
looking for what this means (and looking to add a new entry to the list) will
see it here.

[gab, 2013/12/18 20:15:43]

Done.

+    TRACKING_STRATEGY_SPLIT,
+  };
+
+  struct TrackedPreference {
+    size_t reporting_id;
+    const char* name;
+    PrefTrackingStrategy strategy;
+  };
+
+  // Constructs a PrefHashFilter with the default set of tracked preferences.
   explicit PrefHashFilter(scoped_ptr<PrefHashStore> pref_hash_store);
+
+  // Constructs a PrefHashFilter with the specified |tracked_preferences|. Meant
+  // to be used by tests only.
+  PrefHashFilter(scoped_ptr<PrefHashStore> pref_hash_store,
+                 const TrackedPreference tracked_preferences[],
+                 size_t tracked_preferences_size);
+
   virtual ~PrefHashFilter();
 
   // PrefFilter implementation.
@@ -35,8 +56,33 @@ class PrefHashFilter : public PrefFilter {
                             const base::Value* value) OVERRIDE;
 
  private:
+  struct TrackedPreferenceMetaData {
+    size_t reporting_id;
+    PrefTrackingStrategy strategy;
+  };
+
+  typedef std::map<std::string, const TrackedPreferenceMetaData>
+      TrackedPreferencesMap;
+
+  /// Initializes |tracked_paths_| from |tracked_preferences_|.
+  void InitTrackedPreferences(const TrackedPreference tracked_preferences[],
+                              size_t tracked_preferences_size);
+
+  // Updates the hash(es) in |pref_hash_store_| for |value| at |path| based on
+  // the tracking |strategy|.
+  void StoreHashUsingStrategy(const std::string& path,
+                              const base::Value* value,
+                              PrefTrackingStrategy strategy);
+
+  // Checks the hash(es) in |pref_hash_store_| for |value| at |path| based on
+  // the tracking |strategy|.
+  PrefHashStore::ValueState CheckValueUsingStrategy(
+      const std::string& path,
+      const base::Value* value,
+      PrefTrackingStrategy strategy);
+
   scoped_ptr<PrefHashStore> pref_hash_store_;
-  std::set<std::string> tracked_paths_;
+  TrackedPreferencesMap tracked_paths_;
 
   DISALLOW_COPY_AND_ASSIGN(PrefHashFilter);
 };