Add a StatisticsDeltaReader class and use cases

base/metrics/statistics_recorder.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // StatisticsRecorder holds all Histograms and BucketRanges that are used by
 // Histograms in the system. It provides a general place for
 // Histograms/BucketRanges to register, and supports a global API for accessing
 // (i.e., dumping, or graphing) the data.
 
 #ifndef BASE_METRICS_STATISTICS_RECORDER_H_
@@ skipping 52 matching lines @@
   static void GetHistograms(Histograms* output);
 
   // Method for extracting BucketRanges used by all histograms registered.
   static void GetBucketRanges(std::vector<const BucketRanges*>* output);
 
   // Find a histogram by name. It matches the exact name. This method is thread
   // safe.  It returns NULL if a matching histogram is not found.
   static HistogramBase* FindHistogram(const std::string& name);
 
   // GetSnapshot copies some of the pointers to registered histograms into the
-  // caller supplied vector (Histograms).  Only histograms with names matching
-  // query are returned. The query must be a substring of histogram name for its
-  // pointer to be copied.
+  // caller supplied vector (Histograms). Only histograms which have |query| as
+  // a substring are copied (an empty string will process all registered
+  // histograms).
   static void GetSnapshot(const std::string& query, Histograms* snapshot);
 
  private:
   // We keep all registered histograms in a map, from name to histogram.
   typedef std::map<std::string, HistogramBase*> HistogramMap;
 
   // We keep all |bucket_ranges_| in a map, from checksum to a list of
   // |bucket_ranges_|.  Checksum is calculated from the |ranges_| in
   // |bucket_ranges_|.
   typedef std::map<uint32, std::list<const BucketRanges*>*> RangesMap;
 
   friend struct DefaultLazyInstanceTraits<StatisticsRecorder>;
   friend class HistogramBaseTest;
   friend class HistogramTest;
   friend class SparseHistogramTest;
+  friend class StatisticsDeltaReaderTest;
   friend class StatisticsRecorderTest;
   FRIEND_TEST_ALL_PREFIXES(HistogramDeltaSerializationTest,
                            DeserializeHistogramAndAddSamples);
 
   // The constructor just initializes static members. Usually client code should
   // use Initialize to do this. But in test code, you can friend this class and
   // call destructor/constructor to get a clean StatisticsRecorder.
   StatisticsRecorder();
   ~StatisticsRecorder();
 
   static void DumpHistogramsToVlog(void* instance);
 
   static HistogramMap* histograms_;
   static RangesMap* ranges_;
 
   // Lock protects access to above maps.
   static base::Lock* lock_;
 
   DISALLOW_COPY_AND_ASSIGN(StatisticsRecorder);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_STATISTICS_RECORDER_H_