Merge multiple histogram snapshots into single one for reporting.

base/metrics/histogram_base.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.
 
 #ifndef BASE_METRICS_HISTOGRAM_BASE_H_
 #define BASE_METRICS_HISTOGRAM_BASE_H_
 
 #include <limits.h>
 #include <stddef.h>
 #include <stdint.h>
@@ skipping 142 matching lines @@
   bool SerializeInfo(base::Pickle* pickle) const;
 
   // Try to find out data corruption from histogram and the samples.
   // The returned value is a combination of Inconsistency enum.
   virtual int FindCorruption(const HistogramSamples& samples) const;
 
   // Snapshot the current complete set of sample data.
   // Override with atomic/locked snapshot if needed.
   virtual scoped_ptr<HistogramSamples> SnapshotSamples() const = 0;
 
+  // Calculate delta from the previous call to this method.

[Alexei Svitkine (slow), 2016/02/09 19:46:51]

I don't think the comment is sufficiently clear. Please expand to make it clear
what it's doing.

"delta from previous call to this method" doesn't tell me much - i.e. delta of
what?

[bcwhite, 2016/02/11 16:42:38]

Done.

+  virtual scoped_ptr<HistogramSamples> SnapshotDelta() = 0;
+
   // The following methods provide graphical histogram displays.
   virtual void WriteHTMLGraph(std::string* output) const = 0;
   virtual void WriteAscii(std::string* output) const = 0;
 
   // Produce a JSON representation of the histogram. This is implemented with
   // the help of GetParameters and GetCountAndBucketData; overwrite them to
   // customize the output.
   void WriteJSON(std::string* output) const;
 
  protected:
@@ skipping 31 matching lines @@
  private:
   const std::string histogram_name_;
   AtomicCount flags_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramBase);
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_HISTOGRAM_BASE_H_