Add support for persistent sparse histograms.

base/metrics/sample_map.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.
 
 // SampleMap implements HistogramSamples interface. It is used by the
 // SparseHistogram class to store samples.
 
 #ifndef BASE_METRICS_SAMPLE_MAP_H_
 #define BASE_METRICS_SAMPLE_MAP_H_
 
 #include <stdint.h>
 
 #include <map>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/histogram_samples.h"
 
 namespace base {
 
 class BASE_EXPORT SampleMap : public HistogramSamples {
  public:
   SampleMap();
   explicit SampleMap(uint64_t id);
   ~SampleMap() override;
 
-  // HistogramSamples implementation:
+  // HistogramSamples:
   void Accumulate(HistogramBase::Sample value,
                   HistogramBase::Count count) override;
   HistogramBase::Count GetCount(HistogramBase::Sample value) const override;
   HistogramBase::Count TotalCount() const override;
   scoped_ptr<SampleCountIterator> Iterator() const override;
 
  protected:
-  bool AddSubtractImpl(
-      SampleCountIterator* iter,
-      HistogramSamples::Operator op) override;  // |op| is ADD or SUBTRACT.
+  // Performs arithemetic. |op| is ADD or SUBTRACT.
+  bool AddSubtractImpl(SampleCountIterator* iter, Operator op) override;
 
  private:
   std::map<HistogramBase::Sample, HistogramBase::Count> sample_counts_;
 
   DISALLOW_COPY_AND_ASSIGN(SampleMap);
 };
 
 class BASE_EXPORT SampleMapIterator : public SampleCountIterator {
  public:
   typedef std::map<HistogramBase::Sample, HistogramBase::Count>
       SampleToCountMap;
 
   explicit SampleMapIterator(const SampleToCountMap& sample_counts);
   ~SampleMapIterator() override;
 
-  // SampleCountIterator implementation:
+  // SampleCountIterator:
   bool Done() const override;
   void Next() override;
   void Get(HistogramBase::Sample* min,
            HistogramBase::Sample* max,
            HistogramBase::Count* count) const override;
 
  private:
   void SkipEmptyBuckets();
 
   SampleToCountMap::const_iterator iter_;
   const SampleToCountMap::const_iterator end_;
 };
 
 }  // namespace base
 
 #endif  // BASE_METRICS_SAMPLE_MAP_H_