Add support for persistent sparse histograms.

base/metrics/sample_map.h

Index: base/metrics/sample_map.h
diff --git a/base/metrics/sample_map.h b/base/metrics/sample_map.h
index da536e31e676d1e71516caf5c03c535719168451..6dff4fe13c2a2df2504b935f3fe3add877853c52 100644
--- a/base/metrics/sample_map.h
+++ b/base/metrics/sample_map.h
@@ -17,6 +17,7 @@
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/histogram_samples.h"
+#include "base/metrics/persistent_memory_allocator.h"
 
 namespace base {
 
@@ -34,9 +35,8 @@ class BASE_EXPORT SampleMap : public HistogramSamples {
   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_;
@@ -66,6 +66,73 @@ class BASE_EXPORT SampleMapIterator : public SampleCountIterator {
   const SampleToCountMap::const_iterator end_;
 };
 
+
+// A sample-map similar to the above but holds its samples in persistent memory
+// in order to be live across process restarts or be shared between processes.
+class BASE_EXPORT PersistentSampleMap : public HistogramSamples {

[Alexei Svitkine (slow), 2016/03/03 17:43:38]

I would make a separate file for this.

[bcwhite, 2016/03/07 15:30:39]

Done.

+ public:
+  PersistentSampleMap(uint64_t id,
+                      PersistentMemoryAllocator* allocator,
+                      Metadata* meta);
+  ~PersistentSampleMap() override;
+
+  // HistogramSamples implementation:

[Alexei Svitkine (slow), 2016/03/03 17:43:38]

Nit: // HistogramSamples:

[bcwhite, 2016/03/07 15:30:39]

Done.  Above, too.

+  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:
+  // Performs arithemetic. |op| is ADD or SUBTRACT.
+  bool AddSubtractImpl(SampleCountIterator* iter, Operator op) override;
+
+  // Gets a pointer to a "count" corresponding to a given |value|. It can
+  // optionally create the bucket if it doesn't already exist.
+  HistogramBase::Count* GetSampleCountStorage(HistogramBase::Sample value,
+                                              bool create_if_necessary);
+
+ private:
+  enum : HistogramBase::Sample { kAllSamples = -1 };
+
+  // Imports samples from persistent memory, stopping after importing
+  // |until_value| or when there is nothing left to import. Pass kAllSamples
+  // to import everything. Returns a pointer to the count for |until_value|
+  // if it is found, NULL otherwise.
+  HistogramBase::Count* ImportSamples(HistogramBase::Sample until_value);
+
+  // All created/loaded sample values and their associated counts.
+  std::map<HistogramBase::Sample, HistogramBase::Count*> sample_counts_;
+
+  // The persistent memory allocator holding samples and an iterator through it.
+  PersistentMemoryAllocator* allocator_;
+  PersistentMemoryAllocator::Iterator sample_iter_;
+
+  DISALLOW_COPY_AND_ASSIGN(PersistentSampleMap);
+};
+
+class BASE_EXPORT PersistentSampleMapIterator : public SampleCountIterator {

[Alexei Svitkine (slow), 2016/03/03 17:43:38]

How is this class different than SampleMapIterator?

[bcwhite, 2016/03/07 15:30:39]

It operates on a PersistentSampleMap instead of a SampleMap.  It boils down to a
map to Count* instead of Count and associated code changes.

Deriving it from SampeMapIterator instead of SampleCountIterator wouldn't change
anything; the same set of methods would have to be defined and there would be
duplicated private variables of the same names but slightly different types.

+ public:
+  typedef std::map<HistogramBase::Sample, HistogramBase::Count*>
+      SampleToCountMap;
+
+  explicit PersistentSampleMapIterator(const SampleToCountMap& sample_counts);
+  ~PersistentSampleMapIterator() override;
+
+  // SampleCountIterator implementation:
+  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_