Add support for persistent sparse histograms.

base/metrics/persistent_sample_map.h

+// Copyright (c) 2016 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.
+
+// PersistentSampleMap implements HistogramSamples interface. It is used
+// by the SparseHistogram class to store samples in persistent memory which
+// allows it to be shared between processes or live across restarts.
+
+#ifndef BASE_METRICS_PERSISTENT_SAMPLE_MAP_H_
+#define BASE_METRICS_PERSISTENT_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"
+#include "base/metrics/persistent_memory_allocator.h"
+
+namespace base {
+
+class BASE_EXPORT PersistentSampleMap : public HistogramSamples {
+ public:
+  PersistentSampleMap(uint64_t id,
+                      PersistentMemoryAllocator* allocator,
+                      Metadata* meta);
+  ~PersistentSampleMap() override;
+
+  // 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:
+  // 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|. Returns NULL
+  // if sample does not exist.
+  HistogramBase::Count* GetSampleCountStorage(HistogramBase::Sample value);
+
+  // Gets a pointer to a "count" corresponding to a given |value|, creating
+  // the sample (initialized to zero) if it does not already exists.
+  HistogramBase::Count* GetOrCreateSampleCountStorage(
+      HistogramBase::Sample value);
+
+ private:
+  enum : HistogramBase::Sample { kAllSamples = -1 };
+
+  // Imports samples from persistent memory, stopping after importing

[Alexei Svitkine (slow), 2016/03/08 19:46:25]

I don't understand what Import means in this context - without reading the code.
Can you expand the comment to explain this? i.e. that it iterates over all
persistent memory and fills |sample_counts_| map as it does.

[bcwhite, 2016/03/09 01:16:52]

Done.

+  // |until_value| or when there is nothing left to import. Pass kAllSamples

[Alexei Svitkine (slow), 2016/03/08 19:46:25]

What does "stopping after importing |until_value|" mean? Do you mean
"|until_value| samples" - as in that many samples - or until we get a sample
with that value? If it's the former, can it be named |max_samples|? Also, I'd
just document that passing -1 here explicitly results in all being imported - no
need for the extra constant.

[bcwhite, 2016/03/09 01:16:52]

It's the latter.

I had -1 originally documented in the comment.  Greg specifically asked for the
constant.

[Alexei Svitkine (slow), 2016/03/09 20:35:19]

Okay. I guess it doesn't matter too much either way.

+  // to import everything. Returns a pointer to the count for |until_value|
+  // if it is found, NULL otherwise.

[Alexei Svitkine (slow), 2016/03/08 19:46:25]

Nit: NULL -> null

[bcwhite, 2016/03/09 01:16:52]

Done.

+  HistogramBase::Count* ImportSamples(HistogramBase::Sample until_value);
+
+  // All created/loaded sample values and their associated counts.

[Alexei Svitkine (slow), 2016/03/08 19:46:25]

Expand on ownership.

[bcwhite, 2016/03/09 01:16:52]

Done.

+  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), 2016/03/08 19:46:25]

This class shouldn't be used directly by clients, right? Iterator() above
returns the interface.

So, can this be moved entirely to the .cc?

[bcwhite, 2016/03/09 01:16:52]

That works.  Would you like me to do the same to SampleMapIterator while I'm at
it?

[Alexei Svitkine (slow), 2016/03/09 20:35:19]

Yes, please. :)

[bcwhite, 2016/03/09 22:57:33]

Done.

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