Add support for persistent sparse histograms.

base/metrics/persistent_sample_map.h

Index: base/metrics/persistent_sample_map.h
diff --git a/base/metrics/persistent_sample_map.h b/base/metrics/persistent_sample_map.h
new file mode 100644
index 0000000000000000000000000000000000000000..cecd15aaefe648177f8f2d4f4f9726bb595ea2ca
--- /dev/null
+++ b/base/metrics/persistent_sample_map.h
@@ -0,0 +1,76 @@
+// 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 by iterating over all sample
+  // records found therein, adding them to the sample_counts_ map. If a
+  // count for the sample |until_value| is found, stop the import and return
+  // a pointer to that counter. If that value is not found, null will be
+  // returned after all currently available samples have been loaded. Pass
+  // kAllSamples to force the importing of all available samples.
+  HistogramBase::Count* ImportSamples(HistogramBase::Sample until_value);
+
+  // All created/loaded sample values and their associated counts. The storage
+  // for the actual Count numbers is owned by the |allocator_|.
+  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);
+};
+
+}  // namespace base
+
+#endif  // BASE_METRICS_PERSISTENT_SAMPLE_MAP_H_