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diff --git a/base/metrics/sparse_histogram.h b/base/metrics/sparse_histogram.h
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+++ b/base/metrics/sparse_histogram.h
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+// 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_SPARSE_HISTOGRAM_H_
+#define BASE_METRICS_SPARSE_HISTOGRAM_H_
+
+#include <map>
+#include <string>
+
+#include "base/base_export.h"
+#include "base/basictypes.h"
+#include "base/compiler_specific.h"
+#include "base/metrics/histogram_base.h"
+#include "base/synchronization/lock.h"
+
+namespace base {
+
+class BASE_EXPORT_PRIVATE SparseHistogram : public HistogramBase {
+ public:
+  // If there's one with same name, return the existing one. If not, create a
+  // new one.
+  static HistogramBase* FactoryGet(const std::string& name, int32 flags);
+
+  virtual ~SparseHistogram();
+
+  virtual void Add(Sample value) OVERRIDE;
+
+  virtual void SnapshotSample(std::map<Sample, Count>* sample) const;
+  virtual void WriteHTMLGraph(std::string* output) const OVERRIDE;
+  virtual void WriteAscii(std::string* output) const OVERRIDE;
+
+ protected:
+  // Clients should always use FactoryGet to create SparseHistogram.
+  SparseHistogram(const std::string& name);
+
+ private:
+  friend class SparseHistogramTest;  // For constuctor calling.
+
+  std::map<Sample, Count> samples_;
+
+  // Protects access to above map.
+  mutable base::Lock lock_;
+
+  DISALLOW_COPY_AND_ASSIGN(SparseHistogram);
+};
+
+}  // namespace base
+
+#endif  // BASE_METRICS_SPARSE_HISTOGRAM_H_