Add SampleMap and use it in SparseHistogram

base/metrics/sparse_histogram.cc

 // 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.
 
 #include "base/metrics/sparse_histogram.h"
 
 #include "base/metrics/statistics_recorder.h"
+#include "base/synchronization/lock.h"
 
 using std::string;
 
 namespace base {
 
 // static
 HistogramBase* SparseHistogram::FactoryGet(const string& name,
                                            int32 flags) {
   // TODO(kaiwang): Register and get SparseHistogram with StatisticsRecorder.
   HistogramBase* histogram = new SparseHistogram(name);
   histogram->SetFlags(flags);
   return histogram;
 }
 
 SparseHistogram::~SparseHistogram() {}
 
 void SparseHistogram::Add(Sample value) {
-  base::AutoLock auto_lock(lock_);
-  samples_[value]++;
+  samples_.Accumulate(value, 1);
 }
 
-void SparseHistogram::SnapshotSample(std::map<Sample, Count>* samples) const {
-  base::AutoLock auto_lock(lock_);
-  *samples = samples_;
+scoped_ptr<SampleMap> SparseHistogram::SnapshotSamples() const {
+  base::AutoLock auto_lock(samples_.lock_);
+  scoped_ptr<SampleMap> snapshot(new SampleMap());
+  snapshot->Add(samples_);

[jar (doing other things), 2012/10/02 17:41:11]

Will this induce numerous locks?  Note that we don't really need to lock
|snapshot| since we are the only one that knows about it.

+  return snapshot.Pass();
 }
 
 void SparseHistogram::WriteHTMLGraph(string* output) const {
   // TODO(kaiwang): Implement.
 }
 
 void SparseHistogram::WriteAscii(string* output) const {
   // TODO(kaiwang): Implement.
 }
 
 SparseHistogram::SparseHistogram(const string& name)
     : HistogramBase(name) {}
 
 }  // namespace base