Add SampleMap and use it in SparseHistogram

base/metrics/sparse_histogram.cc

Index: base/metrics/sparse_histogram.cc
diff --git a/base/metrics/sparse_histogram.cc b/base/metrics/sparse_histogram.cc
index 05a68b53540a1f5daf23c62c8ac6b6faa94e4006..684d5dc746ea98c884b497b9462994d8d54f30b7 100644
--- a/base/metrics/sparse_histogram.cc
+++ b/base/metrics/sparse_histogram.cc
@@ -5,6 +5,7 @@
 #include "base/metrics/sparse_histogram.h"
 
 #include "base/metrics/statistics_recorder.h"
+#include "base/synchronization/lock.h"
 
 using std::string;
 
@@ -22,13 +23,14 @@ HistogramBase* SparseHistogram::FactoryGet(const string& name,
 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 {