Merge multiple histogram snapshots into single one for reporting.

base/metrics/histogram.cc

Index: base/metrics/histogram.cc
diff --git a/base/metrics/histogram.cc b/base/metrics/histogram.cc
index c33d2f03c05c769d0dd9bad12c85a26ce3550460..df1368e6b07db5b1a41560ce0171ce32a9846dc7 100644
--- a/base/metrics/histogram.cc
+++ b/base/metrics/histogram.cc
@@ -272,10 +272,11 @@ HistogramBase* Histogram::PersistentGet(const std::string& name,
                                         Sample maximum,
                                         const BucketRanges* ranges,
                                         HistogramBase::AtomicCount* counts,
+                                        HistogramBase::AtomicCount* logged,
                                         uint32_t counts_size,
                                         HistogramSamples::Metadata* meta) {
-  return new Histogram(name, minimum, maximum, ranges, counts, counts_size,
-                       meta);
+  return new Histogram(name, minimum, maximum, ranges, counts, logged,
+                       counts_size, meta);
 }
 
 // Calculate what range of values are held in each bucket.
@@ -430,6 +431,21 @@ scoped_ptr<HistogramSamples> Histogram::SnapshotSamples() const {
   return SnapshotSampleVector();
 }
 
+scoped_ptr<HistogramSamples> Histogram::SnapshotDelta() {
+  scoped_ptr<HistogramSamples> snapshot = SnapshotSampleVector();
+  if (!logged_) {
+    // If nothing has been previously logged, save this one as logged and

[Alexei Svitkine (slow), 2016/02/09 19:46:51]

Nit: save this one as |logged_counts_| and

[bcwhite, 2016/02/11 16:42:38]

Done.

+    // gather another snapshot to return.
+    logged_.swap(snapshot);
+    return SnapshotSampleVector();
+  }
+
+  // Subtract what was previously logged and update that information.
+  snapshot->Subtract(*logged_);
+  logged_->Add(*snapshot);
+  return snapshot;
+}
+
 void Histogram::AddSamples(const HistogramSamples& samples) {
   samples_->Add(samples);
 }
@@ -477,6 +493,7 @@ Histogram::Histogram(const std::string& name,
                      Sample maximum,
                      const BucketRanges* ranges,
                      HistogramBase::AtomicCount* counts,
+                     HistogramBase::AtomicCount* logged,
                      uint32_t counts_size,
                      HistogramSamples::Metadata* meta)
   : HistogramBase(name),
@@ -486,6 +503,8 @@ Histogram::Histogram(const std::string& name,
   if (ranges) {
     samples_.reset(new SampleVector(HashMetricName(name),
                                     counts, counts_size, meta, ranges));
+    logged_.reset(new SampleVector(samples_->id(),
+                                   logged, counts_size, meta, ranges));
   }
 }
 
@@ -776,9 +795,10 @@ HistogramBase* LinearHistogram::PersistentGet(
     Sample maximum,
     const BucketRanges* ranges,
     HistogramBase::AtomicCount* counts,
+    HistogramBase::AtomicCount* logged,
     uint32_t counts_size,
     HistogramSamples::Metadata* meta) {
-  return new LinearHistogram(name, minimum, maximum, ranges, counts,
+  return new LinearHistogram(name, minimum, maximum, ranges, counts, logged,
                              counts_size, meta);
 }
 
@@ -813,6 +833,7 @@ LinearHistogram::LinearHistogram(const std::string& name,
                                  Sample maximum,
                                  const BucketRanges* ranges,
                                  HistogramBase::AtomicCount* counts,
+                                 HistogramBase::AtomicCount* logged,
                                  uint32_t counts_size,
                                  HistogramSamples::Metadata* meta)
     : Histogram(name,
@@ -820,6 +841,7 @@ LinearHistogram::LinearHistogram(const std::string& name,
                 maximum,
                 ranges,
                 counts,
+                logged,
                 counts_size,
                 meta) {}
 
@@ -919,8 +941,9 @@ HistogramBase* BooleanHistogram::PersistentGet(
     const std::string& name,
     const BucketRanges* ranges,
     HistogramBase::AtomicCount* counts,
+    HistogramBase::AtomicCount* logged,
     HistogramSamples::Metadata* meta) {
-  return new BooleanHistogram(name, ranges, counts, meta);
+  return new BooleanHistogram(name, ranges, counts, logged, meta);
 }
 
 HistogramType BooleanHistogram::GetHistogramType() const {
@@ -934,8 +957,9 @@ BooleanHistogram::BooleanHistogram(const std::string& name,
 BooleanHistogram::BooleanHistogram(const std::string& name,
                                    const BucketRanges* ranges,
                                    HistogramBase::AtomicCount* counts,
+                                   HistogramBase::AtomicCount* logged,
                                    HistogramSamples::Metadata* meta)
-    : LinearHistogram(name, 1, 2, ranges, counts, 2, meta) {}
+    : LinearHistogram(name, 1, 2, ranges, counts, logged, 2, meta) {}
 
 HistogramBase* BooleanHistogram::DeserializeInfoImpl(PickleIterator* iter) {
   std::string histogram_name;
@@ -1019,9 +1043,10 @@ HistogramBase* CustomHistogram::PersistentGet(
     const std::string& name,
     const BucketRanges* ranges,
     HistogramBase::AtomicCount* counts,
+    HistogramBase::AtomicCount* logged,
     uint32_t counts_size,
     HistogramSamples::Metadata* meta) {
-  return new CustomHistogram(name, ranges, counts, counts_size, meta);
+  return new CustomHistogram(name, ranges, counts, logged, counts_size, meta);
 }
 
 HistogramType CustomHistogram::GetHistogramType() const {
@@ -1053,6 +1078,7 @@ CustomHistogram::CustomHistogram(const std::string& name,
 CustomHistogram::CustomHistogram(const std::string& name,
                                  const BucketRanges* ranges,
                                  HistogramBase::AtomicCount* counts,
+                                 HistogramBase::AtomicCount* logged,
                                  uint32_t counts_size,
                                  HistogramSamples::Metadata* meta)
     : Histogram(name,
@@ -1060,6 +1086,7 @@ CustomHistogram::CustomHistogram(const std::string& name,
                 ranges->range(ranges->bucket_count() - 1),
                 ranges,
                 counts,
+                logged,
                 counts_size,
                 meta) {}