Merge multiple histogram snapshots into single one for reporting.

base/metrics/histogram_unittest.cc

Index: base/metrics/histogram_unittest.cc
diff --git a/base/metrics/histogram_unittest.cc b/base/metrics/histogram_unittest.cc
index 09f9106d09758837ce79ac15453bb5c198a3b9d8..81fb38c977ece78b7b0b6f006952665f55b1367f 100644
--- a/base/metrics/histogram_unittest.cc
+++ b/base/metrics/histogram_unittest.cc
@@ -208,6 +208,34 @@ TEST_F(HistogramTest, NameMatchTest) {
   EXPECT_EQ(2, samples->GetCount(10));
 }
 
+// Check that delta calculations work correct.
+TEST_F(HistogramTest, DeltaTest) {
+  HistogramBase* histogram =
+      Histogram::FactoryGet("DeltaHistogram", 1, 64, 8,
+                            HistogramBase::kNoFlags);
+  histogram->Add(1);
+  histogram->Add(10);
+  histogram->Add(50);
+
+  scoped_ptr<HistogramSamples> samples = histogram->SnapshotDelta();
+  EXPECT_EQ(3, samples->TotalCount());
+  EXPECT_EQ(1, samples->GetCount(1));
+  EXPECT_EQ(1, samples->GetCount(10));
+  EXPECT_EQ(1, samples->GetCount(50));
+
+  samples = histogram->SnapshotDelta();
+  EXPECT_EQ(0, samples->TotalCount());
+
+  histogram->Add(10);
+  histogram->Add(10);
+  samples = histogram->SnapshotDelta();
+  EXPECT_EQ(2, samples->TotalCount());
+  EXPECT_EQ(2, samples->GetCount(10));
+
+  samples = histogram->SnapshotDelta();
+  EXPECT_EQ(0, samples->TotalCount());
+}
+
 TEST_F(HistogramTest, ExponentialRangesTest) {
   // Check that we got a nice exponential when there was enough room.
   BucketRanges ranges(9);