Support saving browser metrics to disk and reading them during next run.

base/metrics/histogram_unittest.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/histogram.h"
 
 #include <limits.h>
 #include <stddef.h>
 #include <stdint.h>
 
@@ skipping 129 matching lines @@
   LOCAL_HISTOGRAM_PERCENTAGE("DuplicatedHistogram", 10);
   LOCAL_HISTOGRAM_PERCENTAGE("DuplicatedHistogram", 10);
   HistogramBase* histogram = LinearHistogram::FactoryGet(
       "DuplicatedHistogram", 1, 101, 102, HistogramBase::kNoFlags);
 
   std::unique_ptr<HistogramSamples> samples = histogram->SnapshotSamples();
   EXPECT_EQ(2, samples->TotalCount());
   EXPECT_EQ(2, samples->GetCount(10));
 }
 
-// Check that delta calculations work correct.
+// Check that delta calculations work correctly.
 TEST_P(HistogramTest, DeltaTest) {
   HistogramBase* histogram =
       Histogram::FactoryGet("DeltaHistogram", 1, 64, 8,
                             HistogramBase::kNoFlags);
   histogram->Add(1);
   histogram->Add(10);
   histogram->Add(50);
 
   std::unique_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));
   EXPECT_EQ(samples->TotalCount(), samples->redundant_count());
 
   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());
 }
 
+// Check that final-delta calculations work correctly.
+TEST_P(HistogramTest, FinalDeltaTest) {
+  HistogramBase* histogram =
+      Histogram::FactoryGet("FinalDeltaHistogram", 1, 64, 8,
+                            HistogramBase::kNoFlags);
+  histogram->Add(1);
+  histogram->Add(10);
+  histogram->Add(50);
+
+  std::unique_ptr<HistogramSamples> samples = histogram->SnapshotFinalDelta();

[Ilya Sherman, 2016/05/04 06:48:07]

Please include an additional test where the code first calls SnapshotDelta(). 
It's important to test both the codepath where there was no delta previously
recorded, and where there was one.

[bcwhite, 2016/05/04 14:12:50]

Done.

+  EXPECT_EQ(3, samples->TotalCount());
+  EXPECT_EQ(1, samples->GetCount(1));
+  EXPECT_EQ(1, samples->GetCount(10));
+  EXPECT_EQ(1, samples->GetCount(50));
+  EXPECT_EQ(samples->TotalCount(), samples->redundant_count());
+}
+
 TEST_P(HistogramTest, ExponentialRangesTest) {
   // Check that we got a nice exponential when there was enough room.
   BucketRanges ranges(9);
   Histogram::InitializeBucketRanges(1, 64, &ranges);
   EXPECT_EQ(0, ranges.range(0));
   int power_of_2 = 1;
   for (int i = 1; i < 8; i++) {
     EXPECT_EQ(power_of_2, ranges.range(i));
     power_of_2 *= 2;
   }
@@ skipping 527 matching lines @@
   // CustomHistogram needs at least 1 valid range.
   custom_ranges.clear();
   custom_ranges.push_back(0);
   EXPECT_DEATH(CustomHistogram::FactoryGet("BadRangesCustom3", custom_ranges,
                                            HistogramBase::kNoFlags),
                "");
 }
 #endif
 
 }  // namespace base