Rework UMAHistogramHelper and StatisticsDeltaReader into [Chrome]HistogramTester.

base/test/histogram_tester.cc

Index: base/test/histogram_tester.cc
diff --git a/base/test/histogram_tester.cc b/base/test/histogram_tester.cc
new file mode 100644
index 0000000000000000000000000000000000000000..0c6278e1cfea1242a06de1a187f092a6a43031c0
--- /dev/null
+++ b/base/test/histogram_tester.cc
@@ -0,0 +1,116 @@
+// Copyright 2014 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/test/histogram_tester.h"
+
+#include "base/metrics/histogram.h"
+#include "base/metrics/histogram_samples.h"
+#include "base/metrics/statistics_recorder.h"
+#include "base/stl_util.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace base {
+
+HistogramTester::HistogramTester() {
+  StatisticsRecorder::Initialize();  // Safe to call multiple times.
+
+  // Record any histogram data that exists when the object is created so it can
+  // be subtracted later.
+  StatisticsRecorder::Histograms histograms;
+  StatisticsRecorder::GetSnapshot(std::string(), &histograms);
+  for (size_t i = 0; i < histograms.size(); ++i) {
+    histograms_snapshot_[histograms[i]->histogram_name()] =
+        histograms[i]->SnapshotSamples().release();
+  }
+}
+
+HistogramTester::~HistogramTester() {
+  STLDeleteValues(&histograms_snapshot_);
+}
+
+void HistogramTester::ExpectUniqueSample(
+    const std::string& name,
+    base::HistogramBase::Sample sample,
+    base::HistogramBase::Count expected_count) const {
+  base::HistogramBase* histogram =
+      base::StatisticsRecorder::FindHistogram(name);
+  EXPECT_NE(static_cast<base::HistogramBase*>(NULL), histogram)
+      << "Histogram \"" << name << "\" does not exist.";
+
+  if (histogram) {
+    scoped_ptr<base::HistogramSamples> samples(histogram->SnapshotSamples());
+    CheckBucketCount(name, sample, expected_count, *samples);
+    CheckTotalCount(name, expected_count, *samples);
+  }
+}
+
+void HistogramTester::ExpectBucketCount(
+    const std::string& name,
+    base::HistogramBase::Sample sample,
+    base::HistogramBase::Count expected_count) const {
+  base::HistogramBase* histogram =
+      base::StatisticsRecorder::FindHistogram(name);
+  EXPECT_NE(static_cast<base::HistogramBase*>(NULL), histogram)
+      << "Histogram \"" << name << "\" does not exist.";
+
+  if (histogram) {
+    scoped_ptr<base::HistogramSamples> samples(histogram->SnapshotSamples());
+    CheckBucketCount(name, sample, expected_count, *samples);
+  }
+}
+
+void HistogramTester::ExpectTotalCount(const std::string& name,
+                                       base::HistogramBase::Count count) const {
+  base::HistogramBase* histogram =
+      base::StatisticsRecorder::FindHistogram(name);
+  if (histogram) {
+    scoped_ptr<base::HistogramSamples> samples(histogram->SnapshotSamples());
+    CheckTotalCount(name, count, *samples);
+  } else {
+    // No histogram means there were zero samples.
+    EXPECT_EQ(count, 0) << "Histogram \"" << name << "\" does not exist.";
+  }
+}
+
+scoped_ptr<HistogramSamples> HistogramTester::GetHistogramSamplesSinceCreation(
+    const std::string& histogram_name) {
+  HistogramBase* histogram = StatisticsRecorder::FindHistogram(histogram_name);
+  if (!histogram)
+    return scoped_ptr<HistogramSamples>();
+  scoped_ptr<HistogramSamples> named_samples(histogram->SnapshotSamples());
+  HistogramSamples* named_original_samples =
+      histograms_snapshot_[histogram_name];
+  if (named_original_samples)
+    named_samples->Subtract(*named_original_samples);
+  return named_samples.Pass();
+}
+
+void HistogramTester::CheckBucketCount(
+    const std::string& name,
+    base::HistogramBase::Sample sample,
+    base::HistogramBase::Count expected_count,
+    base::HistogramSamples& samples) const {
+  int actual_count = samples.GetCount(sample);
+  if (histograms_snapshot_.count(name))
+    actual_count -= histograms_snapshot_.at(name)->GetCount(sample);

[Ilya Sherman, 2014/07/16 20:01:12]

Please use .find() rather than .at(), as .at() can throw an exception.  (It's
really quite frustrating that there's no const version of operator[]... alas.)

[Mike Lerman, 2014/07/17 15:04:18]

The at() command directly follows a count() which verifies that I do have a
value in the map for my key. Since I'm doing that check, isn't it easier, rather
than declaring a std::map<std::string, HistogramSamples*>::iterator to hold the
return value of find() before I call .second->GetCount()?

[Ilya Sherman, 2014/07/17 18:35:09]

I'd still prefer to use .find() rather than .at().  While it's safe here, it's
better to just not call code that can call an exception.  Otherwise, other folks
might come along and try to write similar code, but not realize this issue, and
then accidentally introduce exceptions.

+  EXPECT_EQ(expected_count, actual_count)
+      << "Histogram \"" << name
+      << "\" does not have the right number of samples (" << expected_count
+      << ") in the expected bucket (" << sample << "). It has (" << actual_count
+      << ").";
+}
+
+void HistogramTester::CheckTotalCount(const std::string& name,
+                                      base::HistogramBase::Count expected_count,
+                                      base::HistogramSamples& samples) const {
+  int actual_count = samples.TotalCount();
+  if (histograms_snapshot_.count(name))
+    actual_count -= histograms_snapshot_.at(name)->TotalCount();

[Ilya Sherman, 2014/07/16 20:01:12]

(here too)

[Mike Lerman, 2014/07/17 15:04:18]

Question above

+  EXPECT_EQ(expected_count, actual_count)
+      << "Histogram \"" << name
+      << "\" does not have the right total number of samples ("
+      << expected_count << "). It has (" << actual_count << ").";
+}
+
+}  // namespace base