Add a StatisticsDeltaReader class and use cases

base/test/histogram_recorder.cc

Index: base/test/histogram_recorder.cc
diff --git a/base/test/histogram_recorder.cc b/base/test/histogram_recorder.cc
new file mode 100644
index 0000000000000000000000000000000000000000..ee852f71c0c4e98ee3c01bed471a4210986b2ecf
--- /dev/null
+++ b/base/test/histogram_recorder.cc
@@ -0,0 +1,52 @@
+// Copyright 2013 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_recorder.h"
+
+#include "base/metrics/histogram.h"
+#include "base/metrics/statistics_recorder.h"
+#include "base/stl_util.h"
+
+namespace base {
+
+// static
+void HistogramRecorder::Initialize() {

[ppi, 2013/12/23 16:37:48]

Should we drop it and call StatisticsRecorder directly?

[lpromero, 2013/12/23 17:22:54]

I liked the encapsulation. Then, when using HistogramDeltaReader, no need to
include statistics_recorder. What do you think?

[ppi, 2013/12/23 17:31:38]

This is interesting point, but note that when using HistogramDeltaReader, one
will probably record some histograms. So, StatisticsRecorder needs to be
initialized anyway for the sake of histograms, and we probably don't need to
pretend that HistogramDeltaReader::Initialize() and
StatisticsRecorder::Initialize() are different things.

Not terribly important, so please feel free to skip if you're not convinced:).

+  // Ensure that StatisticsRecorder is initialized.
+  StatisticsRecorder::Initialize();
+}
+
+HistogramRecorder::HistogramRecorder() {
+  // Record any histogram data that exists when the object is created so it can
+  // be subtracted later.
+  StatisticsRecorder::Histograms histograms;
+  StatisticsRecorder::GetSnapshot("", &histograms);
+  for (size_t i = 0; i < histograms.size(); i++) {
+    original_samples_[histograms[i]->histogram_name()] =
+        histograms[i]->SnapshotSamples().release();
+  }
+}
+
+HistogramRecorder::~HistogramRecorder() {
+  STLDeleteValues(&original_samples_);
+}
+
+// static
+bool HistogramRecorder::IsActive() {

[ppi, 2013/12/23 16:37:48]

Should we drop it and call StatisticsRecorder directly?

[lpromero, 2013/12/23 17:22:54]

Ditto.

+  return StatisticsRecorder::IsActive();
+}
+
+scoped_ptr<HistogramSamples>
+    HistogramRecorder::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 = original_samples_[histogram_name];
+  if (named_original_samples)
+    named_samples->Subtract(*named_original_samples);
+  return named_samples.Pass();
+}
+
+}  // namespace base