Add a StatisticsDeltaReader class and use cases

base/test/statistics_delta_reader.cc

Index: base/test/statistics_delta_reader.cc
diff --git a/base/test/statistics_delta_reader.cc b/base/test/statistics_delta_reader.cc
new file mode 100644
index 0000000000000000000000000000000000000000..0e241c18395148ae14c81f1d97e39923f5de8ef8
--- /dev/null
+++ b/base/test/statistics_delta_reader.cc
@@ -0,0 +1,41 @@
+// 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/statistics_delta_reader.h"
+
+#include "base/metrics/histogram.h"
+#include "base/metrics/statistics_recorder.h"
+#include "base/stl_util.h"
+
+namespace base {
+
+StatisticsDeltaReader::StatisticsDeltaReader() {
+  // 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();
+  }
+}
+
+StatisticsDeltaReader::~StatisticsDeltaReader() {
+  STLDeleteValues(&original_samples_);
+}
+
+scoped_ptr<HistogramSamples>
+    StatisticsDeltaReader::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