Rework UMAHistogramHelper and StatisticsDeltaReader into [Chrome]HistogramTester.

base/test/histogram_tester.h

Index: base/test/histogram_tester.h
diff --git a/base/test/histogram_tester.h b/base/test/histogram_tester.h
new file mode 100644
index 0000000000000000000000000000000000000000..8b1f4ef873b05a99a979c67c291b6dc8ef51b67a
--- /dev/null
+++ b/base/test/histogram_tester.h
@@ -0,0 +1,81 @@
+// 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.
+
+#ifndef BASE_TEST_HISTOGRAM_TESTER_H_
+#define BASE_TEST_HISTOGRAM_TESTER_H_
+
+#include <map>
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/metrics/histogram.h"
+#include "base/metrics/histogram_base.h"
+
+namespace base {
+
+class HistogramSamples;
+
+// HistogramTester provides a simple interface for examining histograms, UMA
+// or otherwise. Tests can use this interface to verify that Histogram data is

[Ilya Sherman, 2014/07/16 18:44:46]

nit: Uppercase "Histogram" should be lowercase.

[Mike Lerman, 2014/07/16 19:36:35]

Done.

+// getting logged as intended.
+class HistogramTester {
+ public:
+  // The constructor will call StatisticsRecorder::Initialize() for you. Also,
+  // this takes a snapshot of all current histograms counts.
+  HistogramTester();
+  ~HistogramTester();
+
+  // We know the exact number of samples in a bucket, and that no other bucket
+  // should have samples. Measures the diff from the snapshot taken when this
+  // object was constructed.
+  const void ExpectUniqueSample(const std::string& name,

[Ilya Sherman, 2014/07/16 18:44:46]

"const void" doesn't make a lot of sense as a return type.  Did you mean to mark
the method as const?

[Mike Lerman, 2014/07/16 19:36:36]

Ummm. Yes. Awkward.

+                                base::HistogramBase::Sample sample,
+                                base::HistogramBase::Count expected_count);
+
+  // We know the exact number of samples in a bucket, but other buckets may
+  // have samples as well. Measures the diff from the snapshot taken when this
+  // object was constructed.
+  const void ExpectBucketCount(const std::string& name,
+                               base::HistogramBase::Sample sample,
+                               base::HistogramBase::Count expected_count);
+
+  // We don't know the values of the samples, but we know how many there are.
+  // This measures the diff from the snapshot taken when this object was
+  // constructed.
+  const void ExpectTotalCount(const std::string& name,
+                              base::HistogramBase::Count count);
+
+  // Access a modified HistogramSamples containing only what has been logged
+  // to the histogram since the creation of this object.
+  scoped_ptr<HistogramSamples> GetHistogramSamplesSinceCreation(
+      const std::string& histogram_name);
+
+ private:
+  // Verifies and asserts that value in the |sample| bucket matches the
+  // |expected_count|. The bucket's current value is determined from |samples|
+  // and is modified based on the snapshot stored for histogram |name|.
+  const void CheckBucketCount(const std::string& name,
+                              base::HistogramBase::Sample sample,
+                              base::Histogram::Count expected_count,
+                              base::HistogramSamples& samples);
+
+  // Verifies that the total number of values recorded for the histogram |name|
+  // is |expected_count|. This is checked against |samples| minus the snapshot
+  // that was taken for |name|.
+  const void CheckTotalCount(const std::string& name,
+                             base::Histogram::Count expected_count,
+                             base::HistogramSamples& samples);
+
+  // Used to determine the histogram changes made during this instance's
+  // lifecycle. This instance takes ownership of the samples, which are deleted
+  // when the instance is destroyed.
+  std::map<std::string, HistogramSamples*> histograms_snapshot_;
+
+  DISALLOW_COPY_AND_ASSIGN(HistogramTester);
+};
+
+}  // namespace base
+
+#endif  // BASE_TEST_HISTOGRAM_TESTER_H_