base/test/histogram_tester.h: Add a way to query a whole family of related

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
index 7ac7ca67e7efac0db0f4d8fe1921b719c77aedef..cfab3c60f24e267c9b6e0a08dcde3421764201f8 100644
--- a/base/test/histogram_tester.h
+++ b/base/test/histogram_tester.h
@@ -26,6 +26,8 @@ class HistogramSamples;
 // getting logged as intended.
 class HistogramTester {
  public:
+  using CountsMap = std::map<std::string, base::HistogramBase::Count>;
+
   // The constructor will call StatisticsRecorder::Initialize() for you. Also,
   // this takes a snapshot of all current histograms counts.
   HistogramTester();
@@ -67,12 +69,32 @@ class HistogramTester {
   // slightly less helpful failure message:
   //   EXPECT_EQ(expected_buckets,
   //             histogram_tester.GetAllSamples("HistogramName"));
-  std::vector<Bucket> GetAllSamples(const std::string& name);
+  std::vector<Bucket> GetAllSamples(const std::string& name) const;
+
+  // Finds histograms whose names start with |query|, and returns them along
+  // with the counts of any samples added since the creation of this object.
+  // Histograms that are unchanged are omitted from the result. The return value
+  // is a map whose keys are the histogram name, and whose values are the sample
+  // count.
+  //
+  // This is useful for cases where the code under test is choosing among a
+  // family of related histograms and incrementing one of them. Typically you
+  // should pass the result of this function directly to EXPECT_THAT.
+  //
+  // Example usage, using gmock (which produces better failure messages):
+  //   #include "testing/gmock/include/gmock/gmock.h"
+  // ...
+  //   base::HistogramTester::CountsMap expected_counts;
+  //   expected_counts["MyMetric.A"] = 1;
+  //   expected_counts["MyMetric.B"] = 1;
+  //   EXPECT_THAT(histogram_tester.GetTotalCountsForPrefix("MyMetric."),
+  //               testing::ContainerEq(expected_counts));
+  CountsMap GetTotalCountsForPrefix(const std::string& query) const;
 
   // 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);
+      const std::string& histogram_name) const;
 
  private:
   // Verifies and asserts that value in the |sample| bucket matches the