base/test/metrics/histogram_tester.h

// 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_METRICS_HISTOGRAM_TESTER_H_
#define BASE_TEST_METRICS_HISTOGRAM_TESTER_H_

#include <functional>
#include <map>
#include <memory>
#include <ostream>
#include <string>
#include <type_traits>
#include <utility>
#include <vector>

#include "base/metrics/histogram.h"
#include "base/metrics/histogram_base.h"
#include "base/strings/string_piece.h"
#include "base/time/time.h"
#include "testing/gmock/include/gmock/gmock.h"

namespace base {

struct Bucket;
class HistogramSamples;

// HistogramTester provides a simple interface for examining histograms, UMA
// or otherwise. Tests can use this interface to verify that histogram data is
// getting logged as intended.
//
// Note: When using this class from a browser test, one might have to call
// SubprocessMetricsProvider::MergeHistogramDeltasForTesting() to sync the
// histogram data between the renderer and browser processes. If it is in a
// content browser test, then content::FetchHistogramsFromChildProcesses()
// should be used to achieve that.
// To test histograms in Java tests, use HistogramTestRule.
class HistogramTester {
 public:
  using CountsMap = std::map<std::string, HistogramBase::Count, std::less<>>;

  // Takes a snapshot of all current histograms counts.
  HistogramTester();

  HistogramTester(const HistogramTester&) = delete;
  HistogramTester& operator=(const HistogramTester&) = delete;

  ~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.
  void ExpectUniqueSample(StringPiece name,
                          HistogramBase::Sample sample,
                          HistogramBase::Count expected_bucket_count) const;
  template <typename T>
  void ExpectUniqueSample(StringPiece name,
                          T sample,
                          HistogramBase::Count expected_bucket_count) const {
    ExpectUniqueSample(name, static_cast<HistogramBase::Sample>(sample),
                       expected_bucket_count);
  }
  void ExpectUniqueTimeSample(StringPiece name,
                              TimeDelta sample,
                              HistogramBase::Count expected_bucket_count) const;

  // 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.
  void ExpectBucketCount(StringPiece name,
                         HistogramBase::Sample sample,
                         HistogramBase::Count expected_count) const;
  template <typename T>
  void ExpectBucketCount(StringPiece name,
                         T sample,
                         HistogramBase::Count expected_count) const {
    ExpectBucketCount(name, static_cast<HistogramBase::Sample>(sample),
                      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.
  void ExpectTotalCount(StringPiece name, HistogramBase::Count count) const;

  // We know exact number of samples for buckets corresponding to a time
  // interval. Other intervals may have samples too.
  void ExpectTimeBucketCount(StringPiece name,
                             TimeDelta sample,
                             HistogramBase::Count count) const;

  // We don't know the values of the samples, but we know their sum.
  // This returns the diff from the snapshot taken when this object was
  // constructed.
  int64_t GetTotalSum(StringPiece name) const;

  // Returns a list of all of the buckets recorded since creation of this
  // object, as vector<Bucket>, where the Bucket represents the min boundary of
  // the bucket and the count of samples recorded to that bucket since creation.
  //
  // Example usage, using gMock:
  //   EXPECT_THAT(histogram_tester.GetAllSamples("HistogramName"),
  //               ElementsAre(Bucket(1, 5), Bucket(2, 10), Bucket(3, 5)));
  //
  // If you want make empty bucket explicit, use the BucketsAre() matcher
  // defined below:
  //   EXPECT_THAT(histogram_tester.GetAllSamples("HistogramName"),
  //               BucketsAre(Bucket(1, 0), Bucket(2, 10), Bucket(3, 5)));
  //
  // If you want to test a superset relation, prefer BucketsInclude() over
  // IsSupersetOf() because the former handles empty buckets as expected:
  //   EXPECT_THAT(histogram_tester.GetAllSamples("HistogramName"),
  //               BucketsInclude(Bucket(1, 0), Bucket(2, 10), Bucket(3, 5)));
  // With IsSupersetOf(), this expectation would always fail because
  // GetAllSamples() does not contain empty buckets.
  //
  // If you build the expected list programmatically, you can use the matchers
  // ElementsAreArray(), BucketsAreArray(), BucketsIncludeArray().
  //
  // If you prefer not to depend on gMock at the expense of a slightly less
  // helpful failure message, use EXPECT_EQ:
  //   EXPECT_EQ(expected_buckets,
  //             histogram_tester.GetAllSamples("HistogramName"));
  std::vector<Bucket> GetAllSamples(StringPiece name) const;

  // Returns the value of the |sample| bucket for ths histogram |name|.
  HistogramBase::Count GetBucketCount(StringPiece name,
                                      HistogramBase::Sample sample) const;
  template <typename T>
  HistogramBase::Count GetBucketCount(StringPiece name, T sample) const {
    return GetBucketCount(name, static_cast<HistogramBase::Sample>(sample));
  }

  // Finds histograms whose names start with |prefix|, 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(StringPiece prefix) const;

  // Access a modified HistogramSamples containing only what has been logged
  // to the histogram since the creation of this object.
  std::unique_ptr<HistogramSamples> GetHistogramSamplesSinceCreation(
      StringPiece histogram_name) const;

  // Dumps all histograms that have had new samples added to them into a string,
  // for debugging purposes. Note: this will dump the entire contents of any
  // modified histograms and not just the modified buckets.
  std::string GetAllHistogramsRecorded() const;

 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|.
  void CheckBucketCount(StringPiece name,
                        HistogramBase::Sample sample,
                        Histogram::Count expected_count,
                        const HistogramSamples& samples) const;

  // Returns the total number of values recorded for histogram |name|. This
  // is calculated as the number from |samples| minus the snapshot that was
  // taken for |name|.
  int GetTotalCountForSamples(StringPiece name,
                              const HistogramSamples& samples) const;

  // 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|.
  void CheckTotalCount(StringPiece name,
                       Histogram::Count expected_count,
                       const HistogramSamples& samples) const;

  // Sets the value for |count| to be the value in the |sample| bucket. The
  // bucket's current value is determined from |samples| and is modified based
  // on the snapshot stored for histogram |name|.
  void GetBucketCountForSamples(StringPiece name,
                                HistogramBase::Sample sample,
                                const HistogramSamples& samples,
                                HistogramBase::Count* count) const;

  // Used to determine the histogram changes made during this instance's
  // lifecycle.
  std::map<std::string, std::unique_ptr<HistogramSamples>, std::less<>>
      histograms_snapshot_;
};

struct Bucket {
  Bucket(HistogramBase::Sample min, HistogramBase::Count count)
      : min(min), count(count) {}

  // A variant of the above constructor that accepts an `EnumType` for the `min`
  // value. Typically, this `EnumType` is the C++ enum (class) that is
  // associated with the metric this bucket is referring to.
  //
  // The constructor forwards to the above non-templated constructor. Therefore,
  // `EnumType` must be implicitly convertible to `HistogramBase::Sample`.
  template <typename MetricEnum,
            typename = std::enable_if_t<std::is_enum_v<MetricEnum>>>
  Bucket(MetricEnum min, HistogramBase::Count count)
      : Bucket(static_cast<std::underlying_type_t<MetricEnum>>(min), count) {}

  bool operator==(const Bucket& other) const;

  HistogramBase::Sample min;
  HistogramBase::Count count;
};

void PrintTo(const Bucket& value, std::ostream* os);

// The BucketsAre[Array]() and BucketsInclude[Array]() matchers are intended to
// match GetAllSamples().
//
// Unlike the standard matchers UnorderedElementsAreArray() and IsSupersetOf(),
// they explicitly support empty buckets (`Bucket::count == 0`). Empty buckets
// need special handling because GetAllSamples() doesn't contain empty ones.

// BucketsAre() and BucketsAreArray() match a container that contains exactly
// the non-empty `buckets`.
//
// For example,
//   EXPECT_THAT(histogram_tester.GetAllSamples("HistogramName"),
//               BucketsAre(Bucket(Enum::A, 0),
//                          Bucket(Enum::B, 1),
//                          Bucket(Enum::C, 2)));
// - matches the actual value `{Bucket(B, 1), Bucket(C, 2)}`;
// - does not match `{Bucket(A, n), Bucket(B, 1), Bucket(C, 2)}` for any `n`
//   (including `n == 0`).
template <typename BucketArray>
auto BucketsAreArray(BucketArray buckets) {
  auto non_empty_buckets = buckets;
  EraseIf(non_empty_buckets, [](Bucket b) { return b.count == 0; });
  return ::testing::UnorderedElementsAreArray(non_empty_buckets);
}

template <typename... BucketTypes>
auto BucketsAre(BucketTypes... buckets) {
  return BucketsAreArray(std::vector<Bucket>{buckets...});
}

// BucketsInclude() and BucketsIncludeArray() are empty-bucket-friendly
// replacements of IsSupersetOf[Array](): they match a container that contains
// all non-empty `buckets` and none of the empty `buckets`.
//
// For example,
//   EXPECT_THAT(histogram_tester.GetAllSamples("HistogramName"),
//               BucketsInclude(Bucket(Enum::A, 0),
//                              Bucket(Enum::B, 1),
//                              Bucket(Enum::C, 2)));
// - matches `{Bucket(B, 1), Bucket(C, 2), Bucket(D, 3)}`;
// - not match `{Bucket(A, n), Bucket(B, 1), Bucket(C, 2), Bucket(D, 3)}` for
//   any `n` (including `n == 0`).
template <typename BucketArray>
auto BucketsIncludeArray(const BucketArray& buckets) {
  // The `empty_buckets` have `count == 0`, so the `HistogramBase::Sample`
  // suffices.
  std::vector<HistogramBase::Sample> empty_buckets;
  std::vector<Bucket> non_empty_buckets;
  for (const Bucket& b : buckets) {
    if (b.count == 0) {
      empty_buckets.push_back(b.min);
    } else {
      non_empty_buckets.push_back(b);
    }
  }
  using ::testing::AllOf;
  using ::testing::AnyOfArray;
  using ::testing::Each;
  using ::testing::Field;
  using ::testing::IsSupersetOf;
  using ::testing::Not;
  return AllOf(
      IsSupersetOf(non_empty_buckets),
      Each(Field("Bucket::min", &Bucket::min, Not(AnyOfArray(empty_buckets)))));
}

template <typename... BucketTypes>
auto BucketsInclude(BucketTypes... buckets) {
  return BucketsIncludeArray(std::vector<Bucket>{buckets...});
}

}  // namespace base

#endif  // BASE_TEST_METRICS_HISTOGRAM_TESTER_H_