Use a different algorithm with the low entropy source for field trials.

chrome/common/metrics/entropy_provider_unittest.cc

+// Copyright (c) 2012 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 <cmath>
+#include <limits>
+#include <numeric>
+
+#include "base/basictypes.h"
+#include "base/guid.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/rand_util.h"
+#include "base/string_number_conversions.h"
+#include "chrome/common/metrics/entropy_provider.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace {
+
+// Computes the standard deviation of distribution |values|.
+double ComputeStandardDeviation(const std::vector<int>& values) {
+  const int sum = std::accumulate(values.begin(), values.end(), 0.0);
+  const int mean = sum / values.size();

[Ilya Sherman, 2012/08/21 03:42:27]

nit: Should this be a double rather than an int?

[Alexei Svitkine (slow), 2012/08/21 14:25:49]

Changed to use doubles throughout the function.

(The previous code was correct but only because the mean was always a full
integer - i.e. for 100000 iterations and 20 buckets, it would always be 5000).

+  const int sum_of_squares = std::inner_product(values.begin(), values.end(),
+                                                values.begin(), 0.0);
+  const double variance = static_cast<double>(sum_of_squares) /
+                          values.size() - (mean * mean);
+  return std::sqrt(variance);
+}
+
+}  // namespace
+
+
+class EntropyProviderTest : public testing::Test {
+ public:
+  // Computes SHA1-based entropy for the given |trial_name| based on
+  // |entropy_source|
+  double GenerateSHA1Entropy(const std::string& entropy_source,
+                             const std::string& trial_name) {
+    SHA1EntropyProvider sha1_provider(entropy_source);
+    return sha1_provider.GetEntropyForTrial(trial_name);
+  }
+
+  // Generates permutation-based entropy for the given |trial_name| based on
+  // |entropy_source| which must be in the range [0, entropy_max).
+  double GeneratePermutedEntropy(uint16 entropy_source,
+                                 size_t entropy_max,
+                                 const std::string& trial_name) {
+    PermutedEntropyProvider permuted_provider(entropy_source, entropy_max);
+    return permuted_provider.GetEntropyForTrial(trial_name);
+  }
+};
+
+
+TEST_F(EntropyProviderTest, UseOneTimeRandomizationSHA1) {
+  // Simply asserts that two trials using one-time randomization
+  // that have different names, normally generate different results.
+  //
+  // Note that depending on the one-time random initialization, they
+  // _might_ actually give the same result, but we know that given
+  // the particular client_id we use for unit tests they won't.
+  base::FieldTrialList field_trial_list(new SHA1EntropyProvider("client_id"));
+  scoped_refptr<base::FieldTrial> trials[] = {
+      base::FieldTrialList::FactoryGetFieldTrial("one", 100, "default",
+          base::FieldTrialList::kExpirationYearInFuture, 1, 1, NULL),
+      base::FieldTrialList::FactoryGetFieldTrial("two", 100, "default",
+          base::FieldTrialList::kExpirationYearInFuture, 1, 1, NULL),
+  };
+
+  for (size_t i = 0; i < arraysize(trials); ++i) {
+    trials[i]->UseOneTimeRandomization();
+
+    for (int j = 0; j < 100; ++j)
+      trials[i]->AppendGroup("", 1);
+  }
+
+  // The trials are most likely to give different results since they have
+  // different names.
+  ASSERT_NE(trials[0]->group(), trials[1]->group());
+  ASSERT_NE(trials[0]->group_name(), trials[1]->group_name());
+}
+
+TEST_F(EntropyProviderTest, UseOneTimeRandomizationPermuted) {
+  // Simply asserts that two trials using one-time randomization
+  // that have different names, normally generate different results.
+  //
+  // Note that depending on the one-time random initialization, they
+  // _might_ actually give the same result, but we know that given
+  // the particular client_id we use for unit tests they won't.
+  const size_t kMaxLowEntropySize = (1 << 13);
+  base::FieldTrialList field_trial_list(
+      new PermutedEntropyProvider(1234, kMaxLowEntropySize));
+  scoped_refptr<base::FieldTrial> trials[] = {
+      base::FieldTrialList::FactoryGetFieldTrial("one", 100, "default",
+          base::FieldTrialList::kExpirationYearInFuture, 1, 1, NULL),
+      base::FieldTrialList::FactoryGetFieldTrial("two", 100, "default",
+          base::FieldTrialList::kExpirationYearInFuture, 1, 1, NULL),
+  };
+
+  for (size_t i = 0; i < arraysize(trials); ++i) {
+    trials[i]->UseOneTimeRandomization();
+
+    for (int j = 0; j < 100; ++j)
+      trials[i]->AppendGroup("", 1);
+  }
+
+  // The trials are most likely to give different results since they have
+  // different names.
+  ASSERT_NE(trials[0]->group(), trials[1]->group());
+  ASSERT_NE(trials[0]->group_name(), trials[1]->group_name());
+}
+
+TEST_F(EntropyProviderTest, SHA1Entropy) {
+  const double results[] = {
+      GenerateSHA1Entropy("hi", "1"),
+      GenerateSHA1Entropy("there", "1"),
+  };
+  ASSERT_NE(results[0], results[1]);
+  for (size_t i = 0; i < arraysize(results); ++i) {
+    ASSERT_LE(0.0, results[i]);
+    ASSERT_GT(1.0, results[i]);
+  }
+
+  ASSERT_EQ(GenerateSHA1Entropy("yo", "1"),
+            GenerateSHA1Entropy("yo", "1"));
+  ASSERT_NE(GenerateSHA1Entropy("yo", "something"),
+            GenerateSHA1Entropy("yo", "else"));
+}
+
+TEST_F(EntropyProviderTest, PermutedEntropy) {
+  const size_t kMaxLowEntropySize = (1 << 13);
+  const double results[] = {
+      GeneratePermutedEntropy(1234, kMaxLowEntropySize, "1"),
+      GeneratePermutedEntropy(4321, kMaxLowEntropySize, "1"),
+  };
+  ASSERT_NE(results[0], results[1]);
+  for (size_t i = 0; i < arraysize(results); ++i) {
+    ASSERT_LE(0.0, results[i]);
+    ASSERT_GT(1.0, results[i]);
+  }
+
+  ASSERT_EQ(GeneratePermutedEntropy(1234, kMaxLowEntropySize, "1"),
+            GeneratePermutedEntropy(1234, kMaxLowEntropySize, "1"));
+  ASSERT_NE(GeneratePermutedEntropy(1234, kMaxLowEntropySize, "something"),
+            GeneratePermutedEntropy(1234, kMaxLowEntropySize, "else"));
+}
+
+TEST_F(EntropyProviderTest, SHA1EntropyIsUniform) {
+  const size_t kMaxLowEntropySize = (1 << 13);
+  const int kBucketCount = 20;
+  const int kIterationCount = 100000;

[Ilya Sherman, 2012/08/21 03:42:27]

nit: size_t throughout?

[Alexei Svitkine (slow), 2012/08/21 14:25:49]

Done.

+
+  const std::string trial_names[] = {
+      "TestTrial",
+      "AnotherTestTrial",
+      "NewTabButton",
+  };
+
+  for (size_t i = 0; i < arraysize(trial_names); ++i) {
+    std::vector<int> distribution(kBucketCount);
+
+    for (int j = 0; j < kIterationCount; ++j) {
+      // Use a random GUID + 13 bits of entropy to match how the
+      // SHA1EntropyProvider is used in metrics_service.cc.
+      const int low_entropy_source =
+          static_cast<uint16>(base::RandInt(0, kMaxLowEntropySize - 1));
+      const std::string high_entropy_source =
+          base::GenerateGUID() + base::IntToString(low_entropy_source);
+      const double entropy_value =
+          GenerateSHA1Entropy(high_entropy_source, trial_names[i]);
+      const int bucket = static_cast<int>(kBucketCount * entropy_value);
+      ASSERT_LT(bucket, kBucketCount);
+      distribution[bucket] += 1;
+    }
+
+    int min_value = std::numeric_limits<int>::max();
+    int max_value = std::numeric_limits<int>::min();
+    for (int j = 0; j < kBucketCount; ++j) {
+      min_value = std::min(min_value, distribution[j]);
+      max_value = std::max(max_value, distribution[j]);
+    }
+
+    // TODO(asvitkine): Figure out pass / fail criteria.
+    printf("min = %d, max = %d, stddev = %f\n", min_value, max_value,
+           ComputeStandardDeviation(distribution));
+  }
+}
+
+TEST_F(EntropyProviderTest, PermutedEntropyIsUniform) {
+  const size_t kMaxLowEntropySize = (1 << 13);
+  const int kBucketCount = 20;
+  const int kIterationCount = 100000;
+
+  const std::string trial_names[] = {
+      "TestTrial",
+      "AnotherTestTrial",
+      "NewTabButton",
+  };
+
+  for (size_t i = 0; i < arraysize(trial_names); ++i) {
+    std::vector<int> distribution(kBucketCount);
+
+    // Note: Given a trial name, the computed mapping will be the same.
+    // As a performance optimization, pre-compute the mapping once per trial
+    // name and index into it each iteration.

[Ilya Sherman, 2012/08/21 03:42:27]

Is this performance optimization needed for the test to run smoothly?  If not,
I'd prefer to test the actual code, so that there's no chance for a bug to get
fixed there and not in the test.  If the optimization is necessary, though,
well, so be it.

[Alexei Svitkine (slow), 2012/08/21 14:25:49]

Unfortunately, yes it is needed. Without it, this test case takes 15s to run on
my machine. With the optimization, it runs in 0.5s.

(Note: That 15s on my beefy machine may very well mean a timeout on our much
slower bots.)

+    std::vector<uint16> mapping(kMaxLowEntropySize);
+    metrics_internal::PermuteMappingUsingTrialName(trial_names[i], &mapping);
+
+    for (int j = 0; j < kIterationCount; ++j) {
+      const int low_entropy_source =
+          static_cast<uint16>(base::RandInt(0, kMaxLowEntropySize - 1));
+      const double entropy_value =
+          mapping[low_entropy_source] / static_cast<double>(kMaxLowEntropySize);
+      const int bucket = static_cast<int>(kBucketCount * entropy_value);
+      ASSERT_LT(bucket, kBucketCount);
+      distribution[bucket] += 1;
+    }
+
+    int min_value = std::numeric_limits<int>::max();
+    int max_value = std::numeric_limits<int>::min();
+    for (int j = 0; j < kBucketCount; ++j) {
+      min_value = std::min(min_value, distribution[j]);
+      max_value = std::max(max_value, distribution[j]);
+    }
+
+    // TODO(asvitkine): Figure out pass / fail criteria.
+    printf("min = %d, max = %d, stddev = %f\n", min_value, max_value,
+           ComputeStandardDeviation(distribution));
+  }
+}
+
+TEST_F(EntropyProviderTest, SeededRandGeneratorIsUniform) {
+  // Verifies that SeededRandGenerator has a uniform distribution.
+  //
+  // Mirrors RandUtilTest.RandGeneratorIsUniform in base/rand_util_unittest.cc.
+
+  const uint32 kTopOfRange = (std::numeric_limits<uint32>::max() / 4ULL) * 3ULL;
+  const uint32 kExpectedAverage = kTopOfRange / 2ULL;
+  const uint32 kAllowedVariance = kExpectedAverage / 50ULL;  // +/- 2%
+  const int kMinAttempts = 1000;
+  const int kMaxAttempts = 1000000;
+
+  const std::string trial_names[] = {
+      "TestTrial",
+      "AnotherTestTrial",
+      "NewTabButton",
+  };
+
+  for (size_t i = 0; i < arraysize(trial_names); ++i) {
+    const uint32 seed = metrics_internal::HashName(trial_names[i]);
+    metrics_internal::SeededRandGenerator rand_generator(seed);
+
+    double cumulative_average = 0.0;
+    int count = 0;
+    while (count < kMaxAttempts) {
+      uint32 value = rand_generator(kTopOfRange);
+      cumulative_average = (count * cumulative_average + value) / (count + 1);
+
+      // Don't quit too quickly for things to start converging, or we may have
+      // a false positive.
+      if (count > kMinAttempts &&
+          kExpectedAverage - kAllowedVariance < cumulative_average &&
+          cumulative_average < kExpectedAverage + kAllowedVariance) {
+        break;
+      }
+
+      ++count;
+    }
+
+    ASSERT_LT(count, kMaxAttempts) << "Expected average was " <<
+        kExpectedAverage << ", average ended at " << cumulative_average <<
+        ", for trial " << trial_names[i];
+  }
+}
+