Simulate field trial assignments with new variations seed.

components/variations/variations_seed_simulator.cc

+// 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 "components/variations/variations_seed_simulator.h"
+
+#include <map>
+
+#include "base/metrics/field_trial.h"
+#include "components/variations/processed_study.h"
+#include "components/variations/proto/study.pb.h"
+#include "components/variations/variations_associated_data.h"
+
+namespace chrome_variations {
+
+namespace {
+
+// Fills in |current_state| with the current process' active field trials, as a
+// map of trial names to group names.
+void GetCurrentTrialState(std::map<std::string, std::string>* current_state) {
+  base::FieldTrial::ActiveGroups trial_groups;
+  base::FieldTrialList::GetActiveFieldTrialGroups(&trial_groups);
+  for (size_t i = 0; i < trial_groups.size(); ++i)
+    (*current_state)[trial_groups[i].trial_name] = trial_groups[i].group_name;
+}
+
+// Simulate group assignment for the specified study with PERMANENT consistency.
+// Returns the experiment group that will be selected. Mirrors logic in
+// VariationsSeedProcessor::CreateTrialFromStudy().
+std::string SimulateGroupAssignment(
+    const base::FieldTrial::EntropyProvider& entropy_provider,
+    const ProcessedStudy& processed_study) {
+  const Study& study = *processed_study.study();
+  DCHECK_EQ(Study_Consistency_PERMANENT, study.consistency());
+
+  const double entropy_value =
+      entropy_provider.GetEntropyForTrial(study.name(),
+                                          study.randomization_seed());
+  scoped_refptr<base::FieldTrial> trial(
+      base::FieldTrial::CreateSimulatedFieldTrial(
+          study.name(), processed_study.total_probability(),
+          study.default_experiment_name(), entropy_value));
+
+  for (int i = 0; i < study.experiment_size(); ++i) {
+    const Study_Experiment& experiment = study.experiment(i);
+    if (!experiment.has_forcing_flag() &&

[Mathieu, 2013/12/30 15:21:37]

what happens if the current version of the running trial is using a forced flag?
We wouldn't want to report that a different group would be chosen at restart.
Did you mean to check for forcing_flag upstream from this class?

[Alexei Svitkine (slow), 2014/01/06 19:22:40]

You're right that it's currently not handled. And its also a bit tricky to
handle - since currently that info isn't saved - we either need to keep the old
seed around and use it for this check or some extra metadata that we save off
during the evaluation of the initial seed. I've added a TODO for this that I'll
address in a follow-up CL.

[mathp, 2014/01/06 19:37:47]

Sounds good. In the meantime it's possible this function will erroneously report
a change in groups, but I guess it's fine since forcing flag populations are
small.

[Alexei Svitkine (slow), 2014/01/06 19:47:58]

Right. Also, even though this CL adds the class, its not yet being used
(something I will do in another CL).

+        experiment.name() != study.default_experiment_name()) {
+      trial->AppendGroup(experiment.name(), experiment.probability_weight());
+    }
+  }
+  if (processed_study.is_expired())
+    trial->Disable();
+  return trial->group_name();
+}
+
+// Finds an experiment in |study| with name |experiment_name| and returns it,
+// or NULL if it does not exist.
+const Study_Experiment* FindExperiment(const Study& study,
+                                       const std::string& experiment_name) {
+  for (int i = 0; i < study.experiment_size(); ++i) {
+    if (study.experiment(i).name() == experiment_name)
+      return &study.experiment(i);
+  }
+  return NULL;
+}
+
+// Checks whether experiment params set for |experiment| on |study| are exactly
+// equal to the params registered for the corresponding field trial in the
+// current process.
+bool VariationParamsAreEqual(const Study& study,
+                             const Study_Experiment& experiment) {
+  std::map<std::string, std::string> params;
+  GetVariationParams(study.name(), &params);
+
+  if (static_cast<int>(params.size()) != experiment.param_size())
+    return false;
+
+  for (int i = 0; i < experiment.param_size(); ++i) {
+    std::map<std::string, std::string>::const_iterator it =
+        params.find(experiment.param(i).name());
+    if (it == params.end() || it->second != experiment.param(i).value())
+      return false;
+  }
+
+  return true;
+}
+
+}  // namespace
+
+VariationsSeedSimulator::VariationsSeedSimulator(
+    const base::FieldTrial::EntropyProvider& entropy_provider)
+    : entropy_provider_(entropy_provider) {
+}
+
+VariationsSeedSimulator::~VariationsSeedSimulator() {
+}
+
+int VariationsSeedSimulator::ComputeDifferences(
+    const std::vector<ProcessedStudy>& processed_studies) {
+  std::map<std::string, std::string> current_state;
+  GetCurrentTrialState(&current_state);
+  int group_change_count = 0;
+
+  for (size_t i = 0; i < processed_studies.size(); ++i) {
+    const Study& study = *processed_studies[i].study();
+    std::map<std::string, std::string>::const_iterator it =
+        current_state.find(study.name());
+
+    // Skip studies that aren't activated in the current state.
+    // TODO(asvitkine): This should be handled more intelligently. There are
+    // several cases that fall into this category:
+    //   1) There's an existing field trial with this name but it is not active.
+    //   2) There's an existing expired field trial with this name, which is
+    //      also not considered as active.
+    //   3) This is a new study config that previously didn't exist.
+    // The above cases should be differentiated and handled explicitly.
+    if (it == current_state.end())
+      continue;
+
+    // Study exists in the current state, check whether its group will change.
+    const std::string& selected_group = it->second;
+    if (study.consistency() == Study_Consistency_PERMANENT) {
+      if (PermanentStudyGroupChanged(processed_studies[i], selected_group))
+        ++group_change_count;
+    } else if (study.consistency() == Study_Consistency_SESSION) {
+      if (SessionStudyGroupChanged(processed_studies[i], selected_group))
+        ++group_change_count;
+    }
+  }
+
+  // TODO(asvitkine): Handle removed studies (i.e. studies that existed in the
+  // old seed, but were removed). This will require tracking the set of studies
+  // that were created from the original seed.
+
+  return group_change_count;
+}
+
+bool VariationsSeedSimulator::PermanentStudyGroupChanged(
+    const ProcessedStudy& processed_study,
+    const std::string& selected_group) {
+  const Study& study = *processed_study.study();
+  DCHECK_EQ(Study_Consistency_PERMANENT, study.consistency());
+
+  const std::string simulated_group = SimulateGroupAssignment(entropy_provider_,
+                                                              processed_study);
+  // TODO(asvitkine): Sometimes group names are changed without changing any
+  // behavior (e.g. if the behavior is control entirely via params). Support a

[Mathieu, 2013/12/30 15:21:37]

*controlled

[Alexei Svitkine (slow), 2014/01/06 19:22:40]

Done.

+  // mechanism to bypass this check.
+  if (simulated_group != selected_group)
+    return true;
+
+  const Study_Experiment* experiment = FindExperiment(study, selected_group);
+  DCHECK(experiment);
+  return !VariationParamsAreEqual(study, *experiment);
+}
+
+bool VariationsSeedSimulator::SessionStudyGroupChanged(
+    const ProcessedStudy& processed_study,
+    const std::string& selected_group) {
+  const Study& study = *processed_study.study();
+  DCHECK_EQ(Study_Consistency_SESSION, study.consistency());
+
+  if (processed_study.is_expired() &&
+      selected_group != study.default_experiment_name()) {

[Mathieu, 2013/12/30 15:21:37]

Tests are well commented but this isn't. Can you add a comment before this check
explaining why this returns true?

[Alexei Svitkine (slow), 2014/01/06 19:22:40]

Done.

+    return true;
+  }
+
+  const Study_Experiment* experiment = FindExperiment(study, selected_group);
+  if (!experiment)
+    return true;
+  if (experiment->probability_weight() == 0 &&
+      !experiment->has_forcing_flag()) {
+    return true;
+  }
+
+  // Current group exists in the study - check whether its params changed.
+  return !VariationParamsAreEqual(study, *experiment);
+}
+
+}  // namespace chrome_variations