Simulate field trial assignments with new variations seed.

components/variations/variations_seed_simulator.cc

Index: components/variations/variations_seed_simulator.cc
===================================================================
--- components/variations/variations_seed_simulator.cc	(revision 0)
+++ components/variations/variations_seed_simulator.cc	(working copy)
@@ -0,0 +1,185 @@
+// 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);
+    // TODO(asvitkine): This needs to properly handle the case where a group was
+    // forced via forcing_flag in the current state, so that it is not treated
+    // as changed.
+    if (!experiment.has_forcing_flag() &&
+        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;
+    }

[jwd, 2014/01/07 18:31:37]

This seems to implicitly do the correct thing when consistency changes. Is it
worth mentioning that?

[Alexei Svitkine (slow), 2014/01/08 15:17:35]

Done.

+  }
+
+  // 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 controlled entirely via params). Support
+  // a 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()) {
+    // An expired study will result in the default group being selected - mark
+    // it as changed if the current group differs from the default.
+    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