Fix inconsistent FieldTrial group assignment due to float errors.

base/metrics/field_trial.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 "base/metrics/field_trial.h"
 
+#include <algorithm>
+
 #include "base/build_time.h"
 #include "base/logging.h"
 #include "base/rand_util.h"
 #include "base/sha1.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/sys_byteorder.h"
 
 namespace base {
@@ skipping 14 matching lines @@
   exploded.day_of_week = 0;  // Should be unused.
   exploded.day_of_month = day_of_month;
   exploded.hour = 0;
   exploded.minute = 0;
   exploded.second = 0;
   exploded.millisecond = 0;
 
   return Time::FromLocalExploded(exploded);
 }
 
+// Returns the boundary value for comparing against the FieldTrial's added
+// groups for a given |divisor| (total probability) and |entropy_value|.
+FieldTrial::Probability GetGroupBoundaryValue(
+    FieldTrial::Probability divisor,
+    double entropy_value) {
+  // Add a tiny epsilon value to get consistent results when converting floating
+  // points to int. Without it, boundary values have inconsistent results, e.g.:
+  //
+  //   static_cast<FieldTrial::Probability>(100 * 0.56) == 56
+  //   static_cast<FieldTrial::Probability>(100 * 0.57) == 56
+  //   static_cast<FieldTrial::Probability>(100 * 0.58) == 57
+  //   static_cast<FieldTrial::Probability>(100 * 0.59) == 59
+  const double kEpsilon = 1e-8;
+  const FieldTrial::Probability result =
+      static_cast<FieldTrial::Probability>(divisor * entropy_value + kEpsilon);
+  // Ensure that adding the epsilon still results in a value < |divisor|.
+  return std::min(result, divisor - 1);
+}
+
 }  // namespace
 
-static const char kHistogramFieldTrialSeparator('_');
-
 // statics
 const int FieldTrial::kNotFinalized = -1;
 const int FieldTrial::kDefaultGroupNumber = 0;
 bool FieldTrial::enable_benchmarking_ = false;
 
 const char FieldTrialList::kPersistentStringSeparator('/');
 int FieldTrialList::kNoExpirationYear = 0;
 
 //------------------------------------------------------------------------------
 // FieldTrial methods and members.
 
 FieldTrial::FieldTrial(const std::string& trial_name,
                        const Probability total_probability,
                        const std::string& default_group_name,
                        double entropy_value)
     : trial_name_(trial_name),
       divisor_(total_probability),
       default_group_name_(default_group_name),
-      random_(static_cast<Probability>(divisor_ * entropy_value)),
+      random_(GetGroupBoundaryValue(total_probability, entropy_value)),
       accumulated_group_probability_(0),
       next_group_number_(kDefaultGroupNumber + 1),
       group_(kNotFinalized),
       enable_field_trial_(true),
       forced_(false),
       group_reported_(false) {
   DCHECK_GT(total_probability, 0);
   DCHECK(!trial_name_.empty());
   DCHECK(!default_group_name_.empty());
 }
@@ skipping 414 matching lines @@
     used_without_global_ = true;
     return;
   }
   AutoLock auto_lock(global_->lock_);
   DCHECK(!global_->PreLockedFind(trial->trial_name()));
   trial->AddRef();
   global_->registered_[trial->trial_name()] = trial;
 }
 
 }  // namespace base