3 files changed, 52 insertions, 3 deletions
diff --git a/base/metrics/field_trial.cc b/base/metrics/field_trial.cc
index 9d4ec72..d500a29 100644
--- a/base/metrics/field_trial.cc
+++ b/base/metrics/field_trial.cc
@@ -4,6 +4,8 @@
 
 #include "base/metrics/field_trial.h"
 
+#include <algorithm>
+
 #include "base/build_time.h"
 #include "base/logging.h"
 #include "base/rand_util.h"
@@ -38,9 +40,26 @@ Time CreateTimeFromParams(int year, int month, int day_of_month) {
   return Time::FromLocalExploded(exploded);
 }
 
-}  // namespace
+// 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);
+}
 
-static const char kHistogramFieldTrialSeparator('_');
+}  // namespace
 
 // statics
 const int FieldTrial::kNotFinalized = -1;
@@ -60,7 +79,7 @@ FieldTrial::FieldTrial(const std::string& trial_name,
     : 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),
diff --git a/base/metrics/field_trial.h b/base/metrics/field_trial.h
index 6dd2d9c..5060a74 100644
--- a/base/metrics/field_trial.h
+++ b/base/metrics/field_trial.h
@@ -173,6 +173,8 @@ class BASE_EXPORT FieldTrial : public RefCounted<FieldTrial> {
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, SetForcedTurnFeatureOn);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, SetForcedChangeDefault_Default);
   FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, SetForcedChangeDefault_NonDefault);
+  FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, FloatBoundariesGiveEqualGroupSizes);
+  FRIEND_TEST_ALL_PREFIXES(FieldTrialTest, DoesNotSurpassTotalProbability);
 
   friend class base::FieldTrialList;
 
diff --git a/base/metrics/field_trial_unittest.cc b/base/metrics/field_trial_unittest.cc
index bfcba0f..10d3c3f 100644
--- a/base/metrics/field_trial_unittest.cc
+++ b/base/metrics/field_trial_unittest.cc
@@ -849,4 +849,32 @@ TEST_F(FieldTrialTest, ExpirationYearNotExpired) {
   EXPECT_EQ(kGroupName, trial->group_name());
 }
 
+TEST_F(FieldTrialTest, FloatBoundariesGiveEqualGroupSizes) {
+  const int kBucketCount = 100;
+
+  // Try each boundary value |i / 100.0| as the entropy value.
+  for (int i = 0; i < kBucketCount; ++i) {
+    const double entropy = i / static_cast<double>(kBucketCount);
+
+    scoped_refptr<base::FieldTrial> trial(
+        new base::FieldTrial("test", kBucketCount, "default", entropy));
+    for (int j = 0; j < kBucketCount; ++j)
+      trial->AppendGroup(base::StringPrintf("%d", j), 1);
+
+    EXPECT_EQ(base::StringPrintf("%d", i), trial->group_name());
+  }
+}
+
+TEST_F(FieldTrialTest, DoesNotSurpassTotalProbability) {
+  const double kEntropyValue = 1.0 - 1e-9;
+  ASSERT_LT(kEntropyValue, 1.0);
+
+  scoped_refptr<base::FieldTrial> trial(
+      new base::FieldTrial("test", 2, "default", kEntropyValue));
+  trial->AppendGroup("1", 1);
+  trial->AppendGroup("2", 1);
+
+  EXPECT_EQ("2", trial->group_name());
+}
+
 }  // namespace base