Add one-time randomization support for FieldTrial, and the ability to

base/metrics/field_trial_unittest.cc

Index: base/metrics/field_trial_unittest.cc
diff --git a/base/metrics/field_trial_unittest.cc b/base/metrics/field_trial_unittest.cc
index 0af6d6082ac897ecb4d542dd82ea9de82575b5ae..10b3dc8baf1954191b6292e32c44ea05f3c429ec 100644
--- a/base/metrics/field_trial_unittest.cc
+++ b/base/metrics/field_trial_unittest.cc
@@ -1,4 +1,4 @@
-// Copyright (c) 2010 The Chromium Authors. All rights reserved.
+// Copyright (c) 2011 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.
 
@@ -6,7 +6,9 @@
 
 #include "base/metrics/field_trial.h"
 
+#include "base/rand_util.h"
 #include "base/stringprintf.h"
+#include "base/string_number_conversions.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace base {
@@ -295,4 +297,70 @@ TEST_F(FieldTrialTest, MakeName) {
             FieldTrial::MakeName("Histogram", "Field Trial"));
 }
 
+TEST_F(FieldTrialTest, MachineIdToUniformDouble) {
+  double results[] = {
+      FieldTrial::MachineIdToUniformDouble("hi"),
+      FieldTrial::MachineIdToUniformDouble("there"),
+  };
+  EXPECT_NE(results[0], results[1]);
+  for (int i = 0; i < arraysize(results); ++i) {
+    EXPECT_LE(0.0, results[i]);
+    EXPECT_GT(1.0, results[i]);
+  }
+
+  EXPECT_EQ(FieldTrial::MachineIdToUniformDouble("yo"),
+            FieldTrial::MachineIdToUniformDouble("yo"));
+}
+
+// Not marking flaky as it should be incredibly rare for it to
+// not pass, and the diagnostics we print in this case would help
+// determine whether it is a true failure.
+TEST_F(FieldTrialTest, ProbabilisticMachineIdToUniformDoubleIsUniform) {
+  // A uniform distribution should result in an expected average value
+  // of 0.5.  The actual average for a large number of samples should,
+  // with extremely high probability, be very close to this.
+  const int kNumSamples = 5000;
+  double total_value = 0.0;
+  for (int i = 0; i < kNumSamples; ++i) {
+    total_value += FieldTrial::MachineIdToUniformDouble(
+        IntToString(RandInt(0, MAXINT)));
+  }
+
+  double average = total_value / kNumSamples;
+  double kExpectedMin = 0.45;
+  double kExpectedMax = 0.55;
+  EXPECT_LT(kExpectedMin, average);
+  EXPECT_GT(kExpectedMax, average);

[jar (doing other things), 2011/04/21 01:03:50]

Flaky tests are not acceptable, even with an explanation message.
You *can* re-run the test until you pass (by some definition), but you can't
have an Expect() that can fail with any detectable probability.  

The common trick is to "run it again" if it fails... and again... and again.  If
it can't pass in about 5 minutes, the test harness will finally kill you with a
failed-to-complete.

Looking at the test, even if this was very non random, it would "eventually"
pass.

Maybe it would be better to just put results into some array of possible ranges,
and run until you have data in all N range buckets??

[Jói, 2011/04/21 19:50:33]

Fixed.

+
+  if (average < kExpectedMin || average > kExpectedMax) {
+    printf("Average was %f, outside the expected range (%f, %f).\n"
+           "Values far outside the range may indicate a real problem,\n"
+           "whereas values just outside the range are likely just flukes.\n",
+           average, kExpectedMin, kExpectedMax);
+  }
+}
+
+TEST_F(FieldTrialTest, UseOneTimeRandomization) {
+  // Simply asserts that two trials with the same machine_id and the
+  // same set of groups should return the same results.
+
+  scoped_refptr<FieldTrial> trials[] = {
+    new FieldTrial("first", 100, "default", 2050, 1, 1),

[jar (doing other things), 2011/04/21 01:03:50]

use (... next_year_, 12, 31) for the date.  See line 294 as an example.

[Jói, 2011/04/21 19:50:33]

Done.

+    new FieldTrial("second", 100, "default", 2050, 1, 1),
+  };
+
+  std::string machine_id = IntToString(RandInt(0, MAXINT));
+  for (int i = 0; i < arraysize(trials); ++i) {
+    // Same machine_id for both trials.
+    trials[i]->UseOneTimeRandomization(machine_id);
+
+    for (int j = 0; j < 10; ++j) {
+      trials[i]->AppendGroup("", 10);
+    }
+  }
+
+  ASSERT_EQ(trials[0]->group(), trials[1]->group());
+  ASSERT_EQ(trials[0]->group_name(), trials[1]->group_name());
+}
+
 }  // namespace base