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..fe6e90328f6a86e1d0fe2ce37a8c70695bc35141 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.
 
@@ -6,9 +6,13 @@
 
 #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"
 
+#include <limits>
+
 namespace base {
 
 class FieldTrialTest : public testing::Test {
@@ -295,4 +299,108 @@ TEST_F(FieldTrialTest, MakeName) {
             FieldTrial::MakeName("Histogram", "Field Trial"));
 }
 
+TEST_F(FieldTrialTest, HashClientId) {
+  double results[] = {
+      FieldTrial::HashClientId("hi", "1"),
+      FieldTrial::HashClientId("there", "1"),
+  };
+  ASSERT_NE(results[0], results[1]);
+  for (size_t i = 0; i < arraysize(results); ++i) {
+    ASSERT_LE(0.0, results[i]);
+    ASSERT_GT(1.0, results[i]);
+  }
+
+  ASSERT_EQ(FieldTrial::HashClientId("yo", "1"),
+            FieldTrial::HashClientId("yo", "1"));
+  ASSERT_NE(FieldTrial::HashClientId("yo", "something"),
+            FieldTrial::HashClientId("yo", "else"));
+}
+
+TEST_F(FieldTrialTest, HashClientIdIsUniform) {
+  // Choose a random start number but go sequentially from there, so
+  // that each test tries a different range but we never provide uniformly
+  // distributed input data.
+  int current_number = RandInt(0, std::numeric_limits<int>::max());
+
+  // The expected value of a random distribution is the average over all
+  // samples as the number of samples approaches infinity.  For a uniform
+  // distribution from [0.0, 1.0) this would be 0.5.
+  //
+  // We do kSamplesBetweenChecks at a time and check if the value has converged
+  // to a narrow interval around 0.5.  A non-uniform distribution would likely
+  // converge at something different, or not converge consistently within this
+  // range (i.e. the test would start timing out occasionally).
+  int kSamplesBetweenChecks = 300;
+  int num_samples = 0;
+  double total_value = 0.0;
+  while (true) {
+    for (int i = 0; i < kSamplesBetweenChecks; ++i) {
+      total_value += FieldTrial::HashClientId(
+          IntToString(current_number++), "salt");
+      num_samples++;
+    }
+
+    double average = total_value / num_samples;
+    double kExpectedMin = 0.48;
+    double kExpectedMax = 0.52;
+
+    if (average < kExpectedMin || average > kExpectedMax) {
+      printf("After %d samples, the average was %f, outside the expected\n"
+             "range (%f, %f). We will add more samples and check after every\n"
+             "%d samples. If the average does not converge, something\n"
+             "is broken. If it does converge, the test will pass.\n",
+             num_samples, average,
+             kExpectedMin, kExpectedMax, kSamplesBetweenChecks);
+    } else {
+      // Success.
+      break;
+    }
+  }
+}
+
+TEST_F(FieldTrialTest, UseOneTimeRandomization) {
+  // Simply asserts that two trials using one-time randomization
+  // that have different names, normally generate different results.
+  //
+  // Note that depending on the one-time random initialization, they
+  // _might_ actually give the same result, but we know that given
+  // this particular initialization they won't.
+  FieldTrialList::EnableOneTimeRandomization("this is cheating");
+
+  scoped_refptr<FieldTrial> trials[] = {
+    new FieldTrial("one", 100, "default", next_year_, 1, 1),
+    new FieldTrial("two", 100, "default", next_year_, 1, 1),
+  };
+
+  for (size_t i = 0; i < arraysize(trials); ++i) {
+    trials[i]->UseOneTimeRandomization();
+
+    for (int j = 0; j < 100; ++j) {
+      trials[i]->AppendGroup("", 1);
+    }
+  }
+
+  // The trials are most likely to give different results since they have
+  // different names.
+  ASSERT_NE(trials[0]->group(), trials[1]->group());
+  ASSERT_NE(trials[0]->group_name(), trials[1]->group_name());
+}
+
+TEST_F(FieldTrialTest, DisableImmediately) {
+  FieldTrial* trial =
+      new FieldTrial("trial", 100, "default", next_year_, 12, 31);
+  trial->Disable();
+  ASSERT_EQ("default", trial->group_name());
+  ASSERT_EQ(FieldTrial::kDefaultGroupNumber, trial->group());
+}
+
+TEST_F(FieldTrialTest, DisableAfterInitialization) {
+  FieldTrial* trial =
+      new FieldTrial("trial", 100, "default", next_year_, 12, 31);
+  trial->AppendGroup("non_default", 100);
+  ASSERT_EQ("non_default", trial->group_name());
+  trial->Disable();
+  ASSERT_EQ("default", trial->group_name());
+}
+
 }  // namespace base