Fix base::RandGenerator bug (it had non-uniform random distribution).

base/rand_util_unittest.cc

Index: base/rand_util_unittest.cc
diff --git a/base/rand_util_unittest.cc b/base/rand_util_unittest.cc
index d7fa37af827e8589299c4a066ef83308ca4a384a..80a65b108144007817f2f86b117ee0c134a888ce 100644
--- a/base/rand_util_unittest.cc
+++ b/base/rand_util_unittest.cc
@@ -61,3 +61,41 @@ TEST(RandUtilTest, RandGeneratorForRandomShuffle) {
   EXPECT_LE(std::numeric_limits<ptrdiff_t>::max(),
             std::numeric_limits<int64>::max());
 }
+
+TEST(RandUtilTest, RandGeneratorIsUniform) {
+  // Verify that RandGenerator has a uniform distribution. This is a
+  // regression test that consistently failed when RandGenerator was
+  // implemented this way:
+  //
+  //   return base::RandUint64() % max;
+  //
+  // The worst case for such an implementation is e.g. a top of range
+  // that is 2/3rds of the way to MAX_UINT64, in which case the bottom
+  // half of the range would be twice as likely to occur as the top
+  // half, assuming a naive modulus implementation of RandGenerator.
+  const uint64 kTopOfRange = (std::numeric_limits<uint64>::max() / 3L) * 2L;
+  const uint64 kExpectedAverage = kTopOfRange / 2L;
+  const uint64 kAllowedVariance = kExpectedAverage / 50L;  // +/- 2%
+  const int kMinAttempts = 1000;
+  const int kMaxAttempts = 1000000;
+
+  double cumulative_average = 0.0;
+  int count = 0;
+  while (count < kMaxAttempts) {
+    uint64 value = base::RandGenerator(kTopOfRange);
+    cumulative_average = (count * cumulative_average + value) / (count + 1);
+
+    // Don't quit too quickly for things to start converging, or we may have
+    // a false positive.
+    if (count > kMinAttempts &&
+        kExpectedAverage - kAllowedVariance < cumulative_average &&
+        cumulative_average < kExpectedAverage + kAllowedVariance) {
+      break;
+    }
+
+    ++count;
+  }
+
+  ASSERT_LT(count, kMaxAttempts) << "Expected average was " <<
+      kExpectedAverage << ", average ended at " << cumulative_average;
+}