From af2e192b4a8d5ff0270b54bda694254e5cd5f8fa Mon Sep 17 00:00:00 2001 From: "joi@chromium.org" Date: Mon, 30 May 2011 17:39:09 +0000 Subject: Fix base::RandGenerator bug (it had non-uniform random distribution). Add test that would have caught bug. Also add a test to verify that our random generators are at least somewhat random. BUG=84221 TEST=base_unittests --gtest_filter=RandUtilTest.* Review URL: http://codereview.chromium.org/7080005 git-svn-id: svn://svn.chromium.org/chrome/trunk/src@87244 0039d316-1c4b-4281-b951-d872f2087c98 --- base/rand_util_unittest.cc | 60 ++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 60 insertions(+) (limited to 'base/rand_util_unittest.cc') diff --git a/base/rand_util_unittest.cc b/base/rand_util_unittest.cc index d7fa37a..b81e2ef 100644 --- a/base/rand_util_unittest.cc +++ b/base/rand_util_unittest.cc @@ -61,3 +61,63 @@ TEST(RandUtilTest, RandGeneratorForRandomShuffle) { EXPECT_LE(std::numeric_limits::max(), std::numeric_limits::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; + // + // A degenerate 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. A bit of calculus care of jar@ shows that the largest + // measurable delta is when the top of the range is 3/4ths of the + // way, so that's what we use in the test. + const uint64 kTopOfRange = (std::numeric_limits::max() / 4ULL) * 3ULL; + const uint64 kExpectedAverage = kTopOfRange / 2ULL; + const uint64 kAllowedVariance = kExpectedAverage / 50ULL; // +/- 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; +} + +TEST(RandUtilTest, RandUint64ProducesBothValuesOfAllBits) { + // This tests to see that our underlying random generator is good + // enough, for some value of good enough. + uint64 kAllZeros = 0ULL; + uint64 kAllOnes = ~kAllZeros; + uint64 found_ones = kAllZeros; + uint64 found_zeros = kAllOnes; + + for (size_t i = 0; i < 1000; ++i) { + uint64 value = base::RandUint64(); + found_ones |= value; + found_zeros &= value; + + if (found_zeros == kAllZeros && found_ones == kAllOnes) + return; + } + + FAIL() << "Didn't achieve all bit values in maximum number of tries."; +} -- cgit v1.1