text: // 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.
  
  #include "base/rand_util.h"
  
  #include <algorithm>
  #include <limits>
  
  #include "base/logging.h"
  #include "base/memory/scoped_ptr.h"
  #include "base/time/time.h"
  #include "testing/gtest/include/gtest/gtest.h"
  
  namespace {
  
  const int kIntMin = std::numeric_limits<int>::min();
  const int kIntMax = std::numeric_limits<int>::max();
  
  }  // namespace
  
  TEST(RandUtilTest, SameMinAndMax) {
    EXPECT_EQ(base::RandInt(0, 0), 0);
    EXPECT_EQ(base::RandInt(kIntMin, kIntMin), kIntMin);
    EXPECT_EQ(base::RandInt(kIntMax, kIntMax), kIntMax);
  }
  
  TEST(RandUtilTest, RandDouble) {
    // Force 64-bit precision, making sure we're not in a 80-bit FPU register.
    volatile double number = base::RandDouble();
    EXPECT_GT(1.0, number);
    EXPECT_LE(0.0, number);
  }
  
  TEST(RandUtilTest, RandBytes) {
    const size_t buffer_size = 50;
    char buffer[buffer_size];
    memset(buffer, 0, buffer_size);
    base::RandBytes(buffer, buffer_size);
    std::sort(buffer, buffer + buffer_size);
    // Probability of occurrence of less than 25 unique bytes in 50 random bytes
    // is below 10^-25.
    EXPECT_GT(std::unique(buffer, buffer + buffer_size) - buffer, 25);
  }
  
  TEST(RandUtilTest, RandBytesAsString) {
    std::string random_string = base::RandBytesAsString(1);
    EXPECT_EQ(1U, random_string.size());
    random_string = base::RandBytesAsString(145);
    EXPECT_EQ(145U, random_string.size());
    char accumulator = 0;
    for (size_t i = 0; i < random_string.size(); ++i)
      accumulator |= random_string[i];
    // In theory this test can fail, but it won't before the universe dies of
    // heat death.
    EXPECT_NE(0, accumulator);
  }
  
  // Make sure that it is still appropriate to use RandGenerator in conjunction
  // with std::random_shuffle().
  TEST(RandUtilTest, RandGeneratorForRandomShuffle) {
    EXPECT_EQ(base::RandGenerator(1), 0U);
    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;
    //
    // 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<uint64>::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.";
  }
  
  // Benchmark test for RandBytes().  Disabled since it's intentionally slow and
  // does not test anything that isn't already tested by the existing RandBytes()
  // tests.
  TEST(RandUtilTest, DISABLED_RandBytesPerf) {
    // Benchmark the performance of |kTestIterations| of RandBytes() using a
    // buffer size of |kTestBufferSize|.
    const int kTestIterations = 10;
    const size_t kTestBufferSize = 1 * 1024 * 1024;
  
    scoped_ptr<uint8[]> buffer(new uint8[kTestBufferSize]);
    const base::TimeTicks now = base::TimeTicks::HighResNow();
    for (int i = 0; i < kTestIterations; ++i)
      base::RandBytes(buffer.get(), kTestBufferSize);
    const base::TimeTicks end = base::TimeTicks::HighResNow();
  
    LOG(INFO) << "RandBytes(" << kTestBufferSize << ") took: "
              << (end - now).InMicroseconds() << "µs";
  }
  