base/rand_util_unittest.cc

   1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "base/rand_util.h"
   6
   7 #include <limits>
   8
   9 #include "testing/gtest/include/gtest/gtest.h"
  10
  11 namespace {
  12
  13 const int kIntMin = std::numeric_limits<int>::min();
  14 const int kIntMax = std::numeric_limits<int>::max();
  15
  16 }  // namespace
  17
  18 TEST(RandUtilTest, SameMinAndMax) {
  19   EXPECT_EQ(base::RandInt(0, 0), 0);
  20   EXPECT_EQ(base::RandInt(kIntMin, kIntMin), kIntMin);
  21   EXPECT_EQ(base::RandInt(kIntMax, kIntMax), kIntMax);
  22 }
  23
  24 TEST(RandUtilTest, RandDouble) {
  25   // Force 64-bit precision, making sure we're not in a 80-bit FPU register.
  26   volatile double number = base::RandDouble();
  27   EXPECT_GT(1.0, number);
  28   EXPECT_LE(0.0, number);
  29 }
  30
  31 TEST(RandUtilTest, RandBytes) {
  32   const size_t buffer_size = 145;
  33   char buffer[buffer_size];
  34   memset(buffer, 0, buffer_size);
  35   base::RandBytes(buffer, buffer_size);
  36   char accumulator = 0;
  37   for(size_t i = 0; i < buffer_size; ++i)
  38     accumulator |= buffer[i];
  39   // In theory this test can fail, but it won't before the universe dies of
  40   // heat death.
  41   EXPECT_NE(0, accumulator);
  42 }
  43
  44 TEST(RandUtilTest, RandBytesAsString) {
  45   std::string random_string = base::RandBytesAsString(0);
  46   EXPECT_EQ(0U, random_string.size());
  47   random_string = base::RandBytesAsString(145);
  48   EXPECT_EQ(145U, random_string.size());
  49   char accumulator = 0;
  50   for (size_t i = 0; i < random_string.size(); ++i)
  51     accumulator |= random_string[i];
  52   // In theory this test can fail, but it won't before the universe dies of
  53   // heat death.
  54   EXPECT_NE(0, accumulator);
  55 }
  56
  57 // Make sure that it is still appropriate to use RandGenerator in conjunction
  58 // with std::random_shuffle().
  59 TEST(RandUtilTest, RandGeneratorForRandomShuffle) {
  60   EXPECT_EQ(base::RandGenerator(1), 0U);
  61   EXPECT_LE(std::numeric_limits<ptrdiff_t>::max(),
  62             std::numeric_limits<int64>::max());
  63 }
  64
  65 TEST(RandUtilTest, RandGeneratorIsUniform) {
  66   // Verify that RandGenerator has a uniform distribution. This is a
  67   // regression test that consistently failed when RandGenerator was
  68   // implemented this way:
  69   //
  70   //   return base::RandUint64() % max;
  71   //
  72   // A degenerate case for such an implementation is e.g. a top of
  73   // range that is 2/3rds of the way to MAX_UINT64, in which case the
  74   // bottom half of the range would be twice as likely to occur as the
  75   // top half. A bit of calculus care of jar@ shows that the largest
  76   // measurable delta is when the top of the range is 3/4ths of the
  77   // way, so that's what we use in the test.
  78   const uint64 kTopOfRange = (std::numeric_limits<uint64>::max() / 4ULL) * 3ULL;
  79   const uint64 kExpectedAverage = kTopOfRange / 2ULL;
  80   const uint64 kAllowedVariance = kExpectedAverage / 50ULL;  // +/- 2%
  81   const int kMinAttempts = 1000;
  82   const int kMaxAttempts = 1000000;
  83
  84   double cumulative_average = 0.0;
  85   int count = 0;
  86   while (count < kMaxAttempts) {
  87     uint64 value = base::RandGenerator(kTopOfRange);
  88     cumulative_average = (count * cumulative_average + value) / (count + 1);
  89
  90     // Don't quit too quickly for things to start converging, or we may have
  91     // a false positive.
  92     if (count > kMinAttempts &&
  93         kExpectedAverage - kAllowedVariance < cumulative_average &&
  94         cumulative_average < kExpectedAverage + kAllowedVariance) {
  95       break;
  96     }
  97
  98     ++count;
  99   }
 100
 101   ASSERT_LT(count, kMaxAttempts) << "Expected average was " <<
 102       kExpectedAverage << ", average ended at " << cumulative_average;
 103 }
 104
 105 TEST(RandUtilTest, RandUint64ProducesBothValuesOfAllBits) {
 106   // This tests to see that our underlying random generator is good
 107   // enough, for some value of good enough.
 108   uint64 kAllZeros = 0ULL;
 109   uint64 kAllOnes = ~kAllZeros;
 110   uint64 found_ones = kAllZeros;
 111   uint64 found_zeros = kAllOnes;
 112
 113   for (size_t i = 0; i < 1000; ++i) {
 114     uint64 value = base::RandUint64();
 115     found_ones |= value;
 116     found_zeros &= value;
 117
 118     if (found_zeros == kAllZeros && found_ones == kAllOnes)
 119       return;
 120   }
 121
 122   FAIL() << "Didn't achieve all bit values in maximum number of tries.";
 123 }