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