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      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 <stddef.h>
      8 #include <stdint.h>
      9 
     10 #include <algorithm>
     11 #include <limits>
     12 
     13 #include <gtest/gtest.h>
     14 
     15 #include "base/logging.h"
     16 #include "base/memory/scoped_ptr.h"
     17 #include "base/time/time.h"
     18 
     19 namespace {
     20 
     21 const int kIntMin = std::numeric_limits<int>::min();
     22 const int kIntMax = std::numeric_limits<int>::max();
     23 
     24 }  // namespace
     25 
     26 TEST(RandUtilTest, RandInt) {
     27   EXPECT_EQ(base::RandInt(0, 0), 0);
     28   EXPECT_EQ(base::RandInt(kIntMin, kIntMin), kIntMin);
     29   EXPECT_EQ(base::RandInt(kIntMax, kIntMax), kIntMax);
     30 
     31   // Check that the DCHECKS in RandInt() don't fire due to internal overflow.
     32   // There was a 50% chance of that happening, so calling it 40 times means
     33   // the chances of this passing by accident are tiny (9e-13).
     34   for (int i = 0; i < 40; ++i)
     35     base::RandInt(kIntMin, kIntMax);
     36 }
     37 
     38 TEST(RandUtilTest, RandDouble) {
     39   // Force 64-bit precision, making sure we're not in a 80-bit FPU register.
     40   volatile double number = base::RandDouble();
     41   EXPECT_GT(1.0, number);
     42   EXPECT_LE(0.0, number);
     43 }
     44 
     45 TEST(RandUtilTest, RandBytes) {
     46   const size_t buffer_size = 50;
     47   char buffer[buffer_size];
     48   memset(buffer, 0, buffer_size);
     49   base::RandBytes(buffer, buffer_size);
     50   std::sort(buffer, buffer + buffer_size);
     51   // Probability of occurrence of less than 25 unique bytes in 50 random bytes
     52   // is below 10^-25.
     53   EXPECT_GT(std::unique(buffer, buffer + buffer_size) - buffer, 25);
     54 }
     55 
     56 TEST(RandUtilTest, RandBytesAsString) {
     57   std::string random_string = base::RandBytesAsString(1);
     58   EXPECT_EQ(1U, random_string.size());
     59   random_string = base::RandBytesAsString(145);
     60   EXPECT_EQ(145U, random_string.size());
     61   char accumulator = 0;
     62   for (size_t i = 0; i < random_string.size(); ++i)
     63     accumulator |= random_string[i];
     64   // In theory this test can fail, but it won't before the universe dies of
     65   // heat death.
     66   EXPECT_NE(0, accumulator);
     67 }
     68 
     69 // Make sure that it is still appropriate to use RandGenerator in conjunction
     70 // with std::random_shuffle().
     71 TEST(RandUtilTest, RandGeneratorForRandomShuffle) {
     72   EXPECT_EQ(base::RandGenerator(1), 0U);
     73   EXPECT_LE(std::numeric_limits<ptrdiff_t>::max(),
     74             std::numeric_limits<int64_t>::max());
     75 }
     76 
     77 TEST(RandUtilTest, RandGeneratorIsUniform) {
     78   // Verify that RandGenerator has a uniform distribution. This is a
     79   // regression test that consistently failed when RandGenerator was
     80   // implemented this way:
     81   //
     82   //   return base::RandUint64() % max;
     83   //
     84   // A degenerate case for such an implementation is e.g. a top of
     85   // range that is 2/3rds of the way to MAX_UINT64, in which case the
     86   // bottom half of the range would be twice as likely to occur as the
     87   // top half. A bit of calculus care of jar@ shows that the largest
     88   // measurable delta is when the top of the range is 3/4ths of the
     89   // way, so that's what we use in the test.
     90   const uint64_t kTopOfRange =
     91       (std::numeric_limits<uint64_t>::max() / 4ULL) * 3ULL;
     92   const uint64_t kExpectedAverage = kTopOfRange / 2ULL;
     93   const uint64_t kAllowedVariance = kExpectedAverage / 50ULL;  // +/- 2%
     94   const int kMinAttempts = 1000;
     95   const int kMaxAttempts = 1000000;
     96 
     97   double cumulative_average = 0.0;
     98   int count = 0;
     99   while (count < kMaxAttempts) {
    100     uint64_t value = base::RandGenerator(kTopOfRange);
    101     cumulative_average = (count * cumulative_average + value) / (count + 1);
    102 
    103     // Don't quit too quickly for things to start converging, or we may have
    104     // a false positive.
    105     if (count > kMinAttempts &&
    106         kExpectedAverage - kAllowedVariance < cumulative_average &&
    107         cumulative_average < kExpectedAverage + kAllowedVariance) {
    108       break;
    109     }
    110 
    111     ++count;
    112   }
    113 
    114   ASSERT_LT(count, kMaxAttempts) << "Expected average was " <<
    115       kExpectedAverage << ", average ended at " << cumulative_average;
    116 }
    117 
    118 TEST(RandUtilTest, RandUint64ProducesBothValuesOfAllBits) {
    119   // This tests to see that our underlying random generator is good
    120   // enough, for some value of good enough.
    121   uint64_t kAllZeros = 0ULL;
    122   uint64_t kAllOnes = ~kAllZeros;
    123   uint64_t found_ones = kAllZeros;
    124   uint64_t found_zeros = kAllOnes;
    125 
    126   for (size_t i = 0; i < 1000; ++i) {
    127     uint64_t value = base::RandUint64();
    128     found_ones |= value;
    129     found_zeros &= value;
    130 
    131     if (found_zeros == kAllZeros && found_ones == kAllOnes)
    132       return;
    133   }
    134 
    135   FAIL() << "Didn't achieve all bit values in maximum number of tries.";
    136 }
    137