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
TEST(RandUtilTest,SameMinAndMax)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
TEST(RandUtilTest,RandDouble)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
TEST(RandUtilTest,RandBytes)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
TEST(RandUtilTest,RandBytesAsString)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().
TEST(RandUtilTest,RandGeneratorForRandomShuffle)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
TEST(RandUtilTest,RandGeneratorIsUniform)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
TEST(RandUtilTest,RandUint64ProducesBothValuesOfAllBits)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 }
123