1 // Copyright 2013 the V8 project 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 "src/base/utils/random-number-generator.h"
6
7 #include <stdio.h>
8 #include <stdlib.h>
9
10 #include <algorithm>
11 #include <new>
12
13 #include "src/base/bits.h"
14 #include "src/base/macros.h"
15 #include "src/base/platform/mutex.h"
16 #include "src/base/platform/time.h"
17
18 namespace v8 {
19 namespace base {
20
21 static LazyMutex entropy_mutex = LAZY_MUTEX_INITIALIZER;
22 static RandomNumberGenerator::EntropySource entropy_source = nullptr;
23
24 // static
SetEntropySource(EntropySource source)25 void RandomNumberGenerator::SetEntropySource(EntropySource source) {
26 LockGuard<Mutex> lock_guard(entropy_mutex.Pointer());
27 entropy_source = source;
28 }
29
30
RandomNumberGenerator()31 RandomNumberGenerator::RandomNumberGenerator() {
32 // Check if embedder supplied an entropy source.
33 { LockGuard<Mutex> lock_guard(entropy_mutex.Pointer());
34 if (entropy_source != nullptr) {
35 int64_t seed;
36 if (entropy_source(reinterpret_cast<unsigned char*>(&seed),
37 sizeof(seed))) {
38 SetSeed(seed);
39 return;
40 }
41 }
42 }
43
44 #if V8_OS_CYGWIN || V8_OS_WIN
45 // Use rand_s() to gather entropy on Windows. See:
46 // https://code.google.com/p/v8/issues/detail?id=2905
47 unsigned first_half, second_half;
48 errno_t result = rand_s(&first_half);
49 DCHECK_EQ(0, result);
50 result = rand_s(&second_half);
51 DCHECK_EQ(0, result);
52 SetSeed((static_cast<int64_t>(first_half) << 32) + second_half);
53 #else
54 // Gather entropy from /dev/urandom if available.
55 FILE* fp = fopen("/dev/urandom", "rb");
56 if (fp != nullptr) {
57 int64_t seed;
58 size_t n = fread(&seed, sizeof(seed), 1, fp);
59 fclose(fp);
60 if (n == 1) {
61 SetSeed(seed);
62 return;
63 }
64 }
65
66 // We cannot assume that random() or rand() were seeded
67 // properly, so instead of relying on random() or rand(),
68 // we just seed our PRNG using timing data as fallback.
69 // This is weak entropy, but it's sufficient, because
70 // it is the responsibility of the embedder to install
71 // an entropy source using v8::V8::SetEntropySource(),
72 // which provides reasonable entropy, see:
73 // https://code.google.com/p/v8/issues/detail?id=2905
74 int64_t seed = Time::NowFromSystemTime().ToInternalValue() << 24;
75 seed ^= TimeTicks::HighResolutionNow().ToInternalValue() << 16;
76 seed ^= TimeTicks::Now().ToInternalValue() << 8;
77 SetSeed(seed);
78 #endif // V8_OS_CYGWIN || V8_OS_WIN
79 }
80
81
NextInt(int max)82 int RandomNumberGenerator::NextInt(int max) {
83 DCHECK_LT(0, max);
84
85 // Fast path if max is a power of 2.
86 if (bits::IsPowerOfTwo(max)) {
87 return static_cast<int>((max * static_cast<int64_t>(Next(31))) >> 31);
88 }
89
90 while (true) {
91 int rnd = Next(31);
92 int val = rnd % max;
93 if (rnd - val + (max - 1) >= 0) {
94 return val;
95 }
96 }
97 }
98
99
NextDouble()100 double RandomNumberGenerator::NextDouble() {
101 XorShift128(&state0_, &state1_);
102 return ToDouble(state0_, state1_);
103 }
104
105
NextInt64()106 int64_t RandomNumberGenerator::NextInt64() {
107 XorShift128(&state0_, &state1_);
108 return bit_cast<int64_t>(state0_ + state1_);
109 }
110
111
NextBytes(void * buffer,size_t buflen)112 void RandomNumberGenerator::NextBytes(void* buffer, size_t buflen) {
113 for (size_t n = 0; n < buflen; ++n) {
114 static_cast<uint8_t*>(buffer)[n] = static_cast<uint8_t>(Next(8));
115 }
116 }
117
ComplementSample(const std::unordered_set<uint64_t> & set,uint64_t max)118 static std::vector<uint64_t> ComplementSample(
119 const std::unordered_set<uint64_t>& set, uint64_t max) {
120 std::vector<uint64_t> result;
121 result.reserve(max - set.size());
122 for (uint64_t i = 0; i < max; i++) {
123 if (!set.count(i)) {
124 result.push_back(i);
125 }
126 }
127 return result;
128 }
129
NextSample(uint64_t max,size_t n)130 std::vector<uint64_t> RandomNumberGenerator::NextSample(uint64_t max,
131 size_t n) {
132 CHECK_LE(n, max);
133
134 if (n == 0) {
135 return std::vector<uint64_t>();
136 }
137
138 // Choose to select or exclude, whatever needs fewer generator calls.
139 size_t smaller_part = static_cast<size_t>(
140 std::min(max - static_cast<uint64_t>(n), static_cast<uint64_t>(n)));
141 std::unordered_set<uint64_t> selected;
142
143 size_t counter = 0;
144 while (selected.size() != smaller_part && counter / 3 < smaller_part) {
145 uint64_t x = static_cast<uint64_t>(NextDouble() * max);
146 CHECK_LT(x, max);
147
148 selected.insert(x);
149 counter++;
150 }
151
152 if (selected.size() == smaller_part) {
153 if (smaller_part != n) {
154 return ComplementSample(selected, max);
155 }
156 return std::vector<uint64_t>(selected.begin(), selected.end());
157 }
158
159 // Failed to select numbers in smaller_part * 3 steps, try different approach.
160 return NextSampleSlow(max, n, selected);
161 }
162
NextSampleSlow(uint64_t max,size_t n,const std::unordered_set<uint64_t> & excluded)163 std::vector<uint64_t> RandomNumberGenerator::NextSampleSlow(
164 uint64_t max, size_t n, const std::unordered_set<uint64_t>& excluded) {
165 CHECK_GE(max - excluded.size(), n);
166
167 std::vector<uint64_t> result;
168 result.reserve(max - excluded.size());
169
170 for (uint64_t i = 0; i < max; i++) {
171 if (!excluded.count(i)) {
172 result.push_back(i);
173 }
174 }
175
176 // Decrease result vector until it contains values to select or exclude,
177 // whatever needs fewer generator calls.
178 size_t larger_part = static_cast<size_t>(
179 std::max(max - static_cast<uint64_t>(n), static_cast<uint64_t>(n)));
180
181 // Excluded set may cause that initial result is already smaller than
182 // larget_part.
183 while (result.size() != larger_part && result.size() > n) {
184 size_t x = static_cast<size_t>(NextDouble() * result.size());
185 CHECK_LT(x, result.size());
186
187 std::swap(result[x], result.back());
188 result.pop_back();
189 }
190
191 if (result.size() != n) {
192 return ComplementSample(
193 std::unordered_set<uint64_t>(result.begin(), result.end()), max);
194 }
195 return result;
196 }
197
Next(int bits)198 int RandomNumberGenerator::Next(int bits) {
199 DCHECK_LT(0, bits);
200 DCHECK_GE(32, bits);
201 XorShift128(&state0_, &state1_);
202 return static_cast<int>((state0_ + state1_) >> (64 - bits));
203 }
204
205
SetSeed(int64_t seed)206 void RandomNumberGenerator::SetSeed(int64_t seed) {
207 initial_seed_ = seed;
208 state0_ = MurmurHash3(bit_cast<uint64_t>(seed));
209 state1_ = MurmurHash3(~state0_);
210 CHECK(state0_ != 0 || state1_ != 0);
211 }
212
213
MurmurHash3(uint64_t h)214 uint64_t RandomNumberGenerator::MurmurHash3(uint64_t h) {
215 h ^= h >> 33;
216 h *= uint64_t{0xFF51AFD7ED558CCD};
217 h ^= h >> 33;
218 h *= uint64_t{0xC4CEB9FE1A85EC53};
219 h ^= h >> 33;
220 return h;
221 }
222
223 } // namespace base
224 } // namespace v8
225