1/*
2 * Copyright 2014-present Facebook, Inc.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#ifndef FOLLY_RANDOM_H_
18#error This file may only be included from folly/Random.h
19#endif
20
21namespace folly {
22
23namespace detail {
24
25// Return the state size needed by RNG, expressed as a number of uint32_t
26// integers. Specialized for all templates specified in the C++11 standard.
27// For some (mersenne_twister_engine), this is exported as a state_size static
28// data member; for others, the standard shows formulas.
29
30template <class RNG, typename = void>
31struct StateSize {
32 // A sane default.
33 using type = std::integral_constant<size_t, 512>;
34};
35
36template <class RNG>
37struct StateSize<RNG, void_t<decltype(RNG::state_size)>> {
38 using type = std::integral_constant<size_t, RNG::state_size>;
39};
40
41template <class UIntType, UIntType a, UIntType c, UIntType m>
42struct StateSize<std::linear_congruential_engine<UIntType, a, c, m>> {
43 // From the standard [rand.eng.lcong], this is ceil(log2(m) / 32) + 3,
44 // which is the same as ceil(ceil(log2(m) / 32) + 3, and
45 // ceil(log2(m)) <= std::numeric_limits<UIntType>::digits
46 using type = std::integral_constant<
47 size_t,
48 (std::numeric_limits<UIntType>::digits + 31) / 32 + 3>;
49};
50
51template <class UIntType, size_t w, size_t s, size_t r>
52struct StateSize<std::subtract_with_carry_engine<UIntType, w, s, r>> {
53 // [rand.eng.sub]: r * ceil(w / 32)
54 using type = std::integral_constant<size_t, r*((w + 31) / 32)>;
55};
56
57template <typename RNG>
58using StateSizeT = _t<StateSize<RNG>>;
59
60template <class RNG>
61struct SeedData {
62 SeedData() {
63 Random::secureRandom(seedData.data(), seedData.size() * sizeof(uint32_t));
64 }
65
66 static constexpr size_t stateSize = StateSizeT<RNG>::value;
67 std::array<uint32_t, stateSize> seedData;
68};
69
70} // namespace detail
71
72template <class RNG, class /* EnableIf */>
73void Random::seed(RNG& rng) {
74 detail::SeedData<RNG> sd;
75 std::seed_seq s(std::begin(sd.seedData), std::end(sd.seedData));
76 rng.seed(s);
77}
78
79template <class RNG, class /* EnableIf */>
80auto Random::create() -> RNG {
81 detail::SeedData<RNG> sd;
82 std::seed_seq s(std::begin(sd.seedData), std::end(sd.seedData));
83 return RNG(s);
84}
85
86} // namespace folly
87