1 // Copyright 2017 The Abseil Authors.
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
7 // https://www.apache.org/licenses/LICENSE-2.0
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
15 // -----------------------------------------------------------------------------
16 // File: seed_sequences.h
17 // -----------------------------------------------------------------------------
19 // This header contains utilities for creating and working with seed sequences
20 // conforming to [rand.req.seedseq]. In general, direct construction of seed
21 // sequences is discouraged, but use-cases for construction of identical bit
22 // generators (using the same seed sequence) may be helpful (e.g. replaying a
23 // simulation whose state is derived from variates of a bit generator).
25 #ifndef ABSL_RANDOM_SEED_SEQUENCES_H_
26 #define ABSL_RANDOM_SEED_SEQUENCES_H_
31 #include "absl/random/internal/salted_seed_seq.h"
32 #include "absl/random/internal/seed_material.h"
33 #include "absl/random/seed_gen_exception.h"
34 #include "absl/types/span.h"
38 // -----------------------------------------------------------------------------
40 // -----------------------------------------------------------------------------
42 // `absl::SeedSeq` constructs a seed sequence according to [rand.req.seedseq]
43 // for use within bit generators. `absl::SeedSeq`, unlike `std::seed_seq`
44 // additionally salts the generated seeds with extra implementation-defined
45 // entropy. For that reason, you can use `absl::SeedSeq` in combination with
46 // standard library bit generators (e.g. `std::mt19937`) to introduce
47 // non-determinism in your seeds.
51 // absl::SeedSeq my_seed_seq({a, b, c});
52 // std::mt19937 my_bitgen(my_seed_seq);
54 using SeedSeq = random_internal::SaltedSeedSeq<std::seed_seq>;
56 // -----------------------------------------------------------------------------
57 // absl::CreateSeedSeqFrom(bitgen*)
58 // -----------------------------------------------------------------------------
60 // Constructs a seed sequence conforming to [rand.req.seedseq] using variates
61 // produced by a provided bit generator.
63 // You should generally avoid direct construction of seed sequences, but
64 // use-cases for reuse of a seed sequence to construct identical bit generators
65 // may be helpful (eg. replaying a simulation whose state is derived from bit
68 // If bitgen == nullptr, then behavior is undefined.
72 // absl::BitGen my_bitgen;
73 // auto seed_seq = absl::CreateSeedSeqFrom(&my_bitgen);
74 // absl::BitGen new_engine(seed_seq); // derived from my_bitgen, but not
77 template <typename URBG>
78 SeedSeq CreateSeedSeqFrom(URBG* urbg) {
80 seed_material[random_internal::kEntropyBlocksNeeded];
82 if (!random_internal::ReadSeedMaterialFromURBG(
83 urbg, absl::MakeSpan(seed_material))) {
84 random_internal::ThrowSeedGenException();
86 return SeedSeq(std::begin(seed_material), std::end(seed_material));
89 // -----------------------------------------------------------------------------
90 // absl::MakeSeedSeq()
91 // -----------------------------------------------------------------------------
93 // Constructs an `absl::SeedSeq` salting the generated values using
94 // implementation-defined entropy. The returned sequence can be used to create
95 // equivalent bit generators correlated using this sequence.
99 // auto my_seed_seq = absl::MakeSeedSeq();
100 // std::mt19937 rng1(my_seed_seq);
101 // std::mt19937 rng2(my_seed_seq);
102 // EXPECT_EQ(rng1(), rng2());
104 SeedSeq MakeSeedSeq();
108 #endif // ABSL_RANDOM_SEED_SEQUENCES_H_