| // Copyright 2017 The Abseil Authors. | |
| // | |
| // Licensed under the Apache License, Version 2.0 (the "License"); | |
| // you may not use this file except in compliance with the License. | |
| // You may obtain a copy of the License at | |
| // | |
| // https://www.apache.org/licenses/LICENSE-2.0 | |
| // | |
| // Unless required by applicable law or agreed to in writing, software | |
| // distributed under the License is distributed on an "AS IS" BASIS, | |
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | |
| // See the License for the specific language governing permissions and | |
| // limitations under the License. | |
| // | |
| // ----------------------------------------------------------------------------- | |
| // File: seed_sequences.h | |
| // ----------------------------------------------------------------------------- | |
| // | |
| // This header contains utilities for creating and working with seed sequences | |
| // conforming to [rand.req.seedseq]. In general, direct construction of seed | |
| // sequences is discouraged, but use-cases for construction of identical bit | |
| // generators (using the same seed sequence) may be helpful (e.g. replaying a | |
| // simulation whose state is derived from variates of a bit generator). | |
| namespace absl { | |
| ABSL_NAMESPACE_BEGIN | |
| // ----------------------------------------------------------------------------- | |
| // absl::SeedSeq | |
| // ----------------------------------------------------------------------------- | |
| // | |
| // `absl::SeedSeq` constructs a seed sequence according to [rand.req.seedseq] | |
| // for use within bit generators. `absl::SeedSeq`, unlike `std::seed_seq` | |
| // additionally salts the generated seeds with extra implementation-defined | |
| // entropy. For that reason, you can use `absl::SeedSeq` in combination with | |
| // standard library bit generators (e.g. `std::mt19937`) to introduce | |
| // non-determinism in your seeds. | |
| // | |
| // Example: | |
| // | |
| // absl::SeedSeq my_seed_seq({a, b, c}); | |
| // std::mt19937 my_bitgen(my_seed_seq); | |
| // | |
| using SeedSeq = random_internal::SaltedSeedSeq<std::seed_seq>; | |
| // ----------------------------------------------------------------------------- | |
| // absl::CreateSeedSeqFrom(bitgen*) | |
| // ----------------------------------------------------------------------------- | |
| // | |
| // Constructs a seed sequence conforming to [rand.req.seedseq] using variates | |
| // produced by a provided bit generator. | |
| // | |
| // You should generally avoid direct construction of seed sequences, but | |
| // use-cases for reuse of a seed sequence to construct identical bit generators | |
| // may be helpful (eg. replaying a simulation whose state is derived from bit | |
| // generator values). | |
| // | |
| // If bitgen == nullptr, then behavior is undefined. | |
| // | |
| // Example: | |
| // | |
| // absl::BitGen my_bitgen; | |
| // auto seed_seq = absl::CreateSeedSeqFrom(&my_bitgen); | |
| // absl::BitGen new_engine(seed_seq); // derived from my_bitgen, but not | |
| // // correlated. | |
| // | |
| template <typename URBG> | |
| SeedSeq CreateSeedSeqFrom(URBG* urbg) { | |
| SeedSeq::result_type | |
| seed_material[random_internal::kEntropyBlocksNeeded]; | |
| if (!random_internal::ReadSeedMaterialFromURBG( | |
| urbg, absl::MakeSpan(seed_material))) { | |
| random_internal::ThrowSeedGenException(); | |
| } | |
| return SeedSeq(std::begin(seed_material), std::end(seed_material)); | |
| } | |
| // ----------------------------------------------------------------------------- | |
| // absl::MakeSeedSeq() | |
| // ----------------------------------------------------------------------------- | |
| // | |
| // Constructs an `absl::SeedSeq` salting the generated values using | |
| // implementation-defined entropy. The returned sequence can be used to create | |
| // equivalent bit generators correlated using this sequence. | |
| // | |
| // Example: | |
| // | |
| // auto my_seed_seq = absl::MakeSeedSeq(); | |
| // std::mt19937 rng1(my_seed_seq); | |
| // std::mt19937 rng2(my_seed_seq); | |
| // EXPECT_EQ(rng1(), rng2()); | |
| // | |
| SeedSeq MakeSeedSeq(); | |
| ABSL_NAMESPACE_END | |
| } // namespace absl | |