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Diffstat (limited to 'absl/random/seed_sequences.h')
| -rw-r--r-- | absl/random/seed_sequences.h | 110 |
1 files changed, 110 insertions, 0 deletions
diff --git a/absl/random/seed_sequences.h b/absl/random/seed_sequences.h new file mode 100644 index 00000000..73d075c0 --- /dev/null +++ b/absl/random/seed_sequences.h @@ -0,0 +1,110 @@ +// 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). + +#ifndef ABSL_RANDOM_SEED_SEQUENCES_H_ +#define ABSL_RANDOM_SEED_SEQUENCES_H_ + +#include <iterator> +#include <random> + +#include "absl/random/internal/salted_seed_seq.h" +#include "absl/random/internal/seed_material.h" +#include "absl/random/seed_gen_exception.h" +#include "absl/types/span.h" + +namespace absl { +inline namespace lts_2019_08_08 { + +// ----------------------------------------------------------------------------- +// 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(); + +} // inline namespace lts_2019_08_08 +} // namespace absl + +#endif // ABSL_RANDOM_SEED_SEQUENCES_H_ |