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