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+// 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.
+
+#ifndef ABSL_RANDOM_INTERNAL_RANDEN_ENGINE_H_
+#define ABSL_RANDOM_INTERNAL_RANDEN_ENGINE_H_
+
+#include <algorithm>
+#include <cinttypes>
+#include <cstdlib>
+#include <iostream>
+#include <iterator>
+#include <limits>
+#include <type_traits>
+
+#include "absl/meta/type_traits.h"
+#include "absl/random/internal/iostream_state_saver.h"
+#include "absl/random/internal/randen.h"
+
+namespace absl {
+namespace random_internal {
+
+// Deterministic pseudorandom byte generator with backtracking resistance
+// (leaking the state does not compromise prior outputs). Based on Reverie
+// (see "A Robust and Sponge-Like PRNG with Improved Efficiency") instantiated
+// with an improved Simpira-like permutation.
+// Returns values of type "T" (must be a built-in unsigned integer type).
+//
+// RANDen = RANDom generator or beetroots in Swiss High German.
+// 'Strong' (well-distributed, unpredictable, backtracking-resistant) random
+// generator, faster in some benchmarks than std::mt19937_64 and pcg64_c32.
+template <typename T>
+class alignas(16) randen_engine {
+ public:
+ // C++11 URBG interface:
+ using result_type = T;
+ static_assert(std::is_unsigned<result_type>::value,
+ "randen_engine template argument must be a built-in unsigned "
+ "integer type");
+
+ static constexpr result_type(min)() {
+ return (std::numeric_limits<result_type>::min)();
+ }
+
+ static constexpr result_type(max)() {
+ return (std::numeric_limits<result_type>::max)();
+ }
+
+ explicit randen_engine(result_type seed_value = 0) { seed(seed_value); }
+
+ template <class SeedSequence,
+ typename = typename absl::enable_if_t<
+ !std::is_same<SeedSequence, randen_engine>::value>>
+ explicit randen_engine(SeedSequence&& seq) {
+ seed(seq);
+ }
+
+ randen_engine(const randen_engine&) = default;
+
+ // Returns random bits from the buffer in units of result_type.
+ result_type operator()() {
+ // Refill the buffer if needed (unlikely).
+ if (next_ >= kStateSizeT) {
+ next_ = kCapacityT;
+ impl_.Generate(state_);
+ }
+
+ return state_[next_++];
+ }
+
+ template <class SeedSequence>
+ typename absl::enable_if_t<
+ !std::is_convertible<SeedSequence, result_type>::value>
+ seed(SeedSequence&& seq) {
+ // Zeroes the state.
+ seed();
+ reseed(seq);
+ }
+
+ void seed(result_type seed_value = 0) {
+ next_ = kStateSizeT;
+ // Zeroes the inner state and fills the outer state with seed_value to
+ // mimics behaviour of reseed
+ std::fill(std::begin(state_), std::begin(state_) + kCapacityT, 0);
+ std::fill(std::begin(state_) + kCapacityT, std::end(state_), seed_value);
+ }
+
+ // Inserts entropy into (part of) the state. Calling this periodically with
+ // sufficient entropy ensures prediction resistance (attackers cannot predict
+ // future outputs even if state is compromised).
+ template <class SeedSequence>
+ void reseed(SeedSequence& seq) {
+ using sequence_result_type = typename SeedSequence::result_type;
+ static_assert(sizeof(sequence_result_type) == 4,
+ "SeedSequence::result_type must be 32-bit");
+
+ constexpr size_t kBufferSize =
+ Randen::kSeedBytes / sizeof(sequence_result_type);
+ alignas(16) sequence_result_type buffer[kBufferSize];
+
+ // Randen::Absorb XORs the seed into state, which is then mixed by a call
+ // to Randen::Generate. Seeding with only the provided entropy is preferred
+ // to using an arbitrary generate() call, so use [rand.req.seed_seq]
+ // size as a proxy for the number of entropy units that can be generated
+ // without relying on seed sequence mixing...
+ const size_t entropy_size = seq.size();
+ if (entropy_size < kBufferSize) {
+ // ... and only request that many values, or 256-bits, when unspecified.
+ const size_t requested_entropy = (entropy_size == 0) ? 8u : entropy_size;
+ std::fill(std::begin(buffer) + requested_entropy, std::end(buffer), 0);
+ seq.generate(std::begin(buffer), std::begin(buffer) + requested_entropy);
+ // The Randen paper suggests preferentially initializing even-numbered
+ // 128-bit vectors of the randen state (there are 16 such vectors).
+ // The seed data is merged into the state offset by 128-bits, which
+ // implies prefering seed bytes [16..31, ..., 208..223]. Since the
+ // buffer is 32-bit values, we swap the corresponding buffer positions in
+ // 128-bit chunks.
+ size_t dst = kBufferSize;
+ while (dst > 7) {
+ // leave the odd bucket as-is.
+ dst -= 4;
+ size_t src = dst >> 1;
+ // swap 128-bits into the even bucket
+ std::swap(buffer[--dst], buffer[--src]);
+ std::swap(buffer[--dst], buffer[--src]);
+ std::swap(buffer[--dst], buffer[--src]);
+ std::swap(buffer[--dst], buffer[--src]);
+ }
+ } else {
+ seq.generate(std::begin(buffer), std::end(buffer));
+ }
+ impl_.Absorb(buffer, state_);
+
+ // Generate will be called when operator() is called
+ next_ = kStateSizeT;
+ }
+
+ void discard(uint64_t count) {
+ uint64_t step = std::min<uint64_t>(kStateSizeT - next_, count);
+ count -= step;
+
+ constexpr uint64_t kRateT = kStateSizeT - kCapacityT;
+ while (count > 0) {
+ next_ = kCapacityT;
+ impl_.Generate(state_);
+ step = std::min<uint64_t>(kRateT, count);
+ count -= step;
+ }
+ next_ += step;
+ }
+
+ bool operator==(const randen_engine& other) const {
+ return next_ == other.next_ &&
+ std::equal(std::begin(state_), std::end(state_),
+ std::begin(other.state_));
+ }
+
+ bool operator!=(const randen_engine& other) const {
+ return !(*this == other);
+ }
+
+ template <class CharT, class Traits>
+ friend std::basic_ostream<CharT, Traits>& operator<<(
+ std::basic_ostream<CharT, Traits>& os, // NOLINT(runtime/references)
+ const randen_engine<T>& engine) { // NOLINT(runtime/references)
+ using numeric_type =
+ typename random_internal::stream_format_type<result_type>::type;
+ auto saver = random_internal::make_ostream_state_saver(os);
+ for (const auto& elem : engine.state_) {
+ // In the case that `elem` is `uint8_t`, it must be cast to something
+ // larger so that it prints as an integer rather than a character. For
+ // simplicity, apply the cast all circumstances.
+ os << static_cast<numeric_type>(elem) << os.fill();
+ }
+ os << engine.next_;
+ return os;
+ }
+
+ template <class CharT, class Traits>
+ friend std::basic_istream<CharT, Traits>& operator>>(
+ std::basic_istream<CharT, Traits>& is, // NOLINT(runtime/references)
+ randen_engine<T>& engine) { // NOLINT(runtime/references)
+ using numeric_type =
+ typename random_internal::stream_format_type<result_type>::type;
+ result_type state[kStateSizeT];
+ size_t next;
+ for (auto& elem : state) {
+ // It is not possible to read uint8_t from wide streams, so it is
+ // necessary to read a wider type and then cast it to uint8_t.
+ numeric_type value;
+ is >> value;
+ elem = static_cast<result_type>(value);
+ }
+ is >> next;
+ if (is.fail()) {
+ return is;
+ }
+ std::memcpy(engine.state_, state, sizeof(engine.state_));
+ engine.next_ = next;
+ return is;
+ }
+
+ private:
+ static constexpr size_t kStateSizeT =
+ Randen::kStateBytes / sizeof(result_type);
+ static constexpr size_t kCapacityT =
+ Randen::kCapacityBytes / sizeof(result_type);
+
+ // First kCapacityT are `inner', the others are accessible random bits.
+ alignas(16) result_type state_[kStateSizeT];
+ size_t next_; // index within state_
+ Randen impl_;
+};
+
+} // namespace random_internal
+} // namespace absl
+
+#endif // ABSL_RANDOM_INTERNAL_RANDEN_ENGINE_H_
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