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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_