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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_NONSECURE_BASE_H_
 #define ABSL_RANDOM_INTERNAL_NONSECURE_BASE_H_
 
 #include <algorithm>
 #include <cstdint>
 #include <iterator>
 #include <type_traits>
 #include <utility>
 #include <vector>
 
 #include "absl/base/macros.h"
 #include "absl/container/inlined_vector.h"
 #include "absl/meta/type_traits.h"
 #include "absl/random/internal/pool_urbg.h"
 #include "absl/random/internal/salted_seed_seq.h"
 #include "absl/random/internal/seed_material.h"
 #include "absl/types/span.h"
 
 namespace absl {
 ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // RandenPoolSeedSeq is a custom seed sequence type where generate() fills the
 // provided buffer via the RandenPool entropy source.
 class RandenPoolSeedSeq {
  private:
   struct ContiguousTag {};
   struct BufferTag {};
 
   // Generate random unsigned values directly into the buffer.
   template <typename Contiguous>
   void generate_impl(ContiguousTag, Contiguous begin, Contiguous end) {
     const size_t n = static_cast<size_t>(std::distance(begin, end));
     auto* a = &(*begin);
     RandenPool<uint8_t>::Fill(
         absl::MakeSpan(reinterpret_cast<uint8_t*>(a), sizeof(*a) * n));
   }
 
   // Construct a buffer of size n and fill it with values, then copy
   // those values into the seed iterators.
   template <typename RandomAccessIterator>
   void generate_impl(BufferTag, RandomAccessIterator begin,
                      RandomAccessIterator end) {
     const size_t n = std::distance(begin, end);
     absl::InlinedVector<uint32_t, 8> data(n, 0);
     RandenPool<uint32_t>::Fill(absl::MakeSpan(data.begin(), data.end()));
     std::copy(std::begin(data), std::end(data), begin);
   }
 
  public:
   using result_type = uint32_t;
 
   size_t size() { return 0; }
 
   template <typename OutIterator>
   void param(OutIterator) const {}
 
   template <typename RandomAccessIterator>
   void generate(RandomAccessIterator begin, RandomAccessIterator end) {
     // RandomAccessIterator must be assignable from uint32_t
     if (begin != end) {
       using U = typename std::iterator_traits<RandomAccessIterator>::value_type;
       // ContiguousTag indicates the common case of a known contiguous buffer,
       // which allows directly filling the buffer. In C++20,
       // std::contiguous_iterator_tag provides a mechanism for testing this
       // capability, however until Abseil's support requirements allow us to
       // assume C++20, limit checks to a few common cases.
       using TagType = absl::conditional_t<
           (std::is_pointer<RandomAccessIterator>::value ||
            std::is_same<RandomAccessIterator,
                         typename std::vector<U>::iterator>::value),
           ContiguousTag, BufferTag>;
 
       generate_impl(TagType{}, begin, end);
     }
   }
 };
 
 // Each instance of NonsecureURBGBase<URBG> will be seeded by variates produced
 // by a thread-unique URBG-instance.
 template <typename URBG, typename Seeder = RandenPoolSeedSeq>
 class NonsecureURBGBase {
  public:
   using result_type = typename URBG::result_type;
 
   // Default constructor
   NonsecureURBGBase() : urbg_(ConstructURBG()) {}
 
   // Copy disallowed, move allowed.
   NonsecureURBGBase(const NonsecureURBGBase&) = delete;
   NonsecureURBGBase& operator=(const NonsecureURBGBase&) = delete;
   NonsecureURBGBase(NonsecureURBGBase&&) = default;
   NonsecureURBGBase& operator=(NonsecureURBGBase&&) = default;
 
   // Constructor using a seed
   template <class SSeq, typename = typename absl::enable_if_t<
                             !std::is_same<SSeq, NonsecureURBGBase>::value>>
   explicit NonsecureURBGBase(SSeq&& seq)
       : urbg_(ConstructURBG(std::forward<SSeq>(seq))) {}
 
   // Note: on MSVC, min() or max() can be interpreted as MIN() or MAX(), so we
   // enclose min() or max() in parens as (min)() and (max)().
   // Additionally, clang-format requires no space before this construction.
 
   // NonsecureURBGBase::min()
   static constexpr result_type(min)() { return (URBG::min)(); }
 
   // NonsecureURBGBase::max()
   static constexpr result_type(max)() { return (URBG::max)(); }
 
   // NonsecureURBGBase::operator()()
   result_type operator()() { return urbg_(); }
 
   // NonsecureURBGBase::discard()
   void discard(unsigned long long values) {  // NOLINT(runtime/int)
     urbg_.discard(values);
   }
 
   bool operator==(const NonsecureURBGBase& other) const {
     return urbg_ == other.urbg_;
   }
 
   bool operator!=(const NonsecureURBGBase& other) const {
     return !(urbg_ == other.urbg_);
   }
 
  private:
   static URBG ConstructURBG() {
     Seeder seeder;
     return URBG(seeder);
   }
 
   template <typename SSeq>
   static URBG ConstructURBG(SSeq&& seq) {  // NOLINT(runtime/references)
     auto salted_seq =
         random_internal::MakeSaltedSeedSeq(std::forward<SSeq>(seq));
     return URBG(salted_seq);
   }
 
   URBG urbg_;
 };
 
 }  // namespace random_internal
 ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_NONSECURE_BASE_H_

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