THUAI6/CAPI/cpp/grpc/include/absl/random/internal/pool_urbg.h

// 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_POOL_URBG_H_
#define ABSL_RANDOM_INTERNAL_POOL_URBG_H_

#include <cinttypes>
#include <limits>

#include "absl/random/internal/traits.h"
#include "absl/types/span.h"

namespace absl
{
    ABSL_NAMESPACE_BEGIN
    namespace random_internal
    {

        // RandenPool is a thread-safe random number generator [random.req.urbg] that
        // uses an underlying pool of Randen generators to generate values.  Each thread
        // has affinity to one instance of the underlying pool generators.  Concurrent
        // access is guarded by a spin-lock.
        template<typename T>
        class RandenPool
        {
        public:
            using result_type = T;
            static_assert(std::is_unsigned<result_type>::value, "RandenPool 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)();
            }

            RandenPool()
            {
            }

            // Returns a single value.
            inline result_type operator()()
            {
                return Generate();
            }

            // Fill data with random values.
            static void Fill(absl::Span<result_type> data);

        protected:
            // Generate returns a single value.
            static result_type Generate();
        };

        extern template class RandenPool<uint8_t>;
        extern template class RandenPool<uint16_t>;
        extern template class RandenPool<uint32_t>;
        extern template class RandenPool<uint64_t>;

        // PoolURBG uses an underlying pool of random generators to implement a
        // thread-compatible [random.req.urbg] interface with an internal cache of
        // values.
        template<typename T, size_t kBufferSize>
        class PoolURBG
        {
            // Inheritance to access the protected static members of RandenPool.
            using unsigned_type = typename make_unsigned_bits<T>::type;
            using PoolType = RandenPool<unsigned_type>;
            using SpanType = absl::Span<unsigned_type>;

            static constexpr size_t kInitialBuffer = kBufferSize + 1;
            static constexpr size_t kHalfBuffer = kBufferSize / 2;

        public:
            using result_type = T;

            static_assert(std::is_unsigned<result_type>::value, "PoolURBG must be parameterized by an unsigned integer type");

            static_assert(kBufferSize > 1, "PoolURBG must be parameterized by a buffer-size > 1");

            static_assert(kBufferSize <= 256, "PoolURBG must be parameterized by a buffer-size <= 256");

            static constexpr result_type(min)()
            {
                return (std::numeric_limits<result_type>::min)();
            }

            static constexpr result_type(max)()
            {
                return (std::numeric_limits<result_type>::max)();
            }

            PoolURBG() :
                next_(kInitialBuffer)
            {
            }

            // copy-constructor does not copy cache.
            PoolURBG(const PoolURBG&) :
                next_(kInitialBuffer)
            {
            }
            const PoolURBG& operator=(const PoolURBG&)
            {
                next_ = kInitialBuffer;
                return *this;
            }

            // move-constructor does move cache.
            PoolURBG(PoolURBG&&) = default;
            PoolURBG& operator=(PoolURBG&&) = default;

            inline result_type operator()()
            {
                if (next_ >= kBufferSize)
                {
                    next_ = (kBufferSize > 2 && next_ > kBufferSize) ? kHalfBuffer : 0;
                    PoolType::Fill(SpanType(reinterpret_cast<unsigned_type*>(state_ + next_), kBufferSize - next_));
                }
                return state_[next_++];
            }

        private:
            // Buffer size.
            size_t next_;  // index within state_
            result_type state_[kBufferSize];
        };

    }  // namespace random_internal
    ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_RANDOM_INTERNAL_POOL_URBG_H_