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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_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 {
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
} // namespace absl
#endif // ABSL_RANDOM_INTERNAL_POOL_URBG_H_
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