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// Copyright 2019 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_BASE_INTERNAL_EXPONENTIAL_BIASED_H_
#define ABSL_BASE_INTERNAL_EXPONENTIAL_BIASED_H_
#include <stdint.h>
namespace absl {
namespace base_internal {
// ExponentialBiased provides a small and fast random number generator for a
// rounded exponential distribution. This generator doesn't requires very little
// state doesn't impose synchronization overhead, which makes it useful in some
// specialized scenarios.
//
// For the generated variable X, X ~ floor(Exponential(1/mean)). The floor
// operation introduces a small amount of bias, but the distribution is useful
// to generate a wait time. That is, if an operation is supposed to happen on
// average to 1/mean events, then the generated variable X will describe how
// many events to skip before performing the operation and computing a new X.
//
// The mathematically precise distribution to use for integer wait times is a
// Geometric distribution, but a Geometric distribution takes slightly more time
// to compute and when the mean is large (say, 100+), the Geometric distribution
// is hard to distinguish from the result of ExponentialBiased.
//
// This class is thread-compatible.
class ExponentialBiased {
public:
// The number of bits set by NextRandom.
static constexpr int kPrngNumBits = 48;
// Generates the floor of an exponentially distributed random variable by
// rounding the value down to the nearest integer. The result will be in the
// range [0, int64_t max / 2].
int64_t Get(int64_t mean);
// Computes a random number in the range [0, 1<<(kPrngNumBits+1) - 1]
//
// This is public to enable testing.
static uint64_t NextRandom(uint64_t rnd);
private:
void Initialize();
uint64_t rng_{0};
bool initialized_{false};
};
// Returns the next prng value.
// pRNG is: aX+b mod c with a = 0x5DEECE66D, b = 0xB, c = 1<<48
// This is the lrand64 generator.
inline uint64_t ExponentialBiased::NextRandom(uint64_t rnd) {
const uint64_t prng_mult = uint64_t{0x5DEECE66D};
const uint64_t prng_add = 0xB;
const uint64_t prng_mod_power = 48;
const uint64_t prng_mod_mask =
~((~static_cast<uint64_t>(0)) << prng_mod_power);
return (prng_mult * rnd + prng_add) & prng_mod_mask;
}
} // namespace base_internal
} // namespace absl
#endif // ABSL_BASE_INTERNAL_EXPONENTIAL_BIASED_H_
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