78 lines
2.8 KiB
C
78 lines
2.8 KiB
C
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// Copyright 2019 The Abseil Authors.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef ABSL_BASE_INTERNAL_EXPONENTIAL_BIASED_H_
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#define ABSL_BASE_INTERNAL_EXPONENTIAL_BIASED_H_
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#include <stdint.h>
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namespace absl {
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namespace base_internal {
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// ExponentialBiased provides a small and fast random number generator for a
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// rounded exponential distribution. This generator doesn't requires very little
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// state doesn't impose synchronization overhead, which makes it useful in some
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// specialized scenarios.
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//
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// For the generated variable X, X ~ floor(Exponential(1/mean)). The floor
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// operation introduces a small amount of bias, but the distribution is useful
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// to generate a wait time. That is, if an operation is supposed to happen on
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// average to 1/mean events, then the generated variable X will describe how
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// many events to skip before performing the operation and computing a new X.
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//
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// The mathematically precise distribution to use for integer wait times is a
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// Geometric distribution, but a Geometric distribution takes slightly more time
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// to compute and when the mean is large (say, 100+), the Geometric distribution
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// is hard to distinguish from the result of ExponentialBiased.
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//
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// This class is thread-compatible.
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class ExponentialBiased {
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public:
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// The number of bits set by NextRandom.
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static constexpr int kPrngNumBits = 48;
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// Generates the floor of an exponentially distributed random variable by
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// rounding the value down to the nearest integer. The result will be in the
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// range [0, int64_t max / 2].
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int64_t Get(int64_t mean);
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// Computes a random number in the range [0, 1<<(kPrngNumBits+1) - 1]
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//
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// This is public to enable testing.
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static uint64_t NextRandom(uint64_t rnd);
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private:
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void Initialize();
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uint64_t rng_{0};
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bool initialized_{false};
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};
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// Returns the next prng value.
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// pRNG is: aX+b mod c with a = 0x5DEECE66D, b = 0xB, c = 1<<48
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// This is the lrand64 generator.
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inline uint64_t ExponentialBiased::NextRandom(uint64_t rnd) {
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const uint64_t prng_mult = uint64_t{0x5DEECE66D};
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const uint64_t prng_add = 0xB;
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const uint64_t prng_mod_power = 48;
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const uint64_t prng_mod_mask =
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~((~static_cast<uint64_t>(0)) << prng_mod_power);
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return (prng_mult * rnd + prng_add) & prng_mod_mask;
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}
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} // namespace base_internal
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} // namespace absl
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#endif // ABSL_BASE_INTERNAL_EXPONENTIAL_BIASED_H_
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