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Diffstat (limited to 'third_party/abseil_cpp/absl/base/internal/exponential_biased.h')
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1 files changed, 0 insertions, 130 deletions
diff --git a/third_party/abseil_cpp/absl/base/internal/exponential_biased.h b/third_party/abseil_cpp/absl/base/internal/exponential_biased.h deleted file mode 100644 index 94f79a3378..0000000000 --- a/third_party/abseil_cpp/absl/base/internal/exponential_biased.h +++ /dev/null @@ -1,130 +0,0 @@ -// 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> - -#include "absl/base/config.h" -#include "absl/base/macros.h" - -namespace absl { -ABSL_NAMESPACE_BEGIN -namespace base_internal { - -// ExponentialBiased provides a small and fast random number generator for a -// rounded exponential distribution. This generator manages very little state, -// and imposes no synchronization overhead. This makes it useful in specialized -// scenarios requiring minimum overhead, such as stride based periodic sampling. -// -// ExponentialBiased provides two closely related functions, GetSkipCount() and -// GetStride(), both returning a rounded integer defining a number of events -// required before some event with a given mean probability occurs. -// -// The distribution is useful to generate a random wait time or some periodic -// event with a given mean probability. For example, if an action is supposed to -// happen on average once every 'N' events, then we can get a random 'stride' -// counting down how long before the event to happen. For example, if we'd want -// to sample one in every 1000 'Frobber' calls, our code could look like this: -// -// Frobber::Frobber() { -// stride_ = exponential_biased_.GetStride(1000); -// } -// -// void Frobber::Frob(int arg) { -// if (--stride == 0) { -// SampleFrob(arg); -// stride_ = exponential_biased_.GetStride(1000); -// } -// ... -// } -// -// The rounding of the return value creates a bias, especially for smaller means -// where the distribution of the fraction is not evenly distributed. We correct -// this bias by tracking the fraction we rounded up or down on each iteration, -// effectively tracking the distance between the cumulative value, and the -// rounded cumulative value. For example, given a mean of 2: -// -// raw = 1.63076, cumulative = 1.63076, rounded = 2, bias = -0.36923 -// raw = 0.14624, cumulative = 1.77701, rounded = 2, bias = 0.14624 -// raw = 4.93194, cumulative = 6.70895, rounded = 7, bias = -0.06805 -// raw = 0.24206, cumulative = 6.95101, rounded = 7, bias = 0.24206 -// etc... -// -// Adjusting with rounding bias is relatively trivial: -// -// double value = bias_ + exponential_distribution(mean)(); -// double rounded_value = std::round(value); -// bias_ = value - rounded_value; -// return rounded_value; -// -// This class is thread-compatible. -class ExponentialBiased { - public: - // The number of bits set by NextRandom. - static constexpr int kPrngNumBits = 48; - - // `GetSkipCount()` returns the number of events to skip before some chosen - // event happens. For example, randomly tossing a coin, we will on average - // throw heads once before we get tails. We can simulate random coin tosses - // using GetSkipCount() as: - // - // ExponentialBiased eb; - // for (...) { - // int number_of_heads_before_tail = eb.GetSkipCount(1); - // for (int flips = 0; flips < number_of_heads_before_tail; ++flips) { - // printf("head..."); - // } - // printf("tail\n"); - // } - // - int64_t GetSkipCount(int64_t mean); - - // GetStride() returns the number of events required for a specific event to - // happen. See the class comments for a usage example. `GetStride()` is - // equivalent to `GetSkipCount(mean - 1) + 1`. When to use `GetStride()` or - // `GetSkipCount()` depends mostly on what best fits the use case. - int64_t GetStride(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}; - double bias_{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 -ABSL_NAMESPACE_END -} // namespace absl - -#endif // ABSL_BASE_INTERNAL_EXPONENTIAL_BIASED_H_ |