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author | Colin Cross <ccross@android.com> | 2022-04-13 02:50:39 +0000 |
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committer | Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com> | 2022-04-13 02:50:39 +0000 |
commit | d3dfe74c4025bacd9741d761815a8cb059e49c68 (patch) | |
tree | 2ce94d7f0804ccb77d1fa9b2a1bca00eecdff1e2 /third_party/abseil-cpp/absl/profiling/internal/exponential_biased.h | |
parent | f60eaea2240ba9e1c508e8e0c91d39ee9fc47be5 (diff) | |
parent | 7563023510bf04108a954596ea9393a4c11ac279 (diff) | |
download | webrtc-android13-mainline-tzdata4-release.tar.gz |
Merge changes I0ab600cd,I1e74c64a am: 798f3afdf6 am: 2f9c4b2c3b am: a9167328fc am: 7563023510aml_tz4_332714070aml_tz4_332714050aml_tz4_332714010aml_tz4_331910000aml_tz4_331314030aml_tz4_331314020aml_tz4_331314010aml_tz4_331012050aml_tz4_331012040aml_tz4_331012000android13-mainline-tzdata4-releaseaml_tz4_332714010
Original change: https://android-review.googlesource.com/c/platform/external/webrtc/+/2062410
Change-Id: I5558645b9d1d98de28e58bd4a9a8aa9d7d0c49da
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>
Diffstat (limited to 'third_party/abseil-cpp/absl/profiling/internal/exponential_biased.h')
-rw-r--r-- | third_party/abseil-cpp/absl/profiling/internal/exponential_biased.h | 130 |
1 files changed, 130 insertions, 0 deletions
diff --git a/third_party/abseil-cpp/absl/profiling/internal/exponential_biased.h b/third_party/abseil-cpp/absl/profiling/internal/exponential_biased.h new file mode 100644 index 0000000000..d31f7782e8 --- /dev/null +++ b/third_party/abseil-cpp/absl/profiling/internal/exponential_biased.h @@ -0,0 +1,130 @@ +// 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_PROFILING_INTERNAL_EXPONENTIAL_BIASED_H_ +#define ABSL_PROFILING_INTERNAL_EXPONENTIAL_BIASED_H_ + +#include <stdint.h> + +#include "absl/base/config.h" +#include "absl/base/macros.h" + +namespace absl { +ABSL_NAMESPACE_BEGIN +namespace profiling_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::rint(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 profiling_internal +ABSL_NAMESPACE_END +} // namespace absl + +#endif // ABSL_PROFILING_INTERNAL_EXPONENTIAL_BIASED_H_ |