From b832dce8489ef7b6231384909fd9b68d5a5ff2b7 Mon Sep 17 00:00:00 2001 From: Abseil Team Date: Tue, 25 Feb 2020 22:27:31 +0100 Subject: Creation of LTS branch "lts_2020_02_25" - 0033c9ea91a52ade7c6b725aa2ef3cbe15463421 Fix build on FreeBSD/powerpc (#616) by kgotlinux <60880393+kgotlinux@users.noreply.github.com> - 0d5ce2797eb695aee7e019e25323251ef6ffc277 Export of internal Abseil changes by Abseil Team - b69c7d880caddfc25bf348dbcfe9d45fdd8bc6e6 Export of internal Abseil changes by Abseil Team - 2a5633fc077a58528cdbfe78720f3f6bfdc6044d Merge "Export of internal Abseil changes" by Xiaoyi Zhang - f9b3d6e493c1b6ab3dbdab71c5f8fa849db4abaf Add RISCV support to GetProgramCounter() (#621) by Khem Raj - 0232c87f21c26718aa3eb2d86678070f3b498a4e Add missing ABSL_HAVE_VDSO_SUPPORT conditional (#622) by Sinan Kaya <41809318+franksinankaya@users.noreply.github.com> - 3c814105108680997d0821077694f663693b5382 Export of internal Abseil changes by Abseil Team - c44657f55692eddf5504156645d1f4ec7b3acabd Export of internal Abseil changes by Abseil Team - 98eb410c93ad059f9bba1bf43f5bb916fc92a5ea Export of internal Abseil changes by Abseil Team - bf78e977309c4cb946914b456404141ddac1c302 Export of internal Abseil changes by Abseil Team - d95d1567165d449e4c213ea31a15cbb112a9865f Export of internal Abseil changes by Abseil Team - 24713a7036a81498334807fa5c7ad3cb7c643711 Export of internal Abseil changes by Abseil Team - 72382c21fefed981b4b8a2a1b82e2d231c2c2e39 Export of internal Abseil changes by Abseil Team - 08a7e7bf972c8451855a5022f2faf3d3655db015 Export of internal Abseil changes by Abseil Team - 36bcd9599b3f48c99357ba61cf33584889306d6a Fix pointer format specifier in documentation (#614) by Andre Nguyen - 0f86336b6939ea673cc1cbe29189286cae67d63a Export of internal Abseil changes by Abseil Team - c512f118dde6ffd51cb7d8ac8804bbaf4d266c3a Export of internal Abseil changes by Abseil Team - 37dd2562ec830d547a1524bb306be313ac3f2556 Export of internal Abseil changes by Abseil Team - 44427702614d7b86b064ba06a390f5eb2f85dbf6 fix: Add support for more ARM processors detection (#608) by Andre Nguyen - 159bf2bf6d1cc8087e02468d071e94d1177d1bae Export of internal Abseil changes by Abseil Team - a2e6adecc294dc4cd98cc285a9134ce58e0f2ad0 Use https links. 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262d74ba81b1fc4d71f459555cde8ecb39786d68 Export of internal Abseil changes by Abseil Team - f0afae0d49af3e15a7169e019634d7719143d94d Export of internal Abseil changes by Abseil Team - 0e7afdcbd24c7e5b7cab4e0217d8886f1525b520 Export of internal Abseil changes by Abseil Team - 9a41ffdd3a0ccbcdd29c4e3886b28e06f2cd9c66 Export of internal Abseil changes by Abseil Team - 36910d3d7e9fccadd6603f232d0c4f54dcd47c7e [bazel] Add fixes for --incompatible_load_cc_rules_from_b... by Yannic - aae8143cf9aa611f70d7ea9b95b8b8b383b2271a Export of internal Abseil changes by Abseil Team - d9aa92d7fb324314f9df487ac23d32a25650b742 Export of internal Abseil changes by Abseil Team - 321ab5303023c86cd15d9ddc5740fb4b4fde32e1 Export of internal Abseil changes by Abseil Team - 4ef574064e75b86f115549e9eb4c7e806781b3ab Export of internal Abseil changes by Abseil Team GitOrigin-RevId: 0033c9ea91a52ade7c6b725aa2ef3cbe15463421 Change-Id: I8a2b70063cb3ab40c6943a6db0fe40cae71ed8d7 --- absl/base/internal/exponential_biased.h | 130 ++++++++++++++++++++++++++++++++ 1 file changed, 130 insertions(+) create mode 100644 absl/base/internal/exponential_biased.h (limited to 'absl/base/internal/exponential_biased.h') diff --git a/absl/base/internal/exponential_biased.h b/absl/base/internal/exponential_biased.h new file mode 100644 index 00000000..94f79a33 --- /dev/null +++ b/absl/base/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_BASE_INTERNAL_EXPONENTIAL_BIASED_H_ +#define ABSL_BASE_INTERNAL_EXPONENTIAL_BIASED_H_ + +#include + +#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(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_ -- cgit v1.2.3