505accda38
Summary: Scoped anonymous enums seem to be better supported than static constexpr at the moment, so this diff replaces the latter with the former. Also, this diff removes an incorrect inclusion of pthread.h. MSVC build was broken starting with D50439. Test Plan: 1. build 2. observe proper skiplist behavior by absence of pathological slowdown 3. push diff to tmp_try_windows branch to tickle AppVeyor 4. wait for contbuild before committing to master Reviewers: sdong Reviewed By: sdong Subscribers: dhruba Differential Revision: https://reviews.facebook.net/D50517
110 lines
3.5 KiB
C++
110 lines
3.5 KiB
C++
// Copyright (c) 2013, Facebook, Inc. All rights reserved.
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// This source code is licensed under the BSD-style license found in the
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// LICENSE file in the root directory of this source tree. An additional grant
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// of patent rights can be found in the PATENTS file in the same directory.
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//
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// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#pragma once
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#include <random>
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#include <stdint.h>
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namespace rocksdb {
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// A very simple random number generator. Not especially good at
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// generating truly random bits, but good enough for our needs in this
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// package.
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class Random {
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private:
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enum : uint32_t {
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M = 2147483647L // 2^31-1
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};
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enum : uint64_t {
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A = 16807 // bits 14, 8, 7, 5, 2, 1, 0
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};
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uint32_t seed_;
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static uint32_t GoodSeed(uint32_t s) { return (s & M) != 0 ? (s & M) : 1; }
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public:
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// This is the largest value that can be returned from Next()
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enum : uint32_t { kMaxNext = M };
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explicit Random(uint32_t s) : seed_(GoodSeed(s)) {}
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void Reset(uint32_t s) { seed_ = GoodSeed(s); }
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uint32_t Next() {
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// We are computing
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// seed_ = (seed_ * A) % M, where M = 2^31-1
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//
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// seed_ must not be zero or M, or else all subsequent computed values
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// will be zero or M respectively. For all other values, seed_ will end
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// up cycling through every number in [1,M-1]
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uint64_t product = seed_ * A;
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// Compute (product % M) using the fact that ((x << 31) % M) == x.
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seed_ = static_cast<uint32_t>((product >> 31) + (product & M));
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// The first reduction may overflow by 1 bit, so we may need to
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// repeat. mod == M is not possible; using > allows the faster
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// sign-bit-based test.
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if (seed_ > M) {
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seed_ -= M;
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}
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return seed_;
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}
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// Returns a uniformly distributed value in the range [0..n-1]
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// REQUIRES: n > 0
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uint32_t Uniform(int n) { return Next() % n; }
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// Randomly returns true ~"1/n" of the time, and false otherwise.
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// REQUIRES: n > 0
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bool OneIn(int n) { return (Next() % n) == 0; }
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// Skewed: pick "base" uniformly from range [0,max_log] and then
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// return "base" random bits. The effect is to pick a number in the
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// range [0,2^max_log-1] with exponential bias towards smaller numbers.
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uint32_t Skewed(int max_log) {
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return Uniform(1 << Uniform(max_log + 1));
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}
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// Returns a Random instance for use by the current thread without
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// additional locking
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static Random* GetTLSInstance();
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};
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// A simple 64bit random number generator based on std::mt19937_64
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class Random64 {
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private:
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std::mt19937_64 generator_;
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public:
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explicit Random64(uint64_t s) : generator_(s) { }
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// Generates the next random number
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uint64_t Next() { return generator_(); }
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// Returns a uniformly distributed value in the range [0..n-1]
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// REQUIRES: n > 0
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uint64_t Uniform(uint64_t n) {
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return std::uniform_int_distribution<uint64_t>(0, n - 1)(generator_);
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}
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// Randomly returns true ~"1/n" of the time, and false otherwise.
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// REQUIRES: n > 0
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bool OneIn(uint64_t n) { return Uniform(n) == 0; }
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// Skewed: pick "base" uniformly from range [0,max_log] and then
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// return "base" random bits. The effect is to pick a number in the
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// range [0,2^max_log-1] with exponential bias towards smaller numbers.
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uint64_t Skewed(int max_log) {
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return Uniform(1 << Uniform(max_log + 1));
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}
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};
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} // namespace rocksdb
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