| // Copyright (c) 2011-present, Facebook, Inc. All rights reserved. |
| // This source code is licensed under both the GPLv2 (found in the |
| // COPYING file in the root directory) and Apache 2.0 License |
| // (found in the LICENSE.Apache file in the root directory). |
| // |
| // Copyright (c) 2011 The LevelDB Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. See the AUTHORS file for names of contributors. |
| |
| #pragma once |
| #include <random> |
| #include <stdint.h> |
| |
| namespace rocksdb { |
| |
| // A very simple random number generator. Not especially good at |
| // generating truly random bits, but good enough for our needs in this |
| // package. |
| class Random { |
| private: |
| enum : uint32_t { |
| M = 2147483647L // 2^31-1 |
| }; |
| enum : uint64_t { |
| A = 16807 // bits 14, 8, 7, 5, 2, 1, 0 |
| }; |
| |
| uint32_t seed_; |
| |
| static uint32_t GoodSeed(uint32_t s) { return (s & M) != 0 ? (s & M) : 1; } |
| |
| public: |
| // This is the largest value that can be returned from Next() |
| enum : uint32_t { kMaxNext = M }; |
| |
| explicit Random(uint32_t s) : seed_(GoodSeed(s)) {} |
| |
| void Reset(uint32_t s) { seed_ = GoodSeed(s); } |
| |
| uint32_t Next() { |
| // We are computing |
| // seed_ = (seed_ * A) % M, where M = 2^31-1 |
| // |
| // seed_ must not be zero or M, or else all subsequent computed values |
| // will be zero or M respectively. For all other values, seed_ will end |
| // up cycling through every number in [1,M-1] |
| uint64_t product = seed_ * A; |
| |
| // Compute (product % M) using the fact that ((x << 31) % M) == x. |
| seed_ = static_cast<uint32_t>((product >> 31) + (product & M)); |
| // The first reduction may overflow by 1 bit, so we may need to |
| // repeat. mod == M is not possible; using > allows the faster |
| // sign-bit-based test. |
| if (seed_ > M) { |
| seed_ -= M; |
| } |
| return seed_; |
| } |
| |
| // Returns a uniformly distributed value in the range [0..n-1] |
| // REQUIRES: n > 0 |
| uint32_t Uniform(int n) { return Next() % n; } |
| |
| // Randomly returns true ~"1/n" of the time, and false otherwise. |
| // REQUIRES: n > 0 |
| bool OneIn(int n) { return (Next() % n) == 0; } |
| |
| // Skewed: pick "base" uniformly from range [0,max_log] and then |
| // return "base" random bits. The effect is to pick a number in the |
| // range [0,2^max_log-1] with exponential bias towards smaller numbers. |
| uint32_t Skewed(int max_log) { |
| return Uniform(1 << Uniform(max_log + 1)); |
| } |
| |
| // Returns a Random instance for use by the current thread without |
| // additional locking |
| static Random* GetTLSInstance(); |
| }; |
| |
| // A simple 64bit random number generator based on std::mt19937_64 |
| class Random64 { |
| private: |
| std::mt19937_64 generator_; |
| |
| public: |
| explicit Random64(uint64_t s) : generator_(s) { } |
| |
| // Generates the next random number |
| uint64_t Next() { return generator_(); } |
| |
| // Returns a uniformly distributed value in the range [0..n-1] |
| // REQUIRES: n > 0 |
| uint64_t Uniform(uint64_t n) { |
| return std::uniform_int_distribution<uint64_t>(0, n - 1)(generator_); |
| } |
| |
| // Randomly returns true ~"1/n" of the time, and false otherwise. |
| // REQUIRES: n > 0 |
| bool OneIn(uint64_t n) { return Uniform(n) == 0; } |
| |
| // Skewed: pick "base" uniformly from range [0,max_log] and then |
| // return "base" random bits. The effect is to pick a number in the |
| // range [0,2^max_log-1] with exponential bias towards smaller numbers. |
| uint64_t Skewed(int max_log) { |
| return Uniform(uint64_t(1) << Uniform(max_log + 1)); |
| } |
| }; |
| |
| } // namespace rocksdb |