1 // Copyright (c) 2011-present, Facebook, Inc. All rights reserved. 2 // This source code is licensed under both the GPLv2 (found in the 3 // COPYING file in the root directory) and Apache 2.0 License 4 // (found in the LICENSE.Apache file in the root directory). 5 // 6 // Copyright (c) 2011 The LevelDB Authors. All rights reserved. 7 // Use of this source code is governed by a BSD-style license that can be 8 // found in the LICENSE file. See the AUTHORS file for names of contributors. 9 10 #pragma once 11 #include <stdint.h> 12 #include <random> 13 14 #include "rocksdb/rocksdb_namespace.h" 15 16 namespace ROCKSDB_NAMESPACE { 17 18 // A very simple random number generator. Not especially good at 19 // generating truly random bits, but good enough for our needs in this 20 // package. 21 class Random { 22 private: 23 enum : uint32_t { 24 M = 2147483647L // 2^31-1 25 }; 26 enum : uint64_t { 27 A = 16807 // bits 14, 8, 7, 5, 2, 1, 0 28 }; 29 30 uint32_t seed_; 31 GoodSeed(uint32_t s)32 static uint32_t GoodSeed(uint32_t s) { return (s & M) != 0 ? (s & M) : 1; } 33 34 public: 35 // This is the largest value that can be returned from Next() 36 enum : uint32_t { kMaxNext = M }; 37 Random(uint32_t s)38 explicit Random(uint32_t s) : seed_(GoodSeed(s)) {} 39 Reset(uint32_t s)40 void Reset(uint32_t s) { seed_ = GoodSeed(s); } 41 Next()42 uint32_t Next() { 43 // We are computing 44 // seed_ = (seed_ * A) % M, where M = 2^31-1 45 // 46 // seed_ must not be zero or M, or else all subsequent computed values 47 // will be zero or M respectively. For all other values, seed_ will end 48 // up cycling through every number in [1,M-1] 49 uint64_t product = seed_ * A; 50 51 // Compute (product % M) using the fact that ((x << 31) % M) == x. 52 seed_ = static_cast<uint32_t>((product >> 31) + (product & M)); 53 // The first reduction may overflow by 1 bit, so we may need to 54 // repeat. mod == M is not possible; using > allows the faster 55 // sign-bit-based test. 56 if (seed_ > M) { 57 seed_ -= M; 58 } 59 return seed_; 60 } 61 62 // Returns a uniformly distributed value in the range [0..n-1] 63 // REQUIRES: n > 0 Uniform(int n)64 uint32_t Uniform(int n) { return Next() % n; } 65 66 // Randomly returns true ~"1/n" of the time, and false otherwise. 67 // REQUIRES: n > 0 OneIn(int n)68 bool OneIn(int n) { return Uniform(n) == 0; } 69 70 // "Optional" one-in-n, where 0 or negative always returns false 71 // (may or may not consume a random value) OneInOpt(int n)72 bool OneInOpt(int n) { return n > 0 && OneIn(n); } 73 74 // Returns random bool that is true for the given percentage of 75 // calls on average. Zero or less is always false and 100 or more 76 // is always true (may or may not consume a random value) PercentTrue(int percentage)77 bool PercentTrue(int percentage) { 78 return static_cast<int>(Uniform(100)) < percentage; 79 } 80 81 // Skewed: pick "base" uniformly from range [0,max_log] and then 82 // return "base" random bits. The effect is to pick a number in the 83 // range [0,2^max_log-1] with exponential bias towards smaller numbers. Skewed(int max_log)84 uint32_t Skewed(int max_log) { 85 return Uniform(1 << Uniform(max_log + 1)); 86 } 87 88 // Returns a Random instance for use by the current thread without 89 // additional locking 90 static Random* GetTLSInstance(); 91 }; 92 93 // A good 32-bit random number generator based on std::mt19937. 94 // This exists in part to avoid compiler variance in warning about coercing 95 // uint_fast32_t from mt19937 to uint32_t. 96 class Random32 { 97 private: 98 std::mt19937 generator_; 99 100 public: Random32(uint32_t s)101 explicit Random32(uint32_t s) : generator_(s) {} 102 103 // Generates the next random number Next()104 uint32_t Next() { return static_cast<uint32_t>(generator_()); } 105 106 // Returns a uniformly distributed value in the range [0..n-1] 107 // REQUIRES: n > 0 Uniform(uint32_t n)108 uint32_t Uniform(uint32_t n) { 109 return static_cast<uint32_t>( 110 std::uniform_int_distribution<std::mt19937::result_type>( 111 0, n - 1)(generator_)); 112 } 113 114 // Returns an *almost* uniformly distributed value in the range [0..n-1]. 115 // Much faster than Uniform(). 116 // REQUIRES: n > 0 Uniformish(uint32_t n)117 uint32_t Uniformish(uint32_t n) { 118 // fastrange (without the header) 119 return static_cast<uint32_t>((uint64_t(generator_()) * uint64_t(n)) >> 32); 120 } 121 122 // Randomly returns true ~"1/n" of the time, and false otherwise. 123 // REQUIRES: n > 0 OneIn(uint32_t n)124 bool OneIn(uint32_t n) { return Uniform(n) == 0; } 125 126 // Skewed: pick "base" uniformly from range [0,max_log] and then 127 // return "base" random bits. The effect is to pick a number in the 128 // range [0,2^max_log-1] with exponential bias towards smaller numbers. Skewed(int max_log)129 uint32_t Skewed(int max_log) { 130 return Uniform(uint32_t{1} << Uniform(max_log + 1)); 131 } 132 133 // Reset the seed of the generator to the given value Seed(uint32_t new_seed)134 void Seed(uint32_t new_seed) { generator_.seed(new_seed); } 135 }; 136 137 // A good 64-bit random number generator based on std::mt19937_64 138 class Random64 { 139 private: 140 std::mt19937_64 generator_; 141 142 public: Random64(uint64_t s)143 explicit Random64(uint64_t s) : generator_(s) { } 144 145 // Generates the next random number Next()146 uint64_t Next() { return generator_(); } 147 148 // Returns a uniformly distributed value in the range [0..n-1] 149 // REQUIRES: n > 0 Uniform(uint64_t n)150 uint64_t Uniform(uint64_t n) { 151 return std::uniform_int_distribution<uint64_t>(0, n - 1)(generator_); 152 } 153 154 // Randomly returns true ~"1/n" of the time, and false otherwise. 155 // REQUIRES: n > 0 OneIn(uint64_t n)156 bool OneIn(uint64_t n) { return Uniform(n) == 0; } 157 158 // Skewed: pick "base" uniformly from range [0,max_log] and then 159 // return "base" random bits. The effect is to pick a number in the 160 // range [0,2^max_log-1] with exponential bias towards smaller numbers. Skewed(int max_log)161 uint64_t Skewed(int max_log) { 162 return Uniform(uint64_t(1) << Uniform(max_log + 1)); 163 } 164 }; 165 166 } // namespace ROCKSDB_NAMESPACE 167