ceph/src/rocksdb/util/random.h

   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 <algorithm>
  13 #include <random>
  14
  15 #include "rocksdb/rocksdb_namespace.h"
  16
  17 namespace ROCKSDB_NAMESPACE {
  18
  19 // A very simple random number generator.  Not especially good at
  20 // generating truly random bits, but good enough for our needs in this
  21 // package.
  22 class Random {
  23  private:
  24   enum : uint32_t {
  25     M = 2147483647L  // 2^31-1
  26   };
  27   enum : uint64_t {
  28     A = 16807  // bits 14, 8, 7, 5, 2, 1, 0
  29   };
  30
  31   uint32_t seed_;
  32
  33   static uint32_t GoodSeed(uint32_t s) { return (s & M) != 0 ? (s & M) : 1; }
  34
  35  public:
  36   // This is the largest value that can be returned from Next()
  37   enum : uint32_t { kMaxNext = M };
  38
  39   explicit Random(uint32_t s) : seed_(GoodSeed(s)) {}
  40
  41   void Reset(uint32_t s) { seed_ = GoodSeed(s); }
  42
  43   uint32_t Next() {
  44     // We are computing
  45     //       seed_ = (seed_ * A) % M,    where M = 2^31-1
  46     //
  47     // seed_ must not be zero or M, or else all subsequent computed values
  48     // will be zero or M respectively.  For all other values, seed_ will end
  49     // up cycling through every number in [1,M-1]
  50     uint64_t product = seed_ * A;
  51
  52     // Compute (product % M) using the fact that ((x << 31) % M) == x.
  53     seed_ = static_cast<uint32_t>((product >> 31) + (product & M));
  54     // The first reduction may overflow by 1 bit, so we may need to
  55     // repeat.  mod == M is not possible; using > allows the faster
  56     // sign-bit-based test.
  57     if (seed_ > M) {
  58       seed_ -= M;
  59     }
  60     return seed_;
  61   }
  62
  63   // Returns a uniformly distributed value in the range [0..n-1]
  64   // REQUIRES: n > 0
  65   uint32_t Uniform(int n) { return Next() % n; }
  66
  67   // Randomly returns true ~"1/n" of the time, and false otherwise.
  68   // REQUIRES: n > 0
  69   bool OneIn(int n) { return Uniform(n) == 0; }
  70
  71   // "Optional" one-in-n, where 0 or negative always returns false
  72   // (may or may not consume a random value)
  73   bool OneInOpt(int n) { return n > 0 && OneIn(n); }
  74
  75   // Returns random bool that is true for the given percentage of
  76   // calls on average. Zero or less is always false and 100 or more
  77   // is always true (may or may not consume a random value)
  78   bool PercentTrue(int percentage) {
  79     return static_cast<int>(Uniform(100)) < percentage;
  80   }
  81
  82   // Skewed: pick "base" uniformly from range [0,max_log] and then
  83   // return "base" random bits.  The effect is to pick a number in the
  84   // range [0,2^max_log-1] with exponential bias towards smaller numbers.
  85   uint32_t Skewed(int max_log) {
  86     return Uniform(1 << Uniform(max_log + 1));
  87   }
  88
  89   // Returns a random string of length "len"
  90   std::string RandomString(int len);
  91
  92   // Generates a random string of len bytes using human-readable characters
  93   std::string HumanReadableString(int len);
  94
  95   // Returns a Random instance for use by the current thread without
  96   // additional locking
  97   static Random* GetTLSInstance();
  98 };
  99
 100 // A good 32-bit random number generator based on std::mt19937.
 101 // This exists in part to avoid compiler variance in warning about coercing
 102 // uint_fast32_t from mt19937 to uint32_t.
 103 class Random32 {
 104  private:
 105   std::mt19937 generator_;
 106
 107  public:
 108   explicit Random32(uint32_t s) : generator_(s) {}
 109
 110   // Generates the next random number
 111   uint32_t Next() { return static_cast<uint32_t>(generator_()); }
 112
 113   // Returns a uniformly distributed value in the range [0..n-1]
 114   // REQUIRES: n > 0
 115   uint32_t Uniform(uint32_t n) {
 116     return static_cast<uint32_t>(
 117         std::uniform_int_distribution<std::mt19937::result_type>(
 118             0, n - 1)(generator_));
 119   }
 120
 121   // Returns an *almost* uniformly distributed value in the range [0..n-1].
 122   // Much faster than Uniform().
 123   // REQUIRES: n > 0
 124   uint32_t Uniformish(uint32_t n) {
 125     // fastrange (without the header)
 126     return static_cast<uint32_t>((uint64_t(generator_()) * uint64_t(n)) >> 32);
 127   }
 128
 129   // Randomly returns true ~"1/n" of the time, and false otherwise.
 130   // REQUIRES: n > 0
 131   bool OneIn(uint32_t n) { return Uniform(n) == 0; }
 132
 133   // Skewed: pick "base" uniformly from range [0,max_log] and then
 134   // return "base" random bits.  The effect is to pick a number in the
 135   // range [0,2^max_log-1] with exponential bias towards smaller numbers.
 136   uint32_t Skewed(int max_log) {
 137     return Uniform(uint32_t{1} << Uniform(max_log + 1));
 138   }
 139
 140   // Reset the seed of the generator to the given value
 141   void Seed(uint32_t new_seed) { generator_.seed(new_seed); }
 142 };
 143
 144 // A good 64-bit random number generator based on std::mt19937_64
 145 class Random64 {
 146  private:
 147   std::mt19937_64 generator_;
 148
 149  public:
 150   explicit Random64(uint64_t s) : generator_(s) { }
 151
 152   // Generates the next random number
 153   uint64_t Next() { return generator_(); }
 154
 155   // Returns a uniformly distributed value in the range [0..n-1]
 156   // REQUIRES: n > 0
 157   uint64_t Uniform(uint64_t n) {
 158     return std::uniform_int_distribution<uint64_t>(0, n - 1)(generator_);
 159   }
 160
 161   // Randomly returns true ~"1/n" of the time, and false otherwise.
 162   // REQUIRES: n > 0
 163   bool OneIn(uint64_t n) { return Uniform(n) == 0; }
 164
 165   // Skewed: pick "base" uniformly from range [0,max_log] and then
 166   // return "base" random bits.  The effect is to pick a number in the
 167   // range [0,2^max_log-1] with exponential bias towards smaller numbers.
 168   uint64_t Skewed(int max_log) {
 169     return Uniform(uint64_t(1) << Uniform(max_log + 1));
 170   }
 171 };
 172
 173 // A seeded replacement for removed std::random_shuffle
 174 template <class RandomIt>
 175 void RandomShuffle(RandomIt first, RandomIt last, uint32_t seed) {
 176   std::mt19937 rng(seed);
 177   std::shuffle(first, last, rng);
 178 }
 179
 180 // A replacement for removed std::random_shuffle
 181 template <class RandomIt>
 182 void RandomShuffle(RandomIt first, RandomIt last) {
 183   RandomShuffle(first, last, std::random_device{}());
 184 }
 185
 186 }  // namespace ROCKSDB_NAMESPACE