1 // Copyright (c) 2011 The LevelDB Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style license that can be 3 // found in the LICENSE file. See the AUTHORS file for names of contributors. 4 5 #ifndef STORAGE_LEVELDB_UTIL_RANDOM_H_ 6 #define STORAGE_LEVELDB_UTIL_RANDOM_H_ 7 8 #include <stdint.h> 9 10 namespace leveldb { 11 12 // A very simple random number generator. Not especially good at 13 // generating truly random bits, but good enough for our needs in this 14 // package. 15 class Random { 16 private: 17 uint32_t seed_; 18 public: 19 explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) { 20 // Avoid bad seeds. 21 if (seed_ == 0 || seed_ == 2147483647L) { 22 seed_ = 1; 23 } 24 } 25 uint32_t Next() { 26 static const uint32_t M = 2147483647L; // 2^31-1 27 static const uint64_t A = 16807; // bits 14, 8, 7, 5, 2, 1, 0 28 // We are computing 29 // seed_ = (seed_ * A) % M, where M = 2^31-1 30 // 31 // seed_ must not be zero or M, or else all subsequent computed values 32 // will be zero or M respectively. For all other values, seed_ will end 33 // up cycling through every number in [1,M-1] 34 uint64_t product = seed_ * A; 35 36 // Compute (product % M) using the fact that ((x << 31) % M) == x. 37 seed_ = static_cast<uint32_t>((product >> 31) + (product & M)); 38 // The first reduction may overflow by 1 bit, so we may need to 39 // repeat. mod == M is not possible; using > allows the faster 40 // sign-bit-based test. 41 if (seed_ > M) { 42 seed_ -= M; 43 } 44 return seed_; 45 } 46 // Returns a uniformly distributed value in the range [0..n-1] 47 // REQUIRES: n > 0 48 uint32_t Uniform(int n) { return Next() % n; } 49 50 // Randomly returns true ~"1/n" of the time, and false otherwise. 51 // REQUIRES: n > 0 52 bool OneIn(int n) { return (Next() % n) == 0; } 53 54 // Skewed: pick "base" uniformly from range [0,max_log] and then 55 // return "base" random bits. The effect is to pick a number in the 56 // range [0,2^max_log-1] with exponential bias towards smaller numbers. 57 uint32_t Skewed(int max_log) { 58 return Uniform(1 << Uniform(max_log + 1)); 59 } 60 }; 61 62 } // namespace leveldb 63 64 #endif // STORAGE_LEVELDB_UTIL_RANDOM_H_ 65
