| // 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. |
| |
| #ifndef STORAGE_LEVELDB_UTIL_RANDOM_H_ |
| #define STORAGE_LEVELDB_UTIL_RANDOM_H_ |
| |
| #include <stdint.h> |
| |
| namespace leveldb { |
| |
| // 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: |
| uint32_t seed_; |
| public: |
| explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) { |
| // Avoid bad seeds. |
| if (seed_ == 0 || seed_ == 2147483647L) { |
| seed_ = 1; |
| } |
| } |
| uint32_t Next() { |
| static const uint32_t M = 2147483647L; // 2^31-1 |
| static const uint64_t A = 16807; // bits 14, 8, 7, 5, 2, 1, 0 |
| // 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)); |
| } |
| }; |
| |
| } // namespace leveldb |
| |
| #endif // STORAGE_LEVELDB_UTIL_RANDOM_H_ |