git / moonbite-core / src/leveldb/util/random.h · raw

// 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_