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
10namespace 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.
15class 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