Inside Bitcoin

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
19 public:
20  explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) {
21    // Avoid bad seeds.
22    if (seed_ == 0 || seed_ == 2147483647L) {
23      seed_ = 1;
24    }
25  }
26  uint32_t Next() {
27    static const uint32_t M = 2147483647L;  // 2^31-1
28    static const uint64_t A = 16807;        // bits 14, 8, 7, 5, 2, 1, 0
29    // We are computing
30    //       seed_ = (seed_ * A) % M,    where M = 2^31-1
31    //
32    // seed_ must not be zero or M, or else all subsequent computed values
33    // will be zero or M respectively.  For all other values, seed_ will end
34    // up cycling through every number in [1,M-1]
35    uint64_t product = seed_ * A;
36
37    // Compute (product % M) using the fact that ((x << 31) % M) == x.
38    seed_ = static_cast<uint32_t>((product >> 31) + (product & M));
39    // The first reduction may overflow by 1 bit, so we may need to
40    // repeat.  mod == M is not possible; using > allows the faster
41    // sign-bit-based test.
42    if (seed_ > M) {
43      seed_ -= M;
44    }
45    return seed_;
46  }
47  // Returns a uniformly distributed value in the range [0..n-1]
48  // REQUIRES: n > 0
49  uint32_t Uniform(int n) { return Next() % n; }
50
51  // Randomly returns true ~"1/n" of the time, and false otherwise.
52  // REQUIRES: n > 0
53  bool OneIn(int n) { return (Next() % n) == 0; }
54
55  // Skewed: pick "base" uniformly from range [0,max_log] and then
56  // return "base" random bits.  The effect is to pick a number in the
57  // range [0,2^max_log-1] with exponential bias towards smaller numbers.
58  uint32_t Skewed(int max_log) { return Uniform(1 << Uniform(max_log + 1)); }
59};
60
61}  // namespace leveldb
62
63#endif  // STORAGE_LEVELDB_UTIL_RANDOM_H_
64