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  
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_