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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  public:
Random(uint32_t s)19   explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) {
20     // Avoid bad seeds.
21     if (seed_ == 0 || seed_ == 2147483647L) {
22       seed_ = 1;
23     }
24   }
Next()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
Uniform(int n)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
OneIn(int n)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.
Skewed(int max_log)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