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_
64