src/leveldb/util/random.h

   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:
  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_