Line data    Source code

       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         518 :   explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) {
      20             :     // Avoid bad seeds.
      21         159 :     if (seed_ == 0 || seed_ == 2147483647L) {
      22           0 :       seed_ = 1;
      23             :     }
      24           0 :   }
      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       37397 :     uint64_t product = seed_ * A;
      35             : 
      36             :     // Compute (product % M) using the fact that ((x << 31) % M) == x.
      37       37397 :     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       37397 :     if (seed_ > M) {
      42           0 :       seed_ -= M;
      43             :     }
      44       37397 :     return seed_;
      45             :   }
      46             :   // Returns a uniformly distributed value in the range [0..n-1]
      47             :   // REQUIRES: n > 0
      48         216 :   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_