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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                 :             : 
      19                 :             :  public:
      20                 :        9616 :   explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) {
      21                 :             :     // Avoid bad seeds.
      22         [ -  + ]:        9616 :     if (seed_ == 0 || seed_ == 2147483647L) {
      23                 :           0 :       seed_ = 1;
      24                 :             :     }
      25                 :             :   }
      26                 :      958125 :   uint32_t Next() {
      27                 :      958125 :     static const uint32_t M = 2147483647L;  // 2^31-1
      28                 :      958125 :     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                 :      958125 :     uint64_t product = seed_ * A;
      36                 :             : 
      37                 :             :     // Compute (product % M) using the fact that ((x << 31) % M) == x.
      38                 :      958125 :     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         [ -  + ]:      958125 :     if (seed_ > M) {
      43                 :           0 :       seed_ -= M;
      44                 :             :     }
      45         [ +  + ]:      958125 :     return seed_;
      46                 :             :   }
      47                 :             :   // Returns a uniformly distributed value in the range [0..n-1]
      48                 :             :   // REQUIRES: n > 0
      49         [ -  + ]:        5659 :   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_