text: // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file. See the AUTHORS file for names of contributors.
  
  #ifndef STORAGE_LEVELDB_UTIL_RANDOM_H_
  #define STORAGE_LEVELDB_UTIL_RANDOM_H_
  
  #include <stdint.h>
  
  namespace leveldb {
  
  // A very simple random number generator.  Not especially good at
  // generating truly random bits, but good enough for our needs in this
  // package.
  class Random {
     private:
      uint32_t seed_;
  
     public:
      explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) {
          // Avoid bad seeds.
          if (seed_ == 0 || seed_ == 2147483647L) {
              seed_ = 1;
          }
      }
      uint32_t Next() {
          static const uint32_t M = 2147483647L;  // 2^31-1
          static const uint64_t A = 16807;        // bits 14, 8, 7, 5, 2, 1, 0
          // We are computing
          //       seed_ = (seed_ * A) % M,    where M = 2^31-1
          //
          // seed_ must not be zero or M, or else all subsequent computed values
          // will be zero or M respectively.  For all other values, seed_ will end
          // up cycling through every number in [1,M-1]
          uint64_t product = seed_ * A;
  
          // Compute (product % M) using the fact that ((x << 31) % M) == x.
          seed_ = static_cast<uint32_t>((product >> 31) + (product & M));
          // The first reduction may overflow by 1 bit, so we may need to
          // repeat.  mod == M is not possible; using > allows the faster
          // sign-bit-based test.
          if (seed_ > M) {
              seed_ -= M;
          }
          return seed_;
      }
      // Returns a uniformly distributed value in the range [0..n-1]
      // REQUIRES: n > 0
      uint32_t Uniform(int n) { return Next() % n; }
  
      // Randomly returns true ~"1/n" of the time, and false otherwise.
      // REQUIRES: n > 0
      bool OneIn(int n) { return (Next() % n) == 0; }
  
      // Skewed: pick "base" uniformly from range [0,max_log] and then
      // return "base" random bits.  The effect is to pick a number in the
      // range [0,2^max_log-1] with exponential bias towards smaller numbers.
      uint32_t Skewed(int max_log) { return Uniform(1 << Uniform(max_log + 1)); }
  };
  
  }  // namespace leveldb
  
  #endif  // STORAGE_LEVELDB_UTIL_RANDOM_H_
  