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) { }
    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));
    }
  
    // Shuffle the array into random order
    void Shuffle(int *array, int n) {
    	if (n > 1) {
  	  int i;
  	  for (i=0; i<n-1; i++) {
  	  	int j = i + Next() / (2147483647 / (n-i) + 1);
  		int t = array[j];
  		array[j] = array[i];
  		array[i] = t;
  	  }
  	}
    }
  };
  
  }  // namespace leveldb
  
  #endif  // STORAGE_LEVELDB_UTIL_RANDOM_H_
  