// 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 <cstdint>

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_