src/kudu/util/random.h

// 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 KUDU_UTIL_RANDOM_H_
#define KUDU_UTIL_RANDOM_H_

#include <stdint.h>

#include <cmath>

#include "kudu/util/locks.h"

namespace kudu {

namespace random_internal {

static const uint32_t M = 2147483647L;   // 2^31-1
const double kTwoPi = 6.283185307179586476925286;

} // namespace random_internal

// A very simple random number generator.  Not especially good at
// generating truly random bits, but good enough for our needs in this
// package. This implementation is not thread-safe.
class Random {
 private:
  uint32_t seed_;
 public:
  explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) {
    // Avoid bad seeds.
    if (seed_ == 0 || seed_ == random_internal::M) {
      seed_ = 1;
    }
  }

  // Next pseudo-random 32-bit unsigned integer.
  // FIXME: This currently only generates 31 bits of randomness.
  // The MSB will always be zero.
  uint32_t Next() {
    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 & random_internal::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_ > random_internal::M) {
      seed_ -= random_internal::M;
    }
    return seed_;
  }

  // Alias for consistency with Next64
  uint32_t Next32() { return Next(); }

  // Next pseudo-random 64-bit unsigned integer.
  // FIXME: This currently only generates 62 bits of randomness due to Next()
  // only giving 31 bits of randomness. The 2 most significant bits will always
  // be zero.
  uint64_t Next64() {
    uint64_t large = Next();
    // Only shift by 31 bits so we end up with zeros in MSB and not scattered
    // throughout the 64-bit word. This is due to the weakness in Next() noted
    // above.
    large <<= 31;
    large |= Next();
    return large;
  }

  // Returns a uniformly distributed value in the range [0..n-1]
  // REQUIRES: n > 0
  uint32_t Uniform(uint32_t n) { return Next() % n; }

  // Alias for consistency with Uniform64
  uint32_t Uniform32(uint32_t n) { return Uniform(n); }

  // Returns a uniformly distributed 64-bit value in the range [0..n-1]
  // REQUIRES: n > 0
  uint64_t Uniform64(uint64_t n) { return Next64() % 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));
  }

  // Creates a normal distribution variable using the
  // Box-Muller transform. See:
  // http://en.wikipedia.org/wiki/Box%E2%80%93Muller_transform
  // Adapted from WebRTC source code at:
  // webrtc/trunk/modules/video_coding/main/test/test_util.cc
  double Normal(double mean, double std_dev) {
    double uniform1 = (Next() + 1.0) / (random_internal::M + 1.0);
    double uniform2 = (Next() + 1.0) / (random_internal::M + 1.0);
    return (mean + std_dev * sqrt(-2 * ::log(uniform1)) * cos(random_internal::kTwoPi * uniform2));
  }

  // Return a random number between 0.0 and 1.0 inclusive.
  double NextDoubleFraction() {
    return Next() / static_cast<double>(random_internal::M + 1.0);
  }
};

// Thread-safe wrapper around Random.
class ThreadSafeRandom {
 public:
  explicit ThreadSafeRandom(uint32_t s)
      : random_(s) {
  }

  uint32_t Next() {
    lock_guard<simple_spinlock> l(&lock_);
    return random_.Next();
  }

  uint32_t Next32() {
    lock_guard<simple_spinlock> l(&lock_);
    return random_.Next32();
  }

  uint64_t Next64() {
    lock_guard<simple_spinlock> l(&lock_);
    return random_.Next64();
  }

  uint32_t Uniform(uint32_t n) {
    lock_guard<simple_spinlock> l(&lock_);
    return random_.Uniform(n);
  }

  uint32_t Uniform32(uint32_t n) {
    lock_guard<simple_spinlock> l(&lock_);
    return random_.Uniform32(n);
  }

  uint64_t Uniform64(uint64_t n) {
    lock_guard<simple_spinlock> l(&lock_);
    return random_.Uniform64(n);
  }

  bool OneIn(int n) {
    lock_guard<simple_spinlock> l(&lock_);
    return random_.OneIn(n);
  }

  uint32_t Skewed(int max_log) {
    lock_guard<simple_spinlock> l(&lock_);
    return random_.Skewed(max_log);
  }

  double Normal(double mean, double std_dev) {
    lock_guard<simple_spinlock> l(&lock_);
    return random_.Normal(mean, std_dev);
  }

 private:
  simple_spinlock lock_;
  Random random_;
};



}  // namespace kudu

#endif  // KUDU_UTIL_RANDOM_H_