prng.h

/**
 * This file is part of the CernVM File System.
 *
 * A simple linear congruential pseudo number generator.  Thread-safe since
 * there is no global state like with random().
 */

#ifndef CVMFS_PRNG_H_
#define CVMFS_PRNG_H_

#include <sys/time.h>
#include <stdint.h>
#include <cassert>
#include <cstdlib>

#ifdef CVMFS_NAMESPACE_GUARD
namespace CVMFS_NAMESPACE_GUARD {
#endif

/**
 *  * Pseudo Random Number Generator.  See: TAoCP, volume 2
 *   */
class Prng {
 public:
  Prng() {
    state_ = 0;
  }

  void InitSeed(const uint64_t seed) {
    state_ = seed;
  }

  void InitLocaltime() {
    struct timeval tv_now;
    int retval = gettimeofday(&tv_now, NULL);
    assert(retval == 0);
    state_ = tv_now.tv_usec;
  }

  /**
 *    * Returns random number in [0..boundary-1]
 *       */
  uint32_t Next(const uint64_t boundary) {
    state_ = a*state_ + c;
    double scaled_val = double(state_) * double(boundary) /
                        double(18446744073709551616.0);
    return (uint32_t)scaled_val % boundary;
  }
 private:
  // Magic numbers from MMIX
  //   //static const uint64_t m = 2^64;
  static const uint64_t a = 6364136223846793005LLU;
  static const uint64_t c = 1442695040888963407LLU;
  uint64_t state_;
};  // class Prng

#ifdef CVMFS_NAMESPACE_GUARD
}
#endif
  
#endif  // CVMFS_PRNG_H_