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boost/random/geometric_distribution.hpp

/* boost random/geometric_distribution.hpp header file
 *
 * Copyright Jens Maurer 2000-2001
 * Distributed under the Boost Software License, Version 1.0. (See
 * accompanying file LICENSE_1_0.txt or copy at
 * http://www.boost.org/LICENSE_1_0.txt)
 *
 * See http://www.boost.org for most recent version including documentation.
 *
 * $Id: geometric_distribution.hpp 60755 2010-03-22 00:45:06Z steven_watanabe $
 *
 * Revision history
 *  2001-02-18  moved to individual header files
 */

#ifndef BOOST_RANDOM_GEOMETRIC_DISTRIBUTION_HPP
#define BOOST_RANDOM_GEOMETRIC_DISTRIBUTION_HPP

#include <boost/config/no_tr1/cmath.hpp>          // std::log
#include <cassert>
#include <iostream>
#include <boost/random/detail/config.hpp>
#include <boost/random/uniform_01.hpp>

namespace boost {

#if defined(__GNUC__) && (__GNUC__ < 3)
// Special gcc workaround: gcc 2.95.x ignores using-declarations
// in template classes (confirmed by gcc author Martin v. Loewis)
  using std::log;
#endif

/**
 * An instantiation of the class template @c geometric_distribution models
 * a \random_distribution.  The distribution produces positive
 * integers which are the number of bernoulli trials
 * with probability @c p required to get one that fails.
 *
 * For the geometric distribution, \f$p(i) = (1-p) p^{i-1}\f$.
 */
template<class IntType = int, class RealType = double>
class geometric_distribution
{
public:
  typedef RealType input_type;
  typedef IntType result_type;

  /**
   * Contructs a new geometric_distribution with the paramter @c p.
   *
   * Requires: 0 < p < 1
   */
  explicit geometric_distribution(const RealType& p = RealType(0.5))
    : _p(p)
  {
    assert(RealType(0) < _p && _p < RealType(1));
    init();
  }

  // compiler-generated copy ctor and assignment operator are fine

  /**
   * Returns: the distribution parameter @c p
   */
  RealType p() const { return _p; }
  void reset() { }

  template<class Engine>
  result_type operator()(Engine& eng)
  {
#ifndef BOOST_NO_STDC_NAMESPACE
    using std::log;
    using std::floor;
#endif
    return IntType(floor(log(RealType(1)-eng()) / _log_p)) + IntType(1);
  }

#ifndef BOOST_RANDOM_NO_STREAM_OPERATORS
  template<class CharT, class Traits>
  friend std::basic_ostream<CharT,Traits>&
  operator<<(std::basic_ostream<CharT,Traits>& os, const geometric_distribution& gd)
  {
    os << gd._p;
    return os;
  }

  template<class CharT, class Traits>
  friend std::basic_istream<CharT,Traits>&
  operator>>(std::basic_istream<CharT,Traits>& is, geometric_distribution& gd)
  {
    is >> std::ws >> gd._p;
    gd.init();
    return is;
  }
#endif

private:

  /// \cond hide_private_functions

  void init()
  {
#ifndef BOOST_NO_STDC_NAMESPACE
    using std::log;
#endif
    _log_p = log(_p);
  }

  /// \endcond

  RealType _p;
  RealType _log_p;
};

} // namespace boost

#endif // BOOST_RANDOM_GEOMETRIC_DISTRIBUTION_HPP