/* boost random/poisson_distribution.hpp header file

 *

 * Copyright Jens Maurer 2002

 * 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: poisson_distribution.hpp,v 1.15 2005/05/21 15:57:00 dgregor Exp $

 *

 */

#ifndef BOOST_RANDOM_POISSON_DISTRIBUTION_HPP

#define BOOST_RANDOM_POISSON_DISTRIBUTION_HPP

#include <cmath>

#include <cassert>

#include <iostream>

#include <boost/limits.hpp>

#include <boost/static_assert.hpp>

namespace boost {

// Knuth

template<class IntType = int, class RealType = double>

class poisson_distribution

{

public:

  typedef RealType input_type;

  typedef IntType result_type;

  explicit poisson_distribution(const RealType& mean = RealType(1))

    : _mean(mean)

  {

#ifndef BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS

    // MSVC fails BOOST_STATIC_ASSERT with std::numeric_limits at class scope

    BOOST_STATIC_ASSERT(std::numeric_limits<IntType>::is_integer);

    BOOST_STATIC_ASSERT(!std::numeric_limits<RealType>::is_integer);

#endif

    assert(mean > RealType(0));

    init();

  }

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

  RealType mean() const { return _mean; }

  void reset() { }

  template<class Engine>

  result_type operator()(Engine& eng)

  {

    // TODO: This is O(_mean), but it should be O(log(_mean)) for large _mean

    RealType product = RealType(1);

    for(result_type m = 0; ; ++m) {

      product *= eng();

      if(product <= _exp_mean)

        return m;

    }

  }

#if !defined(BOOST_NO_OPERATORS_IN_NAMESPACE) && !defined(BOOST_NO_MEMBER_TEMPLATE_FRIENDS)

  template<class CharT, class Traits>

  friend std::basic_ostream<CharT,Traits>&

  operator<<(std::basic_ostream<CharT,Traits>& os, const poisson_distribution& pd)

  {

    os << pd._mean;

    return os;

  }

  template<class CharT, class Traits>

  friend std::basic_istream<CharT,Traits>&

  operator>>(std::basic_istream<CharT,Traits>& is, poisson_distribution& pd)

  {

    is >> std::ws >> pd._mean;

    pd.init();

    return is;

  }

#endif

private:

  void init()

  {

#ifndef BOOST_NO_STDC_NAMESPACE

    // allow for Koenig lookup

    using std::exp;

#endif

    _exp_mean = exp(-_mean);

  }

  RealType _mean;

  // some precomputed data from the parameters

  RealType _exp_mean;

};

} // namespace boost

#endif // BOOST_RANDOM_POISSON_DISTRIBUTION_HPP