/* 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