sl@0: /* boost random/poisson_distribution.hpp header file
sl@0:  *
sl@0:  * Copyright Jens Maurer 2002
sl@0:  * Distributed under the Boost Software License, Version 1.0. (See
sl@0:  * accompanying file LICENSE_1_0.txt or copy at
sl@0:  * http://www.boost.org/LICENSE_1_0.txt)
sl@0:  *
sl@0:  * See http://www.boost.org for most recent version including documentation.
sl@0:  *
sl@0:  * $Id: poisson_distribution.hpp,v 1.15 2005/05/21 15:57:00 dgregor Exp $
sl@0:  *
sl@0:  */
sl@0: 
sl@0: #ifndef BOOST_RANDOM_POISSON_DISTRIBUTION_HPP
sl@0: #define BOOST_RANDOM_POISSON_DISTRIBUTION_HPP
sl@0: 
sl@0: #include <cmath>
sl@0: #include <cassert>
sl@0: #include <iostream>
sl@0: #include <boost/limits.hpp>
sl@0: #include <boost/static_assert.hpp>
sl@0: 
sl@0: namespace boost {
sl@0: 
sl@0: // Knuth
sl@0: template<class IntType = int, class RealType = double>
sl@0: class poisson_distribution
sl@0: {
sl@0: public:
sl@0:   typedef RealType input_type;
sl@0:   typedef IntType result_type;
sl@0: 
sl@0:   explicit poisson_distribution(const RealType& mean = RealType(1))
sl@0:     : _mean(mean)
sl@0:   {
sl@0: #ifndef BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS
sl@0:     // MSVC fails BOOST_STATIC_ASSERT with std::numeric_limits at class scope
sl@0:     BOOST_STATIC_ASSERT(std::numeric_limits<IntType>::is_integer);
sl@0:     BOOST_STATIC_ASSERT(!std::numeric_limits<RealType>::is_integer);
sl@0: #endif
sl@0: 
sl@0:     assert(mean > RealType(0));
sl@0:     init();
sl@0:   }
sl@0: 
sl@0:   // compiler-generated copy ctor and assignment operator are fine
sl@0: 
sl@0:   RealType mean() const { return _mean; }
sl@0:   void reset() { }
sl@0: 
sl@0:   template<class Engine>
sl@0:   result_type operator()(Engine& eng)
sl@0:   {
sl@0:     // TODO: This is O(_mean), but it should be O(log(_mean)) for large _mean
sl@0:     RealType product = RealType(1);
sl@0:     for(result_type m = 0; ; ++m) {
sl@0:       product *= eng();
sl@0:       if(product <= _exp_mean)
sl@0:         return m;
sl@0:     }
sl@0:   }
sl@0: 
sl@0: #if !defined(BOOST_NO_OPERATORS_IN_NAMESPACE) && !defined(BOOST_NO_MEMBER_TEMPLATE_FRIENDS)
sl@0:   template<class CharT, class Traits>
sl@0:   friend std::basic_ostream<CharT,Traits>&
sl@0:   operator<<(std::basic_ostream<CharT,Traits>& os, const poisson_distribution& pd)
sl@0:   {
sl@0:     os << pd._mean;
sl@0:     return os;
sl@0:   }
sl@0: 
sl@0:   template<class CharT, class Traits>
sl@0:   friend std::basic_istream<CharT,Traits>&
sl@0:   operator>>(std::basic_istream<CharT,Traits>& is, poisson_distribution& pd)
sl@0:   {
sl@0:     is >> std::ws >> pd._mean;
sl@0:     pd.init();
sl@0:     return is;
sl@0:   }
sl@0: #endif
sl@0: 
sl@0: private:
sl@0:   void init()
sl@0:   {
sl@0: #ifndef BOOST_NO_STDC_NAMESPACE
sl@0:     // allow for Koenig lookup
sl@0:     using std::exp;
sl@0: #endif
sl@0:     _exp_mean = exp(-_mean);
sl@0:   }
sl@0: 
sl@0:   RealType _mean;
sl@0:   // some precomputed data from the parameters
sl@0:   RealType _exp_mean;
sl@0: };
sl@0: 
sl@0: } // namespace boost
sl@0: 
sl@0: #endif // BOOST_RANDOM_POISSON_DISTRIBUTION_HPP