williamr@2: /* boost random/poisson_distribution.hpp header file
williamr@2:  *
williamr@2:  * Copyright Jens Maurer 2002
williamr@2:  * Distributed under the Boost Software License, Version 1.0. (See
williamr@2:  * accompanying file LICENSE_1_0.txt or copy at
williamr@2:  * http://www.boost.org/LICENSE_1_0.txt)
williamr@2:  *
williamr@2:  * See http://www.boost.org for most recent version including documentation.
williamr@2:  *
williamr@2:  * $Id: poisson_distribution.hpp,v 1.15 2005/05/21 15:57:00 dgregor Exp $
williamr@2:  *
williamr@2:  */
williamr@2: 
williamr@2: #ifndef BOOST_RANDOM_POISSON_DISTRIBUTION_HPP
williamr@2: #define BOOST_RANDOM_POISSON_DISTRIBUTION_HPP
williamr@2: 
williamr@2: #include <cmath>
williamr@2: #include <cassert>
williamr@2: #include <iostream>
williamr@2: #include <boost/limits.hpp>
williamr@2: #include <boost/static_assert.hpp>
williamr@2: 
williamr@2: namespace boost {
williamr@2: 
williamr@2: // Knuth
williamr@2: template<class IntType = int, class RealType = double>
williamr@2: class poisson_distribution
williamr@2: {
williamr@2: public:
williamr@2:   typedef RealType input_type;
williamr@2:   typedef IntType result_type;
williamr@2: 
williamr@2:   explicit poisson_distribution(const RealType& mean = RealType(1))
williamr@2:     : _mean(mean)
williamr@2:   {
williamr@2: #ifndef BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS
williamr@2:     // MSVC fails BOOST_STATIC_ASSERT with std::numeric_limits at class scope
williamr@2:     BOOST_STATIC_ASSERT(std::numeric_limits<IntType>::is_integer);
williamr@2:     BOOST_STATIC_ASSERT(!std::numeric_limits<RealType>::is_integer);
williamr@2: #endif
williamr@2: 
williamr@2:     assert(mean > RealType(0));
williamr@2:     init();
williamr@2:   }
williamr@2: 
williamr@2:   // compiler-generated copy ctor and assignment operator are fine
williamr@2: 
williamr@2:   RealType mean() const { return _mean; }
williamr@2:   void reset() { }
williamr@2: 
williamr@2:   template<class Engine>
williamr@2:   result_type operator()(Engine& eng)
williamr@2:   {
williamr@2:     // TODO: This is O(_mean), but it should be O(log(_mean)) for large _mean
williamr@2:     RealType product = RealType(1);
williamr@2:     for(result_type m = 0; ; ++m) {
williamr@2:       product *= eng();
williamr@2:       if(product <= _exp_mean)
williamr@2:         return m;
williamr@2:     }
williamr@2:   }
williamr@2: 
williamr@2: #if !defined(BOOST_NO_OPERATORS_IN_NAMESPACE) && !defined(BOOST_NO_MEMBER_TEMPLATE_FRIENDS)
williamr@2:   template<class CharT, class Traits>
williamr@2:   friend std::basic_ostream<CharT,Traits>&
williamr@2:   operator<<(std::basic_ostream<CharT,Traits>& os, const poisson_distribution& pd)
williamr@2:   {
williamr@2:     os << pd._mean;
williamr@2:     return os;
williamr@2:   }
williamr@2: 
williamr@2:   template<class CharT, class Traits>
williamr@2:   friend std::basic_istream<CharT,Traits>&
williamr@2:   operator>>(std::basic_istream<CharT,Traits>& is, poisson_distribution& pd)
williamr@2:   {
williamr@2:     is >> std::ws >> pd._mean;
williamr@2:     pd.init();
williamr@2:     return is;
williamr@2:   }
williamr@2: #endif
williamr@2: 
williamr@2: private:
williamr@2:   void init()
williamr@2:   {
williamr@2: #ifndef BOOST_NO_STDC_NAMESPACE
williamr@2:     // allow for Koenig lookup
williamr@2:     using std::exp;
williamr@2: #endif
williamr@2:     _exp_mean = exp(-_mean);
williamr@2:   }
williamr@2: 
williamr@2:   RealType _mean;
williamr@2:   // some precomputed data from the parameters
williamr@2:   RealType _exp_mean;
williamr@2: };
williamr@2: 
williamr@2: } // namespace boost
williamr@2: 
williamr@2: #endif // BOOST_RANDOM_POISSON_DISTRIBUTION_HPP