1 /* boost random/poisson_distribution.hpp header file

     2  *

     3  * Copyright Jens Maurer 2002

     4  * Distributed under the Boost Software License, Version 1.0. (See

     5  * accompanying file LICENSE_1_0.txt or copy at

     6  * http://www.boost.org/LICENSE_1_0.txt)

     7  *

     8  * See http://www.boost.org for most recent version including documentation.

     9  *

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

    11  *

    12  */

    13 

    14 #ifndef BOOST_RANDOM_POISSON_DISTRIBUTION_HPP

    15 #define BOOST_RANDOM_POISSON_DISTRIBUTION_HPP

    16 

    17 #include <cmath>

    18 #include <cassert>

    19 #include <iostream>

    20 #include <boost/limits.hpp>

    21 #include <boost/static_assert.hpp>

    22 

    23 namespace boost {

    24 

    25 // Knuth

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

    27 class poisson_distribution

    28 {

    29 public:

    30   typedef RealType input_type;

    31   typedef IntType result_type;

    32 

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

    34     : _mean(mean)

    35   {

    36 #ifndef BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS

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

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

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

    40 #endif

    41 

    42     assert(mean > RealType(0));

    43     init();

    44   }

    45 

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

    47 

    48   RealType mean() const { return _mean; }

    49   void reset() { }

    50 

    51   template<class Engine>

    52   result_type operator()(Engine& eng)

    53   {

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

    55     RealType product = RealType(1);

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

    57       product *= eng();

    58       if(product <= _exp_mean)

    59         return m;

    60     }

    61   }

    62 

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

    64   template<class CharT, class Traits>

    65   friend std::basic_ostream<CharT,Traits>&

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

    67   {

    68     os << pd._mean;

    69     return os;

    70   }

    71 

    72   template<class CharT, class Traits>

    73   friend std::basic_istream<CharT,Traits>&

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

    75   {

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

    77     pd.init();

    78     return is;

    79   }

    80 #endif

    81 

    82 private:

    83   void init()

    84   {

    85 #ifndef BOOST_NO_STDC_NAMESPACE

    86     // allow for Koenig lookup

    87     using std::exp;

    88 #endif

    89     _exp_mean = exp(-_mean);

    90   }

    91 

    92   RealType _mean;

    93   // some precomputed data from the parameters

    94   RealType _exp_mean;

    95 };

    96 

    97 } // namespace boost

    98 

    99 #endif // BOOST_RANDOM_POISSON_DISTRIBUTION_HPP