boost/random/poisson_distribution.hpp

   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  * $Id$
  11  *
  12  */
  13
  14 #ifndef BOOST_RANDOM_POISSON_DISTRIBUTION_HPP
  15 #define BOOST_RANDOM_POISSON_DISTRIBUTION_HPP
  16
  17 #include <boost/config/no_tr1/cmath.hpp>
  18 #include <cassert>
  19 #include <iostream>
  20 #include <boost/limits.hpp>
  21 #include <boost/static_assert.hpp>
  22 #include <boost/random/detail/config.hpp>
  23
  24 namespace boost {
  25
  26 // Knuth
  27 template<class IntType = int, class RealType = double>
  28 class poisson_distribution
  29 {
  30 public:
  31   typedef RealType input_type;
  32   typedef IntType result_type;
  33
  34   explicit poisson_distribution(const RealType& mean_arg = RealType(1))
  35     : _mean(mean_arg)
  36   {
  37 #ifndef BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS
  38     // MSVC fails BOOST_STATIC_ASSERT with std::numeric_limits at class scope
  39     BOOST_STATIC_ASSERT(std::numeric_limits<IntType>::is_integer);
  40     BOOST_STATIC_ASSERT(!std::numeric_limits<RealType>::is_integer);
  41 #endif
  42
  43     assert(_mean > RealType(0));
  44     init();
  45   }
  46
  47   // compiler-generated copy ctor and assignment operator are fine
  48
  49   RealType mean() const { return _mean; }
  50   void reset() { }
  51
  52   template<class Engine>
  53   result_type operator()(Engine& eng)
  54   {
  55     // TODO: This is O(_mean), but it should be O(log(_mean)) for large _mean
  56     RealType product = RealType(1);
  57     for(result_type m = 0; ; ++m) {
  58       product *= eng();
  59       if(product <= _exp_mean)
  60         return m;
  61     }
  62   }
  63
  64 #ifndef BOOST_RANDOM_NO_STREAM_OPERATORS
  65   template<class CharT, class Traits>
  66   friend std::basic_ostream<CharT,Traits>&
  67   operator<<(std::basic_ostream<CharT,Traits>& os, const poisson_distribution& pd)
  68   {
  69     os << pd._mean;
  70     return os;
  71   }
  72
  73   template<class CharT, class Traits>
  74   friend std::basic_istream<CharT,Traits>&
  75   operator>>(std::basic_istream<CharT,Traits>& is, poisson_distribution& pd)
  76   {
  77     is >> std::ws >> pd._mean;
  78     pd.init();
  79     return is;
  80   }
  81 #endif
  82
  83 private:
  84   void init()
  85   {
  86 #ifndef BOOST_NO_STDC_NAMESPACE
  87     // allow for Koenig lookup
  88     using std::exp;
  89 #endif
  90     _exp_mean = exp(-_mean);
  91   }
  92
  93   RealType _mean;
  94   // some precomputed data from the parameters
  95   RealType _exp_mean;
  96 };
  97
  98 } // namespace boost
  99
 100 #endif // BOOST_RANDOM_POISSON_DISTRIBUTION_HPP