| /* 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 60755 2010-03-22 00:45:06Z steven_watanabe $ |
| * |
| */ |
| |
| #ifndef BOOST_RANDOM_POISSON_DISTRIBUTION_HPP |
| #define BOOST_RANDOM_POISSON_DISTRIBUTION_HPP |
| |
| #include <boost/config/no_tr1/cmath.hpp> |
| #include <cassert> |
| #include <iostream> |
| #include <boost/limits.hpp> |
| #include <boost/static_assert.hpp> |
| #include <boost/random/detail/config.hpp> |
| |
| namespace boost { |
| |
| // Knuth |
| |
| /** |
| * An instantiation of the class template @c poisson_distribution is a |
| * model of \random_distribution. The poisson distribution has |
| * \f$p(i) = \frac{e^{-\lambda}\lambda^i}{i!}\f$ |
| */ |
| template<class IntType = int, class RealType = double> |
| class poisson_distribution |
| { |
| public: |
| typedef RealType input_type; |
| typedef IntType result_type; |
| |
| /** |
| * Constructs a @c poisson_distribution with the parameter @c mean. |
| * |
| * Requires: mean > 0 |
| */ |
| explicit poisson_distribution(const RealType& mean_arg = RealType(1)) |
| : _mean(mean_arg) |
| { |
| #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 |
| |
| /** |
| * Returns: the "mean" parameter of the distribution. |
| */ |
| 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; |
| } |
| } |
| |
| #ifndef BOOST_RANDOM_NO_STREAM_OPERATORS |
| 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: |
| /// \cond hide_private_members |
| void init() |
| { |
| #ifndef BOOST_NO_STDC_NAMESPACE |
| // allow for Koenig lookup |
| using std::exp; |
| #endif |
| _exp_mean = exp(-_mean); |
| } |
| /// \endcond |
| |
| RealType _mean; |
| // some precomputed data from the parameters |
| RealType _exp_mean; |
| }; |
| |
| } // namespace boost |
| |
| #endif // BOOST_RANDOM_POISSON_DISTRIBUTION_HPP |