/src/contrib/boost/random/poisson_distribution.hpp
http://pythonocc.googlecode.com/ · C++ Header · 116 lines · 67 code · 16 blank · 33 comment · 2 complexity · f177ef8cc15de3adbd2a42ea0320af9c MD5 · raw file
- /* 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