100 lines
2.5 KiB
C++
Executable File
100 lines
2.5 KiB
C++
Executable File
/* boost random/poisson_distribution.hpp header file
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*
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* Copyright Jens Maurer 2002
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* Distributed under the Boost Software License, Version 1.0. (See
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* accompanying file LICENSE_1_0.txt or copy at
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* http://www.boost.org/LICENSE_1_0.txt)
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*
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* See http://www.boost.org for most recent version including documentation.
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*
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* $Id: poisson_distribution.hpp 41369 2007-11-25 18:07:19Z bemandawes $
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*
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*/
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#ifndef BOOST_RANDOM_POISSON_DISTRIBUTION_HPP
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#define BOOST_RANDOM_POISSON_DISTRIBUTION_HPP
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#include <cmath>
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#include <cassert>
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#include <iostream>
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#include <boost/limits.hpp>
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#include <boost/static_assert.hpp>
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namespace boost {
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// Knuth
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template<class IntType = int, class RealType = double>
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class poisson_distribution
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{
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public:
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typedef RealType input_type;
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typedef IntType result_type;
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explicit poisson_distribution(const RealType& mean_arg = RealType(1))
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: _mean(mean_arg)
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{
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#ifndef BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS
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// MSVC fails BOOST_STATIC_ASSERT with std::numeric_limits at class scope
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BOOST_STATIC_ASSERT(std::numeric_limits<IntType>::is_integer);
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BOOST_STATIC_ASSERT(!std::numeric_limits<RealType>::is_integer);
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#endif
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assert(_mean > RealType(0));
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init();
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}
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// compiler-generated copy ctor and assignment operator are fine
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RealType mean() const { return _mean; }
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void reset() { }
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template<class Engine>
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result_type operator()(Engine& eng)
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{
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// TODO: This is O(_mean), but it should be O(log(_mean)) for large _mean
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RealType product = RealType(1);
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for(result_type m = 0; ; ++m) {
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product *= eng();
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if(product <= _exp_mean)
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return m;
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}
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}
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#if !defined(BOOST_NO_OPERATORS_IN_NAMESPACE) && !defined(BOOST_NO_MEMBER_TEMPLATE_FRIENDS)
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template<class CharT, class Traits>
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friend std::basic_ostream<CharT,Traits>&
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operator<<(std::basic_ostream<CharT,Traits>& os, const poisson_distribution& pd)
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{
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os << pd._mean;
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return os;
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}
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template<class CharT, class Traits>
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friend std::basic_istream<CharT,Traits>&
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operator>>(std::basic_istream<CharT,Traits>& is, poisson_distribution& pd)
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{
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is >> std::ws >> pd._mean;
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pd.init();
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return is;
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}
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#endif
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private:
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void init()
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{
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#ifndef BOOST_NO_STDC_NAMESPACE
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// allow for Koenig lookup
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using std::exp;
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#endif
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_exp_mean = exp(-_mean);
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}
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RealType _mean;
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// some precomputed data from the parameters
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RealType _exp_mean;
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};
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} // namespace boost
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#endif // BOOST_RANDOM_POISSON_DISTRIBUTION_HPP
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