/* 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 |