sl@0: /* boost random/exponential_distribution.hpp header file
sl@0:  *
sl@0:  * Copyright Jens Maurer 2000-2001
sl@0:  * Distributed under the Boost Software License, Version 1.0. (See
sl@0:  * accompanying file LICENSE_1_0.txt or copy at
sl@0:  * http://www.boost.org/LICENSE_1_0.txt)
sl@0:  *
sl@0:  * See http://www.boost.org for most recent version including documentation.
sl@0:  *
sl@0:  * $Id: exponential_distribution.hpp,v 1.16 2004/07/27 03:43:32 dgregor Exp $
sl@0:  *
sl@0:  * Revision history
sl@0:  *  2001-02-18  moved to individual header files
sl@0:  */
sl@0: 
sl@0: #ifndef BOOST_RANDOM_EXPONENTIAL_DISTRIBUTION_HPP
sl@0: #define BOOST_RANDOM_EXPONENTIAL_DISTRIBUTION_HPP
sl@0: 
sl@0: #include <cmath>
sl@0: #include <cassert>
sl@0: #include <iostream>
sl@0: #include <boost/limits.hpp>
sl@0: #include <boost/static_assert.hpp>
sl@0: 
sl@0: namespace boost {
sl@0: 
sl@0: // exponential distribution: p(x) = lambda * exp(-lambda * x)
sl@0: template<class RealType = double>
sl@0: class exponential_distribution
sl@0: {
sl@0: public:
sl@0:   typedef RealType input_type;
sl@0:   typedef RealType result_type;
sl@0: 
sl@0: #if !defined(BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS) && !(defined(BOOST_MSVC) && BOOST_MSVC <= 1300)
sl@0:   BOOST_STATIC_ASSERT(!std::numeric_limits<RealType>::is_integer);
sl@0: #endif
sl@0: 
sl@0:   explicit exponential_distribution(result_type lambda = result_type(1))
sl@0:     : _lambda(lambda) { assert(lambda > result_type(0)); }
sl@0: 
sl@0:   // compiler-generated copy ctor and assignment operator are fine
sl@0: 
sl@0:   result_type lambda() const { return _lambda; }
sl@0: 
sl@0:   void reset() { }
sl@0: 
sl@0:   template<class Engine>
sl@0:   result_type operator()(Engine& eng)
sl@0:   { 
sl@0: #ifndef BOOST_NO_STDC_NAMESPACE
sl@0:     using std::log;
sl@0: #endif
sl@0:     return -result_type(1) / _lambda * log(result_type(1)-eng());
sl@0:   }
sl@0: 
sl@0: #if !defined(BOOST_NO_OPERATORS_IN_NAMESPACE) && !defined(BOOST_NO_MEMBER_TEMPLATE_FRIENDS)
sl@0:   template<class CharT, class Traits>
sl@0:   friend std::basic_ostream<CharT,Traits>&
sl@0:   operator<<(std::basic_ostream<CharT,Traits>& os, const exponential_distribution& ed)
sl@0:   {
sl@0:     os << ed._lambda;
sl@0:     return os;
sl@0:   }
sl@0: 
sl@0:   template<class CharT, class Traits>
sl@0:   friend std::basic_istream<CharT,Traits>&
sl@0:   operator>>(std::basic_istream<CharT,Traits>& is, exponential_distribution& ed)
sl@0:   {
sl@0:     is >> std::ws >> ed._lambda;
sl@0:     return is;
sl@0:   }
sl@0: #endif
sl@0: 
sl@0: private:
sl@0:   result_type _lambda;
sl@0: };
sl@0: 
sl@0: } // namespace boost
sl@0: 
sl@0: #endif // BOOST_RANDOM_EXPONENTIAL_DISTRIBUTION_HPP