/* boost random/exponential_distribution.hpp header file

  *

  * Copyright Jens Maurer 2000-2001

  * 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: exponential_distribution.hpp,v 1.16 2004/07/27 03:43:32 dgregor Exp $

  *

  * Revision history

  *  2001-02-18  moved to individual header files

  */

 #ifndef BOOST_RANDOM_EXPONENTIAL_DISTRIBUTION_HPP

 #define BOOST_RANDOM_EXPONENTIAL_DISTRIBUTION_HPP

 #include <cmath>

 #include <cassert>

 #include <iostream>

 #include <boost/limits.hpp>

 #include <boost/static_assert.hpp>

 namespace boost {

 // exponential distribution: p(x) = lambda * exp(-lambda * x)

 template<class RealType = double>

 class exponential_distribution

 {

 public:

   typedef RealType input_type;

   typedef RealType result_type;

 #if !defined(BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS) && !(defined(BOOST_MSVC) && BOOST_MSVC <= 1300)

   BOOST_STATIC_ASSERT(!std::numeric_limits<RealType>::is_integer);

 #endif

   explicit exponential_distribution(result_type lambda = result_type(1))

     : _lambda(lambda) { assert(lambda > result_type(0)); }

   // compiler-generated copy ctor and assignment operator are fine

   result_type lambda() const { return _lambda; }

   void reset() { }

   template<class Engine>

   result_type operator()(Engine& eng)

   { 

 #ifndef BOOST_NO_STDC_NAMESPACE

     using std::log;

 #endif

     return -result_type(1) / _lambda * log(result_type(1)-eng());

   }

 #if !defined(BOOST_NO_OPERATORS_IN_NAMESPACE) && !defined(BOOST_NO_MEMBER_TEMPLATE_FRIENDS)

   template<class CharT, class Traits>

   friend std::basic_ostream<CharT,Traits>&

   operator<<(std::basic_ostream<CharT,Traits>& os, const exponential_distribution& ed)

   {

     os << ed._lambda;

     return os;

   }

   template<class CharT, class Traits>

   friend std::basic_istream<CharT,Traits>&

   operator>>(std::basic_istream<CharT,Traits>& is, exponential_distribution& ed)

   {

     is >> std::ws >> ed._lambda;

     return is;

   }

 #endif

 private:

   result_type _lambda;

 };

 } // namespace boost

 #endif // BOOST_RANDOM_EXPONENTIAL_DISTRIBUTION_HPP