/* 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