/* boost random/exponential_distribution.hpp header file
 *
 * Copyright Jens Maurer 2000-2001
 * Permission to use, copy, modify, sell, and distribute this software
 * is hereby granted without fee provided that the above copyright notice
 * appears in all copies and that both that copyright notice and this
 * permission notice appear in supporting documentation,
 *
 * Jens Maurer makes no representations about the suitability of this
 * software for any purpose. It is provided "as is" without express or
 * implied warranty.
 *
 * See http://www.boost.org for most recent version including documentation.
 *
 * $Id: exponential_distribution.hpp,v 1.2 2003/12/05 22:04:59 tshead 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 <boost/limits.hpp>
#include <boost/static_assert.hpp>
#include <boost/random/uniform_01.hpp>

namespace boost {

// exponential distribution: p(x) = lambda * exp(-lambda * x)
template<class UniformRandomNumberGenerator, class RealType = double,
        class Adaptor = uniform_01<UniformRandomNumberGenerator, RealType> >
class exponential_distribution
{
public:
  typedef Adaptor adaptor_type;
  typedef UniformRandomNumberGenerator base_type;
  typedef RealType result_type;

#ifndef BOOST_NO_LIMITS_COMPILE_TIME_CONSTANTS
  BOOST_STATIC_ASSERT(!std::numeric_limits<RealType>::is_integer);
#endif

  explicit exponential_distribution(base_type& rng,
                                    result_type lambda = result_type(1))
    : _rng(rng), _lambda(lambda) { assert(lambda > result_type(0)); }

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

  adaptor_type& adaptor() { return _rng; }
  base_type& base() const { return _rng.base(); }
  void reset() { _rng.reset(); }

  result_type operator()()
  { 
#ifndef BOOST_NO_STDC_NAMESPACE
    using std::log;
#endif
    return -result_type(1) / _lambda * log(result_type(1)-_rng());
  }

#ifndef BOOST_NO_OPERATORS_IN_NAMESPACE
  friend bool operator==(const exponential_distribution& x, 
                         const exponential_distribution& y)
  { return x._lambda == y._lambda && x._rng == y._rng; }

#ifndef 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

#else
  // Use a member function
  bool operator==(const exponential_distribution& rhs) const
  { return _lambda == rhs._lambda && _rng == rhs._rng;  }
#endif
private:
  adaptor_type _rng;
  result_type _lambda;
};

} // namespace boost

#endif // BOOST_RANDOM_EXPONENTIAL_DISTRIBUTION_HPP
