boost/random/discrete_distribution.hpp - platform/external/boost - Git at Google

 /* boost random/discrete_distribution.hpp header file
  *
  * Copyright Steven Watanabe 2009-2011
  * 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: discrete_distribution.hpp 71018 2011-04-05 21:27:52Z steven_watanabe $
  */

 #ifndef BOOST_RANDOM_DISCRETE_DISTRIBUTION_HPP_INCLUDED
 #define BOOST_RANDOM_DISCRETE_DISTRIBUTION_HPP_INCLUDED

 #include <vector>
 #include <limits>
 #include <numeric>
 #include <utility>
 #include <iterator>
 #include <boost/assert.hpp>
 #include <boost/random/uniform_01.hpp>
 #include <boost/random/uniform_int.hpp>
 #include <boost/random/detail/config.hpp>
 #include <boost/random/detail/operators.hpp>
 #include <boost/random/detail/vector_io.hpp>

 #ifndef BOOST_NO_INITIALIZER_LISTS
 #include <initializer_list>
 #endif

 #include <boost/range/begin.hpp>
 #include <boost/range/end.hpp>

 #include <boost/random/detail/disable_warnings.hpp>

 namespace boost {
 namespace random {

 /**
  * The class @c discrete_distribution models a \random_distribution.
  * It produces integers in the range [0, n) with the probability
  * of producing each value is specified by the parameters of the
  * distribution.
  */
 template<class IntType = int, class WeightType = double>
 class discrete_distribution {
 public:
     typedef WeightType input_type;
     typedef IntType result_type;

     class param_type {
     public:

         typedef discrete_distribution distribution_type;

         /**
          * Constructs a @c param_type object, representing a distribution
          * with \f$p(0) = 1\f$ and \f$p(k|k>0) = 0\f$.
          */
         param_type() : _probabilities(1, static_cast<WeightType>(1)) {}
         /**
          * If @c first == @c last, equivalent to the default constructor.
          * Otherwise, the values of the range represent weights for the
          * possible values of the distribution.
          */
         template<class Iter>
         param_type(Iter first, Iter last) : _probabilities(first, last)
         {
             normalize();
         }
 #ifndef BOOST_NO_INITIALIZER_LISTS
         /**
          * If wl.size() == 0, equivalent to the default constructor.
          * Otherwise, the values of the @c initializer_list represent
          * weights for the possible values of the distribution.
          */
         param_type(const std::initializer_list<WeightType>& wl)
           : _probabilities(wl)
         {
             normalize();
         }
 #endif
         /**
          * If the range is empty, equivalent to the default constructor.
          * Otherwise, the elements of the range represent
          * weights for the possible values of the distribution.
          */
         template<class Range>
         explicit param_type(const Range& range)
           : _probabilities(boost::begin(range), boost::end(range))
         {
             normalize();
         }

         /**
          * If nw is zero, equivalent to the default constructor.
          * Otherwise, the range of the distribution is [0, nw),
          * and the weights are found by  calling fw with values
          * evenly distributed between \f$\mbox{xmin} + \delta/2\f$ and
          * \f$\mbox{xmax} - \delta/2\f$, where
          * \f$\delta = (\mbox{xmax} - \mbox{xmin})/\mbox{nw}\f$.
          */
         template<class Func>
         param_type(std::size_t nw, double xmin, double xmax, Func fw)
         {
             std::size_t n = (nw == 0) ? 1 : nw;
             double delta = (xmax - xmin) / n;
             BOOST_ASSERT(delta > 0);
             for(std::size_t k = 0; k < n; ++k) {
                 _probabilities.push_back(fw(xmin + k*delta + delta/2));
             }
             normalize();
         }

         /**
          * Returns a vector containing the probabilities of each possible
          * value of the distribution.
          */
         std::vector<WeightType> probabilities() const
         {
             return _probabilities;
         }

         /** Writes the parameters to a @c std::ostream. */
         BOOST_RANDOM_DETAIL_OSTREAM_OPERATOR(os, param_type, parm)
         {
             detail::print_vector(os, parm._probabilities);
             return os;
         }

         /** Reads the parameters from a @c std::istream. */
         BOOST_RANDOM_DETAIL_ISTREAM_OPERATOR(is, param_type, parm)
         {
             std::vector<WeightType> temp;
             detail::read_vector(is, temp);
             if(is) {
                 parm._probabilities.swap(temp);
             }
             return is;
         }

         /** Returns true if the two sets of parameters are the same. */
         BOOST_RANDOM_DETAIL_EQUALITY_OPERATOR(param_type, lhs, rhs)
         {
             return lhs._probabilities == rhs._probabilities;
         }
         /** Returns true if the two sets of parameters are different. */
         BOOST_RANDOM_DETAIL_INEQUALITY_OPERATOR(param_type)
     private:
         /// @cond show_private
         friend class discrete_distribution;
         explicit param_type(const discrete_distribution& dist)
           : _probabilities(dist.probabilities())
         {}
         void normalize()
         {
             WeightType sum =
                 std::accumulate(_probabilities.begin(), _probabilities.end(),
                                 static_cast<WeightType>(0));
             for(typename std::vector<WeightType>::iterator
                     iter = _probabilities.begin(),
                     end = _probabilities.end();
                     iter != end; ++iter)
             {
                 *iter /= sum;
             }
         }
         std::vector<WeightType> _probabilities;
         /// @endcond
     };

     /**
      * Creates a new @c discrete_distribution object that has
      * \f$p(0) = 1\f$ and \f$p(i|i>0) = 0\f$.
      */
     discrete_distribution()
     {
         _alias_table.push_back(std::make_pair(static_cast<WeightType>(1),
                                               static_cast<IntType>(0)));
     }
     /**
      * Constructs a discrete_distribution from an iterator range.
      * If @c first == @c last, equivalent to the default constructor.
      * Otherwise, the values of the range represent weights for the
      * possible values of the distribution.
      */
     template<class Iter>
     discrete_distribution(Iter first, Iter last)
     {
         init(first, last);
     }
 #ifndef BOOST_NO_INITIALIZER_LISTS
     /**
      * Constructs a @c discrete_distribution from a @c std::initializer_list.
      * If the @c initializer_list is empty, equivalent to the default
      * constructor.  Otherwise, the values of the @c initializer_list
      * represent weights for the possible values of the distribution.
      * For example, given the distribution
      *
      * @code
      * discrete_distribution<> dist{1, 4, 5};
      * @endcode
      *
      * The probability of a 0 is 1/10, the probability of a 1 is 2/5,
      * the probability of a 2 is 1/2, and no other values are possible.
      */
     discrete_distribution(std::initializer_list<WeightType> wl)
     {
         init(wl.begin(), wl.end());
     }
 #endif
     /**
      * Constructs a discrete_distribution from a Boost.Range range.
      * If the range is empty, equivalent to the default constructor.
      * Otherwise, the values of the range represent weights for the
      * possible values of the distribution.
      */
     template<class Range>
     explicit discrete_distribution(const Range& range)
     {
         init(boost::begin(range), boost::end(range));
     }
     /**
      * Constructs a discrete_distribution that approximates a function.
      * If nw is zero, equivalent to the default constructor.
      * Otherwise, the range of the distribution is [0, nw),
      * and the weights are found by  calling fw with values
      * evenly distributed between \f$\mbox{xmin} + \delta/2\f$ and
      * \f$\mbox{xmax} - \delta/2\f$, where
      * \f$\delta = (\mbox{xmax} - \mbox{xmin})/\mbox{nw}\f$.
      */
     template<class Func>
     discrete_distribution(std::size_t nw, double xmin, double xmax, Func fw)
     {
         std::size_t n = (nw == 0) ? 1 : nw;
         double delta = (xmax - xmin) / n;
         BOOST_ASSERT(delta > 0);
         std::vector<WeightType> weights;
         for(std::size_t k = 0; k < n; ++k) {
             weights.push_back(fw(xmin + k*delta + delta/2));
         }
         init(weights.begin(), weights.end());
     }
     /**
      * Constructs a discrete_distribution from its parameters.
      */
     explicit discrete_distribution(const param_type& parm)
     {
         param(parm);
     }

     /**
      * Returns a value distributed according to the parameters of the
      * discrete_distribution.
      */
     template<class URNG>
     IntType operator()(URNG& urng) const
     {
         BOOST_ASSERT(!_alias_table.empty());
         WeightType test = uniform_01<WeightType>()(urng);
         IntType result = uniform_int<IntType>((min)(), (max)())(urng);
         if(test < _alias_table[result].first) {
             return result;
         } else {
             return(_alias_table[result].second);
         }
     }

     /**
      * Returns a value distributed according to the parameters
      * specified by param.
      */
     template<class URNG>
     IntType operator()(URNG& urng, const param_type& parm) const
     {
         while(true) {
             WeightType val = uniform_01<WeightType>()(urng);
             WeightType sum = 0;
             std::size_t result = 0;
             for(typename std::vector<WeightType>::const_iterator
                 iter = parm._probabilities.begin(),
                 end = parm._probabilities.end();
                 iter != end; ++iter, ++result)
             {
                 sum += *iter;
                 if(sum > val) {
                     return result;
                 }
             }
         }
     }

     /** Returns the smallest value that the distribution can produce. */
     result_type min BOOST_PREVENT_MACRO_SUBSTITUTION () const { return 0; }
     /** Returns the largest value that the distribution can produce. */
     result_type max BOOST_PREVENT_MACRO_SUBSTITUTION () const
     { return static_cast<result_type>(_alias_table.size() - 1); }

     /**
      * Returns a vector containing the probabilities of each
      * value of the distribution.  For example, given
      *
      * @code
      * discrete_distribution<> dist = { 1, 4, 5 };
      * std::vector<double> p = dist.param();
      * @endcode
      *
      * the vector, p will contain {0.1, 0.4, 0.5}.
      */
     std::vector<WeightType> probabilities() const
     {
         std::vector<WeightType> result(_alias_table.size());
         const WeightType mean =
             static_cast<WeightType>(1) / _alias_table.size();
         std::size_t i = 0;
         for(typename alias_table_t::const_iterator
                 iter = _alias_table.begin(),
                 end = _alias_table.end();
                 iter != end; ++iter, ++i)
         {
             WeightType val = iter->first * mean;
             result[i] += val;
             result[iter->second] += mean - val;
         }
         return(result);
     }

     /** Returns the parameters of the distribution. */
     param_type param() const
     {
         return param_type(*this);
     }
     /** Sets the parameters of the distribution. */
     void param(const param_type& parm)
     {
         init(parm._probabilities.begin(), parm._probabilities.end());
     }

     /**
      * Effects: Subsequent uses of the distribution do not depend
      * on values produced by any engine prior to invoking reset.
      */
     void reset() {}

     /** Writes a distribution to a @c std::ostream. */
     BOOST_RANDOM_DETAIL_OSTREAM_OPERATOR(os, discrete_distribution, dd)
     {
         os << dd.param();
         return os;
     }

     /** Reads a distribution from a @c std::istream */
     BOOST_RANDOM_DETAIL_ISTREAM_OPERATOR(is, discrete_distribution, dd)
     {
         param_type parm;
         if(is >> parm) {
             dd.param(parm);
         }
         return is;
     }

     /**
      * Returns true if the two distributions will return the
      * same sequence of values, when passed equal generators.
      */
     BOOST_RANDOM_DETAIL_EQUALITY_OPERATOR(discrete_distribution, lhs, rhs)
     {
         return lhs._alias_table == rhs._alias_table;
     }
     /**
      * Returns true if the two distributions may return different
      * sequences of values, when passed equal generators.
      */
     BOOST_RANDOM_DETAIL_INEQUALITY_OPERATOR(discrete_distribution)

 private:

     /// @cond show_private

     template<class Iter>
     void init(Iter first, Iter last, std::input_iterator_tag)
     {
         std::vector<WeightType> temp(first, last);
         init(temp.begin(), temp.end());
     }
     template<class Iter>
     void init(Iter first, Iter last, std::forward_iterator_tag)
     {
         std::vector<std::pair<WeightType, IntType> > below_average;
         std::vector<std::pair<WeightType, IntType> > above_average;
         std::size_t size = std::distance(first, last);
         WeightType weight_sum =
             std::accumulate(first, last, static_cast<WeightType>(0));
         WeightType weight_average = weight_sum / size;
         std::size_t i = 0;
         for(; first != last; ++first, ++i) {
             WeightType val = *first / weight_average;
             std::pair<WeightType, IntType> elem(val, static_cast<IntType>(i));
             if(val < static_cast<WeightType>(1)) {
                 below_average.push_back(elem);
             } else {
                 above_average.push_back(elem);
             }
         }

         _alias_table.resize(size);
         typename alias_table_t::iterator
             b_iter = below_average.begin(),
             b_end = below_average.end(),
             a_iter = above_average.begin(),
             a_end = above_average.end()
             ;
         while(b_iter != b_end && a_iter != a_end) {
             _alias_table[b_iter->second] =
                 std::make_pair(b_iter->first, a_iter->second);
             a_iter->first -= (static_cast<WeightType>(1) - b_iter->first);
             if(a_iter->first < static_cast<WeightType>(1)) {
                 *b_iter = *a_iter++;
             } else {
                 ++b_iter;
             }
         }
         for(; b_iter != b_end; ++b_iter) {
             _alias_table[b_iter->second].first = static_cast<WeightType>(1);
         }
         for(; a_iter != a_end; ++a_iter) {
             _alias_table[a_iter->second].first = static_cast<WeightType>(1);
         }
     }
     template<class Iter>
     void init(Iter first, Iter last)
     {
         if(first == last) {
             _alias_table.clear();
             _alias_table.push_back(std::make_pair(static_cast<WeightType>(1),
                                                   static_cast<IntType>(0)));
         } else {
             typename std::iterator_traits<Iter>::iterator_category category;
             init(first, last, category);
         }
     }
     typedef std::vector<std::pair<WeightType, IntType> > alias_table_t;
     alias_table_t _alias_table;
     /// @endcond
 };

 }
 }

 #include <boost/random/detail/enable_warnings.hpp>

 #endif
	/* boost random/discrete_distribution.hpp header file
	*
	* Copyright Steven Watanabe 2009-2011
	* 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: discrete_distribution.hpp 71018 2011-04-05 21:27:52Z steven_watanabe $
	*/

	#ifndef BOOST_RANDOM_DISCRETE_DISTRIBUTION_HPP_INCLUDED
	#define BOOST_RANDOM_DISCRETE_DISTRIBUTION_HPP_INCLUDED

	#include <vector>
	#include <limits>
	#include <numeric>
	#include <utility>
	#include <iterator>
	#include <boost/assert.hpp>
	#include <boost/random/uniform_01.hpp>
	#include <boost/random/uniform_int.hpp>
	#include <boost/random/detail/config.hpp>
	#include <boost/random/detail/operators.hpp>
	#include <boost/random/detail/vector_io.hpp>

	#ifndef BOOST_NO_INITIALIZER_LISTS
	#include <initializer_list>
	#endif

	#include <boost/range/begin.hpp>
	#include <boost/range/end.hpp>

	#include <boost/random/detail/disable_warnings.hpp>

	namespace boost {
	namespace random {

	/**
	* The class @c discrete_distribution models a \random_distribution.
	* It produces integers in the range [0, n) with the probability
	* of producing each value is specified by the parameters of the
	* distribution.
	*/
	template<class IntType = int, class WeightType = double>
	class discrete_distribution {
	public:
	typedef WeightType input_type;
	typedef IntType result_type;

	class param_type {
	public:

	typedef discrete_distribution distribution_type;

	/**
	* Constructs a @c param_type object, representing a distribution
	* with \f$p(0) = 1\f$ and \f$p(k\|k>0) = 0\f$.
	*/
	param_type() : _probabilities(1, static_cast<WeightType>(1)) {}
	/**
	* If @c first == @c last, equivalent to the default constructor.
	* Otherwise, the values of the range represent weights for the
	* possible values of the distribution.
	*/
	template<class Iter>
	param_type(Iter first, Iter last) : _probabilities(first, last)
	{
	normalize();
	}
	#ifndef BOOST_NO_INITIALIZER_LISTS
	/**
	* If wl.size() == 0, equivalent to the default constructor.
	* Otherwise, the values of the @c initializer_list represent
	* weights for the possible values of the distribution.
	*/
	param_type(const std::initializer_list<WeightType>& wl)
	: _probabilities(wl)
	{
	normalize();
	}
	#endif
	/**
	* If the range is empty, equivalent to the default constructor.
	* Otherwise, the elements of the range represent
	* weights for the possible values of the distribution.
	*/
	template<class Range>
	explicit param_type(const Range& range)
	: _probabilities(boost::begin(range), boost::end(range))
	{
	normalize();
	}

	/**
	* If nw is zero, equivalent to the default constructor.
	* Otherwise, the range of the distribution is [0, nw),
	* and the weights are found by calling fw with values
	* evenly distributed between \f$\mbox{xmin} + \delta/2\f$ and
	* \f$\mbox{xmax} - \delta/2\f$, where
	* \f$\delta = (\mbox{xmax} - \mbox{xmin})/\mbox{nw}\f$.
	*/
	template<class Func>
	param_type(std::size_t nw, double xmin, double xmax, Func fw)
	{
	std::size_t n = (nw == 0) ? 1 : nw;
	double delta = (xmax - xmin) / n;
	BOOST_ASSERT(delta > 0);
	for(std::size_t k = 0; k < n; ++k) {
	_probabilities.push_back(fw(xmin + k*delta + delta/2));
	}
	normalize();
	}

	/**
	* Returns a vector containing the probabilities of each possible
	* value of the distribution.
	*/
	std::vector<WeightType> probabilities() const
	{
	return _probabilities;
	}

	/** Writes the parameters to a @c std::ostream. */
	BOOST_RANDOM_DETAIL_OSTREAM_OPERATOR(os, param_type, parm)
	{
	detail::print_vector(os, parm._probabilities);
	return os;
	}

	/** Reads the parameters from a @c std::istream. */
	BOOST_RANDOM_DETAIL_ISTREAM_OPERATOR(is, param_type, parm)
	{
	std::vector<WeightType> temp;
	detail::read_vector(is, temp);
	if(is) {
	parm._probabilities.swap(temp);
	}
	return is;
	}

	/** Returns true if the two sets of parameters are the same. */
	BOOST_RANDOM_DETAIL_EQUALITY_OPERATOR(param_type, lhs, rhs)
	{
	return lhs._probabilities == rhs._probabilities;
	}
	/** Returns true if the two sets of parameters are different. */
	BOOST_RANDOM_DETAIL_INEQUALITY_OPERATOR(param_type)
	private:
	/// @cond show_private
	friend class discrete_distribution;
	explicit param_type(const discrete_distribution& dist)
	: _probabilities(dist.probabilities())
	{}
	void normalize()
	{
	WeightType sum =
	std::accumulate(_probabilities.begin(), _probabilities.end(),
	static_cast<WeightType>(0));
	for(typename std::vector<WeightType>::iterator
	iter = _probabilities.begin(),
	end = _probabilities.end();
	iter != end; ++iter)
	{
	*iter /= sum;
	}
	}
	std::vector<WeightType> _probabilities;
	/// @endcond
	};

	/**
	* Creates a new @c discrete_distribution object that has
	* \f$p(0) = 1\f$ and \f$p(i\|i>0) = 0\f$.
	*/
	discrete_distribution()
	{
	_alias_table.push_back(std::make_pair(static_cast<WeightType>(1),
	static_cast<IntType>(0)));
	}
	/**
	* Constructs a discrete_distribution from an iterator range.
	* If @c first == @c last, equivalent to the default constructor.
	* Otherwise, the values of the range represent weights for the
	* possible values of the distribution.
	*/
	template<class Iter>
	discrete_distribution(Iter first, Iter last)
	{
	init(first, last);
	}
	#ifndef BOOST_NO_INITIALIZER_LISTS
	/**
	* Constructs a @c discrete_distribution from a @c std::initializer_list.
	* If the @c initializer_list is empty, equivalent to the default
	* constructor. Otherwise, the values of the @c initializer_list
	* represent weights for the possible values of the distribution.
	* For example, given the distribution
	*
	* @code
	* discrete_distribution<> dist{1, 4, 5};
	* @endcode
	*
	* The probability of a 0 is 1/10, the probability of a 1 is 2/5,
	* the probability of a 2 is 1/2, and no other values are possible.
	*/
	discrete_distribution(std::initializer_list<WeightType> wl)
	{
	init(wl.begin(), wl.end());
	}
	#endif
	/**
	* Constructs a discrete_distribution from a Boost.Range range.
	* If the range is empty, equivalent to the default constructor.
	* Otherwise, the values of the range represent weights for the
	* possible values of the distribution.
	*/
	template<class Range>
	explicit discrete_distribution(const Range& range)
	{
	init(boost::begin(range), boost::end(range));
	}
	/**
	* Constructs a discrete_distribution that approximates a function.
	* If nw is zero, equivalent to the default constructor.
	* Otherwise, the range of the distribution is [0, nw),
	* and the weights are found by calling fw with values
	* evenly distributed between \f$\mbox{xmin} + \delta/2\f$ and
	* \f$\mbox{xmax} - \delta/2\f$, where
	* \f$\delta = (\mbox{xmax} - \mbox{xmin})/\mbox{nw}\f$.
	*/
	template<class Func>
	discrete_distribution(std::size_t nw, double xmin, double xmax, Func fw)
	{
	std::size_t n = (nw == 0) ? 1 : nw;
	double delta = (xmax - xmin) / n;
	BOOST_ASSERT(delta > 0);
	std::vector<WeightType> weights;
	for(std::size_t k = 0; k < n; ++k) {
	weights.push_back(fw(xmin + k*delta + delta/2));
	}
	init(weights.begin(), weights.end());
	}
	/**
	* Constructs a discrete_distribution from its parameters.
	*/
	explicit discrete_distribution(const param_type& parm)
	{
	param(parm);
	}

	/**
	* Returns a value distributed according to the parameters of the
	* discrete_distribution.
	*/
	template<class URNG>
	IntType operator()(URNG& urng) const
	{
	BOOST_ASSERT(!_alias_table.empty());
	WeightType test = uniform_01<WeightType>()(urng);
	IntType result = uniform_int<IntType>((min)(), (max)())(urng);
	if(test < _alias_table[result].first) {
	return result;
	} else {
	return(_alias_table[result].second);
	}
	}

	/**
	* Returns a value distributed according to the parameters
	* specified by param.
	*/
	template<class URNG>
	IntType operator()(URNG& urng, const param_type& parm) const
	{
	while(true) {
	WeightType val = uniform_01<WeightType>()(urng);
	WeightType sum = 0;
	std::size_t result = 0;
	for(typename std::vector<WeightType>::const_iterator
	iter = parm._probabilities.begin(),
	end = parm._probabilities.end();
	iter != end; ++iter, ++result)
	{
	sum += *iter;
	if(sum > val) {
	return result;
	}
	}
	}
	}

	/** Returns the smallest value that the distribution can produce. */
	result_type min BOOST_PREVENT_MACRO_SUBSTITUTION () const { return 0; }
	/** Returns the largest value that the distribution can produce. */
	result_type max BOOST_PREVENT_MACRO_SUBSTITUTION () const
	{ return static_cast<result_type>(_alias_table.size() - 1); }

	/**
	* Returns a vector containing the probabilities of each
	* value of the distribution. For example, given
	*
	* @code
	* discrete_distribution<> dist = { 1, 4, 5 };
	* std::vector<double> p = dist.param();
	* @endcode
	*
	* the vector, p will contain {0.1, 0.4, 0.5}.
	*/
	std::vector<WeightType> probabilities() const
	{
	std::vector<WeightType> result(_alias_table.size());
	const WeightType mean =
	static_cast<WeightType>(1) / _alias_table.size();
	std::size_t i = 0;
	for(typename alias_table_t::const_iterator
	iter = _alias_table.begin(),
	end = _alias_table.end();
	iter != end; ++iter, ++i)
	{
	WeightType val = iter->first * mean;
	result[i] += val;
	result[iter->second] += mean - val;
	}
	return(result);
	}

	/** Returns the parameters of the distribution. */
	param_type param() const
	{
	return param_type(*this);
	}
	/** Sets the parameters of the distribution. */
	void param(const param_type& parm)
	{
	init(parm._probabilities.begin(), parm._probabilities.end());
	}

	/**
	* Effects: Subsequent uses of the distribution do not depend
	* on values produced by any engine prior to invoking reset.
	*/
	void reset() {}

	/** Writes a distribution to a @c std::ostream. */
	BOOST_RANDOM_DETAIL_OSTREAM_OPERATOR(os, discrete_distribution, dd)
	{
	os << dd.param();
	return os;
	}

	/** Reads a distribution from a @c std::istream */
	BOOST_RANDOM_DETAIL_ISTREAM_OPERATOR(is, discrete_distribution, dd)
	{
	param_type parm;
	if(is >> parm) {
	dd.param(parm);
	}
	return is;
	}

	/**
	* Returns true if the two distributions will return the
	* same sequence of values, when passed equal generators.
	*/
	BOOST_RANDOM_DETAIL_EQUALITY_OPERATOR(discrete_distribution, lhs, rhs)
	{
	return lhs._alias_table == rhs._alias_table;
	}
	/**
	* Returns true if the two distributions may return different
	* sequences of values, when passed equal generators.
	*/
	BOOST_RANDOM_DETAIL_INEQUALITY_OPERATOR(discrete_distribution)

	private:

	/// @cond show_private

	template<class Iter>
	void init(Iter first, Iter last, std::input_iterator_tag)
	{
	std::vector<WeightType> temp(first, last);
	init(temp.begin(), temp.end());
	}
	template<class Iter>
	void init(Iter first, Iter last, std::forward_iterator_tag)
	{
	std::vector<std::pair<WeightType, IntType> > below_average;
	std::vector<std::pair<WeightType, IntType> > above_average;
	std::size_t size = std::distance(first, last);
	WeightType weight_sum =
	std::accumulate(first, last, static_cast<WeightType>(0));
	WeightType weight_average = weight_sum / size;
	std::size_t i = 0;
	for(; first != last; ++first, ++i) {
	WeightType val = *first / weight_average;
	std::pair<WeightType, IntType> elem(val, static_cast<IntType>(i));
	if(val < static_cast<WeightType>(1)) {
	below_average.push_back(elem);
	} else {
	above_average.push_back(elem);
	}
	}

	_alias_table.resize(size);
	typename alias_table_t::iterator
	b_iter = below_average.begin(),
	b_end = below_average.end(),
	a_iter = above_average.begin(),
	a_end = above_average.end()
	;
	while(b_iter != b_end && a_iter != a_end) {
	_alias_table[b_iter->second] =
	std::make_pair(b_iter->first, a_iter->second);
	a_iter->first -= (static_cast<WeightType>(1) - b_iter->first);
	if(a_iter->first < static_cast<WeightType>(1)) {
	b_iter = a_iter++;
	} else {
	++b_iter;
	}
	}
	for(; b_iter != b_end; ++b_iter) {
	_alias_table[b_iter->second].first = static_cast<WeightType>(1);
	}
	for(; a_iter != a_end; ++a_iter) {
	_alias_table[a_iter->second].first = static_cast<WeightType>(1);
	}
	}
	template<class Iter>
	void init(Iter first, Iter last)
	{
	if(first == last) {
	_alias_table.clear();
	_alias_table.push_back(std::make_pair(static_cast<WeightType>(1),
	static_cast<IntType>(0)));
	} else {
	typename std::iterator_traits<Iter>::iterator_category category;
	init(first, last, category);
	}
	}
	typedef std::vector<std::pair<WeightType, IntType> > alias_table_t;
	alias_table_t _alias_table;
	/// @endcond
	};

	}
	}

	#include <boost/random/detail/enable_warnings.hpp>

	#endif