boost/random/discrete_distribution.hpp
/* 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 81851 2012-12-11 14:42:26Z marshall $ */ #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_CXX11_HDR_INITIALIZER_LIST #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_CXX11_HDR_INITIALIZER_LIST /** * 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_CXX11_HDR_INITIALIZER_LIST /** * 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