boost/random/uniform_smallint.hpp
/* boost random/uniform_smallint.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: uniform_smallint.hpp 71018 2011-04-05 21:27:52Z steven_watanabe $ * * Revision history * 2001-04-08 added min<max assertion (N. Becker) * 2001-02-18 moved to individual header files */ #ifndef BOOST_RANDOM_UNIFORM_SMALLINT_HPP #define BOOST_RANDOM_UNIFORM_SMALLINT_HPP #include <istream> #include <iosfwd> #include <boost/assert.hpp> #include <boost/config.hpp> #include <boost/limits.hpp> #include <boost/type_traits/is_integral.hpp> #include <boost/random/detail/config.hpp> #include <boost/random/detail/operators.hpp> #include <boost/random/detail/signed_unsigned_tools.hpp> #include <boost/random/uniform_01.hpp> #include <boost/detail/workaround.hpp> namespace boost { namespace random { // uniform integer distribution on a small range [min, max] /** * The distribution function uniform_smallint models a \random_distribution. * On each invocation, it returns a random integer value uniformly distributed * in the set of integer numbers {min, min+1, min+2, ..., max}. It assumes * that the desired range (max-min+1) is small compared to the range of the * underlying source of random numbers and thus makes no attempt to limit * quantization errors. * * Let \f$r_{\mathtt{out}} = (\mbox{max}-\mbox{min}+1)\f$ the desired range of * integer numbers, and * let \f$r_{\mathtt{base}}\f$ be the range of the underlying source of random * numbers. Then, for the uniform distribution, the theoretical probability * for any number i in the range \f$r_{\mathtt{out}}\f$ will be * \f$\displaystyle p_{\mathtt{out}}(i) = \frac{1}{r_{\mathtt{out}}}\f$. * Likewise, assume a uniform distribution on \f$r_{\mathtt{base}}\f$ for * the underlying source of random numbers, i.e. * \f$\displaystyle p_{\mathtt{base}}(i) = \frac{1}{r_{\mathtt{base}}}\f$. * Let \f$p_{\mathtt{out\_s}}(i)\f$ denote the random * distribution generated by @c uniform_smallint. Then the sum over all * i in \f$r_{\mathtt{out}}\f$ of * \f$\displaystyle * \left(\frac{p_{\mathtt{out\_s}}(i)}{p_{\mathtt{out}}(i)} - 1\right)^2\f$ * shall not exceed * \f$\displaystyle \frac{r_{\mathtt{out}}}{r_{\mathtt{base}}^2} * (r_{\mathtt{base}} \mbox{ mod } r_{\mathtt{out}}) * (r_{\mathtt{out}} - r_{\mathtt{base}} \mbox{ mod } r_{\mathtt{out}})\f$. * * The template parameter IntType shall denote an integer-like value type. * * @xmlnote * The property above is the square sum of the relative differences * in probabilities between the desired uniform distribution * \f$p_{\mathtt{out}}(i)\f$ and the generated distribution * \f$p_{\mathtt{out\_s}}(i)\f$. * The property can be fulfilled with the calculation * \f$(\mbox{base\_rng} \mbox{ mod } r_{\mathtt{out}})\f$, as follows: * Let \f$r = r_{\mathtt{base}} \mbox{ mod } r_{\mathtt{out}}\f$. * The base distribution on \f$r_{\mathtt{base}}\f$ is folded onto the * range \f$r_{\mathtt{out}}\f$. The numbers i < r have assigned * \f$\displaystyle * \left\lfloor\frac{r_{\mathtt{base}}}{r_{\mathtt{out}}}\right\rfloor+1\f$ * numbers of the base distribution, the rest has only \f$\displaystyle * \left\lfloor\frac{r_{\mathtt{base}}}{r_{\mathtt{out}}}\right\rfloor\f$. * Therefore, * \f$\displaystyle p_{\mathtt{out\_s}}(i) = * \left(\left\lfloor\frac{r_{\mathtt{base}}} * {r_{\mathtt{out}}}\right\rfloor+1\right) / * r_{\mathtt{base}}\f$ for i < r and \f$\displaystyle p_{\mathtt{out\_s}}(i) = * \left\lfloor\frac{r_{\mathtt{base}}} * {r_{\mathtt{out}}}\right\rfloor/r_{\mathtt{base}}\f$ otherwise. * Substituting this in the * above sum formula leads to the desired result. * @endxmlnote * * Note: The upper bound for * \f$(r_{\mathtt{base}} \mbox{ mod } r_{\mathtt{out}}) * (r_{\mathtt{out}} - r_{\mathtt{base}} \mbox{ mod } r_{\mathtt{out}})\f$ is * \f$\displaystyle \frac{r_{\mathtt{out}}^2}{4}\f$. Regarding the upper bound * for the square sum of the relative quantization error of * \f$\displaystyle \frac{r_\mathtt{out}^3}{4r_{\mathtt{base}}^2}\f$, it * seems wise to either choose \f$r_{\mathtt{base}}\f$ so that * \f$r_{\mathtt{base}} > 10r_{\mathtt{out}}^2\f$ or ensure that * \f$r_{\mathtt{base}}\f$ is * divisible by \f$r_{\mathtt{out}}\f$. */ template<class IntType = int> class uniform_smallint { public: typedef IntType input_type; typedef IntType result_type; class param_type { public: typedef uniform_smallint distribution_type; /** constructs the parameters of a @c uniform_smallint distribution. */ param_type(IntType min_arg = 0, IntType max_arg = 9) : _min(min_arg), _max(max_arg) { BOOST_ASSERT(_min <= _max); } /** Returns the minimum value. */ IntType a() const { return _min; } /** Returns the maximum value. */ IntType b() const { return _max; } /** Writes the parameters to a @c std::ostream. */ BOOST_RANDOM_DETAIL_OSTREAM_OPERATOR(os, param_type, parm) { os << parm._min << " " << parm._max; return os; } /** Reads the parameters from a @c std::istream. */ BOOST_RANDOM_DETAIL_ISTREAM_OPERATOR(is, param_type, parm) { is >> parm._min >> std::ws >> parm._max; return is; } /** Returns true if the two sets of parameters are equal. */ BOOST_RANDOM_DETAIL_EQUALITY_OPERATOR(param_type, lhs, rhs) { return lhs._min == rhs._min && lhs._max == rhs._max; } /** Returns true if the two sets of parameters are different. */ BOOST_RANDOM_DETAIL_INEQUALITY_OPERATOR(param_type) private: IntType _min; IntType _max; }; /** * Constructs a @c uniform_smallint. @c min and @c max are the * lower and upper bounds of the output range, respectively. */ explicit uniform_smallint(IntType min_arg = 0, IntType max_arg = 9) : _min(min_arg), _max(max_arg) {} /** * Constructs a @c uniform_smallint from its parameters. */ explicit uniform_smallint(const param_type& parm) : _min(parm.a()), _max(parm.b()) {} /** Returns the minimum value of the distribution. */ result_type a() const { return _min; } /** Returns the maximum value of the distribution. */ result_type b() const { return _max; } /** Returns the minimum value of the distribution. */ result_type min BOOST_PREVENT_MACRO_SUBSTITUTION () const { return _min; } /** Returns the maximum value of the distribution. */ result_type max BOOST_PREVENT_MACRO_SUBSTITUTION () const { return _max; } /** Returns the parameters of the distribution. */ param_type param() const { return param_type(_min, _max); } /** Sets the parameters of the distribution. */ void param(const param_type& parm) { _min = parm.a(); _max = parm.b(); } /** * Effects: Subsequent uses of the distribution do not depend * on values produced by any engine prior to invoking reset. */ void reset() { } /** Returns a value uniformly distributed in the range [min(), max()]. */ template<class Engine> result_type operator()(Engine& eng) const { typedef typename Engine::result_type base_result; return generate(eng, boost::is_integral<base_result>()); } /** Returns a value uniformly distributed in the range [param.a(), param.b()]. */ template<class Engine> result_type operator()(Engine& eng, const param_type& parm) const { return uniform_smallint(parm)(eng); } /** Writes the distribution to a @c std::ostream. */ BOOST_RANDOM_DETAIL_OSTREAM_OPERATOR(os, uniform_smallint, ud) { os << ud._min << " " << ud._max; return os; } /** Reads the distribution from a @c std::istream. */ BOOST_RANDOM_DETAIL_ISTREAM_OPERATOR(is, uniform_smallint, ud) { is >> ud._min >> std::ws >> ud._max; return is; } /** * Returns true if the two distributions will produce identical * sequences of values given equal generators. */ BOOST_RANDOM_DETAIL_EQUALITY_OPERATOR(uniform_smallint, lhs, rhs) { return lhs._min == rhs._min && lhs._max == rhs._max; } /** * Returns true if the two distributions may produce different * sequences of values given equal generators. */ BOOST_RANDOM_DETAIL_INEQUALITY_OPERATOR(uniform_smallint) private: // \cond show_private template<class Engine> result_type generate(Engine& eng, boost::mpl::true_) const { // equivalent to (eng() - eng.min()) % (_max - _min + 1) + _min, // but guarantees no overflow. typedef typename Engine::result_type base_result; typedef typename boost::make_unsigned<base_result>::type base_unsigned; typedef typename boost::make_unsigned<result_type>::type range_type; range_type range = random::detail::subtract<result_type>()(_max, _min); base_unsigned base_range = random::detail::subtract<result_type>()((eng.max)(), (eng.min)()); base_unsigned val = random::detail::subtract<base_result>()(eng(), (eng.min)()); if(range >= base_range) { return boost::random::detail::add<range_type, result_type>()( static_cast<range_type>(val), _min); } else { base_unsigned modulus = static_cast<base_unsigned>(range) + 1; return boost::random::detail::add<range_type, result_type>()( static_cast<range_type>(val % modulus), _min); } } template<class Engine> result_type generate(Engine& eng, boost::mpl::false_) const { typedef typename Engine::result_type base_result; typedef typename boost::make_unsigned<result_type>::type range_type; range_type range = random::detail::subtract<result_type>()(_max, _min); base_result val = boost::uniform_01<base_result>()(eng); // what is the worst that can possibly happen here? // base_result may not be able to represent all the values in [0, range] // exactly. If this happens, it will cause round off error and we // won't be able to produce all the values in the range. We don't // care about this because the user has already told us not to by // using uniform_smallint. However, we do need to be careful // to clamp the result, or floating point rounding can produce // an out of range result. range_type offset = static_cast<range_type>(val * (static_cast<base_result>(range) + 1)); if(offset > range) return _max; return boost::random::detail::add<range_type, result_type>()(offset , _min); } // \endcond result_type _min; result_type _max; }; } // namespace random using random::uniform_smallint; } // namespace boost #endif // BOOST_RANDOM_UNIFORM_SMALLINT_HPP