boost/random/uniform_int_distribution.hpp
/* boost random/uniform_int_distribution.hpp header file * * Copyright Jens Maurer 2000-2001 * Copyright Steven Watanabe 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: uniform_int_distribution.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_INT_DISTRIBUTION_HPP #define BOOST_RANDOM_UNIFORM_INT_DISTRIBUTION_HPP #include <iosfwd> #include <ios> #include <istream> #include <boost/config.hpp> #include <boost/limits.hpp> #include <boost/assert.hpp> #include <boost/random/detail/config.hpp> #include <boost/random/detail/operators.hpp> #include <boost/random/detail/uniform_int_float.hpp> #include <boost/random/detail/signed_unsigned_tools.hpp> #include <boost/type_traits/make_unsigned.hpp> #include <boost/type_traits/is_integral.hpp> namespace boost { namespace random { namespace detail { #ifdef BOOST_MSVC #pragma warning(push) // disable division by zero warning, since we can't // actually divide by zero. #pragma warning(disable:4723) #endif template<class Engine, class T> T generate_uniform_int( Engine& eng, T min_value, T max_value, boost::mpl::true_ /** is_integral<Engine::result_type> */) { typedef T result_type; typedef typename make_unsigned<T>::type range_type; typedef typename Engine::result_type base_result; // ranges are always unsigned typedef typename make_unsigned<base_result>::type base_unsigned; const range_type range = random::detail::subtract<result_type>()(max_value, min_value); const base_result bmin = (eng.min)(); const base_unsigned brange = random::detail::subtract<base_result>()((eng.max)(), (eng.min)()); if(range == 0) { return min_value; } else if(brange == range) { // this will probably never happen in real life // basically nothing to do; just take care we don't overflow / underflow base_unsigned v = random::detail::subtract<base_result>()(eng(), bmin); return random::detail::add<base_unsigned, result_type>()(v, min_value); } else if(brange < range) { // use rejection method to handle things like 0..3 --> 0..4 for(;;) { // concatenate several invocations of the base RNG // take extra care to avoid overflows // limit == floor((range+1)/(brange+1)) // Therefore limit*(brange+1) <= range+1 range_type limit; if(range == (std::numeric_limits<range_type>::max)()) { limit = range/(range_type(brange)+1); if(range % (range_type(brange)+1) == range_type(brange)) ++limit; } else { limit = (range+1)/(range_type(brange)+1); } // We consider "result" as expressed to base (brange+1): // For every power of (brange+1), we determine a random factor range_type result = range_type(0); range_type mult = range_type(1); // loop invariants: // result < mult // mult <= range while(mult <= limit) { // Postcondition: result <= range, thus no overflow // // limit*(brange+1)<=range+1 def. of limit (1) // eng()-bmin<=brange eng() post. (2) // and mult<=limit. loop condition (3) // Therefore mult*(eng()-bmin+1)<=range+1 by (1),(2),(3) (4) // Therefore mult*(eng()-bmin)+mult<=range+1 rearranging (4) (5) // result<mult loop invariant (6) // Therefore result+mult*(eng()-bmin)<range+1 by (5), (6) (7) // // Postcondition: result < mult*(brange+1) // // result<mult loop invariant (1) // eng()-bmin<=brange eng() post. (2) // Therefore result+mult*(eng()-bmin) < // mult+mult*(eng()-bmin) by (1) (3) // Therefore result+(eng()-bmin)*mult < // mult+mult*brange by (2), (3) (4) // Therefore result+(eng()-bmin)*mult < // mult*(brange+1) by (4) result += static_cast<range_type>(random::detail::subtract<base_result>()(eng(), bmin) * mult); // equivalent to (mult * (brange+1)) == range+1, but avoids overflow. if(mult * range_type(brange) == range - mult + 1) { // The destination range is an integer power of // the generator's range. return(result); } // Postcondition: mult <= range // // limit*(brange+1)<=range+1 def. of limit (1) // mult<=limit loop condition (2) // Therefore mult*(brange+1)<=range+1 by (1), (2) (3) // mult*(brange+1)!=range+1 preceding if (4) // Therefore mult*(brange+1)<range+1 by (3), (4) (5) // // Postcondition: result < mult // // See the second postcondition on the change to result. mult *= range_type(brange)+range_type(1); } // loop postcondition: range/mult < brange+1 // // mult > limit loop condition (1) // Suppose range/mult >= brange+1 Assumption (2) // range >= mult*(brange+1) by (2) (3) // range+1 > mult*(brange+1) by (3) (4) // range+1 > (limit+1)*(brange+1) by (1), (4) (5) // (range+1)/(brange+1) > limit+1 by (5) (6) // limit < floor((range+1)/(brange+1)) by (6) (7) // limit==floor((range+1)/(brange+1)) def. of limit (8) // not (2) reductio (9) // // loop postcondition: (range/mult)*mult+(mult-1) >= range // // (range/mult)*mult + range%mult == range identity (1) // range%mult < mult def. of % (2) // (range/mult)*mult+mult > range by (1), (2) (3) // (range/mult)*mult+(mult-1) >= range by (3) (4) // // Note that the maximum value of result at this point is (mult-1), // so after this final step, we generate numbers that can be // at least as large as range. We have to really careful to avoid // overflow in this final addition and in the rejection. Anything // that overflows is larger than range and can thus be rejected. // range/mult < brange+1 -> no endless loop range_type result_increment = generate_uniform_int( eng, static_cast<range_type>(0), static_cast<range_type>(range/mult), boost::mpl::true_()); if((std::numeric_limits<range_type>::max)() / mult < result_increment) { // The multiplcation would overflow. Reject immediately. continue; } result_increment *= mult; // unsigned integers are guaranteed to wrap on overflow. result += result_increment; if(result < result_increment) { // The addition overflowed. Reject. continue; } if(result > range) { // Too big. Reject. continue; } return random::detail::add<range_type, result_type>()(result, min_value); } } else { // brange > range base_unsigned bucket_size; // it's safe to add 1 to range, as long as we cast it first, // because we know that it is less than brange. However, // we do need to be careful not to cause overflow by adding 1 // to brange. if(brange == (std::numeric_limits<base_unsigned>::max)()) { bucket_size = brange / (static_cast<base_unsigned>(range)+1); if(brange % (static_cast<base_unsigned>(range)+1) == static_cast<base_unsigned>(range)) { ++bucket_size; } } else { bucket_size = (brange+1) / (static_cast<base_unsigned>(range)+1); } for(;;) { base_unsigned result = random::detail::subtract<base_result>()(eng(), bmin); result /= bucket_size; // result and range are non-negative, and result is possibly larger // than range, so the cast is safe if(result <= static_cast<base_unsigned>(range)) return random::detail::add<base_unsigned, result_type>()(result, min_value); } } } #ifdef BOOST_MSVC #pragma warning(pop) #endif template<class Engine, class T> inline T generate_uniform_int( Engine& eng, T min_value, T max_value, boost::mpl::false_ /** is_integral<Engine::result_type> */) { uniform_int_float<Engine> wrapper(eng); return generate_uniform_int(wrapper, min_value, max_value, boost::mpl::true_()); } template<class Engine, class T> inline T generate_uniform_int(Engine& eng, T min_value, T max_value) { typedef typename Engine::result_type base_result; return generate_uniform_int(eng, min_value, max_value, boost::is_integral<base_result>()); } } /** * The class template uniform_int_distribution models a \random_distribution. * On each invocation, it returns a random integer value uniformly * distributed in the set of integers {min, min+1, min+2, ..., max}. * * The template parameter IntType shall denote an integer-like value type. */ template<class IntType = int> class uniform_int_distribution { public: typedef IntType input_type; typedef IntType result_type; class param_type { public: typedef uniform_int_distribution distribution_type; /** * Constructs the parameters of a uniform_int_distribution. * * Requires min <= max */ explicit param_type( IntType min_arg = 0, IntType max_arg = (std::numeric_limits<IntType>::max)()) : _min(min_arg), _max(max_arg) { BOOST_ASSERT(_min <= _max); } /** Returns the minimum value of the distribution. */ IntType a() const { return _min; } /** Returns the maximum value of the distribution. */ 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) { IntType min_in, max_in; if(is >> min_in >> std::ws >> max_in) { if(min_in <= max_in) { parm._min = min_in; parm._max = max_in; } else { is.setstate(std::ios_base::failbit); } } 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 uniform_int_distribution. @c min and @c max are * the parameters of the distribution. * * Requires: min <= max */ explicit uniform_int_distribution( IntType min_arg = 0, IntType max_arg = (std::numeric_limits<IntType>::max)()) : _min(min_arg), _max(max_arg) { BOOST_ASSERT(min_arg <= max_arg); } /** Constructs a uniform_int_distribution from its parameters. */ explicit uniform_int_distribution(const param_type& parm) : _min(parm.a()), _max(parm.b()) {} /** Returns the minimum value of the distribution */ IntType min BOOST_PREVENT_MACRO_SUBSTITUTION () const { return _min; } /** Returns the maximum value of the distribution */ IntType max BOOST_PREVENT_MACRO_SUBSTITUTION () const { return _max; } /** Returns the minimum value of the distribution */ IntType a() const { return _min; } /** Returns the maximum value of the distribution */ IntType b() 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 an integer uniformly distributed in the range [min, max]. */ template<class Engine> result_type operator()(Engine& eng) const { return detail::generate_uniform_int(eng, _min, _max); } /** * Returns an integer uniformly distributed in the range * [param.a(), param.b()]. */ template<class Engine> result_type operator()(Engine& eng, const param_type& parm) const { return detail::generate_uniform_int(eng, parm.a(), parm.b()); } /** Writes the distribution to a @c std::ostream. */ BOOST_RANDOM_DETAIL_OSTREAM_OPERATOR(os, uniform_int_distribution, ud) { os << ud.param(); return os; } /** Reads the distribution from a @c std::istream. */ BOOST_RANDOM_DETAIL_ISTREAM_OPERATOR(is, uniform_int_distribution, ud) { param_type parm; if(is >> parm) { ud.param(parm); } return is; } /** * Returns true if the two distributions will produce identical sequences * of values given equal generators. */ BOOST_RANDOM_DETAIL_EQUALITY_OPERATOR(uniform_int_distribution, 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_int_distribution) private: IntType _min; IntType _max; }; } // namespace random } // namespace boost #endif // BOOST_RANDOM_UNIFORM_INT_HPP