uniform_int_dist.h

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// Class template uniform_int_distribution -*- C++ -*-

// Copyright (C) 2009-2016 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/**
 * @file bits/uniform_int_dist.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{random}
 */

#ifndef _GLIBCXX_BITS_UNIFORM_INT_DIST_H
#define _GLIBCXX_BITS_UNIFORM_INT_DIST_H

#include <type_traits>
#include <limits>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  namespace __detail
  {
    /* Determine whether number is a power of 2.  */
    template<typename _Tp>
      inline bool
      _Power_of_2(_Tp __x)
      {
        return ((__x - 1) & __x) == 0;
      };
  }

  /**
   * @brief Uniform discrete distribution for random numbers.
   *
   * A discrete random distribution on the range @f$[min, max]@f$ with equal
   * probability throughout the range.
   *
   * @ingroup random_distributions_uniform
   */
  template<typename _IntType = int>
    class uniform_int_distribution
    {
      static_assert(std::is_integral<_IntType>::value,
                    "template argument not an integral type");

    public:
      /** The type of the range of the distribution. */
      typedef _IntType result_type;
      /** Parameter type. */
      struct param_type
      {
        typedef uniform_int_distribution<_IntType> distribution_type;

        explicit
        param_type(_IntType __a = 0,
                   _IntType __b = std::numeric_limits<_IntType>::max())
        : _M_a(__a), _M_b(__b)
        {
          _GLIBCXX_DEBUG_ASSERT(_M_a <= _M_b);
        }

        result_type
        a() const
        { return _M_a; }

        result_type
        b() const
        { return _M_b; }

        friend bool
        operator==(const param_type& __p1, const param_type& __p2)
        { return __p1._M_a == __p2._M_a && __p1._M_b == __p2._M_b; }

      private:
        _IntType _M_a;
        _IntType _M_b;
      };

    public:
      /**
       * @brief Constructs a uniform distribution object.
       */
      explicit
      uniform_int_distribution(_IntType __a = 0,
                           _IntType __b = std::numeric_limits<_IntType>::max())
      : _M_param(__a, __b)
      { }

      explicit
      uniform_int_distribution(const param_type& __p)
      : _M_param(__p)
      { }

      /**
       * @brief Resets the distribution state.
       *
       * Does nothing for the uniform integer distribution.
       */
      void
      reset() { }

      result_type
      a() const
      { return _M_param.a(); }

      result_type
      b() const
      { return _M_param.b(); }

      /**
       * @brief Returns the parameter set of the distribution.
       */
      param_type
      param() const
      { return _M_param; }

      /**
       * @brief Sets the parameter set of the distribution.
       * @param __param The new parameter set of the distribution.
       */
      void
      param(const param_type& __param)
      { _M_param = __param; }

      /**
       * @brief Returns the inclusive lower bound of the distribution range.
       */
      result_type
      min() const
      { return this->a(); }

      /**
       * @brief Returns the inclusive upper bound of the distribution range.
       */
      result_type
      max() const
      { return this->b(); }

      /**
       * @brief Generating functions.
       */
      template<typename _UniformRandomNumberGenerator>
        result_type
        operator()(_UniformRandomNumberGenerator& __urng)
        { return this->operator()(__urng, _M_param); }

      template<typename _UniformRandomNumberGenerator>
        result_type
        operator()(_UniformRandomNumberGenerator& __urng,
                   const param_type& __p);

      template<typename _ForwardIterator,
               typename _UniformRandomNumberGenerator>
        void
        __generate(_ForwardIterator __f, _ForwardIterator __t,
                   _UniformRandomNumberGenerator& __urng)
        { this->__generate(__f, __t, __urng, _M_param); }

      template<typename _ForwardIterator,
               typename _UniformRandomNumberGenerator>
        void
        __generate(_ForwardIterator __f, _ForwardIterator __t,
                   _UniformRandomNumberGenerator& __urng,
                   const param_type& __p)
        { this->__generate_impl(__f, __t, __urng, __p); }

      template<typename _UniformRandomNumberGenerator>
        void
        __generate(result_type* __f, result_type* __t,
                   _UniformRandomNumberGenerator& __urng,
                   const param_type& __p)
        { this->__generate_impl(__f, __t, __urng, __p); }

      /**
       * @brief Return true if two uniform integer distributions have
       *        the same parameters.
       */
      friend bool
      operator==(const uniform_int_distribution& __d1,
                 const uniform_int_distribution& __d2)
      { return __d1._M_param == __d2._M_param; }

    private:
      template<typename _ForwardIterator,
               typename _UniformRandomNumberGenerator>
        void
        __generate_impl(_ForwardIterator __f, _ForwardIterator __t,
                        _UniformRandomNumberGenerator& __urng,
                        const param_type& __p);

      param_type _M_param;
    };

  template<typename _IntType>
    template<typename _UniformRandomNumberGenerator>
      typename uniform_int_distribution<_IntType>::result_type
      uniform_int_distribution<_IntType>::
      operator()(_UniformRandomNumberGenerator& __urng,
                 const param_type& __param)
      {
        typedef typename _UniformRandomNumberGenerator::result_type
          _Gresult_type;
        typedef typename std::make_unsigned<result_type>::type __utype;
        typedef typename std::common_type<_Gresult_type, __utype>::type
          __uctype;

        const __uctype __urngmin = __urng.min();
        const __uctype __urngmax = __urng.max();
        const __uctype __urngrange = __urngmax - __urngmin;
        const __uctype __urange
          = __uctype(__param.b()) - __uctype(__param.a());

        __uctype __ret;

        if (__urngrange > __urange)
          {
            // downscaling
            const __uctype __uerange = __urange + 1; // __urange can be zero
            const __uctype __scaling = __urngrange / __uerange;
            const __uctype __past = __uerange * __scaling;
            do
              __ret = __uctype(__urng()) - __urngmin;
            while (__ret >= __past);
            __ret /= __scaling;
          }
        else if (__urngrange < __urange)
          {
            // upscaling
            /*
              Note that every value in [0, urange]
              can be written uniquely as

              (urngrange + 1) * high + low

              where

              high in [0, urange / (urngrange + 1)]

              and

              low in [0, urngrange].
            */
            __uctype __tmp; // wraparound control
            do
              {
                const __uctype __uerngrange = __urngrange + 1;
                __tmp = (__uerngrange * operator()
                         (__urng, param_type(0, __urange / __uerngrange)));
                __ret = __tmp + (__uctype(__urng()) - __urngmin);
              }
            while (__ret > __urange || __ret < __tmp);
          }
        else
          __ret = __uctype(__urng()) - __urngmin;

        return __ret + __param.a();
      }


  template<typename _IntType>
    template<typename _ForwardIterator,
             typename _UniformRandomNumberGenerator>
      void
      uniform_int_distribution<_IntType>::
      __generate_impl(_ForwardIterator __f, _ForwardIterator __t,
                      _UniformRandomNumberGenerator& __urng,
                      const param_type& __param)
      {
        __glibcxx_function_requires(_ForwardIteratorConcept<_ForwardIterator>)
        typedef typename _UniformRandomNumberGenerator::result_type
          _Gresult_type;
        typedef typename std::make_unsigned<result_type>::type __utype;
        typedef typename std::common_type<_Gresult_type, __utype>::type
          __uctype;

        const __uctype __urngmin = __urng.min();
        const __uctype __urngmax = __urng.max();
        const __uctype __urngrange = __urngmax - __urngmin;
        const __uctype __urange
          = __uctype(__param.b()) - __uctype(__param.a());

        __uctype __ret;

        if (__urngrange > __urange)
          {
            if (__detail::_Power_of_2(__urngrange + 1)
                && __detail::_Power_of_2(__urange + 1))
              {
                while (__f != __t)
                  {
                    __ret = __uctype(__urng()) - __urngmin;
                    *__f++ = (__ret & __urange) + __param.a();
                  }
              }
            else
              {
                // downscaling
                const __uctype __uerange = __urange + 1; // __urange can be zero
                const __uctype __scaling = __urngrange / __uerange;
                const __uctype __past = __uerange * __scaling;
                while (__f != __t)
                  {
                    do
                      __ret = __uctype(__urng()) - __urngmin;
                    while (__ret >= __past);
                    *__f++ = __ret / __scaling + __param.a();
                  }
              }
          }
        else if (__urngrange < __urange)
          {
            // upscaling
            /*
              Note that every value in [0, urange]
              can be written uniquely as

              (urngrange + 1) * high + low

              where

              high in [0, urange / (urngrange + 1)]

              and

              low in [0, urngrange].
            */
            __uctype __tmp; // wraparound control
            while (__f != __t)
              {
                do
                  {
                    const __uctype __uerngrange = __urngrange + 1;
                    __tmp = (__uerngrange * operator()
                             (__urng, param_type(0, __urange / __uerngrange)));
                    __ret = __tmp + (__uctype(__urng()) - __urngmin);
                  }
                while (__ret > __urange || __ret < __tmp);
                *__f++ = __ret;
              }
          }
        else
          while (__f != __t)
            *__f++ = __uctype(__urng()) - __urngmin + __param.a();
      }

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif