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uniform_int_dist.h

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

// Copyright (C) 2009-2018 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.
   */
  template<typename _IntType = int>
    class uniform_int_distribution
    {
      static_assert(std::is_integral<_IntType>::value,
		    "template argument must be 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_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; }

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

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;

__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();
      }

// operator!= and operator<< and operator>> are defined in <bits/random.h>

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif

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