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[EDIT] FILE: parse_numbers.h

// Components for compile-time parsing of numbers -*- C++ -*-

// Copyright (C) 2013-2018 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/parse_numbers.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{chrono}
 */

#ifndef _GLIBCXX_PARSE_NUMBERS_H
#define _GLIBCXX_PARSE_NUMBERS_H 1

#pragma GCC system_header

// From n3642.pdf except I added binary literals and digit separator '\''.

#if __cplusplus > 201103L

#include <limits>

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

namespace __parse_int
{
  template<unsigned _Base, char _Dig>
    struct _Digit;

template<unsigned _Base>
    struct _Digit<_Base, '0'> : integral_constant<unsigned, 0>
    {
      using __valid = true_type;
    };

template<unsigned _Base>
    struct _Digit<_Base, '1'> : integral_constant<unsigned, 1>
    {
      using __valid = true_type;
    };

template<unsigned _Base, unsigned _Val>
    struct _Digit_impl : integral_constant<unsigned, _Val>
    {
      static_assert(_Base > _Val, "invalid digit");
      using __valid = true_type;
    };

template<unsigned _Base>
    struct _Digit<_Base, '2'> : _Digit_impl<_Base, 2>
    { };

template<unsigned _Base>
    struct _Digit<_Base, '3'> : _Digit_impl<_Base, 3>
    { };

template<unsigned _Base>
    struct _Digit<_Base, '4'> : _Digit_impl<_Base, 4>
    { };

template<unsigned _Base>
    struct _Digit<_Base, '5'> : _Digit_impl<_Base, 5>
    { };

template<unsigned _Base>
    struct _Digit<_Base, '6'> : _Digit_impl<_Base, 6>
    { };

template<unsigned _Base>
    struct _Digit<_Base, '7'> : _Digit_impl<_Base, 7>
    { };

template<unsigned _Base>
    struct _Digit<_Base, '8'> : _Digit_impl<_Base, 8>
    { };

template<unsigned _Base>
    struct _Digit<_Base, '9'> : _Digit_impl<_Base, 9>
    { };

template<unsigned _Base>
    struct _Digit<_Base, 'a'> : _Digit_impl<_Base, 0xa>
    { };

template<unsigned _Base>
    struct _Digit<_Base, 'A'> : _Digit_impl<_Base, 0xa>
    { };

template<unsigned _Base>
    struct _Digit<_Base, 'b'> : _Digit_impl<_Base, 0xb>
    { };

template<unsigned _Base>
    struct _Digit<_Base, 'B'> : _Digit_impl<_Base, 0xb>
    { };

template<unsigned _Base>
    struct _Digit<_Base, 'c'> : _Digit_impl<_Base, 0xc>
    { };

template<unsigned _Base>
    struct _Digit<_Base, 'C'> : _Digit_impl<_Base, 0xc>
    { };

template<unsigned _Base>
    struct _Digit<_Base, 'd'> : _Digit_impl<_Base, 0xd>
    { };

template<unsigned _Base>
    struct _Digit<_Base, 'D'> : _Digit_impl<_Base, 0xd>
    { };

template<unsigned _Base>
    struct _Digit<_Base, 'e'> : _Digit_impl<_Base, 0xe>
    { };

template<unsigned _Base>
    struct _Digit<_Base, 'E'> : _Digit_impl<_Base, 0xe>
    { };

template<unsigned _Base>
    struct _Digit<_Base, 'f'> : _Digit_impl<_Base, 0xf>
    { };

template<unsigned _Base>
    struct _Digit<_Base, 'F'> : _Digit_impl<_Base, 0xf>
    { };

//  Digit separator
  template<unsigned _Base>
    struct _Digit<_Base, '\''> : integral_constant<unsigned, 0>
    {
      using __valid = false_type;
    };

//------------------------------------------------------------------------------

template<unsigned long long _Val>
    using __ull_constant = integral_constant<unsigned long long, _Val>;

template<unsigned _Base, char _Dig, char... _Digs>
    struct _Power_help
    {
      using __next = typename _Power_help<_Base, _Digs...>::type;
      using __valid_digit = typename _Digit<_Base, _Dig>::__valid;
      using type
	= __ull_constant<__next::value * (__valid_digit{} ? _Base : 1ULL)>;
    };

template<unsigned _Base, char _Dig>
    struct _Power_help<_Base, _Dig>
    {
      using __valid_digit = typename _Digit<_Base, _Dig>::__valid;
      using type = __ull_constant<__valid_digit::value>;
    };

template<unsigned _Base, char... _Digs>
    struct _Power : _Power_help<_Base, _Digs...>::type
    { };

template<unsigned _Base>
    struct _Power<_Base> : __ull_constant<0>
    { };

//------------------------------------------------------------------------------

template<unsigned _Base, unsigned long long _Pow, char _Dig, char... _Digs>
    struct _Number_help
    {
      using __digit = _Digit<_Base, _Dig>;
      using __valid_digit = typename __digit::__valid;
      using __next = _Number_help<_Base,
				  __valid_digit::value ? _Pow / _Base : _Pow,
				  _Digs...>;
      using type = __ull_constant<_Pow * __digit::value + __next::type::value>;
      static_assert((type::value / _Pow) == __digit::value,
		    "integer literal does not fit in unsigned long long");
    };

// Skip past digit separators:
  template<unsigned _Base, unsigned long long _Pow, char _Dig, char..._Digs>
    struct _Number_help<_Base, _Pow, '\'', _Dig, _Digs...>
    : _Number_help<_Base, _Pow, _Dig, _Digs...>
    { };

// Terminating case for recursion:
  template<unsigned _Base, char _Dig>
    struct _Number_help<_Base, 1ULL, _Dig>
    {
      using type = __ull_constant<_Digit<_Base, _Dig>::value>;
    };

template<unsigned _Base, char... _Digs>
    struct _Number
    : _Number_help<_Base, _Power<_Base, _Digs...>::value, _Digs...>::type
    { };

template<unsigned _Base>
    struct _Number<_Base>
    : __ull_constant<0>
    { };

//------------------------------------------------------------------------------

template<char... _Digs>
    struct _Parse_int;

template<char... _Digs>
    struct _Parse_int<'0', 'b', _Digs...>
    : _Number<2U, _Digs...>::type
    { };

template<char... _Digs>
    struct _Parse_int<'0', 'B', _Digs...>
    : _Number<2U, _Digs...>::type
    { };

template<char... _Digs>
    struct _Parse_int<'0', 'x', _Digs...>
    : _Number<16U, _Digs...>::type
    { };

template<char... _Digs>
    struct _Parse_int<'0', 'X', _Digs...>
    : _Number<16U, _Digs...>::type
    { };

template<char... _Digs>
    struct _Parse_int<'0', _Digs...>
    : _Number<8U, _Digs...>::type
    { };

template<char... _Digs>
    struct _Parse_int
    : _Number<10U, _Digs...>::type
    { };

} // namespace __parse_int

namespace __select_int
{
  template<unsigned long long _Val, typename... _Ints>
    struct _Select_int_base;

template<unsigned long long _Val, typename _IntType, typename... _Ints>
    struct _Select_int_base<_Val, _IntType, _Ints...>
    : conditional_t<(_Val <= std::numeric_limits<_IntType>::max()),
		    integral_constant<_IntType, _Val>,
		    _Select_int_base<_Val, _Ints...>>
    { };

template<unsigned long long _Val>
    struct _Select_int_base<_Val>
    { };

template<char... _Digs>
    using _Select_int = typename _Select_int_base<
	__parse_int::_Parse_int<_Digs...>::value,
	unsigned char,
	unsigned short,
	unsigned int,
	unsigned long,
	unsigned long long
      >::type;

} // namespace __select_int

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // __cplusplus > 201103L

#endif // _GLIBCXX_PARSE_NUMBERS_H

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