libstdc++-v3/include/bits/charconv.h

   1 // Numeric conversions (to_string, to_chars) -*- C++ -*-
   2
   3 // Copyright (C) 2017-2024 Free Software Foundation, Inc.
   4 //
   5 // This file is part of the GNU ISO C++ Library.  This library is free
   6 // software; you can redistribute it and/or modify it under the
   7 // terms of the GNU General Public License as published by the
   8 // Free Software Foundation; either version 3, or (at your option)
   9 // any later version.
  10
  11 // This library is distributed in the hope that it will be useful,
  12 // but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 // GNU General Public License for more details.
  15
  16 // Under Section 7 of GPL version 3, you are granted additional
  17 // permissions described in the GCC Runtime Library Exception, version
  18 // 3.1, as published by the Free Software Foundation.
  19
  20 // You should have received a copy of the GNU General Public License and
  21 // a copy of the GCC Runtime Library Exception along with this program;
  22 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
  23 // <http://www.gnu.org/licenses/>.
  24
  25 /** @file bits/charconv.h
  26  *  This is an internal header file, included by other library headers.
  27  *  Do not attempt to use it directly. @headername{charconv}
  28  */
  29
  30 #ifndef _GLIBCXX_CHARCONV_H
  31 #define _GLIBCXX_CHARCONV_H 1
  32
  33 #pragma GCC system_header
  34
  35 #if __cplusplus >= 201103L
  36
  37 #include <type_traits>
  38 #include <ext/numeric_traits.h>
  39
  40 namespace std _GLIBCXX_VISIBILITY(default)
  41 {
  42 _GLIBCXX_BEGIN_NAMESPACE_VERSION
  43 namespace __detail
  44 {
  45 #if __cpp_variable_templates
  46   // This accepts 128-bit integers even in strict mode.
  47   template<typename _Tp>
  48     constexpr bool __integer_to_chars_is_unsigned
  49       = ! __gnu_cxx::__int_traits<_Tp>::__is_signed;
  50 #endif
  51
  52   // Generic implementation for arbitrary bases.
  53   template<typename _Tp>
  54     _GLIBCXX14_CONSTEXPR unsigned
  55     __to_chars_len(_Tp __value, int __base = 10) noexcept
  56     {
  57 #if __cpp_variable_templates
  58       static_assert(__integer_to_chars_is_unsigned<_Tp>, "implementation bug");
  59 #endif
  60
  61       unsigned __n = 1;
  62       const unsigned __b2 = __base  * __base;
  63       const unsigned __b3 = __b2 * __base;
  64       const unsigned long __b4 = __b3 * __base;
  65       for (;;)
  66         {
  67           if (__value < (unsigned)__base) return __n;
  68           if (__value < __b2) return __n + 1;
  69           if (__value < __b3) return __n + 2;
  70           if (__value < __b4) return __n + 3;
  71           __value /= __b4;
  72           __n += 4;
  73         }
  74     }
  75
  76   // Write an unsigned integer value to the range [first,first+len).
  77   // The caller is required to provide a buffer of exactly the right size
  78   // (which can be determined by the __to_chars_len function).
  79   template<typename _Tp>
  80     _GLIBCXX23_CONSTEXPR void
  81     __to_chars_10_impl(char* __first, unsigned __len, _Tp __val) noexcept
  82     {
  83 #if __cpp_variable_templates
  84       static_assert(__integer_to_chars_is_unsigned<_Tp>, "implementation bug");
  85 #endif
  86
  87       constexpr char __digits[201] =
  88         "0001020304050607080910111213141516171819"
  89         "2021222324252627282930313233343536373839"
  90         "4041424344454647484950515253545556575859"
  91         "6061626364656667686970717273747576777879"
  92         "8081828384858687888990919293949596979899";
  93       unsigned __pos = __len - 1;
  94       while (__val >= 100)
  95         {
  96           auto const __num = (__val % 100) * 2;
  97           __val /= 100;
  98           __first[__pos] = __digits[__num + 1];
  99           __first[__pos - 1] = __digits[__num];
 100           __pos -= 2;
 101         }
 102       if (__val >= 10)
 103         {
 104           auto const __num = __val * 2;
 105           __first[1] = __digits[__num + 1];
 106           __first[0] = __digits[__num];
 107         }
 108       else
 109         __first[0] = '0' + __val;
 110     }
 111
 112 } // namespace __detail
 113 _GLIBCXX_END_NAMESPACE_VERSION
 114 } // namespace std
 115 #endif // C++11
 116 #endif // _GLIBCXX_CHARCONV_H