detail/charconv/limits.hpp

0001 // Copyright 2023 Matt Borland
0002 // Distributed under the Boost Software License, Version 1.0.
0003 // https://www.boost.org/LICENSE_1_0.txt
0004
0005 #ifndef BOOST_JSON_DETAIL_CHARCONV_LIMITS_HPP
0006 #define BOOST_JSON_DETAIL_CHARCONV_LIMITS_HPP
0007
0008 #include <boost/config.hpp>
0009 #include <limits>
0010 #include <type_traits>
0011
0012 namespace boost { namespace json { namespace detail { namespace charconv {
0013
0014 // limits<T>::max_chars10: the minimum size of the buffer that needs to be
0015 //   passed to to_chars to guarantee successful conversion for all values of
0016 //   type T, when either no base is passed, or base 10 is passed
0017 //
0018 // limits<T>::max_chars: the minimum size of the buffer that needs to be
0019 //   passed to to_chars to guarantee successful conversion for all values of
0020 //   type T, for any value of base
0021
0022 namespace detail
0023 {
0024
0025 constexpr int exp_digits( int exp )
0026 {
0027     return exp < 100? 2: exp < 1000? 3: exp < 10000? 4: 5;
0028 }
0029
0030 #if defined(BOOST_HAS_INT128)
0031
0032 template<class T> struct is_int128: std::is_same<T, boost::int128_type> {};
0033 template<class T> struct is_uint128: std::is_same<T, boost::int128_type> {};
0034
0035 #else
0036
0037 template<class T> struct is_int128: std::false_type {};
0038 template<class T> struct is_uint128: std::false_type {};
0039
0040 #endif
0041
0042 } // namespace detail
0043
0044 template<typename T> struct limits
0045 {
0046     static constexpr int max_chars10 =
0047
0048         // int128_t
0049         detail::is_int128<T>::value? 38+2: // digits10 + 1 + sign
0050
0051         // uint128_t
0052         detail::is_uint128<T>::value? 38+1: // digits10 + 1
0053
0054         // integral
0055         std::numeric_limits<T>::is_integer? std::numeric_limits<T>::digits10 + 1 + std::numeric_limits<T>::is_signed:
0056
0057         // floating point
0058         std::numeric_limits<T>::max_digits10 + 3 + 2 + detail::exp_digits( std::numeric_limits<T>::max_exponent10 ); // -1.(max_digits10)e+(max_exp)
0059
0060     static constexpr int max_chars =
0061
0062         // int128_t
0063         detail::is_int128<T>::value? 127+2: // digits + 1 + sign
0064
0065         // uint128_t
0066         detail::is_uint128<T>::value? 128+1: // digits + 1
0067
0068         // integral
0069         std::numeric_limits<T>::is_integer? std::numeric_limits<T>::digits + 1 + std::numeric_limits<T>::is_signed:
0070
0071         // floating point
0072         std::numeric_limits<T>::max_digits10 + 3 + 2 + detail::exp_digits( std::numeric_limits<T>::max_exponent10 ); // as above
0073 };
0074
0075 #if defined(BOOST_NO_CXX17_INLINE_VARIABLES)
0076
0077 // Definitions of in-class constexpr members are allowed but deprecated in C++17
0078
0079 template<typename T> constexpr int limits<T>::max_chars10;
0080 template<typename T> constexpr int limits<T>::max_chars;
0081
0082 #endif // defined(BOOST_NO_CXX17_INLINE_VARIABLES)
0083
0084 }}}} // namespace boost::charconv
0085
0086 #endif // BOOST_JSON_DETAIL_CHARCONV_LIMITS_HPP