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Dragonbox is used to serialize doubles #1109

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164 changes: 164 additions & 0 deletions include/boost/json/detail/dragonbox/bit_layouts.hpp
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// Copyright 2023 Matt Borland
// Distributed under the Boost Software License, Version 1.0.
// https://www.boost.org/LICENSE_1_0.txt

#ifndef BOOST_JSON_DETAIL_DRAGONBOX_BIT_LAYOUTS_HPP
#define BOOST_JSON_DETAIL_DRAGONBOX_BIT_LAYOUTS_HPP

// #include <boost/charconv/detail/config.hpp>
// #include <boost/charconv/detail/emulated128.hpp>
#include <cstdint>
#include <cfloat>

// Layouts of floating point types as specified by IEEE 754
// See page 23 of IEEE 754-2008

namespace boost {
namespace json {
namespace detail {

struct ieee754_binary16
{
static constexpr int significand_bits = 10;
static constexpr int exponent_bits = 5;
static constexpr int min_exponent = -14;
static constexpr int max_exponent = 15;
static constexpr int exponent_bias = -15;
static constexpr int decimal_digits = 5;
};

struct brainfloat16
{
static constexpr int significand_bits = 7;
static constexpr int exponent_bits = 8;
static constexpr int min_exponent = -126;
static constexpr int max_exponent = 127;
static constexpr int exponent_bias = -127;
static constexpr int decimal_digits = 4;
};

struct ieee754_binary32
{
static constexpr int significand_bits = 23;
static constexpr int exponent_bits = 8;
static constexpr int min_exponent = -126;
static constexpr int max_exponent = 127;
static constexpr int exponent_bias = -127;
static constexpr int decimal_digits = 9;
};

struct ieee754_binary64
{
static constexpr int significand_bits = 52;
static constexpr int exponent_bits = 11;
static constexpr int min_exponent = -1022;
static constexpr int max_exponent = 1023;
static constexpr int exponent_bias = -1023;
static constexpr int decimal_digits = 17;
};

// 80 bit long double (e.g. x86-64)
#if LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384

struct IEEEl2bits
{
#if BOOST_CHARCONV_ENDIAN_LITTLE_BYTE
std::uint64_t mantissa_l : 64;
std::uint32_t exponent : 15;
std::uint32_t sign : 1;
std::uint64_t pad : 48;
#else // Big endian
std::uint64_t pad : 48;
std::uint32_t sign : 1;
std::uint32_t exponent : 15;
std::uint64_t mantissa_h : 64;
#endif
};

struct ieee754_binary80
{
static constexpr int significand_bits = 64; // Fraction is 63 and 1 integer bit
static constexpr int exponent_bits = 15;
static constexpr int min_exponent = -16382;
static constexpr int max_exponent = 16383;
static constexpr int exponent_bias = -16383;
static constexpr int decimal_digits = 18;
};

#define BOOST_CHARCONV_LDBL_BITS 80

// 128 bit long double (e.g. s390x, ppcle64)
#elif LDBL_MANT_DIG == 113 && LDBL_MAX_EXP == 16384

struct IEEEl2bits
{
#if BOOST_CHARCONV_ENDIAN_LITTLE_BYTE
std::uint64_t mantissa_l : 64;
std::uint64_t mantissa_h : 48;
std::uint32_t exponent : 15;
std::uint32_t sign : 1;
#else // Big endian
std::uint32_t sign : 1;
std::uint32_t exponent : 15;
std::uint64_t mantissa_h : 48;
std::uint64_t mantissa_l : 64;
#endif
};

#define BOOST_CHARCONV_LDBL_BITS 128

// 64 bit long double (double == long double on ARM)
#elif LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024

struct IEEEl2bits
{
#if BOOST_CHARCONV_ENDIAN_LITTLE_BYTE
std::uint32_t mantissa_l : 32;
std::uint32_t mantissa_h : 20;
std::uint32_t exponent : 11;
std::uint32_t sign : 1;
#else // Big endian
std::uint32_t sign : 1;
std::uint32_t exponent : 11;
std::uint32_t mantissa_h : 20;
std::uint32_t mantissa_l : 32;
#endif
};

#define BOOST_CHARCONV_LDBL_BITS 64

#else // Unsupported long double representation
# define BOOST_CHARCONV_UNSUPPORTED_LONG_DOUBLE
# define BOOST_CHARCONV_LDBL_BITS -1
#endif

struct IEEEbinary128
{
#if BOOST_CHARCONV_ENDIAN_LITTLE_BYTE
std::uint64_t mantissa_l : 64;
std::uint64_t mantissa_h : 48;
std::uint32_t exponent : 15;
std::uint32_t sign : 1;
#else // Big endian
std::uint32_t sign : 1;
std::uint32_t exponent : 15;
std::uint64_t mantissa_h : 48;
std::uint64_t mantissa_l : 64;
#endif
};

struct ieee754_binary128
{
static constexpr int significand_bits = 112;
static constexpr int exponent_bits = 15;
static constexpr int min_exponent = -16382;
static constexpr int max_exponent = 16383;
static constexpr int exponent_bias = 16383;
static constexpr int decimal_digits = 33;
};

} // namespace detail
} // namespace json
} // namespace boost

#endif // BOOST_JSON_DETAIL_DRAGONBOX_BIT_LAYOUTS_HPP
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