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Make to_chars constexpr
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@jk-jeon
jk-jeon committed May 7, 2024
1 parent 7d7e589 commit 0d273a1
Showing 1 changed file with 204 additions and 9 deletions.
213 changes: 204 additions & 9 deletions include/dragonbox/dragonbox_to_chars.h
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Original file line number Diff line number Diff line change
Expand Up @@ -20,6 +20,38 @@

#include "dragonbox.h"

////////////////////////////////////////////////////////////////////////////////////////
// Language feature detections.
////////////////////////////////////////////////////////////////////////////////////////

// C++14 constexpr
#if defined(__cpp_constexpr) && __cpp_constexpr >= 201304L
#define JKJ_HAS_CONSTEXPR14 1
#elif __cplusplus >= 201402L
#define JKJ_HAS_CONSTEXPR14 1
#elif defined(_MSC_VER) && _MSC_VER >= 1910 && _MSVC_LANG >= 201402L
#define JKJ_HAS_CONSTEXPR14 1
#else
#define JKJ_HAS_CONSTEXPR14 0
#endif

#if JKJ_HAS_CONSTEXPR14
#define JKJ_CONSTEXPR14 constexpr
#else
#define JKJ_CONSTEXPR14
#endif

// C++17 constexpr lambdas
#if defined(__cpp_constexpr) && __cpp_constexpr >= 201603L
#define JKJ_HAS_CONSTEXPR17 1
#elif __cplusplus >= 201703L
#define JKJ_HAS_CONSTEXPR17 1
#elif defined(_MSC_VER) && _MSC_VER >= 1911 && _MSVC_LANG >= 201703L
#define JKJ_HAS_CONSTEXPR17 1
#else
#define JKJ_HAS_CONSTEXPR17 0
#endif

// C++17 inline variables
#if defined(__cpp_inline_variables) && __cpp_inline_variables >= 201606L
#define JKJ_HAS_INLINE_VARIABLE 1
Expand All @@ -37,16 +69,146 @@
#define JKJ_INLINE_VARIABLE static constexpr
#endif

// C++17 if constexpr
#if defined(__cpp_if_constexpr) && __cpp_if_constexpr >= 201606L
#define JKJ_HAS_IF_CONSTEXPR 1
#elif __cplusplus >= 201703L
#define JKJ_HAS_IF_CONSTEXPR 1
#elif defined(_MSC_VER) && _MSC_VER >= 1911 && _MSVC_LANG >= 201703L
#define JKJ_HAS_IF_CONSTEXPR 1
#else
#define JKJ_HAS_IF_CONSTEXPR 0
#endif

#if JKJ_HAS_IF_CONSTEXPR
#define JKJ_IF_CONSTEXPR if constexpr
#else
#define JKJ_IF_CONSTEXPR if
#endif

// C++20 std::bit_cast
#if JKJ_STD_REPLACEMENT_NAMESPACE_DEFINED
#if JKJ_STD_REPLACEMENT_HAS_BIT_CAST
#define JKJ_HAS_BIT_CAST 1
#else
#define JKJ_HAS_BIT_CAST 0
#endif
#elif defined(__cpp_lib_bit_cast) && __cpp_lib_bit_cast >= 201806L
#include <bit>
#define JKJ_HAS_BIT_CAST 1
#else
#define JKJ_HAS_BIT_CAST 0
#endif

// C++23 if consteval or C++20 std::is_constant_evaluated
#if defined(__cpp_if_consteval) && __cpp_is_consteval >= 202106L
#define JKJ_IF_CONSTEVAL if consteval
#define JKJ_IF_NOT_CONSTEVAL if !consteval
#define JKJ_CAN_BRANCH_ON_CONSTEVAL 1
#define JKJ_USE_IS_CONSTANT_EVALUATED 0
#elif JKJ_STD_REPLACEMENT_NAMESPACE_DEFINED
#if JKJ_STD_REPLACEMENT_HAS_IS_CONSTANT_EVALUATED
#define JKJ_IF_CONSTEVAL if (stdr::is_constant_evaluated())
#define JKJ_IF_NOT_CONSTEVAL if (!stdr::is_constant_evaluated())
#define JKJ_CAN_BRANCH_ON_CONSTEVAL 1
#define JKJ_USE_IS_CONSTANT_EVALUATED 1
#elif JKJ_HAS_IF_CONSTEXPR
#define JKJ_IF_CONSTEVAL if constexpr (false)
#define JKJ_IF_NOT_CONSTEVAL if constexpr (true)
#define JKJ_CAN_BRANCH_ON_CONSTEVAL 0
#define JKJ_USE_IS_CONSTANT_EVALUATED 0
#else
#define JKJ_IF_CONSTEVAL if (false)
#define JKJ_IF_NOT_CONSTEVAL if (true)
#define JKJ_CAN_BRANCH_ON_CONSTEVAL 0
#define JKJ_USE_IS_CONSTANT_EVALUATED 0
#endif
#else
#if defined(__cpp_lib_is_constant_evaluated) && __cpp_lib_is_constant_evaluated >= 201811L
#define JKJ_IF_CONSTEVAL if (stdr::is_constant_evaluated())
#define JKJ_IF_NOT_CONSTEVAL if (!stdr::is_constant_evaluated())
#define JKJ_CAN_BRANCH_ON_CONSTEVAL 1
#define JKJ_USE_IS_CONSTANT_EVALUATED 1
#elif JKJ_HAS_IF_CONSTEXPR
#define JKJ_IF_CONSTEVAL if constexpr (false)
#define JKJ_IF_NOT_CONSTEVAL if constexpr (true)
#define JKJ_CAN_BRANCH_ON_CONSTEVAL 0
#define JKJ_USE_IS_CONSTANT_EVALUATED 0
#else
#define JKJ_IF_CONSTEVAL if (false)
#define JKJ_IF_NOT_CONSTEVAL if (true)
#define JKJ_CAN_BRANCH_ON_CONSTEVAL 0
#define JKJ_USE_IS_CONSTANT_EVALUATED 0
#endif
#endif

#if JKJ_CAN_BRANCH_ON_CONSTEVAL && JKJ_HAS_BIT_CAST
#define JKJ_CONSTEXPR20 constexpr
#else
#define JKJ_CONSTEXPR20
#endif

namespace jkj {
namespace dragonbox {
namespace detail {
template <class FloatFormat, class CarrierUInt>
extern char* to_chars(CarrierUInt significand, int exponent, char* buffer) noexcept;

template <stdr::size_t max_digits, class UInt>
JKJ_CONSTEXPR14 char* print_integer_naive(UInt n, char* buffer) noexcept {
char temp[max_digits]{};
auto ptr = temp + sizeof(temp) - 1;
do {
*ptr = char('0' + n % 10);
n /= 10;
--ptr;
} while (n != 0);
while (++ptr != temp + sizeof(temp)) {
*buffer = *ptr;
++buffer;
}
return buffer;
}

template <class FloatFormat, class CarrierUInt>
JKJ_CONSTEXPR14 char* to_chars_naive(CarrierUInt significand, int exponent,
char* buffer) noexcept {
// Print significand.
{
auto ptr = print_integer_naive<FloatFormat::decimal_significand_digits>(significand,
buffer);

// Insert decimal dot.
if (ptr > buffer + 1) {
auto next = *++buffer;
++exponent;
*buffer = '.';
while (++buffer != ptr) {
auto const temp = *buffer;
*buffer = next;
next = temp;
++exponent;
}
*buffer = next;
++buffer;
}
}

*buffer = 'E';
++buffer;
if (exponent < 0) {
*buffer = '-';
++buffer;
exponent = -exponent;
}
return print_integer_naive<FloatFormat::decimal_exponent_digits>(unsigned(exponent),
buffer);
}

// Avoid needless ABI overhead incurred by tag dispatch.
template <class DecimalToBinaryRoundingPolicy, class BinaryToDecimalRoundingPolicy,
class CachePolicy, class PreferredIntegerTypesPolicy, class FormatTraits>
char* to_chars_n_impl(float_bits<FormatTraits> br, char* buffer) noexcept {
JKJ_CONSTEXPR20 char* to_chars_n_impl(float_bits<FormatTraits> br, char* buffer) noexcept {
auto const exponent_bits = br.extract_exponent_bits();
auto const s = br.remove_exponent_bits();

Expand All @@ -56,16 +218,29 @@ namespace jkj {
++buffer;
}
if (br.is_nonzero()) {
JKJ_IF_CONSTEVAL {
auto result = to_decimal_ex(s, exponent_bits, policy::sign::ignore,
policy::trailing_zero::remove_compact,
DecimalToBinaryRoundingPolicy{},
BinaryToDecimalRoundingPolicy{}, CachePolicy{},
PreferredIntegerTypesPolicy{});

return to_chars_naive<typename FormatTraits::format>(
result.significand, result.exponent, buffer);
}

auto result = to_decimal_ex(
s, exponent_bits, policy::sign::ignore, policy::trailing_zero::ignore,
DecimalToBinaryRoundingPolicy{}, BinaryToDecimalRoundingPolicy{},
CachePolicy{}, PreferredIntegerTypesPolicy{});
return to_chars<typename FormatTraits::format,
typename FormatTraits::carrier_uint>(result.significand,
result.exponent, buffer);

return to_chars<typename FormatTraits::format>(result.significand,
result.exponent, buffer);
}
else {
stdr::memcpy(buffer, "0E0", 3);
buffer[0] = '0';
buffer[1] = 'E';
buffer[2] = '0';
return buffer + 3;
}
}
Expand All @@ -75,11 +250,20 @@ namespace jkj {
*buffer = '-';
++buffer;
}
stdr::memcpy(buffer, "Infinity", 8);
buffer[0] = 'I';
buffer[1] = 'n';
buffer[2] = 'f';
buffer[3] = 'i';
buffer[4] = 'n';
buffer[5] = 'i';
buffer[6] = 't';
buffer[7] = 'y';
return buffer + 8;
}
else {
stdr::memcpy(buffer, "NaN", 3);
buffer[0] = 'N';
buffer[1] = 'a';
buffer[2] = 'N';
return buffer + 3;
}
}
Expand All @@ -92,7 +276,7 @@ namespace jkj {
class FormatTraits = ieee754_binary_traits<typename ConversionTraits::format,
typename ConversionTraits::carrier_uint>,
class... Policies>
char* to_chars_n(Float x, char* buffer, Policies...) noexcept {
JKJ_CONSTEXPR20 char* to_chars_n(Float x, char* buffer, Policies...) noexcept {
using policy_holder = detail::make_policy_holder<
detail::detector_default_pair_list<
detail::detector_default_pair<
Expand All @@ -118,7 +302,7 @@ namespace jkj {
class FormatTraits = ieee754_binary_traits<typename ConversionTraits::format,
typename ConversionTraits::carrier_uint>,
class... Policies>
char* to_chars(Float x, char* buffer, Policies... policies) noexcept {
JKJ_CONSTEXPR20 char* to_chars(Float x, char* buffer, Policies... policies) noexcept {
auto ptr = to_chars_n<Float, ConversionTraits, FormatTraits>(x, buffer, policies...);
*ptr = '\0';
return ptr;
Expand All @@ -133,7 +317,18 @@ namespace jkj {
}
}

#undef JKJ_CONSTEXPR20
#undef JKJ_USE_IS_CONSTANT_EVALUATED
#undef JKJ_CAN_BRANCH_ON_CONSTEVAL
#undef JKJ_IF_NOT_CONSTEVAL
#undef JKJ_IF_CONSTEVAL
#undef JKJ_HAS_BIT_CAST
#undef JKJ_IF_CONSTEXPR
#undef JKJ_HAS_IF_CONSTEXPR
#undef JKJ_INLINE_VARIABLE
#undef JKJ_HAS_INLINE_VARIABLE
#undef JKJ_HAS_CONSTEXPR17
#undef JKJ_CONSTEXPR14
#undef JKJ_HAS_CONSTEXPR14

#endif

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