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charconv/src/to_chars.cpp
Matt Borland 96bb503485 Switch on precision rather than format 
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2023-04-12 14:46:48 +02:00

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// Copyright 2020-2023 Junekey Jeon
// Copyright 2022 Peter Dimov
// Copyright 2023 Matt Borland
// Distributed under the Boost Software License, Version 1.0.
// https://www.boost.org/LICENSE_1_0.txt
#include <boost/charconv/detail/dragonbox.hpp>
#include <boost/charconv/detail/floff.hpp>
#include <boost/charconv/to_chars.hpp>
#include <limits>
#include <cstdio>
#include <cstring>
#include <cstdint>
#include <cmath>
namespace boost { namespace charconv { namespace detail {
// These "//"'s are to prevent clang-format to ruin this nice alignment.
// Thanks to reddit user u/mcmcc:
// https://www.reddit.com/r/cpp/comments/so3wx9/dragonbox_110_is_released_a_fast_floattostring/hw8z26r/?context=3
/*
static constexpr char radix_100_table[] = {
'0', '0', '0', '1', '0', '2', '0', '3', '0', '4', //
'0', '5', '0', '6', '0', '7', '0', '8', '0', '9', //
'1', '0', '1', '1', '1', '2', '1', '3', '1', '4', //
'1', '5', '1', '6', '1', '7', '1', '8', '1', '9', //
'2', '0', '2', '1', '2', '2', '2', '3', '2', '4', //
'2', '5', '2', '6', '2', '7', '2', '8', '2', '9', //
'3', '0', '3', '1', '3', '2', '3', '3', '3', '4', //
'3', '5', '3', '6', '3', '7', '3', '8', '3', '9', //
'4', '0', '4', '1', '4', '2', '4', '3', '4', '4', //
'4', '5', '4', '6', '4', '7', '4', '8', '4', '9', //
'5', '0', '5', '1', '5', '2', '5', '3', '5', '4', //
'5', '5', '5', '6', '5', '7', '5', '8', '5', '9', //
'6', '0', '6', '1', '6', '2', '6', '3', '6', '4', //
'6', '5', '6', '6', '6', '7', '6', '8', '6', '9', //
'7', '0', '7', '1', '7', '2', '7', '3', '7', '4', //
'7', '5', '7', '6', '7', '7', '7', '8', '7', '9', //
'8', '0', '8', '1', '8', '2', '8', '3', '8', '4', //
'8', '5', '8', '6', '8', '7', '8', '8', '8', '9', //
'9', '0', '9', '1', '9', '2', '9', '3', '9', '4', //
'9', '5', '9', '6', '9', '7', '9', '8', '9', '9' //
};
static constexpr char radix_100_head_table[] = {
'0', '.', '1', '.', '2', '.', '3', '.', '4', '.', //
'5', '.', '6', '.', '7', '.', '8', '.', '9', '.', //
'1', '.', '1', '.', '1', '.', '1', '.', '1', '.', //
'1', '.', '1', '.', '1', '.', '1', '.', '1', '.', //
'2', '.', '2', '.', '2', '.', '2', '.', '2', '.', //
'2', '.', '2', '.', '2', '.', '2', '.', '2', '.', //
'3', '.', '3', '.', '3', '.', '3', '.', '3', '.', //
'3', '.', '3', '.', '3', '.', '3', '.', '3', '.', //
'4', '.', '4', '.', '4', '.', '4', '.', '4', '.', //
'4', '.', '4', '.', '4', '.', '4', '.', '4', '.', //
'5', '.', '5', '.', '5', '.', '5', '.', '5', '.', //
'5', '.', '5', '.', '5', '.', '5', '.', '5', '.', //
'6', '.', '6', '.', '6', '.', '6', '.', '6', '.', //
'6', '.', '6', '.', '6', '.', '6', '.', '6', '.', //
'7', '.', '7', '.', '7', '.', '7', '.', '7', '.', //
'7', '.', '7', '.', '7', '.', '7', '.', '7', '.', //
'8', '.', '8', '.', '8', '.', '8', '.', '8', '.', //
'8', '.', '8', '.', '8', '.', '8', '.', '8', '.', //
'9', '.', '9', '.', '9', '.', '9', '.', '9', '.', //
'9', '.', '9', '.', '9', '.', '9', '.', '9', '.' //
};
void print_1_digit(std::uint32_t n, char* buffer) noexcept
{
*buffer = char('0' | n);
}
void print_2_digits(std::uint32_t n, char* buffer) noexcept
{
std::memcpy(buffer, radix_100_table + n * 2, 2);
}
// These digit generation routines are inspired by James Anhalt's itoa algorithm:
// https://github.com/jeaiii/itoa
// The main idea is for given n, find y such that floor(10^k * y / 2^32) = n holds,
// where k is an appropriate integer depending on the length of n.
// For example, if n = 1234567, we set k = 6. In this case, we have
// floor(y / 2^32) = 1,
// floor(10^2 * ((10^0 * y) mod 2^32) / 2^32) = 23,
// floor(10^2 * ((10^2 * y) mod 2^32) / 2^32) = 45, and
// floor(10^2 * ((10^4 * y) mod 2^32) / 2^32) = 67.
// See https://jk-jeon.github.io/posts/2022/02/jeaiii-algorithm/ for more explanation.
void print_9_digits(std::uint32_t s32, int& exponent, char* buffer) noexcept
{
// -- IEEE-754 binary32
// Since we do not cut trailing zeros in advance, s32 must be of 6~9 digits
// unless the original input was subnormal.
// In particular, when it is of 9 digits it shouldn't have any trailing zeros.
// -- IEEE-754 binary64
// In this case, s32 must be of 7~9 digits unless the input is subnormal,
// and it shouldn't have any trailing zeros if it is of 9 digits.
if (s32 >= 100000000)
{
// 9 digits.
// 1441151882 = ceil(2^57 / 1'0000'0000) + 1
auto prod = s32 * UINT64_C(1441151882);
prod >>= 25;
std::memcpy(buffer, radix_100_head_table + UINT32_C(prod >> 32) * 2, 2);
prod = static_cast<std::uint32_t>(prod) * UINT64_C(100);
print_2_digits(static_cast<std::uint32_t>(prod >> 32), buffer + 2);
prod = static_cast<std::uint32_t>(prod) * UINT64_C(100);
print_2_digits(static_cast<std::uint32_t>(prod >> 32), buffer + 4);
prod = static_cast<std::uint32_t>(prod) * UINT64_C(100);
print_2_digits(static_cast<std::uint32_t>(prod >> 32), buffer + 6);
prod = static_cast<std::uint32_t>(prod) * UINT64_C(100);
print_2_digits(static_cast<std::uint32_t>(prod >> 32), buffer + 8);
exponent += 8;
buffer += 10;
}
else if (s32 >= 1000000)
{
// 7 or 8 digits.
// 281474978 = ceil(2^48 / 100'0000) + 1
auto prod = s32 * UINT64_C(281474978);
prod >>= 16;
auto const head_digits = static_cast<std::uint32_t>(prod >> 32);
// If s32 is of 8 digits, increase the exponent by 7.
// Otherwise, increase it by 6.
exponent += (6 + unsigned(head_digits >= 10));
// Write the first digit and the decimal point.
std::memcpy(buffer, radix_100_head_table + head_digits * 2, 2);
// This third character may be overwritten later but we don't care.
buffer[2] = radix_100_table[head_digits * 2 + 1];
// Remaining 6 digits are all zero?
if (static_cast<std::uint32_t>(prod) <= static_cast<std::uint32_t>((UINT64_C(1) << 32) / 1000000))
{
// The number of characters actually need to be written is:
// 1, if only the first digit is nonzero, which means that either s32 is of 7
// digits or it is of 8 digits but the second digit is zero, or
// 3, otherwise.
// Note that buffer[2] is never '0' if s32 is of 7 digits, because the input is
// never zero.
buffer += (1 + (unsigned(head_digits >= 10) & unsigned(buffer[2] > '0')) * 2);
}
else
{
// At least one of the remaining 6 digits are nonzero.
// After this adjustment, now the first destination becomes buffer + 2.
buffer += unsigned(head_digits >= 10);
// Obtain the next two digits.
prod = static_cast<std::uint32_t>(prod) * UINT64_C(100);
print_2_digits(static_cast<std::uint32_t>(prod >> 32), buffer + 2);
// Remaining 4 digits are all zero?
if (static_cast<std::uint32_t>(prod) <= static_cast<std::uint32_t>((UINT64_C(1) << 32) / 10000))
{
buffer += (3 + unsigned(buffer[3] > '0'));
}
else
{
// At least one of the remaining 4 digits are nonzero.
// Obtain the next two digits.
prod = static_cast<std::uint32_t>(prod) * UINT64_C(100);
print_2_digits(static_cast<std::uint32_t>(prod >> 32), buffer + 4);
// Remaining 2 digits are all zero?
if (static_cast<std::uint32_t>(prod) <= static_cast<std::uint32_t>((UINT64_C(1) << 32) / 100))
{
buffer += (5 + unsigned(buffer[5] > '0'));
}
else
{
// Obtain the last two digits.
prod = static_cast<std::uint32_t>(prod) * UINT64_C(100);
print_2_digits(static_cast<std::uint32_t>(prod >> 32), buffer + 6);
buffer += (7 + unsigned(buffer[7] > '0'));
}
}
}
}
else if (s32 >= 10000)
{
// 5 or 6 digits.
// 429497 = ceil(2^32 / 1'0000)
auto prod = s32 * UINT64_C(429497);
auto const head_digits = static_cast<std::uint32_t>(prod >> 32);
// If s32 is of 6 digits, increase the exponent by 5.
// Otherwise, increase it by 4.
exponent += (4 + static_cast<unsigned>(head_digits >= 10));
// Write the first digit and the decimal point.
std::memcpy(buffer, radix_100_head_table + head_digits * 2, 2);
// This third character may be overwritten later but we don't care.
buffer[2] = radix_100_table[head_digits * 2 + 1];
// Remaining 4 digits are all zero?
if (static_cast<std::uint32_t>(prod) <= static_cast<std::uint32_t>((UINT64_C(1) << 32) / 10000))
{
// The number of characters actually written is 1 or 3, similarly to the case of
// 7 or 8 digits.
buffer += (1 + (unsigned(head_digits >= 10) & unsigned(buffer[2] > '0')) * 2);
}
else
{
// At least one of the remaining 4 digits are nonzero.
// After this adjustment, now the first destination becomes buffer + 2.
buffer += unsigned(head_digits >= 10);
// Obtain the next two digits.
prod = static_cast<std::uint32_t>(prod) * UINT64_C(100);
print_2_digits(static_cast<std::uint32_t>(prod >> 32), buffer + 2);
// Remaining 2 digits are all zero?
if (static_cast<std::uint32_t>(prod) <= static_cast<std::uint32_t>((UINT64_C(1) << 32) / 100))
{
buffer += (3 + unsigned(buffer[3] > '0'));
}
else
{
// Obtain the last two digits.
prod = static_cast<std::uint32_t>(prod) * UINT64_C(100);
print_2_digits(static_cast<std::uint32_t>(prod >> 32), buffer + 4);
buffer += (5 + unsigned(buffer[5] > '0'));
}
}
}
else if (s32 >= 100)
{
// 3 or 4 digits.
// 42949673 = ceil(2^32 / 100)
auto prod = s32 * UINT64_C(42949673);
auto const head_digits = static_cast<std::uint32_t>(prod >> 32);
// If s32 is of 4 digits, increase the exponent by 3.
// Otherwise, increase it by 2.
exponent += (2 + int(head_digits >= 10));
// Write the first digit and the decimal point.
std::memcpy(buffer, radix_100_head_table + head_digits * 2, 2);
// This third character may be overwritten later but we don't care.
buffer[2] = radix_100_table[head_digits * 2 + 1];
// Remaining 2 digits are all zero?
if (static_cast<std::uint32_t>(prod) <= static_cast<std::uint32_t>((UINT64_C(1) << 32) / 100))
{
// The number of characters actually written is 1 or 3, similarly to the case of
// 7 or 8 digits.
buffer += (1 + (unsigned(head_digits >= 10) & unsigned(buffer[2] > '0')) * 2);
}
else
{
// At least one of the remaining 2 digits are nonzero.
// After this adjustment, now the first destination becomes buffer + 2.
buffer += unsigned(head_digits >= 10);
// Obtain the last two digits.
prod = static_cast<std::uint32_t>(prod) * UINT64_C(100);
print_2_digits(static_cast<std::uint32_t>(prod >> 32), buffer + 2);
buffer += (3 + unsigned(buffer[3] > '0'));
}
}
else
{
// 1 or 2 digits.
// If s32 is of 2 digits, increase the exponent by 1.
exponent += int(s32 >= 10);
// Write the first digit and the decimal point.
std::memcpy(buffer, radix_100_head_table + s32 * 2, 2);
// This third character may be overwritten later but we don't care.
buffer[2] = radix_100_table[s32 * 2 + 1];
// The number of characters actually written is 1 or 3, similarly to the case of
// 7 or 8 digits.
buffer += (1 + (unsigned(s32 >= 10) & unsigned(buffer[2] > '0')) * 2);
}
}
*/
}}} // Namespaces
boost::charconv::to_chars_result boost::charconv::to_chars(char* first, char* last, float value, boost::charconv::chars_format fmt, int precision) noexcept
{
if (fmt == boost::charconv::chars_format::general || fmt == boost::charconv::chars_format::fixed)
{
const auto abs_value = std::abs(value);
constexpr float max_value = static_cast<float>((std::numeric_limits<std::uint32_t>::max)());
constexpr float min_value = 1.0f / max_value;
if (abs_value < max_value && abs_value > min_value)
{
if (value < 0)
{
*first++ = '-';
}
return boost::charconv::to_chars(first, last, static_cast<std::uint32_t>(abs_value));
}
else
{
auto* ptr = boost::charconv::detail::floff<boost::charconv::detail::main_cache_full, boost::charconv::detail::extended_cache_long>(value, std::numeric_limits<float>::max_digits10 - 1, first, fmt);
return { ptr, 0 };
}
}
else if (fmt == boost::charconv::chars_format::scientific)
{
if (precision == -1)
{
precision = std::numeric_limits<float>::max_digits10;
}
if (precision > static_cast<std::ptrdiff_t>(last - first))
{
return { first, EOVERFLOW };
}
auto* ptr = boost::charconv::detail::floff<boost::charconv::detail::main_cache_full, boost::charconv::detail::extended_cache_long>(value, precision, first, fmt);
return { ptr, 0 };
}
else if (fmt == boost::charconv::chars_format::hex)
{
return boost::charconv::detail::to_chars_hex(first, last, value, precision);
}
return { first + std::strlen(first), 0 };
}
boost::charconv::to_chars_result boost::charconv::to_chars(char* first, char* last, double value, boost::charconv::chars_format fmt, int precision) noexcept
{
// Unspecified precision so we always go with shortest representation
if (precision == -1)
{
if (fmt == boost::charconv::chars_format::general || fmt == boost::charconv::chars_format::fixed)
{
const auto abs_value = std::abs(value);
if (abs_value >= 1 && abs_value < 1e18) // 1 x 10^(max_digits10 + 1)
{
auto value_struct = jkj::dragonbox::to_decimal(value);
if (value_struct.is_negative)
{
*first++ = '-';
}
auto r = boost::charconv::to_chars(first, last, value_struct.significand);
if (r.ec != 0)
{
return r;
}
// Bounds check
if (value_struct.exponent < 0)
{
std::memmove(r.ptr + value_struct.exponent + 1, r.ptr + value_struct.exponent, -value_struct.exponent);
std::memset(r.ptr + value_struct.exponent, '.', 1);
}
return { r.ptr, 0 };
}
else
{
auto* ptr = jkj::dragonbox::to_chars(value, first);
return { ptr, 0 };
}
}
else if (fmt == boost::charconv::chars_format::scientific)
{
auto* ptr = jkj::dragonbox::to_chars(value, first);
return { ptr, 0 };
}
}
else
{
if (fmt != boost::charconv::chars_format::hex)
{
auto* ptr = boost::charconv::detail::floff<boost::charconv::detail::main_cache_full, boost::charconv::detail::extended_cache_long>(value, precision, first, fmt);
return { ptr, 0 };
}
}
// Hex handles both cases already
return boost::charconv::detail::to_chars_hex(first, last, value, precision);
}
boost::charconv::to_chars_result boost::charconv::to_chars( char* first, char* last, long double value ) noexcept
{
std::snprintf( first, last - first, "%.*Lg", std::numeric_limits<long double>::max_digits10, value );
return { first + std::strlen(first), 0 };
}
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