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make number serialzation precision configurable #4591

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a509d42
implement settable floating point precision output
maddanio Jul 1, 2024
9d7aa81
add test for precision setting
maddanio Jan 9, 2025
0132248
amalgamate
maddanio Jan 9, 2025
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15 changes: 10 additions & 5 deletions include/nlohmann/detail/conversions/to_chars.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -978,10 +978,11 @@ notation. Otherwise it will be printed in exponential notation.
JSON_HEDLEY_NON_NULL(1)
JSON_HEDLEY_RETURNS_NON_NULL
inline char* format_buffer(char* buf, int len, int decimal_exponent,
int min_exp, int max_exp)
int min_exp, int max_exp, size_t precision)
{
JSON_ASSERT(min_exp < 0);
JSON_ASSERT(max_exp > 0);
precision = (std::min)<size_t>(precision, std::numeric_limits<FloatType>::max_digits10);

const int k = len;
const int n = len + decimal_exponent;
Expand Down Expand Up @@ -1009,6 +1010,9 @@ inline char* format_buffer(char* buf, int len, int decimal_exponent,

JSON_ASSERT(k > n);

// truncate the digits by the precision
k = (std::min)(static_cast<size_t>(n) + precision, static_cast<size_t>(k));

std::memmove(buf + (static_cast<size_t>(n) + 1), buf + n, static_cast<size_t>(k) - static_cast<size_t>(n));
buf[n] = '.';
return buf + (static_cast<size_t>(k) + 1U);
Expand All @@ -1023,7 +1027,8 @@ inline char* format_buffer(char* buf, int len, int decimal_exponent,
buf[0] = '0';
buf[1] = '.';
std::memset(buf + 2, '0', static_cast<size_t>(-n));
return buf + (2U + static_cast<size_t>(-n) + static_cast<size_t>(k));
// truncate the reported buffer end by the precision
return buf + (std::min)(precision + 2, (2U + static_cast<size_t>(-n) + static_cast<size_t>(k)));
}

if (k == 1)
Expand All @@ -1040,7 +1045,7 @@ inline char* format_buffer(char* buf, int len, int decimal_exponent,

std::memmove(buf + 2, buf + 1, static_cast<size_t>(k) - 1);
buf[1] = '.';
buf += 1 + static_cast<size_t>(k);
buf += 1 + (std::min)(precision, static_cast<size_t>(k));
}

*buf++ = 'e';
Expand All @@ -1062,7 +1067,7 @@ format. Returns an iterator pointing past-the-end of the decimal representation.
template<typename FloatType>
JSON_HEDLEY_NON_NULL(1, 2)
JSON_HEDLEY_RETURNS_NON_NULL
char* to_chars(char* first, const char* last, FloatType value)
char* to_chars(char* first, const char* last, FloatType value, size_t precision = std::numeric_limits<FloatType>::max_digits10)
{
static_cast<void>(last); // maybe unused - fix warning
JSON_ASSERT(std::isfinite(value));
Expand Down Expand Up @@ -1111,7 +1116,7 @@ char* to_chars(char* first, const char* last, FloatType value)
JSON_ASSERT(last - first >= 2 + (-kMinExp - 1) + std::numeric_limits<FloatType>::max_digits10);
JSON_ASSERT(last - first >= std::numeric_limits<FloatType>::max_digits10 + 6);

return dtoa_impl::format_buffer(first, len, decimal_exponent, kMinExp, kMaxExp);
return dtoa_impl::format_buffer(first, len, decimal_exponent, kMinExp, kMaxExp, precision);
}

} // namespace detail
Expand Down
11 changes: 7 additions & 4 deletions include/nlohmann/detail/output/serializer.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -64,13 +64,14 @@ class serializer
@param[in] ichar indentation character to use
@param[in] error_handler_ how to react on decoding errors
*/
serializer(output_adapter_t<char> s, const char ichar,
serializer(output_adapter_t<char> s, const char ichar, std::size_t prec = 1000,
error_handler_t error_handler_ = error_handler_t::strict)
: o(std::move(s))
, loc(std::localeconv())
, thousands_sep(loc->thousands_sep == nullptr ? '\0' : std::char_traits<char>::to_char_type(* (loc->thousands_sep)))
, decimal_point(loc->decimal_point == nullptr ? '\0' : std::char_traits<char>::to_char_type(* (loc->decimal_point)))
, indent_char(ichar)
, precision(prec)
, indent_string(512, indent_char)
, error_handler(error_handler_)
{}
Expand Down Expand Up @@ -820,18 +821,18 @@ class serializer
void dump_float(number_float_t x, std::true_type /*is_ieee_single_or_double*/)
{
auto* begin = number_buffer.data();
auto* end = ::nlohmann::detail::to_chars(begin, begin + number_buffer.size(), x);
auto* end = ::nlohmann::detail::to_chars(begin, begin + number_buffer.size(), x, precision);

o->write_characters(begin, static_cast<size_t>(end - begin));
}

void dump_float(number_float_t x, std::false_type /*is_ieee_single_or_double*/)
{
// get number of digits for a float -> text -> float round-trip
static constexpr auto d = std::numeric_limits<number_float_t>::max_digits10;
static constexpr int d_max = std::numeric_limits<number_float_t>::max_digits10;
int d = static_cast<int>((std::min)(precision, static_cast<std::size_t>(d_max)));

// the actual conversion
// NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
std::ptrdiff_t len = (std::snprintf)(number_buffer.data(), number_buffer.size(), "%.*g", d, x);

// negative value indicates an error
Expand Down Expand Up @@ -977,6 +978,8 @@ class serializer

/// the indentation character
const char indent_char;
/// precision for floating point output
std::size_t precision;
/// the indentation string
string_t indent_string;

Expand Down
8 changes: 5 additions & 3 deletions include/nlohmann/json.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1272,10 +1272,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
string_t dump(const int indent = -1,
const char indent_char = ' ',
const bool ensure_ascii = false,
const error_handler_t error_handler = error_handler_t::strict) const
const error_handler_t error_handler = error_handler_t::strict,
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Similar to #4513 (comment) where a bjdata_config_t type was discussed to control the BJData serialization, I think it is also long time to have such a time for the JSON serialization as the parameters start piling up. I'm hesitant to say if this is the last parameter to add before such a config type is introduced, or the first one where we stop the madness...

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so mine is one too much? :) but sure, I understand, will look into it

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ah, hang on, isnt that a public api breaking change? so you want me to change the public api? or add an overload?

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so mine is one too much? :) but sure, I understand, will look into it

Many others before you have been "one too much" and thus haven't been done. :)

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There has been so many discussions about tweaking every last aspect of serializations that adding parameters is getting out of hand.

const size_t precision = std::numeric_limits<size_t>::max()) const
{
string_t result;
serializer s(detail::output_adapter<char, string_t>(result), indent_char, error_handler);
serializer s(detail::output_adapter<char, string_t>(result), indent_char, precision, error_handler);

if (indent >= 0)
{
Expand Down Expand Up @@ -3982,8 +3983,9 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
o.width(0);

// do the actual serialization
serializer s(detail::output_adapter<char>(o), o.fill());
serializer s(detail::output_adapter<char>(o), o.fill(), static_cast<size_t>(o.precision())).;
s.dump(j, pretty_print, false, static_cast<unsigned int>(indentation));

return o;
}

Expand Down
35 changes: 22 additions & 13 deletions single_include/nlohmann/json.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -17886,10 +17886,11 @@ notation. Otherwise it will be printed in exponential notation.
JSON_HEDLEY_NON_NULL(1)
JSON_HEDLEY_RETURNS_NON_NULL
inline char* format_buffer(char* buf, int len, int decimal_exponent,
int min_exp, int max_exp)
int min_exp, int max_exp, size_t precision)
{
JSON_ASSERT(min_exp < 0);
JSON_ASSERT(max_exp > 0);
precision = std::min<size_t>(precision, 1000);

const int k = len;
const int n = len + decimal_exponent;
Expand Down Expand Up @@ -17919,7 +17920,7 @@ inline char* format_buffer(char* buf, int len, int decimal_exponent,

std::memmove(buf + (static_cast<size_t>(n) + 1), buf + n, static_cast<size_t>(k) - static_cast<size_t>(n));
buf[n] = '.';
return buf + (static_cast<size_t>(k) + 1U);
return buf + (std::min(n + precision, static_cast<size_t>(k)) + 1U);
}

if (min_exp < n && n <= 0)
Expand All @@ -17931,7 +17932,7 @@ inline char* format_buffer(char* buf, int len, int decimal_exponent,
buf[0] = '0';
buf[1] = '.';
std::memset(buf + 2, '0', static_cast<size_t>(-n));
return buf + (2U + static_cast<size_t>(-n) + static_cast<size_t>(k));
return buf + std::min(precision + 2, (2U + static_cast<size_t>(-n) + static_cast<size_t>(k)));
}

if (k == 1)
Expand All @@ -17948,7 +17949,7 @@ inline char* format_buffer(char* buf, int len, int decimal_exponent,

std::memmove(buf + 2, buf + 1, static_cast<size_t>(k) - 1);
buf[1] = '.';
buf += 1 + static_cast<size_t>(k);
buf += 1 + std::min(precision, static_cast<size_t>(k));
}

*buf++ = 'e';
Expand All @@ -17970,7 +17971,7 @@ format. Returns an iterator pointing past-the-end of the decimal representation.
template<typename FloatType>
JSON_HEDLEY_NON_NULL(1, 2)
JSON_HEDLEY_RETURNS_NON_NULL
char* to_chars(char* first, const char* last, FloatType value)
char* to_chars(char* first, const char* last, FloatType value, size_t precision = std::numeric_limits<FloatType>::max_digits10)
{
static_cast<void>(last); // maybe unused - fix warning
JSON_ASSERT(std::isfinite(value));
Expand Down Expand Up @@ -18019,7 +18020,7 @@ char* to_chars(char* first, const char* last, FloatType value)
JSON_ASSERT(last - first >= 2 + (-kMinExp - 1) + std::numeric_limits<FloatType>::max_digits10);
JSON_ASSERT(last - first >= std::numeric_limits<FloatType>::max_digits10 + 6);

return dtoa_impl::format_buffer(first, len, decimal_exponent, kMinExp, kMaxExp);
return dtoa_impl::format_buffer(first, len, decimal_exponent, kMinExp, kMaxExp, precision);
}

} // namespace detail
Expand Down Expand Up @@ -18073,13 +18074,14 @@ class serializer
@param[in] ichar indentation character to use
@param[in] error_handler_ how to react on decoding errors
*/
serializer(output_adapter_t<char> s, const char ichar,
serializer(output_adapter_t<char> s, const char ichar, size_t precision = 1000,
error_handler_t error_handler_ = error_handler_t::strict)
: o(std::move(s))
, loc(std::localeconv())
, thousands_sep(loc->thousands_sep == nullptr ? '\0' : std::char_traits<char>::to_char_type(* (loc->thousands_sep)))
, decimal_point(loc->decimal_point == nullptr ? '\0' : std::char_traits<char>::to_char_type(* (loc->decimal_point)))
, indent_char(ichar)
, precision(precision)
, indent_string(512, indent_char)
, error_handler(error_handler_)
{}
Expand Down Expand Up @@ -18829,7 +18831,7 @@ class serializer
void dump_float(number_float_t x, std::true_type /*is_ieee_single_or_double*/)
{
auto* begin = number_buffer.data();
auto* end = ::nlohmann::detail::to_chars(begin, begin + number_buffer.size(), x);
auto* end = ::nlohmann::detail::to_chars(begin, begin + number_buffer.size(), x, precision);

o->write_characters(begin, static_cast<size_t>(end - begin));
}
Expand All @@ -18841,6 +18843,8 @@ class serializer

// the actual conversion
// NOLINTNEXTLINE(cppcoreguidelines-pro-type-vararg,hicpp-vararg)
char format[100];
snprintf(format, 100, "%%.%dg", precision);
std::ptrdiff_t len = (std::snprintf)(number_buffer.data(), number_buffer.size(), "%.*g", d, x);

// negative value indicates an error
Expand Down Expand Up @@ -18986,6 +18990,8 @@ class serializer

/// the indentation character
const char indent_char;
/// precision for floating point output
size_t precision;
/// the indentation string
string_t indent_string;

Expand Down Expand Up @@ -20575,10 +20581,11 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
string_t dump(const int indent = -1,
const char indent_char = ' ',
const bool ensure_ascii = false,
const error_handler_t error_handler = error_handler_t::strict) const
const error_handler_t error_handler = error_handler_t::strict,
const size_t precision = std::numeric_limits<size_t>::max()) const
{
string_t result;
serializer s(detail::output_adapter<char, string_t>(result), indent_char, error_handler);
serializer s(detail::output_adapter<char, string_t>(result), indent_char, precision, error_handler);

if (indent >= 0)
{
Expand Down Expand Up @@ -23278,15 +23285,17 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
friend std::ostream& operator<<(std::ostream& o, const basic_json& j)
{
// read width member and use it as indentation parameter if nonzero
const bool pretty_print = o.width() > 0;
const auto indentation = pretty_print ? o.width() : 0;
const auto width = o.width();
const bool pretty_print = width > 0;
const auto indentation = pretty_print ? width : 0;

// reset width to 0 for subsequent calls to this stream
o.width(0);

// do the actual serialization
serializer s(detail::output_adapter<char>(o), o.fill());
serializer s(detail::output_adapter<char>(o), o.fill(), o.precision());
s.dump(j, pretty_print, false, static_cast<unsigned int>(indentation));

return o;
}

Expand Down
57 changes: 57 additions & 0 deletions tests/src/unit-precision.cpp
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@@ -0,0 +1,57 @@
/*
__ _____ _____ _____
__| | __| | | | JSON for Modern C++ (test suite)
| | |__ | | | | | | version 3.10.5
|_____|_____|_____|_|___| https://github.com/nlohmann/json

Licensed under the MIT License <http://opensource.org/licenses/MIT>.
SPDX-License-Identifier: MIT
Copyright (c) 2013-2022 Niels Lohmann <http://nlohmann.me>.

Permission is hereby granted, free of charge, to any person obtaining a copy
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The MIT license does not need to be quoted verbatim here.

Just take the header that is used in all other tests:

//     __ _____ _____ _____
//  __|  |   __|     |   | |  JSON for Modern C++ (supporting code)
// |  |  |__   |  |  | | | |  version 3.11.3
// |_____|_____|_____|_|___|  https://github.com/nlohmann/json
//
// SPDX-FileCopyrightText: 2013 - 2024 Niels Lohmann <https://nlohmann.me>
// SPDX-License-Identifier: MIT

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copy pasted from another test, but yes

of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

#include "doctest_compatibility.h"

#include <cmath>
#include <iomanip>
#include <nlohmann/json.hpp>
#include <sstream>
#include <string>
using json = nlohmann::json;

#include <set>

TEST_CASE("precision<nlohmann::json>")
{
json j;
j = M_PI;
auto to_string = [](const nlohmann::json & j, size_t precision)
{
std::stringstream ss;
ss << std::setprecision(precision) << j;
return ss.str();
};
auto s1 = to_string(j, 1);
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For clarity, I would also add a test here for the default behavior of no explicitly set precision.

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ok, but tbh I will then just adapt the test to what I observer, since I have no expectation for that case

auto s2 = to_string(j, 2);
auto s3 = to_string(j, 3);
CHECK(s1 == "3.1");
CHECK(s2 == "3.14");
CHECK(s3 == "3.141");
}
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