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Compare integers with floats exactly instead of widening the integer (#5459)

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The mixed number arms of JSON_IMPLEMENT_OPERATOR cast the integer to
number_float_t before comparing. Past the float's mantissa that cast is lossy:
2^63-2 and 2^63-1 both round to 2^63, so each compares equal to that float
while differing from each other. Equality is therefore intransitive and the
ordering is not a strict weak ordering, which makes std::sort over such values,
or using them as keys in std::set or std::map, undefined behavior.

Compare the two exactly instead. The integer's range is a power of two the
float represents exactly, so a float outside it is ordered by magnitude alone;
inside it, truncating the float is exact, and the integer parts and then any
fractional part decide. The helper hands back a pair whose comparison with the
original operator reproduces that ordering, which keeps every operator's return
type as it was, including partial_ordering for the spaceship.

A NaN operand is returned in both members, so NaN stays false for the
relational operators and unordered for <=>. Values a float represents exactly
still compare equal, so json(1) == json(1.0) is unchanged.

Signed-off-by: qatcod <79017227+qatcod@users.noreply.github.com>
This commit is contained in:
Qatadaha Bin Matloob
2026-09-05 09:43:07 +02:00
committed by GitHub
parent 19386dd14a
commit 137a40b9aa
4 changed files with 189 additions and 8 deletions
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single_include/nlohmann/json.hpp
+69 -4
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@@ -223,9 +223,12 @@
#include <array> // array
#include <cmath> // isnan, ldexp, trunc
#include <cstddef> // size_t
#include <cstdint> // uint8_t
#include <limits> // numeric_limits
#include <string> // string
#include <type_traits> // is_signed
// #include <nlohmann/detail/macro_scope.hpp>
// __ _____ _____ _____
@@ -3283,6 +3286,68 @@ inline bool operator<(const value_t lhs, const value_t rhs) noexcept
}
#endif
/*!
@brief compare an integer with a floating point number without precision loss
Widening the integer to the floating point type loses precision beyond the
float's mantissa, which makes equality intransitive: both 2^63-2 and 2^63-1
round to 2^63, so each compares equal to that float while differing from each
other. Ordering built on that is not a strict weak ordering, so sorting such
values, or using them as keys in an ordered container, is undefined behavior.
Returns a value to be compared against zero with the original operator, which
reproduces the exact ordering. A NaN operand is returned as is, so comparing it
against zero keeps NaN's semantics: false for the relational operators and
unordered for `<=>`.
*/
template<typename IntegerType, typename FloatType>
FloatType compare_integer_with_float(const IntegerType i, const FloatType f) noexcept
{
const auto ordered = [](int c) noexcept
{
return static_cast<FloatType>(c);
};
if (std::isnan(f))
{
return f;
}
// values of IntegerType lie in [-bound, bound) when signed and in
// [0, bound) when unsigned; digits excludes the sign bit, so bound is a
// power of two that the float represents exactly
const FloatType bound = std::ldexp(static_cast<FloatType>(1), std::numeric_limits<IntegerType>::digits);
if (f >= bound)
{
return ordered(-1);
}
if (std::is_signed<IntegerType>::value ? (f < -bound) : (f < static_cast<FloatType>(0)))
{
return ordered(1);
}
// f is now within the integer's range, so truncating it is exact
const FloatType truncated = std::trunc(f);
const auto as_integer = static_cast<IntegerType>(truncated);
if (i != as_integer)
{
return ordered(i < as_integer ? -1 : 1);
}
// the integer parts agree, so any fractional part decides
const FloatType fraction = f - truncated;
if (fraction > static_cast<FloatType>(0))
{
return ordered(-1);
}
if (fraction < static_cast<FloatType>(0))
{
return ordered(1);
}
return ordered(0);
}
} // namespace detail
NLOHMANN_JSON_NAMESPACE_END
@@ -25057,19 +25122,19 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
} \
else if (lhs_type == value_t::number_integer && rhs_type == value_t::number_float) \
{ \
return static_cast<number_float_t>(lhs.m_data.m_value.number_integer) op rhs.m_data.m_value.number_float; \
return (detail::compare_integer_with_float(lhs.m_data.m_value.number_integer, rhs.m_data.m_value.number_float)) op (static_cast<number_float_t>(0)); \
} \
else if (lhs_type == value_t::number_float && rhs_type == value_t::number_integer) \
{ \
return lhs.m_data.m_value.number_float op static_cast<number_float_t>(rhs.m_data.m_value.number_integer); \
return (static_cast<number_float_t>(0)) op (detail::compare_integer_with_float(rhs.m_data.m_value.number_integer, lhs.m_data.m_value.number_float)); \
} \
else if (lhs_type == value_t::number_unsigned && rhs_type == value_t::number_float) \
{ \
return static_cast<number_float_t>(lhs.m_data.m_value.number_unsigned) op rhs.m_data.m_value.number_float; \
return (detail::compare_integer_with_float(lhs.m_data.m_value.number_unsigned, rhs.m_data.m_value.number_float)) op (static_cast<number_float_t>(0)); \
} \
else if (lhs_type == value_t::number_float && rhs_type == value_t::number_unsigned) \
{ \
return lhs.m_data.m_value.number_float op static_cast<number_float_t>(rhs.m_data.m_value.number_unsigned); \
return (static_cast<number_float_t>(0)) op (detail::compare_integer_with_float(rhs.m_data.m_value.number_unsigned, lhs.m_data.m_value.number_float)); \
} \
else if (lhs_type == value_t::number_unsigned && rhs_type == value_t::number_integer) \
{ \