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Make exponent threshold depend on representation (#3649)

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Victor Zverovich
2025-01-26 11:51:56 -08:00
parent 9cf9f38ede
commit 52eeeb52a6
3 changed files with 22 additions and 10 deletions
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26
include/fmt/format.h
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@ -2332,7 +2332,7 @@ template <typename Char, typename OutputIt, typename DecimalFP,
typename Grouping = digit_grouping<Char>>
FMT_CONSTEXPR20 auto do_write_float(OutputIt out, const DecimalFP& f,
const format_specs& specs, sign s,
locale_ref loc) -> OutputIt {
int exp_upper, locale_ref loc) -> OutputIt {
auto significand = f.significand;
int significand_size = get_significand_size(f);
const Char zero = static_cast<Char>('0');
@ -2348,7 +2348,7 @@ FMT_CONSTEXPR20 auto do_write_float(OutputIt out, const DecimalFP& f,
if (specs.type() == presentation_type::fixed) return false;
// Use the fixed notation if the exponent is in [exp_lower, exp_upper),
// e.g. 0.0001 instead of 1e-04. Otherwise use the exponent notation.
const int exp_lower = -4, exp_upper = 16;
const int exp_lower = -4;
return output_exp < exp_lower ||
output_exp >= (specs.precision > 0 ? specs.precision : exp_upper);
};
@ -2451,12 +2451,13 @@ template <typename Char> class fallback_digit_grouping {
template <typename Char, typename OutputIt, typename DecimalFP>
FMT_CONSTEXPR20 auto write_float(OutputIt out, const DecimalFP& f,
const format_specs& specs, sign s,
locale_ref loc) -> OutputIt {
int exp_upper, locale_ref loc) -> OutputIt {
if (is_constant_evaluated()) {
return do_write_float<Char, OutputIt, DecimalFP,
fallback_digit_grouping<Char>>(out, f, specs, s, loc);
fallback_digit_grouping<Char>>(out, f, specs, s,
exp_upper, loc);
} else {
return do_write_float<Char>(out, f, specs, s, loc);
return do_write_float<Char>(out, f, specs, s, exp_upper, loc);
}
}
@ -3288,6 +3289,14 @@ FMT_CONSTEXPR20 auto format_float(Float value, int precision,
return exp;
}
// Numbers with exponents greater or equal to the returned value will use
// the exponential notation.
template <typename T> constexpr auto exp_upper() -> int {
return std::numeric_limits<T>::digits10 != 0
? min_of(16, std::numeric_limits<T>::digits10 + 1)
: 16;
}
template <typename Char, typename OutputIt, typename T>
FMT_CONSTEXPR20 auto write_float(OutputIt out, T value, format_specs specs,
locale_ref loc) -> OutputIt {
@ -3303,6 +3312,7 @@ FMT_CONSTEXPR20 auto write_float(OutputIt out, T value, format_specs specs,
if (specs.width != 0) --specs.width;
}
constexpr int exp_upper = detail::exp_upper<T>();
int precision = specs.precision;
if (precision < 0) {
if (specs.type() != presentation_type::none) {
@ -3311,7 +3321,7 @@ FMT_CONSTEXPR20 auto write_float(OutputIt out, T value, format_specs specs,
// Use Dragonbox for the shortest format.
using floaty = conditional_t<sizeof(T) >= sizeof(double), double, float>;
auto dec = dragonbox::to_decimal(static_cast<floaty>(value));
return write_float<Char>(out, dec, specs, s, loc);
return write_float<Char>(out, dec, specs, s, exp_upper, loc);
}
}
@ -3339,7 +3349,7 @@ FMT_CONSTEXPR20 auto write_float(OutputIt out, T value, format_specs specs,
specs.precision = precision;
auto f = big_decimal_fp{buffer.data(), static_cast<int>(buffer.size()), exp};
return write_float<Char>(out, f, specs, s, loc);
return write_float<Char>(out, f, specs, s, exp_upper, loc);
}
template <typename Char, typename OutputIt, typename T,
@ -3366,7 +3376,7 @@ FMT_CONSTEXPR20 auto write(OutputIt out, T value) -> OutputIt {
return write_nonfinite<Char>(out, std::isnan(value), specs, s);
auto dec = dragonbox::to_decimal(static_cast<floaty>(value));
return write_float<Char>(out, dec, specs, s, {});
return write_float<Char>(out, dec, specs, s, exp_upper<T>(), {});
}
template <typename Char, typename OutputIt, typename T,

3
test/format-impl-test.cc
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@ -312,6 +312,7 @@ template <> struct numeric_limits<double_double> {
// is_iec559 is true for double-double in libstdc++.
static constexpr bool is_iec559 = true;
static constexpr int digits = 106;
static constexpr int digits10 = 33;
};
template <> struct is_floating_point<slow_float> : std::true_type {};
@ -341,7 +342,7 @@ TEST(format_impl_test, write_dragon_even) {
auto s = std::string();
fmt::detail::write<char>(std::back_inserter(s), slow_float(33554450.0f), {});
// Specializing is_floating_point is broken in MSVC.
if (!FMT_MSC_VERSION) EXPECT_EQ(s, "33554450");
if (!FMT_MSC_VERSION) EXPECT_EQ(s, "3.355445e+07");
}
#if defined(_WIN32) && !defined(FMT_USE_WRITE_CONSOLE)

3
test/format-test.cc
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@ -1068,7 +1068,8 @@ TEST(format_test, precision) {
EXPECT_EQ(fmt::format("{:#.0f}", 123.0), "123.");
EXPECT_EQ(fmt::format("{:.02f}", 1.234), "1.23");
EXPECT_EQ(fmt::format("{:.1g}", 0.001), "0.001");
EXPECT_EQ(fmt::format("{}", 1019666432.0f), "1019666400");
EXPECT_EQ(fmt::format("{}", 123456789.0f), "1.2345679e+08");
EXPECT_EQ(fmt::format("{}", 1019666432.0f), "1.0196664e+09");
EXPECT_EQ(fmt::format("{:.0e}", 9.5), "1e+01");
EXPECT_EQ(fmt::format("{:.1e}", 1e-34), "1.0e-34");