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Optimize the default FP formatting

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This commit is contained in:
Victor Zverovich
2025-05-30 13:01:45 -07:00
parent befbc5fdb8
commit d9d50495ac
1 changed files with 36 additions and 15 deletions
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51
include/fmt/format.h
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@ -1488,6 +1488,13 @@ template <typename Float> constexpr auto exponent_bias() -> int {
: std::numeric_limits<Float>::max_exponent - 1;
}
FMT_CONSTEXPR inline auto compute_exp_size(int exp) -> int {
auto prefix_size = 2; // sign + 'e'
auto abs_exp = exp >= 0 ? exp : -exp;
if (exp < 100) return prefix_size + 2;
return prefix_size + (abs_exp >= 1000 ? 4 : 3);
}
// Writes the exponent exp in the form "[+-]d{2,3}" to buffer.
template <typename Char, typename OutputIt>
FMT_CONSTEXPR auto write_exponent(int exp, OutputIt out) -> OutputIt {
@ -2348,6 +2355,16 @@ FMT_CONSTEXPR20 auto write_significand(OutputIt out, T significand,
buffer.end(), out);
}
enum { exp_lower = -4 };
// 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 DecimalFP,
typename Grouping = digit_grouping<Char>>
FMT_CONSTEXPR20 auto do_write_float(OutputIt out, const DecimalFP& f,
@ -2368,7 +2385,6 @@ 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;
return output_exp < exp_lower ||
output_exp >= (specs.precision > 0 ? specs.precision : exp_upper);
};
@ -2381,11 +2397,7 @@ FMT_CONSTEXPR20 auto do_write_float(OutputIt out, const DecimalFP& f,
} else if (significand_size == 1) {
decimal_point = Char();
}
auto abs_output_exp = output_exp >= 0 ? output_exp : -output_exp;
int exp_digits = 2;
if (abs_output_exp >= 100) exp_digits = abs_output_exp >= 1000 ? 4 : 3;
size += to_unsigned((decimal_point ? 1 : 0) + 2 + exp_digits);
size += to_unsigned((decimal_point ? 1 : 0) + compute_exp_size(output_exp));
char exp_char = specs.upper() ? 'E' : 'e';
auto write = [=](iterator it) {
if (s != sign::none) *it++ = detail::getsign<Char>(s);
@ -3309,14 +3321,6 @@ 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 {
@ -3396,7 +3400,24 @@ 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, exp_upper<T>(), {});
int significand_size = count_digits(dec.significand);
int exp = dec.exponent + significand_size - 1;
int exp_upper = detail::exp_upper<T>();
if (exp < exp_lower || exp >= exp_upper) {
auto has_decimal_point = significand_size != 1;
size_t size =
to_unsigned((s != sign::none ? 1 : 0) + significand_size +
(has_decimal_point ? 1 : 0) + compute_exp_size(exp));
auto it = reserve(out, size);
if (s != sign::none) *it++ = detail::getsign<Char>(s);
// Insert a decimal point after the first digit and add an exponent.
it = write_significand(it, dec.significand, significand_size, 1,
has_decimal_point ? '.' : Char());
*it++ = static_cast<Char>('e');
it = write_exponent<Char>(exp, it);
return base_iterator(out, it);
}
return write_float<Char>(out, dec, specs, s, exp_upper, {});
}
template <typename Char, typename OutputIt, typename T,