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Optimize the default FP formatting

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This commit is contained in:
Victor Zverovich
2025-08-03 10:13:13 -07:00
parent 23059d558e
commit add164f6b3
1 changed files with 50 additions and 8 deletions
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58
include/fmt/format.h
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@@ -519,6 +519,11 @@ template <typename T, typename OutputIt>
constexpr auto to_pointer(OutputIt, size_t) -> T* {
return nullptr;
}
template <typename T> FMT_CONSTEXPR auto to_pointer(T*& ptr, size_t n) -> T* {
T* begin = ptr;
ptr += n;
return begin;
}
template <typename T>
FMT_CONSTEXPR20 auto to_pointer(basic_appender<T> it, size_t n) -> T* {
buffer<T>& buf = get_container(it);
@@ -1615,6 +1620,15 @@ constexpr auto convert_float(T value) -> convert_float_result<T> {
return static_cast<convert_float_result<T>>(value);
}
template <bool C, typename T, typename F, FMT_ENABLE_IF(C)>
auto select(T true_value, F) -> T {
return true_value;
}
template <bool C, typename T, typename F, FMT_ENABLE_IF(!C)>
auto select(T, F false_value) -> F {
return false_value;
}
template <typename Char, typename OutputIt>
FMT_CONSTEXPR FMT_NOINLINE auto fill(OutputIt it, size_t n,
const basic_specs& specs) -> OutputIt {
@@ -3455,25 +3469,53 @@ FMT_CONSTEXPR20 auto write(OutputIt out, T value) -> OutputIt {
return write_nonfinite<Char>(out, std::isnan(value), {}, s);
auto dec = dragonbox::to_decimal(static_cast<fast_float_t<T>>(value));
int significand_size = count_digits(dec.significand);
int exp = dec.exponent + significand_size - 1;
if (use_fixed(exp, detail::exp_upper<T>())) {
auto significand = dec.significand;
int significand_size = count_digits(significand);
int exponent = dec.exponent + significand_size - 1;
if (use_fixed(exponent, detail::exp_upper<T>())) {
return write_fixed<Char, fallback_digit_grouping<Char>>(
out, dec, significand_size, Char('.'), {}, s);
}
// Write value in the exponential format.
const char* prefix = "e+";
int abs_exponent = exponent;
if (exponent < 0) {
abs_exponent = -exponent;
prefix = "e-";
}
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));
size_t size = std::is_pointer<OutputIt>::value
? 0u
: to_unsigned((s != sign::none ? 1 : 0) + significand_size +
(has_decimal_point ? 1 : 0) +
(abs_exponent >= 100 ? 5 : 4));
if (auto ptr = to_pointer<Char>(out, size)) {
if (s != sign::none) *ptr++ = Char('-');
if (has_decimal_point) {
auto begin = ptr;
ptr = format_decimal<Char>(ptr, significand, significand_size + 1);
*begin = begin[1];
begin[1] = '.';
} else {
*ptr++ = static_cast<Char>('0' + significand);
}
memcpy(ptr, prefix, 2);
ptr += 2;
if (abs_exponent >= 100) {
*ptr++ = static_cast<Char>('0' + abs_exponent / 100);
abs_exponent %= 100;
}
write2digits(ptr, static_cast<unsigned>(abs_exponent));
return select<std::is_pointer<OutputIt>::value>(ptr + 2, out);
}
auto it = reserve(out, size);
if (s != sign::none) *it++ = Char('-');
// Insert a decimal point after the first digit and add an exponent.
it = write_significand(it, dec.significand, significand_size, 1,
it = write_significand(it, significand, significand_size, 1,
has_decimal_point ? Char('.') : Char());
*it++ = Char('e');
it = write_exponent<Char>(exp, it);
it = write_exponent<Char>(exponent, it);
return base_iterator(out, it);
}