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finalize and document parse_digits_until_19: it additionally slightly faster now.

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HedgehogInTheCPP 2026-08-20 20:55:28 +03:00
parent e7a29e0915
commit add21b76d1
1 changed files with 6 additions and 9 deletions
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15
include/fast_float/ascii_number.h
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@ -271,10 +271,7 @@ fastfloat_really_inline FASTFLOAT_CONSTEXPR20 void
parse_digits_until_19(char const *&p, char const *pend, am_mant_t &mantissa) {
#if FASTFLOAT_X86_SIMD >= 31
if (!is_constant_evaluated()) {
/*
* If mantissa has fewer than two digits, a complete
* 16-digit block cannot exceed 10^18 - 1.
*/
// If mantissa < 10^2, a 16-digit block is guaranteed < 10^18 - 1.
while (std::distance(p, pend) >= 16 && mantissa < 100) {
auto const value = parse_16_digits(p);
mantissa = mantissa * 10000000000000000ULL + value;
@ -282,24 +279,24 @@ parse_digits_until_19(char const *&p, char const *pend, am_mant_t &mantissa) {
}
}
#endif
// An 8-digit block is safe while: mantissa < 10^10 because the result is
// guaranteed < 10^18.
// If mantissa < 10^10, a 8-digit block is guaranteed < 10^18 - 1.
while (std::distance(p, pend) >= 8 && mantissa < 10000000000ULL) {
auto const value = parse_8_digits(p);
mantissa = mantissa * 100000000ULL + value;
p += 8;
}
// ?????
while (std::distance(p, pend) >= 4 && mantissa < 10000000000ULL) {
// If mantissa < 10^12, a 4-digit block is guaranteed < 10^18 - 1.
while (std::distance(p, pend) >= 4 && mantissa < 1000000000000ULL) {
auto const value = read_chars_to_unsigned<uint32_t>(p);
mantissa = mantissa * 10000 + parse_4_digits(value);
p += 4;
}
// Finalizer
// If mantissa < 10^19, we should parse digits one by one.
while (p != pend && mantissa < minimal_nineteen_digit_integer) {
mantissa = mantissa * 10 + static_cast<uint8_t>(*p - '0');
++p;
}
// While mantissa >= 10^19, we should stop parsing digits.
}
template <typename UC, FASTFLOAT_ENABLE_IF(!std::is_same<UC, char>::value) = 0>