Function ieee754_parse_decimal

src/fl/stl/ieee754_string.cpp.hpp:261–427

} // namespace

u32 ieee754_parse_decimal(const char* s, fl::size len, fl::size* consumed) FL_NOEXCEPT {
    fl::size i = 0;

    auto fail = [&]() -> u32 {
        if (consumed) *consumed = 0;
        return 0;
    };

    // Skip leading whitespace.
    while (i < len && (s[i] == ' ' || s[i] == '\t' || s[i] == '\n' ||
                       s[i] == '\r' || s[i] == '\f' || s[i] == '\v')) {
        ++i;
    }

    // Sign.
    u32 sign_bit = 0;
    if (i < len && s[i] == '-') { sign_bit = kSignBitMask; ++i; }
    else if (i < len && s[i] == '+') { ++i; }

    // Mantissa: integer + optional fractional. Cap at 19 significant decimal
    // digits -- that is the max u64 can hold (10**19 < 2**64). Beyond that we
    // drop digits but keep tracking the decimal exponent so values like
    // "100000000000000000000" (1e20) still round to the right magnitude.
    fl::u64 mantissa = 0;
    int decimal_exp = 0;
    int significant_digits = 0;
    bool seen_decimal = false;
    bool seen_digit = false;
    constexpr int kMaxSignificantDigits = 19;

    while (i < len) {
        const char c = s[i];
        if (c >= '0' && c <= '9') {
            seen_digit = true;
            if (significant_digits < kMaxSignificantDigits) {
                // u64 max is ~1.8e19, mantissa * 10 + 9 stays below for 18 digits.
                mantissa = mantissa * 10u + static_cast<fl::u64>(c - '0');
                if (significant_digits != 0 || c != '0') {
                    ++significant_digits;
                }
                if (seen_decimal) {
                    --decimal_exp;
                }
            } else {
                // Out of mantissa precision -- track magnitude via decimal_exp only.
                if (!seen_decimal) {
                    ++decimal_exp;
                }
            }
            ++i;
        } else if (c == '.' && !seen_decimal) {
            seen_decimal = true;
            ++i;
        } else {
            break;
        }
    }