Function FASTFLOAT_CONSTEXPR14 parse_infnan

Source/external/fast_float.h:3128–3172

 **/
template <typename T>
from_chars_result FASTFLOAT_CONSTEXPR14
parse_infnan(const char *first, const char *last, T &value)  noexcept  {
  from_chars_result answer{};
  answer.ptr = first;
  answer.ec = std::errc(); // be optimistic
  bool minusSign = false;
  if (*first == '-') { // assume first < last, so dereference without checks; C++17 20.19.3.(7.1) explicitly forbids '+' here
	  minusSign = true;
	  ++first;
  }
#if FASTFLOAT_ALLOWS_LEADING_PLUS // disabled by default
  if (*first == '+') {
	  ++first;
  }
#endif
  if (last - first >= 3) {
	if (fastfloat_strncasecmp(first, "nan", 3)) {
	  answer.ptr = (first += 3);
	  value = minusSign ? -std::numeric_limits<T>::quiet_NaN() : std::numeric_limits<T>::quiet_NaN();
	  // Check for possible nan(n-char-seq-opt), C++17 20.19.3.7, C11 7.20.1.3.3. At least MSVC produces nan(ind) and nan(snan).
	  if(first != last && *first == '(') {
		for(const char* ptr = first + 1; ptr != last; ++ptr) {
		  if (*ptr == ')') {
			answer.ptr = ptr + 1; // valid nan(n-char-seq-opt)
			break;
		  }
		  else if(!(('a' <= *ptr && *ptr <= 'z') || ('A' <= *ptr && *ptr <= 'Z') || ('0' <= *ptr && *ptr <= '9') || *ptr == '_'))
			break; // forbidden char, not nan(n-char-seq-opt)
		}
	  }
	  return answer;
	}
	if (fastfloat_strncasecmp(first, "inf", 3)) {
	  if ((last - first >= 8) && fastfloat_strncasecmp(first + 3, "inity", 5)) {
		answer.ptr = first + 8;
	  } else {
		answer.ptr = first + 3;
	  }
	  value = minusSign ? -std::numeric_limits<T>::infinity() : std::numeric_limits<T>::infinity();
	  return answer;
	}
  }
  answer.ec = std::errc::invalid_argument;
  return answer;
}

/**
 * Returns true if the floating-pointing rounding mode is to 'nearest'.