exponent64 function
Implementation
int exponent64(final double x) {
  final bias = maxexponent(x) - 1;
  final d = digits(x) - 1;
  return ((Float64List.fromList([x]).buffer.asUint32List(0).last &
              0x7fffffff) >>>
          (d - 32)) -
      bias +
      1;
}