double sign(double v) => v > 0 ? 1 : -1;
HSLuvColor lerpHsluv(HSLuvColor a, HSLuvColor b, double p) {
  // find the shortest route through the hue circle
  var forwards = (b.hue - a.hue);
  var backwards = -(360 - forwards.abs()) * sign(forwards);
  var huePath = forwards.abs() < backwards.abs() ? forwards : backwards;
  return HSLuvColor.fromHSL(
      (a.hue   p * huePath) % 360,
      lerpDouble(a.saturation, b.saturation, p)!,
      lerpDouble(a.lightness, b.lightness, p)!);
}