in src/main/java/com/uber/h3core/H3Core.java [921:931]
public double greatCircleDistance(LatLng a, LatLng b, LengthUnit unit) {
double lat1 = toRadians(a.lat);
double lng1 = toRadians(a.lng);
double lat2 = toRadians(b.lat);
double lng2 = toRadians(b.lng);
if (unit == LengthUnit.rads) return h3Api.greatCircleDistanceRads(lat1, lng1, lat2, lng2);
else if (unit == LengthUnit.km) return h3Api.greatCircleDistanceKm(lat1, lng1, lat2, lng2);
else if (unit == LengthUnit.m) return h3Api.greatCircleDistanceM(lat1, lng1, lat2, lng2);
else throw new IllegalArgumentException(String.format("Invalid unit: %s", unit));
}