Union show hole as multipolygon

[samneefs]

When I arrange multiple polygons in a way so that they should form a polygon with a hole, the resulting geoJson is formated as if the hole is another polygon in the multipolygon
[
[ [ outer polygon ] ],
[ [ inner polygon/hole ] ]
]
Whereas it should be
[
[ [ outer polygon ],
[ inner polygon/hole ] ]
]
I have an example (strangly it shows correct, in the viewer, but with F12 you'll see the wrong geoJson
https://codepen.io/Sakke/pen/aYbeEM?editors=0010

The following example is related and does show the error in the viewer:
When I add an additional polygon (making it a true multipolygon), then I also goes wrong.
https://codepen.io/Sakke/pen/dmPoMp?editors=0010

[samneefs]

I was trying to find the reason and it seems to be with if(!resultEvents[i].isExteriorRing)
the original is an exterior ring, and because of that isExteriorRing=true.
Then it does result.push(contour) where I guess it should have done result[result.length - 1].push(contour[0]);
However I have no idea how I could validate it otherwise. Maybe with a method where I can check if one polygon is completely contained in the other, then I could make it a hole. However that would seem to be a workaround.

[samneefs]

It seems this is inherent to the Martinez algorithm: you still have to figure out which polygon is a hole and to which polygon the hole belongs.
In my code I sweep trough the polygons to try and figure this out, however my implementation probably isn't quite performant.

[rowanwins]

Resolved by #113