Hi all
While writing some program that works with OSM relations, I found cycles in the containment graph. For example, relation/2913585 https://www.openstreetmap.org/relation/2913585 is part of relation/12762833 https://www.openstreetmap.org/relation/12762833, which is again part of the same relation where we started. If a naive implementation were to traverse this relationship graph, it would run into an infinite loop. This isn’t a serious problem though; checking for cycles is easy enough. But it seems like a particularly interesting type of tagging mistake, so I thought why not share the fun with you.
In Switzerland, there’s currently three such cycles: relation/2913585 https://www.openstreetmap.org/relation/2913585 ⟶ {relation/12762833, relation/12762834} ⟶ relation/2913585 relation/4012524 https://www.openstreetmap.org/relation/4012524 ⟶ {relation/12762831, relation/12762832} ⟶ relation/4012524 relation/9202820 https://www.openstreetmap.org/relation/9202820 ⟶ relation/13987412 ⟶ relation/9202820
Best,
— Sascha