I suspect you're referring to https://kurviger.de/en https://kurviger.de/en for motorcycling. This uses Graphhopper's builtin round-trip functionality (it was i

I suspect you're referring to https://kurviger.de/en https://kurviger.de/en for motorcycling. This uses Graphhopper's builtin round-trip functionality (it was incorporated by the author of that motorcycling route planner). You can find more about the Graphhopper API here https://docs.graphhopper.com/#operation/getRoute https://docs.graphhopper.com/#operation/getRoute, and it's completely open source at https://github.com/graphhopper/graphhopper https://github.com/graphhopper/graphhopper The motorcycling round-trip algorithm is quite crude, it relies on a user to pick a heading and then points N points in a circle on that heading and builds a route between them. It does not take into account any features of the map (e.g. forests, rivers, obstructions, etc). It's sufficient for longer distances though, but struggles on shorter distances. My project isn't open source yet, but I hope to make it so if/when I finish it!