Support for permalinks

[kresp0]

It would be nice to be able to share the current map view and a specific route result with a permalink.

[ellenhp]

I do intend to support this eventually with some caveats. I don't want to add a URL-shortening service to Headway because I'm trying to make sure the backend doesn't store any user data.

Instead, I'm planning on putting the route endpoints and a hash of the selected route's polyline in the URL. When the link is opened, a routing request is made to get a list of routes, and all of the routes are checked against the hash in the URL. If one of the hashes matches, that alternate is automatically selected. Otherwise, an error is shown.

Since valhalla doesn't have access to traffic or any other dynamic data, this error will happen mostly in areas where Valhalla knows about road closures that are based on time of day.