How it works
First, a few rectangles are calculated (route_calc_selection):
- One that does include the position and destination point with an additional 25% at the ends with an order of 4
- One square-shaped of about 80 km edge length around each waypoint with an order of 8
- One square-shaped of about 20 km edge length around each waypoint with an order of 18
Waypoints are the position, destination and any waypoints in between (if routing with waypoints. The "order" of a road depends on which tile level it is placed in. Refer to binfile, specifically the How An object Is placed in a tile section, for details.
Then the map is queried with this rectangles and from the result a graph is built, consisting of points and segments. Then the graph gets flooded, beginning with the destination. This makes the graph re-useable when the position changes. The used algorithm is LPA* together with Fibonacci-Heaps to quickly get the lowest value point.
When no points are left (this could be optimized, because once the position is reached flooding the graph could stop) the graph is followed from the position back to the destination and a route path is build. This route path will be displayed just like a map.
LPA* is an evolution of the Dijkstra algorithm, which Navit has previously used. LPA* adds the ability to change the cost of individual segments and recalculate the affected parts of the graph. This was added in order to support dynamic traffic information but could be expanded to implement other functionality. For example, we could build and flood the route graph in an iterative manner: add a few segments, flood the graph, determine where we should continue expanding the graph and start the cycle again. We could also use this to speed up recalculation if the user deviates from the calculated route (which may require adding elements to the route graph).
Diagnosing Routing Problems
Turn on the route graph map in the Settings/Maps menu (Or from the Map menu in GTK). You will see lots of arrows and numbers. The numbers indicate the estimate of time in 10ths of seconds to reach your destination. The arrows will show the direction to the destination. If two very different numbers are close together but there should be a connection, there is most likely none.