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I'm actually developping a system for bus ticket reservation. The provider has many routes and different trips. I've setup a rather comprehensive database that maps all this together, but i'm having trouble getting the pathing algorithm working when i comes to cross route reservation.

For example, the user wants to go from Montreal to Sherbrooke, he'll only take what we call here Route #47. But in the event he goes to Sutton instead of Sherbrooke, he now has to transfer into route #53 at some point.

Now, it isn't too hard to detect one and only one transfer. But when i comes to detecting what are the options he can do to cross multiple routes, i'm kinda scared. I've devised a cute and relatively efficient way to do so on 1-3 hops using only SQL but i'm wondering how i should organize all this in a much broader spectrum as the client will probably not stay with 2 routes for the rest of is life.

Example of what i've thought of so far:

StartingStop
joins to Route
joins to StopsOfTheRoute
joins to TransfersOnThatStop
joins to TargetStopOfThatTransfer
joins to RouteOfThatStop
joins to StopsOfThatNewRoute
[wash rince repeat for more hops]
where StopsOFThatNewRoute = EndingStop

Problem is, if i have more than 3 hops, i'm sure my SQL server will choke rather fast under the pressure, even if i correctly index my database, i can easily predict a major failure eventually...

Thank you

Mathieu Dumoulin
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1 Answers1

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My understanding of your problem: You are looking for an algorithm that will help you identify a suitable path (covering segments of one or more routes).

This is as Jason correctly points out, a pathfinding problem. For this kind of problem, you should probably start by having a look at Wikipedia's article on pathfinding, and then dig into the details of Djikstra's algorithm. That will probably get you started.

You will however most likely soon realise that your data model might pose a problem, both from a structure and performance perspective. Typical example would be if you need to manage time constraints: which path has the shortest travel time, assuming you find several? One path might be shortest, but only provide one ride per day, while another path might be longer but provide several rides per day.

A possible way of handling this is to create a graph where each node corresponds to a particular stop, at a particular time. An edge would connect from this stop in room-time to both other geographical stops, as well as itself at the next point in time.

My suggestion would be to start by reading up on the pathfinding algorithms, then revisit your data model with regards to any constraints you might have. Then, focus on the structure for storing the data, and the structure for seaching for paths.

Suggestion (not efficient, but could work if you have a sufficient amount of RAM to spare). Use the relational database server for storing the basics: stops, which routes are connected to which stops and so on. Seems you have this covered already. Then build an in-memory representation of a graph given the constraints that you have. You could probably build your own library for this pretty fast (I am not aware of any such libraries for PHP).

Another alternative could be to use a graph database such as Neo4j and its REST interface. I guess this will require significant some redesign of your application.

Hope this gives you some helpful pointers.

David Pettersson
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    Thank you, actually, i'm not looking the most efficient path but how to list all possible results within a certain timeframe that match the expected path. I thank you for your information, since you are the only one that actually gave a concise answer, i'll mark you as the answerer. Good luck for the future... – Mathieu Dumoulin Oct 17 '12 at 01:15
  • The data model suggested will support this, but you'll need to adjust the algorithm. The stop criteria is not one single optimal solution, but rather the time limit. – David Pettersson Oct 17 '12 at 07:26