VehicleMovementIfc

What a fleet needs from whatever moves its vehicles.

A dispatcher, a tour and a manifest have nothing to say about how a vehicle gets from one place to another. They name locations, ask how far away one is, command a journey and wait for it to end. This is that, and nothing else: everything above it can then be written once and run over more than one movement substrate.

Three substrates are in view and they differ only in how finely they discretise a journey. A zone-based guide path makes a decision at every zone boundary, which the network's geometry fixes. A continuous projection makes one at every interpolation step, which the modeller chooses. A free path makes none, because a move there is a single delay. What they share is this interface.

The contract

• positionNow is where the vehicle is at this instant, not where it last arrived. A substrate that cannot say must not answer with a stale location: every distance-based decision reads this, and a decision made on where a vehicle set off from is a decision made on the wrong fleet.

• pathDistanceTo is distance along the path the vehicle would actually take, by the substrate's own metric — never straight-line separation, unless the substrate is a plane and the two coincide. On a one-way loop a vehicle standing just past a point must go all the way round, and a rule scoring proximity would send the one furthest away.

• beginTravelTo commands a journey and does not suspend. It hands back the queue the caller must wait in, so that the two ends of a wait cannot disagree about where the wake will come from. Null means the vehicle was already there and no journey started.

• A second beginTravelTo while one is in progress is a redirection, not an error. Where a substrate cannot turn round instantly it may defer the change to its next decision point.

• isHalted means stopped short of the destination with nothing scheduled: whatever stopped it owns starting it again, through resumeHalted. It is not the same as blocked, which resolves itself when somebody else moves.

• distanceTravelled and operatingTime never decrease within a replication, and both are continuous across a redirection: a vehicle that turns round has still covered the ground it covered.

Implementations pass a common conformance suite, which is where these sentences are enforced rather than merely stated.

Inheritors