Route Optimization Issue: Unexpected Behavior with dimension evaluator of 0

[oscaroca]

What version of OR-Tools and what language are you using?
Version: main (9.11.4210)
Language: Python

Which solver are you using
Routing Solver

What operating system?
Ubuntu 22.04.4 LTS

What did you do?
Steps to reproduce the behavior:

Working on a route optimisation problem involving depots and deliveries, using multiple vehicles. The implementation concept is to make the depots have the vehicle load 0 units, and use a very large slack so that it can load everything necessary.
In the case of deliveries the slack is reduced to 0 so that the corresponding load is reduced.

def add_dimension_evaluator(self):
        if "dimensions" in self.drivers[0]: # Iterate over different dimensions
            capacity_dimensions = []
            for i in range(len(self.drivers[0]["dimensions"])): # Create a different dimension for each driver
                max_capacities = [driver["dimensions"][i] for driver in self.drivers]
                capacity_evaluator_index = self.routing.RegisterUnaryTransitCallback(
                    self.create_dimensions_evaluator(i),
                )

                self.routing.AddDimensionWithVehicleCapacity(
                    capacity_evaluator_index,
                    999999,
                    max_capacities,  # vehicle maximum capacities
                    True,  # start cumul to zero
                    f"Capacity-{i}",
                )
                capacity_dimension = self.routing.GetDimensionOrDie(f"Capacity-{i}")
                capacity_dimensions.append(capacity_dimension)

                for x in self.starts:
                    index = self.manager.NodeToIndex(x)
                    capacity_dimension.SlackVar(index).SetValue(0)
                for x in self.stops:
                    index = self.manager.NodeToIndex(x["destination"])
                    capacity_dimension.SlackVar(index).SetValue(0)

            return capacity_dimensions



    def create_dimensions_evaluator(self, dimension_index):
        # Define the dimension counters
        dimension = Counter()
        for stop in self.stops:
            if stop["kind"] == "pickup":
                dimension[stop["destination"]] += stop["dimensions"][dimension_index]
            elif stop["kind"] == "delivery":
                dimension[stop["destination"]] -= stop["dimensions"][dimension_index]

        for depot in self.depots:
            dimension[depot["destination"]] = 0

        def capacity_evaluator(from_index):
            """Returns the demand of the node."""
            # Convert from routing variable Index to demands NodeIndex.
            from_node = self.manager.IndexToNode(from_index)
            return dimension.get(from_node, 0)

        return capacity_evaluator

What did you expect to see

Ideally, vehicles arriving at the depot would load what they need and deliveries would be made.

What did you see instead?

The problem is that it only generates the route for 1 vehicle.
I have tried changing the load that a depot gives to 1 and everything works as expected, (the problem is that it generates errors with vehicles without capacities, or that already have the maximum load. That's why it should be 0).
I was wondering why it behaves so differently using 0 or a number other than 0.

[lperron]

0 is not a node. You need to use StartVar(vehicle).

[Mizux]

If you want to use a callback per vehicle type you need to first register them all THEN call this method once after the loop

or-tools/ortools/routing/routing.h

Lines 698 to 701 in c330aec

	bool AddDimensionWithVehicleTransitAndCapacity(
	const std::vector<int>& evaluator_indices, int64_t slack_max,
	std::vector<int64_t> vehicle_capacities, bool fix_start_cumul_to_zero,
	const std::string& name);

notice the first argument, an array of registered callback indices...

note: since it is an array of callback indices you could use the same callback for vehicle sharing the same properties...

[oscaroca]

I understand the proposal, but the problem remains the same. I am sorry if I have explained myself badly.

The problem lies in the value that is put to the depots in the callback. The original idea is to put 0 and let the slack load what is necessary for the stops. The problem is that it only creates routes for 1 driver.

If we use as a value for the depots in the callback the maximum capacity of the vehicle (for example), it creates the different routes, but unless the vehicle can unload 100% of its capacity it will not be able to return to the depot to load. As far as I know (and have tested), it is not possible to set the slack range to negative values.