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The trolley is a resource shared by all the jobs in the problem. Each activity consisting of loading/unloading an item on/from the trolley requires the trolley to be in the same area as the item. Notice that the trolley cannot be represented as a unary resource (class IloUnaryResource) because there is no limit on the number of load/unload activities that can be performed in parallel on the same trolley provided that the trolley is in a suitable area. We want to associate a state variable with the trolley (its position), and express the constraint that two loading or unloading activities that require the trolley to be at different positions cannot overlap.

An instance of the class IloStateResource is used to express this constraint. A state resource represents a resource, by default of infinite capacity, whose state can vary over time. Each activity that uses a state resource may require a state resource to be in a specific state or in any of a given set of states. Two activities cannot overlap if they require incompatible states during their execution.

const IloInt MACH1 = 1; // Machine 1 area
const IloInt MACH2 = 2; // Machine 2 area
const IloInt MACH3 = 3; // Machine 3 area
const IloInt AREAA = 4; // Arrival area
const IloInt AREAS = 5; // Stock area

  /* CREATE THE TROLLEY POSITION RESOURCE */
  IloAnyArray arrayOfPossibleAreas(env,
                                   5,
                                   (IloAny)AREAA,
                                   (IloAny)MACH1,
                                   (IloAny)MACH2,
                                   (IloAny)MACH3,
                                   (IloAny)AREAS);
  IloAnySet setOfPossibleAreas(env,arrayOfPossibleAreas);
  trolley = IloStateResource(env,setOfPossibleAreas, "trolley");

As in the job-shop scheduling problem, each machine is represented as a unary resource (class IloUnaryResource).

  /* CREATE THE MACHINES RESOURCES */
  char buffer[256];
  machines = IloArray<IloUnaryResource>(env, 3);
  for (i = 0; i < 3; i++) {
    sprintf(buffer, "machine%ld", i + 1);
    machines[i] = IloUnaryResource(env, buffer);
  }