Variable Demand in Reservoir Constraints

[variola-m]

What language and solver does this apply to?
Python

Describe the problem you are trying to solve.
We're trying to simulate the inventory of multiple batch-machines. The in-/out-flow at each time step is variable and depends on each other: if the total production-batch is 100kg and machine A produces 30kg, then machine B needs to produce 70kg. Because we have other machines that consume these batches, we need a reservoir to track the inventories over time and make sure that it doesn't become negative.

This seems only possible with a reservoir constraint if the demands can contain integer variables. If I understood the code correctly, it already contains an implementation of a reservoir expanding algorithm that can handle variable demands, namely ExpandReservoirUsingCircuit, which could in theory be enabled with the sat-parameter expand_reservoir_using_circuit=True. Setting this to True still runs in an error during model validation in ValidateReservoirConstraint, as you do a hard check that demands are constant.

Describe the solution you'd like
Setting expand_reservoir_using_circuit in sat_parameters.proto to True should adjust the model validation and not run the part in ValidateReservoirConstraint that checks for constants.

Describe alternatives you've considered
A setting exists with which the model validation can be disabled by the user, as the rest of the code should run fine.

Additional context

expand_reservoir_using_circuit setting in ortools.sat.sat_paramters.proto with comment that explains variable demands are supported

  // Mainly useful for testing.
  //
  // If this and expand_reservoir_constraints is true, we use a different
  // encoding of the reservoir constraint using circuit instead of precedences.
  // Note that this is usually slower, but can exercise different part of the
  // solver. Note that contrary to the precedence encoding, this easily support
  // variable demands.
  //
  // WARNING: with this encoding, the constraint takes a slightly different
  // meaning. There must exist a permutation of the events occurring at the same
  // time such that the level is within the reservoir after each of these events
  // (in this permuted order). So we cannot have +100 and -100 at the same time
  // if the level must be between 0 and 10 (as authorized by the reservoir
  // constraint).
  optional bool expand_reservoir_using_circuit = 288 [default = false];

There already exists a todo comment to allow the user to disable the validation ortools.sat.cp_model_solver.cc:

  // Validate model_proto.
  // TODO(user): provide an option to skip this step for speed?