Allow finding intermediate solutions with Minizinc

[timvandam]

I am trying to find all intermediate solutions for an optimization problem. CPM_minizinc does not allow this, as solve() will only return the last solution, and solveAll() does not work for optimization problems. I did not encounter this problem for the CPM_ortools model, where I can provide a callback to solve() to store all intermediate solutions myself

[timvandam]

For some additional context, I pass intermediate_solutions=True to the solve() function to get multiple solutions. I am then met with the Warning: multiple solutions found, only returning last one log

[timvandam]

The easiest solution would be to simply have an argument change this behavior of solve(). Or otherwise, a callback function argument that is called for each solution that was found (I suppose this is the best option). I am willing to make a PR for this if this sounds good. Let me know

[timvandam]

For those looking for a workaround, I extended the CPM_minizinc class to store objective values the following way:

class MinizincSolver(CPM_minizinc):
    """
    Wrapper around CPM_minizinc that allows capturing all (intermediate) solutions,
    because CPM_minizinc abandons all but the last solution
    """

    objective_values: List[int]

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        self.objective_values = []

    def _post_solve(self, mzn_result):
        solutions = mzn_result.solution if is_any_list(mzn_result.solution) else [mzn_result.solution]
        for sol in solutions:
            self.objective_values.append(sol.objective)
        return super()._post_solve(mzn_result)

I then call solver.solve(intermediate_solutions=True), and use solver.objective_values to access the information I need

[tias]

Hi Tim,

I think that is a very elegant solution.

To have something to main CPMpy would indeed be best with a callback, but if the above solution suits you well then I think we can leave it at that?

[timvandam]

Hi Tim,

I think that is a very elegant solution.

To have something to main CPMpy would indeed be best with a callback, but if the above solution suits you well then I think we can leave it at that?

Yes, this works for me. We can leave it at that, but it may be nice for future users. However, I am not sure how common this requirement is