Ndtyjky

I am trying to use get familiar with google or-tools. I tried a simplified version of the Employee scheduling python example.

from __future__ import print_function

import sys

from ortools.constraint_solver import pywrapcp





def main():

    # Creates the solver.

    solver = pywrapcp.Solver("employee_scheduling")



    num_nurses = 3

    num_shifts = 3

    num_days = 1



    # [START]

    # Create shift variables.

    shifts = {}



    for j in range(num_nurses):

        for i in range(num_days):

            shifts[(j, i)] = solver.IntVar(

                0, num_shifts - 1, "shifts(%i,%i)" % (j, i))

    shifts_flat = [shifts[(j, i)] for j in range(num_nurses)

                for i in range(num_days)]



    # Create nurse variables.

    nurses = {}



    for j in range(num_shifts):

        for i in range(num_days):

            nurses[(j, i)] = solver.IntVar(

                0, num_nurses - 1, "shift%d day%d" % (j, i))

    # Set relationships between shifts and nurses.

    for day in range(num_days):

        nurses_for_day = [nurses[(j, day)] for j in range(num_shifts)]



        for j in range(num_nurses):

            s = shifts[(j, day)]

            solver.Add(s.IndexOf(nurses_for_day) == j)



    # Create the decision builder.

    db = solver.Phase(shifts_flat, solver.CHOOSE_FIRST_UNBOUND,

                    solver.ASSIGN_MIN_VALUE)

    # Create the solution collector.

    solution = solver.Assignment()

    solution.Add(shifts_flat)

    collector = solver.AllSolutionCollector(solution)



    solver.Solve(db, [collector])

    print("Solutions found:", collector.SolutionCount())

    print("Time:", solver.WallTime(), "ms")

    print()





if __name__ == "__main__":

    main()

As you can see the only constraints I have kept is the relationships between shifts and nurses.
With num_nurses = 3, num_shifts = 3 and num_days = 1, the solver is able to find 6 solutions. However, if I change num_shifts to 2, the solver returns 0 solutions.
Shouldn't this have 3 solutions as well (assign one nurse, leave the other two unassigned) ?

As it turns out, this is a limitation with the way employee_scheduling is written right now. A rewrite of it is currently underway and should be done in a couple of weeks.

https://github.com/google/or-tools/issues/932

I recommend this version of shift scheduling that implements a few different constraints:

https://github.com/google/or-tools/blob/master/examples/python/shift_scheduling_sat.py