or-tools : Python 如果 `num_nurses` 不等于 `num_shifts` 员工调度不给出任何解决方案

Question

我正在尝试使用熟悉 google 或-工具。我尝试了 Employee scheduling python 示例的简化版本。

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()

如您所见，我保留的唯一限制条件是轮班和护士之间的关系。在 num_nurses = 3、num_shifts = 3 和 num_days = 1 的情况下，求解器能够找到 6 个解。但是，如果我将 num_shifts 更改为 2，求解器将 returns 0 个解决方案。这不应该也有 3 个解决方案吗（分配一个护士，让其他两个未分配）？

Answer 1

事实证明，这是 employee_scheduling 现在编写方式的限制。目前正在进行重写，应该会在几周内完成。

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

Answer 2

我推荐这个版本的班次安排，它实现了一些不同的约束：

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

or-tools : Python 如果 `num_nurses` 不等于 `num_shifts` 员工调度不给出任何解决方案

or-tools : Python employee scheduling does not give any solutions if `num_nurses` is not equal to `num_shifts`

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