bus_schedule.cs

//
// Copyright 2012 Hakan Kjellerstrand
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

using System;
using System.Collections;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;
using Google.OrTools.ConstraintSolver;

public class BusSchedule
{


  /**
   *
   * Bus scheduling.
   *
   * Minimize number of buses in timeslots.
   *
   * Problem from Taha "Introduction to Operations Research", page 58.
   * 
   * This is a slightly more general model than Taha's.
   *
   * Also see, http://www.hakank.org/or-tools/bus_schedule.py
   *
   */
  private static long Solve(long num_buses_check = 0)
  {

    Solver solver = new Solver("BusSchedule");

    //
    // data
    //
    int time_slots = 6;
    int[] demands = {8, 10, 7, 12, 4, 4};
    int max_num = demands.Sum();

    //
    // Decision variables
    //

    // How many buses start the schedule at time slot t
    IntVar[] x = solver.MakeIntVarArray(time_slots, 0, max_num, "x");
    // Total number of buses
    IntVar num_buses = x.Sum().VarWithName("num_buses");

    //
    // Constraints
    //

    // Meet the demands for this and the next time slot.
    for(int i = 0; i < time_slots - 1; i++) {
      solver.Add(x[i]+x[i+1] >= demands[i]);
    }

    // The demand "around the clock"
    solver.Add(x[time_slots-1] + x[0] - demands[time_slots-1] == 0);

    // For showing all solutions of minimal number of buses
    if (num_buses_check > 0) {
      solver.Add(num_buses == num_buses_check);
    }


    //
    // Search
    //
    DecisionBuilder db = solver.MakePhase(x,
                                          Solver.CHOOSE_FIRST_UNBOUND,
                                          Solver.ASSIGN_MIN_VALUE);

    if (num_buses_check == 0) {

      // Minimize num_buses
      OptimizeVar obj = num_buses.Minimize(1);
      solver.NewSearch(db, obj);

    } else {

      solver.NewSearch(db);

    }

    long result = 0;
    while (solver.NextSolution()) {
      result = num_buses.Value();
      Console.Write("x: ");
      for(int i = 0; i < time_slots; i++) {
        Console.Write("{0,2} ", x[i].Value());
      }
      Console.WriteLine("num_buses: " + num_buses.Value());
    }

    Console.WriteLine("\nSolutions: {0}", solver.Solutions());
    Console.WriteLine("WallTime: {0}ms", solver.WallTime());
    Console.WriteLine("Failures: {0}", solver.Failures());
    Console.WriteLine("Branches: {0} ", solver.Branches());

    solver.EndSearch();

    return result;

  }



  public static void Main(String[] args)
  {

    Console.WriteLine("Check for minimum number of buses: ");
    long num_buses = Solve();
    Console.WriteLine("\n... got {0} as minimal value.", num_buses);
    Console.WriteLine("\nAll solutions: ", num_buses);
    num_buses = Solve(num_buses);

  }
}