secret_santa.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.Collections.Generic;
using System.Linq;
using Google.OrTools.ConstraintSolver;


public class SecretSanta
{

  /**
   *
   * Secret Santa problem in Google CP Solver.
   *
   * From Ruby Quiz Secret Santa
   * http://www.rubyquiz.com/quiz2.html
   * """
   * Honoring a long standing tradition started by my wife's dad, my friends
   * all play a Secret Santa game around Christmas time. We draw names and
   * spend a week sneaking that person gifts and clues to our identity. On the
   * last night of the game, we get together, have dinner, share stories, and,
   * most importantly, try to guess who our Secret Santa was. It's a crazily
   * fun way to enjoy each other's company during the holidays.
   *
   * To choose Santas, we use to draw names out of a hat. This system was
   * tedious, prone to many 'Wait, I got myself...' problems. This year, we
   * made a change to the rules that further complicated picking and we knew
   * the hat draw would not stand up to the challenge. Naturally, to solve
   * this problem, I scripted the process. Since that turned out to be more
   * interesting than I had expected, I decided to share.
   *
   * This weeks Ruby Quiz is to implement a Secret Santa selection script.
   * *  Your script will be fed a list of names on STDIN.
   * ...
   * Your script should then choose a Secret Santa for every name in the list.
   * Obviously, a person cannot be their own Secret Santa. In addition, my friends
   * no longer allow people in the same family to be Santas for each other and your
   * script should take this into account.
   * """
   *
   *  Comment: This model skips the file input and mail parts. We
   *        assume that the friends are identified with a number from 1..n,
   *        and the families is identified with a number 1..num_families.
   *
   * Also see http://www.hakank.org/or-tools/secret_santa.py 
   * Also see http://www.hakank.org/or-tools/secret_santa2.cs 
   *
   */
  private static void Solve()
  {

    Solver solver = new Solver("SecretSanta");

    int[] family = {1,1,1,1, 2, 3,3,3,3,3, 4,4};
    int n = family.Length;

    Console.WriteLine("n = {0}", n);

    IEnumerable<int> RANGE = Enumerable.Range(0, n);

    //
    // Decision variables
    //
    IntVar[] x = solver.MakeIntVarArray(n, 0, n-1, "x");


    //
    // Constraints
    //
    solver.Add(x.AllDifferent());

    // Can't be one own"s Secret Santa
    // (i.e. ensure that there are no fix-point in the array.)
    foreach(int i in RANGE) {
      solver.Add(x[i] != i);
    }


    // No Secret Santa to a person in the same family
    foreach(int i in RANGE) {
      solver.Add(solver.MakeIntConst(family[i]) != family.Element(x[i]));
    }

    //
    // Search
    //
    DecisionBuilder db = solver.MakePhase(x,
                                          Solver.INT_VAR_SIMPLE,
                                          Solver.INT_VALUE_SIMPLE);

    solver.NewSearch(db);

    while (solver.NextSolution()) {
      Console.Write("x:  ");
      foreach(int i in RANGE) {
        Console.Write(x[i].Value() + " ");
      }
      Console.WriteLine();
    }

    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();

  }

  public static void Main(String[] args)
  {
    Solve();
  }
}