/source/library/Interlace/Collections/TopologicalSort.cs

#region Using Directives and Copyright Notice



// Copyright (c) 2007-2010, Computer Consultancy Pty Ltd

// All rights reserved.

// 

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// modification, are permitted provided that the following conditions are met:

//     * Redistributions of source code must retain the above copyright

//       notice, this list of conditions and the following disclaimer.

//     * Redistributions in binary form must reproduce the above copyright

//       notice, this list of conditions and the following disclaimer in the

//       documentation and/or other materials provided with the distribution.

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//       names of its contributors may be used to endorse or promote products

//       derived from this software without specific prior written permission.

// 

// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" 

// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 

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using System;

using System.Collections.Generic;

using System.Text;



#endregion



namespace Interlace.Collections

{

    public delegate IEnumerable<T> EdgeGetter<T>(T forObject);



    public class TopologicalSort

    {

        enum VertexState

        {

            Unexplored = 0,

            Discovered,

            Finished

        }



        public static ICollection<T> Sort<T>(ICollection<T> objects, EdgeGetter<T> edgeGetter)

        {

            List<T> finishedObjects = new List<T>();

            Dictionary<T, VertexState> states = new Dictionary<T, VertexState>(objects.Count);



            foreach (T o in objects)

            {

                if (states.ContainsKey(o)) 

                {

                    throw new TopologicalSortException("An object in a list to be topologically " +

                        "sorted is equal to another object in the list, which is not valid.");

                }



                states[o] = VertexState.Unexplored;

            }



            foreach (T visiting in objects) 

            {

                if (states[visiting] == VertexState.Unexplored)

                {

                    DepthFirstSearchVisit(objects, edgeGetter, states, visiting, finishedObjects);

                }

            }



            finishedObjects.Reverse();



            return finishedObjects;

        }



        static void DepthFirstSearchVisit<T>(ICollection<T> objects, EdgeGetter<T> edgeGetter, Dictionary<T, VertexState> states, T visiting, List<T> finishedObjects)

        {

            states[visiting] = VertexState.Discovered;



            foreach (T edge in edgeGetter(visiting))

            {

                if (states[edge] == VertexState.Unexplored)

                {

                    DepthFirstSearchVisit(objects, edgeGetter, states, edge, finishedObjects);

                } 

                else if (states[edge] == VertexState.Discovered)

                {

                    // The edge is a back-edge; the graph is cyclic:



                    throw new TopologicalSortException("The supplied graph is cyclic, and therefore has " +

                        "no topological sort.");

                }

            }

            

            states[visiting] = VertexState.Finished;



            finishedObjects.Add(visiting);

        }

    }

}