Dfs.cs

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#region License
/*This file is part of Satsuma Graph Library
Copyright © 2013 Balázs Szalkai

This software is provided 'as-is', without any express or implied
warranty. In no event will the authors be held liable for any damages
arising from the use of this software.

Permission is granted to anyone to use this software for any purpose,
including commercial applications, and to alter it and redistribute it
freely, subject to the following restrictions:

   1. The origin of this software must not be misrepresented; you must not
   claim that you wrote the original software. If you use this software
   in a product, an acknowledgment in the product documentation would be
   appreciated but is not required.

   2. Altered source versions must be plainly marked as such, and must not be
   misrepresented as being the original software.

   3. This notice may not be removed or altered from any source
   distribution.*/
#endregion

using System;
using System.Collections.Generic;
using System.Linq;

namespace Satsuma
{
    public abstract class Dfs
    {
        public enum Direction
        {
            Undirected,
            Forward,
            Backward
        }

        protected IGraph Graph { get; private set; }
        private HashSet<Node> traversed;
        private ArcFilter arcFilter;

        protected int Level { get; private set; }

        public void Run(IGraph graph, IEnumerable<Node> roots = null)
        {
            Graph = graph;

            Direction direction;
            Start(out direction);
            switch (direction)
            {
                case Direction.Undirected: arcFilter = ArcFilter.All; break;
                case Direction.Forward: arcFilter = ArcFilter.Forward; break;
                default: arcFilter = ArcFilter.Backward; break;
            }

            traversed = new HashSet<Node>();
            foreach (var node in (roots ?? Graph.Nodes()))
            {
                if (traversed.Contains(node)) continue;

                Level = 0;
                if (!Traverse(node, Arc.Invalid)) break;
            }
            traversed = null;

            StopSearch();
        }

        private bool Traverse(Node node, Arc arc)
        {
            traversed.Add(node);
            if (!NodeEnter(node, arc)) return false;

            foreach (var b in Graph.Arcs(node, arcFilter))
            {
                if (b == arc) continue;

                Node other = Graph.Other(b, node);
                if (traversed.Contains(other))
                {
                    if (!BackArc(node, b)) return false;
                    continue;
                }

                Level++;
                if (!Traverse(other, b)) return false;
                Level--;
            }

            return NodeExit(node, arc);
        }

        protected abstract void Start(out Direction direction);

        protected virtual bool NodeEnter(Node node, Arc arc) { return true; }

        protected virtual bool NodeExit(Node node, Arc arc) { return true; }

        protected virtual bool BackArc(Node node, Arc arc) { return true; }

        protected virtual void StopSearch() { }
    }
}