How to sort depended objects by dependency

I have a collection:

List<VPair<Item, List<Item>> dependencyHierarchy;

The first item in pair is some object (item) and the second one is a collection of the same type objects that the first one depends on. I want to get a List<Item> in order of dependency, so there are not items that depend on the first element and so on (no cycled dependency!).

Input:

Item4 depends on Item3 and Item5
Item3 depends on Item1
Item1 does not depend on any one
Item2 depends on Item4
Item5 does not depend on any one

Result:

Item1
Item5
Item3
Item4
Item2

Thank you.

SOLUTION:

Topological Sorting (thanks to Loïc Février for idea)

and

example on C#, example on Java (thanks to xcud for great examples)

Having struggled with this for a while, here's my attempt at a Linq style TSort extension method:

public static IEnumerable<T> TSort<T>( this IEnumerable<T> source, Func<T, IEnumerable<T>> dependencies, bool throwOnCycle = false )
{
	var sorted = new List<T>();
	var visited = new HashSet<T>();

	foreach( var item in source )
		Visit( item, visited, sorted, dependencies, throwOnCycle );

	return sorted;
}

private static void Visit<T>( T item, HashSet<T> visited, List<T> sorted, Func<T, IEnumerable<T>> dependencies, bool throwOnCycle )
{
	if( !visited.Contains( item ) )
	{
		visited.Add( item );

		foreach( var dep in dependencies( item ) )
			Visit( dep, visited, sorted, dependencies, throwOnCycle );

		sorted.Add( item );
	}
	else
	{
		if( throwOnCycle && !sorted.Contains( item ) )
			throw new Exception( "Cyclic dependency found" );
	}
}

Perfect example to use a topological sort :

http://en.wikipedia.org/wiki/Topological_sorting

It will give you exactly what you need.

There's a nuget for that.

For those of us who prefer not to re-invent the wheel: use nuget to install the QuickGraph .NET library, which includes multiple graph algorithms including topological sort.

To use it, you need to create an instance of AdjacencyGraph<,> such as AdjacencyGraph<String, SEdge<String>>. Then, if you include the appropriate extensions:

using QuickGraph.Algorithms;

You can call:

var sorted = myGraph.TopologicalSort();

To get a list of sorted nodes.

I liked DMM's answer, but it assumes that the input nodes are leaves (which may or may not be what is expected).

I am posting an alternate solution using LINQ that does not make this assumption. In addition, this solution uses yield return to be able to quickly return the leaves (using e.g. TakeWhile).

public static IEnumerable<T> TopologicalSort<T>(this IEnumerable<T> nodes, 
                                                Func<T, IEnumerable<T>> connected)
{
    var elems = nodes.ToDictionary(node => node, 
                                   node => new HashSet<T>(connected(node)));
    while (elems.Count > 0)
    {
        var elem = elems.FirstOrDefault(x => x.Value.Count == 0);
        if (elem.Key == null)
        {
            throw new ArgumentException("Cyclic connections are not allowed");
        }
        elems.Remove(elem.Key);
        foreach (var selem in elems)
        {
            selem.Value.Remove(elem.Key);
        }
        yield return elem.Key;
    }
}

This is my own re-implementation of Topological sorting, the idea is based on http://tawani.blogspot.com/2009/02/topological-sorting-and-cyclic.html (The ported Java source code consumes too much memory, checking 50k objects costs 50k50k4 = 10GB which is unacceptable. In addition, it still has Java coding convention some places)

using System.Collections.Generic;
using System.Diagnostics;

namespace Modules
{
    /// <summary>
    /// Provides fast-algorithm and low-memory usage to sort objects based on their dependencies. 
    /// </summary>
    /// <remarks>
    /// Definition: http://en.wikipedia.org/wiki/Topological_sorting
    /// Source code credited to: http://tawani.blogspot.com/2009/02/topological-sorting-and-cyclic.html    
    /// Original Java source code: http://www.java2s.com/Code/Java/Collections-Data-Structure/Topologicalsorting.htm
    /// </remarks>
    /// <author>ThangTran</author>
    /// <history>
    /// 2012.03.21 - ThangTran: rewritten based on <see cref="TopologicalSorter"/>.
    /// </history>
    public class DependencySorter<T>
    {
        //**************************************************
        //
        // Private members
        //
        //**************************************************

        #region Private members

        /// <summary>
        /// Gets the dependency matrix used by this instance.
        /// </summary>
        private readonly Dictionary<T, Dictionary<T, object>> _matrix = new Dictionary<T, Dictionary<T, object>>();

        #endregion


        //**************************************************
        //
        // Public methods
        //
        //**************************************************

        #region Public methods

        /// <summary>
        /// Adds a list of objects that will be sorted.
        /// </summary>
        public void AddObjects(params T[] objects)
        {
            // --- Begin parameters checking code -----------------------------
            Debug.Assert(objects != null);
            Debug.Assert(objects.Length > 0);
            // --- End parameters checking code -------------------------------

            // add to matrix
            foreach (T obj in objects)
            {
                // add to dictionary
                _matrix.Add(obj, new Dictionary<T, object>());
            }
        }

        /// <summary>
        /// Sets dependencies of given object.
        /// This means <paramref name="obj"/> depends on these <paramref name="dependsOnObjects"/> to run.
        /// Please make sure objects given in the <paramref name="obj"/> and <paramref name="dependsOnObjects"/> are added first.
        /// </summary>
        public void SetDependencies(T obj, params T[] dependsOnObjects)
        {
            // --- Begin parameters checking code -----------------------------
            Debug.Assert(dependsOnObjects != null);
            // --- End parameters checking code -------------------------------

            // set dependencies
            Dictionary<T, object> dependencies = _matrix[obj];
            dependencies.Clear();

            // for each depended objects, add to dependencies
            foreach (T dependsOnObject in dependsOnObjects)
            {
                dependencies.Add(dependsOnObject, null);
            }
        }

        /// <summary>
        /// Sorts objects based on this dependencies.
        /// Note: because of the nature of algorithm and memory usage efficiency, this method can be used only one time.
        /// </summary>
        public T[] Sort()
        {
            // prepare result
            List<T> result = new List<T>(_matrix.Count);

            // while there are still object to get
            while (_matrix.Count > 0)
            {
                // get an independent object
                T independentObject;
                if (!this.GetIndependentObject(out independentObject))
                {
                    // circular dependency found
                    throw new CircularReferenceException();
                }

                // add to result
                result.Add(independentObject);

                // delete processed object
                this.DeleteObject(independentObject);
            }

            // return result
            return result.ToArray();
        }
        
        #endregion


        //**************************************************
        //
        // Private methods
        //
        //**************************************************

        #region Private methods

        /// <summary>
        /// Returns independent object or returns NULL if no independent object is found.
        /// </summary>
        private bool GetIndependentObject(out T result)
        {
            // for each object
            foreach (KeyValuePair<T, Dictionary<T, object>> pair in _matrix)
            {
                // if the object contains any dependency
                if (pair.Value.Count > 0)
                {
                    // has dependency, skip it
                    continue;
                }

                // found
                result = pair.Key;
                return true;
            }

            // not found
            result = default(T);
            return false;
        }

        /// <summary>
        /// Deletes given object from the matrix.
        /// </summary>
        private void DeleteObject(T obj)
        {
            // delete object from matrix
            _matrix.Remove(obj);

            // for each object, remove the dependency reference
            foreach (KeyValuePair<T, Dictionary<T, object>> pair in _matrix)
            {
                // if current object depends on deleting object
                pair.Value.Remove(obj);
            }
        }


        #endregion
    }

    /// <summary>
    /// Represents a circular reference exception when sorting dependency objects.
    /// </summary>
    public class CircularReferenceException : Exception
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="CircularReferenceException"/> class.
        /// </summary>
        public CircularReferenceException()
            : base("Circular reference found.")
        {            
        }
    }
}

I don't like recursive methods so DMM is out. Krumelur looks good but seems to use a lot of memory? Made an alternative stack based method that seems to work. Uses same DFS logic as DMM 's and I used this solutions as comparison when testing.

	public static IEnumerable<T> TopogicalSequenceDFS<T>(this IEnumerable<T> source, Func<T, IEnumerable<T>> deps)
	{
		HashSet<T> yielded = new HashSet<T>();
		HashSet<T> visited = new HashSet<T>();
		Stack<Tuple<T, IEnumerator<T>>> stack = new Stack<Tuple<T, IEnumerator<T>>>();

		foreach (T t in source)
		{
			stack.Clear();
			if (visited.Add(t))
				stack.Push(new Tuple<T, IEnumerator<T>>(t, deps(t).GetEnumerator()));

			while (stack.Count > 0)
			{
				var p = stack.Peek();
				bool depPushed = false;
				while (p.Item2.MoveNext())
				{
					var curr = p.Item2.Current;
					if (visited.Add(curr))
					{
						stack.Push(new Tuple<T, IEnumerator<T>>(curr, deps(curr).GetEnumerator()));
						depPushed = true;
						break;
					}
					else if (!yielded.Contains(curr))
						throw new Exception("cycle");
				}

				if (!depPushed)
				{
					p = stack.Pop();
					if (!yielded.Add(p.Item1))
						throw new Exception("bug");
					yield return p.Item1;
				}
			}
		}
	}

Here is also a simpler stack based BFS variant. It will produce different result than the above, but still valid. I'm not sure if there is any advantage to using the DFS variant above, but it was fun creating it.

    public static IEnumerable<T> TopologicalSequenceBFS<T>(this IEnumerable<T> source, Func<T, IEnumerable<T>> dependencies)
	{
		var yielded = new HashSet<T>();
		var visited = new HashSet<T>();
		var stack = new Stack<Tuple<T, bool>>(source.Select(s => new Tuple<T, bool>(s, false))); // bool signals Add to sorted

		while (stack.Count > 0)
		{
			var item = stack.Pop();
			if (!item.Item2)
			{
				if (visited.Add(item.Item1))
				{
					stack.Push(new Tuple<T, bool>(item.Item1, true)); // To be added after processing the dependencies
					foreach (var dep in dependencies(item.Item1))
						stack.Push(new Tuple<T, bool>(dep, false));
				}
				else if (!yielded.Contains(item.Item1))
					throw new Exception("cyclic");
			}
			else
			{
				if (!yielded.Add(item.Item1))
					throw new Exception("bug");
				yield return item.Item1;
			}
		}
	}

For .NET 4.7+ I suggest replacing Tuple with ValueTuple for lower memory use. In older .NET versions Tuple can be replaced with KeyValuePair.

I would make this easier on myself by storing the dependencies of an Item within the Item itself:

public class Item
{
    private List<Item> m_Dependencies = new List<Item>();

    protected AddDependency(Item _item) { m_Dependencies.Add(_item); }

    public Item()
    {
    }; // eo ctor

    public List<Item> Dependencies {get{return(m_Dependencies);};}
} // eo class Item

Then, given this you can implement a custom Sort delegate for List that sorts based on whether the given Item is contained within the other's list of dependencies:

int CompareItem(Item _1, Item _2)
{
    if(_2.Dependencies.Contains(_1))
        return(-1);
    else if(_1.Dependencies.Contains(_2))
        return(1);
    else
        return(0);
}

A different idea, for cases with only one "parent" depending:

Instead of deps, you'd store the parents.
So you can tell very easily whether an issue is a dependency of some other.
And then use Comparable<T>, which would claim the dependencies "lesser" and the dependency "greater".
And then simply call Collections.sort( List<T>, ParentComparator<T>);

For multi-parent scenario, a tree search would be needed which would result in slow execution. But that could be solved by a cache in a form of A* sort matrix.

I merged DMM's idea with the depth-first-search algorithm on Wikipedia. It works perfect for what I needed.

public static class TopologicalSorter
{
public static List<string> LastCyclicOrder = new List<string>(); //used to see what caused the cycle

sealed class ItemTag
{
  public enum SortTag
  {
    NotMarked,
    TempMarked,
    Marked
  }

  public string Item { get; set; }
  public SortTag Tag { get; set; }

  public ItemTag(string item)
  {
    Item = item;
    Tag = SortTag.NotMarked;
  }
}

public static IEnumerable<string> TSort(this IEnumerable<string> source, Func<string, IEnumerable<string>> dependencies)
{
  TopologicalSorter.LastCyclicOrder.Clear();

  List<ItemTag> allNodes = new List<ItemTag>();
  HashSet<string> sorted = new HashSet<string>(StringComparer.OrdinalIgnoreCase);

  foreach (string item in source)
  {
    if (!allNodes.Where(n => string.Equals(n.Item, item, StringComparison.OrdinalIgnoreCase)).Any())
    {
      allNodes.Add(new ItemTag(item)); //don't insert duplicates
    }
    foreach (string dep in dependencies(item))
    {
      if (allNodes.Where(n => string.Equals(n.Item, dep, StringComparison.OrdinalIgnoreCase)).Any()) continue; //don't insert duplicates
      allNodes.Add(new ItemTag(dep));
    }
  }

  foreach (ItemTag tag in allNodes)
  {
    Visit(tag, allNodes, dependencies, sorted);
  }

  return sorted;
}

static void Visit(ItemTag tag, List<ItemTag> allNodes, Func<string, IEnumerable<string>> dependencies, HashSet<string> sorted)
{
  if (tag.Tag == ItemTag.SortTag.TempMarked)
  {
    throw new GraphIsCyclicException();
  }
  else if (tag.Tag == ItemTag.SortTag.NotMarked)
  {
    tag.Tag = ItemTag.SortTag.TempMarked;
    LastCyclicOrder.Add(tag.Item);

    foreach (ItemTag dep in dependencies(tag.Item).Select(s => allNodes.Where(t => string.Equals(s, t.Item, StringComparison.OrdinalIgnoreCase)).First())) //get item tag which falls with used string
      Visit(dep, allNodes, dependencies, sorted);

    LastCyclicOrder.Remove(tag.Item);
    tag.Tag = ItemTag.SortTag.Marked;
    sorted.Add(tag.Item);
  }
}
}

This is refactored code from post https://stackoverflow.com/a/9991916/4805491.

// Version 1
public static class TopologicalSorter<T> where T : class {

    public struct Item {
        public readonly T Object;
        public readonly T Dependency;
        public Item(T @object, T dependency) {
            Object = @object;
            Dependency = dependency;
        }
    }


    public static T[] Sort(T[] objects, Func<T, T, bool> isDependency) {
        return Sort( objects.ToList(), isDependency ).ToArray();
    }

    public static T[] Sort(T[] objects, Item[] dependencies) {
        return Sort( objects.ToList(), dependencies.ToList() ).ToArray();
    }


    private static List<T> Sort(List<T> objects, Func<T, T, bool> isDependency) {
        return Sort( objects, GetDependencies( objects, isDependency ) );
    }

    private static List<T> Sort(List<T> objects, List<Item> dependencies) {
        var result = new List<T>( objects.Count );

        while (objects.Any()) {
            var obj = GetIndependentObject( objects, dependencies );
            RemoveObject( obj, objects, dependencies );
            result.Add( obj );
        }

        return result;
    }

    private static List<Item> GetDependencies(List<T> objects, Func<T, T, bool> isDependency) {
        var dependencies = new List<Item>();

        for (var i = 0; i < objects.Count; i++) {
            var obj1 = objects[i];
            for (var j = i + 1; j < objects.Count; j++) {
                var obj2 = objects[j];
                if (isDependency( obj1, obj2 )) dependencies.Add( new Item( obj1, obj2 ) ); // obj2 is dependency of obj1
                if (isDependency( obj2, obj1 )) dependencies.Add( new Item( obj2, obj1 ) ); // obj1 is dependency of obj2
            }
        }

        return dependencies;
    }


    private static T GetIndependentObject(List<T> objects, List<Item> dependencies) {
        foreach (var item in objects) {
            if (!GetDependencies( item, dependencies ).Any()) return item;
        }
        throw new Exception( "Circular reference found" );
    }

    private static IEnumerable<Item> GetDependencies(T obj, List<Item> dependencies) {
        return dependencies.Where( i => i.Object == obj );
    }

    private static void RemoveObject(T obj, List<T> objects, List<Item> dependencies) {
        objects.Remove( obj );
        dependencies.RemoveAll( i => i.Object == obj || i.Dependency == obj );
    }

}


// Version 2
public class TopologicalSorter {

    public static T[] Sort<T>(T[] source, Func<T, T, bool> isDependency) {
        var list = new LinkedList<T>( source );
        var result = new List<T>();

        while (list.Any()) {
            var obj = GetIndependentObject( list, isDependency );
            list.Remove( obj );
            result.Add( obj );
        }

        return result.ToArray();
    }

    private static T GetIndependentObject<T>(IEnumerable<T> list, Func<T, T, bool> isDependency) {
        return list.First( i => !GetDependencies( i, list, isDependency ).Any() );
    }

    private static IEnumerable<T> GetDependencies<T>(T obj, IEnumerable<T> list, Func<T, T, bool> isDependency) {
        return list.Where( i => isDependency( obj, i ) ); // i is dependency of obj
    }

}