Flatten (with level numbers) a hierarchical list

Question

I have a collection of class X instances that point to themselves (self-referencing).
As an example, my class could look like this:

public class X {
   string Name {get;set;}
   List<X> Children {get;}
}

Now lets say I have a list of instances of class X, which can self-reference themselves N levels down.

My question is: How do get the instances of X from Nth level in my list?

Basically I am trying to do in C# what a recursive self-referencing common table expression would do in SQL, which is to flatten out a hierarchical list adding level numbers.

I have found this example: https://stackoverflow.com/a/2118192/1171461 That works great but I still cant figure out how to get the elements ONLY from Nth level.

Answer 1

Well, you can modify the Flatten method in the linked example to include a level number as well, along these lines:

public class Leveled<T> 
{
    public T Item {get; set;}
    public int Level {get; set;}
}

public static IEnumerable<Leveled<T>> ToLeveled<T>(this IEnumerable<T> sequence,
                                            int level)
{
   return sequence.Select(item => new Leveled<T>{ Item = item, Level = level});
}

public static IEnumerable<Leveled<T>> FlattenLeveled<T>(this IEnumerable<T> sequence, 
                                            Func<T, IEnumerable<T>> childFetcher)
{
    var itemsToYield = new Queue<Leveled<T>>(sequence.ToLeveled(0));
    while (itemsToYield.Count > 0)
    {
        var leveledItem = itemsToYield.Dequeue();
        yield return leveledItem;

        var children = childFetcher(leveledItem.Item).ToLeveled(leveledItem.Level + 1);
        if (children != null)
        { 
            foreach (var child in children) 
               itemsToYield.Enqueue(child);
        }
    }
}

after this, you can just filter out the required level:

var thirdLevel = myCollection
           .FlattenLeveled(item => item.Children)
           .Where(leveledItem => leveledItem.Level == 2)
           .Select(leveledItem => leveledItem.Item)

Also, from @Servy's comment, since this is a breadth first approach (all of 1st level get done before any of the 2nd level is processed), we can use Skip/TakeWhile, like this:

public static IEnumerable<T> GetHierarchyLevel<T>(this IEnumerable<T> sequence, Func<T, IEnumerable<T>> childFetcher, int level)
{
  return sequence.FlattenLeveled(childFetcher)
                 .SkipWhile(li => li.Level < level)
                 .TakeWhile(li => li.Level == level)
                 .Select(li => li.Item);
}

This will enumerate lazily, so any level further down the hierarchy will not be processed at all.