Reverse lookup a nested dictionary

Question

Dictionary<string, Dictionary<string, ... >...> nestedDictionary;

Above Dictionary has a one-to-many relationship at each level from top to bottom. Adding an item is pretty easy since we have the leaf object and we start from bottom, creating dictionaries and adding each to the relevant parent...

My problem is when I want to find an item at the inner Dictionaries. There are two options:

1. Nested foreach and find the item then snapshot all the loops at the moment we found the item and exit all loops. Then we know item pedigree is string1->string2->...->stringN. Problems with this solution is A) Performance B) Thread-safety (since I want to remove the item, the parent if it has no child and it's parent if it has no child...)
2. Creating a reverse look-up dictionary and indexing added items. Something like a Tuple for all outer dictionaries. Then add the item as key and all the outer parents as Tuple members. Problem: A) Redundancy B) Keeping synchronized reverse look-up Dictionary with main Dictionary.

Any idea for a fast and thread-safe solution?

Answer 1

It looks like you actually have more than two levels of Dictionary. Since you cannot support a variable number of dictionaries using this type syntax:

 Dictionary<string, Dictionary<string, ... >...> nestedDictionary;

I can only assume that it is some number greater than two. Let's say that it's three. For any data structure you construct, you have an intended use and operations that you want to perform efficiently.

I'm going to assume you need calls like this:

var dictionary = new ThreeLevelDictionary();
dictionary.Add(string1, string2, string3, value);
var value = dictionary[string1, string2, string3];
dictionary.Remove(string1, string2, string3);

And (critical to the question) the reverse lookup you are describing:

var strings = dictionary.FindKeys(value);

If these are the operations that you need to perform and to perform quickly, then one data structure that you can use is a Dictionary with a Tuple key:

public class ThreeLevelDictionary<TValue> : Dictionary<Tuple<string, string, string>, TValue>
{
    public void Add(string s1, string s2, string s3, TValue value)
    {
        Add(Tuple.Create(s1, s2, s3), value);
    }

    public TValue this[string s1, string s2, string s3]
    {
        get { return this[Tuple.Create(s1, s2, s3)]; }
        set { value = this[Tuple.Create(s1, s2, s3)]; }
    }

    public void Remove(string s1, string s2, string s3)
    {
        Remove(Tuple.Create(s1, s2, s3);
    }

    public IEnumerable<string> FindKeys(TValue value)
    {
        foreach (var key in Keys)
        {
            if (EqualityComparer<TValue>.Default.Equals(this[key], value))
                return new string[] { key.Item1, key.Item2, key.Item3 };
        }
        throw new InvalidOperationException("missing value");
    }
}

Now you are perfectly positioned to create a reverse-lookup hashtable using another Dictionary if performance indicates that this is a bottleneck.

If the previous liked operations are not the ones you want to perform, then this data structure might not meet your needs. Either way, if you describe the interface first that summarizes what you want the data structure to do, then it's easier to see if there are other alternatives.