Question

Currently I am using a Dictionary<int,node> to store around 10,000 nodes. The key is used as an ID number for later look up and the ‘node’ is a class that contains some data. Other classes within the program use the ID number as a pointer to the node. (this may sound inefficient. However, explaining my reasoning for using a dictionary for this is beyond the scope of my question.)

However, 20% of the nodes are duplicate.
What i want to do is when i add a node check to see if it all ready exists. if it does then use that ID number. If not create a new one.

This is my current solution to the problem:

public class nodeDictionary 
{

    Dictionary<int, node> dict = new Dictionary<int, node>( );
    public int addNewNode( latLng ll )
    {
        node n = new node( ll );
        if ( dict.ContainsValue( n ) )
        {
            foreach ( KeyValuePair<int, node> kv in dict )
            {
                if ( kv.Value == n )
                {
                    return kv.Key;
                }
            }
        }
        else
        {
            if ( dict.Count != 0 )
            {
                dict.Add( dict.Last( ).Key + 1, n );
                return dict.Last( ).Key + 1;
            }
            else
            {
                dict.Add( 0, n );
                return 0;
            }
        }
        throw new Exception( );
    }//end add new node
}

The problem with this is when trying to add a new node to a list of 100,000 nodes it takes 78 milliseconds to add the node. This is unacceptable because i could be adding an additional 1,000 nodes at any given time.

So, is there a better way do do this? I am not looking for someone to write the code for me, I am just looking for guidance.

Answer 1

It sounds like you want to

• make sure that LatLng overrides Equals/GetHashCode (preferrably implement the IEquatable<LatLng> interface)
• stuff all the items directly into a HashSet<LatLng>

For implementing GetHashCode, see here: Why is it important to override GetHashCode when Equals method is overridden?

If you need to generate ‘artificial’ unique IDs in some fashion, I suggest you use the dictionary approach again, but ‘in reverse’:

// uses the same hash function for speedy lookup/insertion
IDictionary<LatLng, int> idMap = new Dictionary<LatLng, int>(); 

foreach (LatLng latLng in LatLngCoords)
{
    if (!idMap.ContainsKey(latLng))
        idMap.Add(latLng, idMap.Count+1); // to start with 1
}

You can have the idMap replace the HashSet<>; the implementation (and performance characteristics) is essentially the same but as an associative container.

Here’s a lookup function to get from LatLng to Id:

int IdLookup(LatLng latLng)
{
     int id;
     if (idMap.TryGetValue(latLng, id))
         return id;
     throw new InvalidArgumentException("Coordinate not in idMap");
}

You could just-in-time add it:

int IdFor(LatLng latLng)
{
     int id;
     if (idMap.TryGetValue(latLng, id))
         return id;

     id = idMap.Count+1;
     idMap.Add(latLng, id);
     return id;
}