Bend the .NET Object to your Will!

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

namespace CodeProject.Batte.ObjectMastery
{
	/*
	 * GraphRegistry.cs
	 * Author: John Batte
	 * Last Modified: 1/5/2009
	 * See http://www.codeproject.com/info/cpol10.aspx for license details
	 */

	/// <summary>
	/// Provides a reference cache for identifying previously enumerated
	/// object instances within the <see cref="ObjectGraph"/>.
	/// </summary>
	internal class GraphRegistry
	{
		private readonly IDictionary<int, IDictionary<object, ObjectGraph>> _registry;

		/// <summary>
		/// Initializes a new instance of the <see cref="GraphRegistry"/> class.
		/// </summary>
		public GraphRegistry()
		{
			_registry = new Dictionary<int, IDictionary<object, ObjectGraph>>();
		}

		/// <summary>
		/// Registers or identifies the target reference.
		/// </summary>
		/// <param name="target">The object reference to register.</param>
		/// <param name="graph">The object graph requesting the registration.</param>
		/// <returns>
		/// Null if the target has not been previously registered; otherwise,
		/// a reference to the <see cref="ObjectGraph"/> that originally registered the target.
		/// </returns>
		public ObjectGraph Register(object target, ObjectGraph graph)
		{
			// I'd rather not do a brute force scan of all previously
			// registered objects in the graph... and I can't rely on
			// GetHashCode across multiple types / instances... so I'm 
			// using one to short circuit the other.  Sort of like putting
			// an index on a checksum column.  Doesn't guarantee uniqueness
			// but it gets us close.

			int key = target.GetHashCode();

			IDictionary<object, ObjectGraph> hashMatches = _registry.ContainsKey(key)
			                                               	? _registry[key] ?? new Dictionary<object, ObjectGraph>()
			                                               	: new Dictionary<object, ObjectGraph>();

			ObjectGraph existingGraph = (from match in hashMatches
			                             where ReferenceEquals(match.Key, target)
			                             select match.Value).FirstOrDefault();

			if (existingGraph != null) return existingGraph;

			hashMatches[target] = graph;
			_registry[key] = hashMatches;
			return null;
		}

		/// <summary>
		/// Clears the registry of all object references.
		/// </summary>
		public void Clear()
		{
			foreach (var key in _registry.Keys) if (_registry[key] != null) _registry[key].Clear();
			_registry.Clear();
		}
	}
}