Completed Second Step of Maze Generation

Completed the second step of maze generation. In this step, a graph is
built that describes which rooms can be connected to each other by
doorways. A maze is created by removing nodes (rooms) from the graph.
The algorithm is incomplete, but it already produces interesting mazes
comparable to DD's hand-made mazes.

A few details for the future:
1. Doorways are currently carved into walls at default locations. This
is just for testing and placement should be improved later. Some
doorways should be removed and redundant doorways should be possible.
2. The size of a section should be assessed and the section should be
discarded if it has too few rooms.
3. An NPE occurs every so often when a maze is generated. It's possible
that it happens because of removing a node from the graph that is
coincidentally the current node for LinkedList's iterator. The solution
would be to add nodes to a list and defer removals until after the
iteration is done.

src/main/java/StevenDimDoors/experimental/DirectedGraph.java

@@ -0,0 +1,225 @@
+package StevenDimDoors.experimental;
+
+/**
+ * Provides a complete implementation of a directed graph.
+ * @author SenseiKiwi
+ *
+ * @param <U> The type of data to store in the graph's nodes
+ * @param <V> The type of data to store in the graph's edges
+ */
+public class DirectedGraph<U, V>
+{
+	private static class GraphNode<P, Q> implements IGraphNode<P, Q>
+	{
+		private LinkedList<Edge<P, Q>> inbound;
+		private LinkedList<Edge<P, Q>> outbound;
+		private ILinkedListNode<GraphNode<P, Q>> graphEntry;
+		private P data;
+		
+		public GraphNode(P data, LinkedList<GraphNode<P, Q>> graphList)
+		{
+			this.data = data;
+			this.inbound = new LinkedList<Edge<P, Q>>();
+			this.outbound = new LinkedList<Edge<P, Q>>();
+			this.graphEntry = graphList.addLast(this);
+		}
+		
+		public int indegree()
+		{
+			return inbound.size();
+		}
+		
+		public int outdegree()
+		{
+			return outbound.size();
+		}
+		
+		public Iterable<Edge<P, Q>> inbound()
+		{
+			return inbound;
+		}
+		
+		public Iterable<Edge<P, Q>> outbound()
+		{
+			return outbound;
+		}
+		
+		public P data()
+		{
+			return data;
+		}
+		
+		public void remove()
+		{
+			graphEntry.remove();
+			graphEntry = null;
+			
+			for (Edge<P, Q> edge : inbound)
+				edge.remove();
+
+			for (Edge<P, Q> edge : outbound)
+				edge.remove();
+			
+			inbound = null;
+			outbound = null;
+			data = null;
+		}
+	}
+	
+	private static class Edge<P, Q> implements IEdge<P, Q>
+	{
+		private GraphNode<P, Q> head;
+		private GraphNode<P, Q> tail;
+		private ILinkedListNode<Edge<P, Q>> headEntry;
+		private ILinkedListNode<Edge<P, Q>> tailEntry;
+		private ILinkedListNode<Edge<P, Q>> graphEntry;
+		private Q data;
+		
+		public Edge(GraphNode<P, Q> head, GraphNode<P, Q> tail, Q data, LinkedList<Edge<P, Q>> graphList)
+		{
+			this.head = head;
+			this.tail = tail;
+			this.data = data;
+			this.graphEntry = graphList.addLast(this);
+			this.headEntry = head.outbound.addLast(this);
+			this.tailEntry = tail.inbound.addLast(this);
+		}
+		
+		public IGraphNode<P, Q> head()
+		{
+			return head;
+		}
+		
+		public IGraphNode<P, Q> tail()
+		{
+			return tail;
+		}
+		
+		public Q data()
+		{
+			return data;
+		}
+		
+		public void remove()
+		{
+			headEntry.remove();
+			tailEntry.remove();
+			graphEntry.remove();
+			headEntry = null;
+			tailEntry = null;
+			graphEntry = null;
+			head = null;
+			tail = null;
+			data = null;
+		}
+	}
+	
+	private LinkedList<GraphNode<U, V>> nodes;
+	private LinkedList<Edge<U, V>> edges;
+	
+	public DirectedGraph()
+	{
+		nodes = new LinkedList<GraphNode<U, V>>();
+		edges = new LinkedList<Edge<U, V>>();
+	}
+	
+	public int nodeCount()
+	{
+		return nodes.size();
+	}
+	
+	public int edgeCount()
+	{
+		return edges.size();
+	}
+	
+	public boolean isEmpty()
+	{
+		return nodes.isEmpty();
+	}
+	
+	public Iterable<? extends IGraphNode<U, V>> nodes()
+	{
+		return nodes;
+	}
+	
+	public Iterable<? extends IEdge<U, V>> edges()
+	{
+		return edges;
+	}
+	
+	private GraphNode<U, V> checkNode(IGraphNode<U, V> node)
+	{
+		GraphNode<U, V> innerNode = (GraphNode<U, V>) node;
+
+		// Check that this node actually belongs to this graph instance.
+		// Accepting foreign nodes could corrupt the graph's internal state.
+		if (innerNode.graphEntry.owner() != nodes)
+		{
+			throw new IllegalArgumentException("The specified node does not belong to this graph.");
+		}
+		return innerNode;
+	}
+	
+	private Edge<U, V> checkEdge(IEdge<U, V> edge)
+	{
+		Edge<U, V> innerEdge = (Edge<U, V>) edge;
+
+		// Check that this node actually belongs to this graph instance.
+		// Accepting foreign nodes could corrupt the graph's internal state.
+		if (innerEdge.graphEntry.owner() != edges)
+		{
+			throw new IllegalArgumentException("The specified edge does not belong to this graph.");
+		}
+		return innerEdge;
+	}
+	
+	public IGraphNode<U, V> addNode(U data)
+	{
+		return new GraphNode<U, V>(data, nodes);
+	}
+	
+	public IEdge<U, V> addEdge(IGraphNode<U, V> head, IGraphNode<U, V> tail, V data)
+	{
+		GraphNode<U, V> innerHead = checkNode(head);
+		GraphNode<U, V> innerTail = checkNode(tail);
+		return new Edge<U, V>(innerHead, innerTail, data, edges);
+	}
+	
+	public U removeNode(IGraphNode<U, V> node)
+	{
+		GraphNode<U, V> innerNode = checkNode(node);
+		U data = innerNode.data();
+		innerNode.remove();
+		return data;
+	}
+	
+	public V removeEdge(IEdge<U, V> edge)
+	{
+		Edge<U, V> innerEdge = checkEdge(edge);
+		V data = innerEdge.data();
+		innerEdge.remove();
+		return data;
+	}
+	
+	public IEdge<U, V> findEdge(IGraphNode<U, V> head, IGraphNode<U, V> tail)
+	{
+		for (IEdge<U, V> edge : head.outbound())
+		{
+			if (edge.tail() == tail)
+				return edge;
+		}
+		return null;
+	}
+	
+	public void clear()
+	{
+		// Remove each node individually to guarantee that all external
+		// references are invalidated. That'll prevent memory leaks and
+		// keep external code from using removed nodes or edges.
+		for (GraphNode<U, V> node : nodes)
+		{
+			node.remove();
+		}
+	}
+}