Some minor fixes.

src/main/java/graphgen/generator/GraphGenerator.java

@@ -218,7 +218,9 @@ public final class GraphGenerator {
 		edgeIncluder.setASAPTimesMap(asapTimes);
 		backedgeIncluder.setASAPTimesMap(asapTimes);
 
-		property.generateProperty(layers, Collections.unmodifiableSet(nodes), nodeTable, edgeCreator);
+		property.generateProperty(layers, Collections.unmodifiableSet(nodes), Collections.unmodifiableMap(nodeTable),
+								  edgeCreator
+		);
 
 		if (verbose) {
 			out.println("\tLayer structure post property generation: " + layers.toString());
@@ -328,7 +330,7 @@ public final class GraphGenerator {
 	 * <p>
 	 * Use <code>new PrintStream(new FileOutputStream(new File(filename))</code> to immediately write to a file.<br>
 	 * <b>Warning</b>: The resulting file might be excessively large due to the
-	 * amount of edges possible in large graphs (O(2* V^2)).
+	 * amount of edges possible in large graphs (O(2*V<sup>2</sup>)).
 	 *
 	 * @param out the print stream
 	 */

src/main/java/graphgen/generator/components/edges/EdgeCreator.java

@@ -180,7 +180,6 @@ public class EdgeCreator extends Observable<Edge<ResourceNode>> implements EdgeC
 		outgoingBackedges.get(src).add(edge);
 		incomingBackedges.get(dst).add(edge);
 
-		// TODO: is this check necessary?
 		if (src != GraphGenerator.SOURCE && dst != GraphGenerator.SINK) {
 			notifyObservers(edge);
 		}

src/main/java/graphgen/graph/LayerStructure.java

@@ -21,7 +21,6 @@ import graphgen.generator.exceptions.IllegalSwapException;
 import graphgen.generator.exceptions.NodeDoesNotExistException;
 import graphgen.generator.exceptions.StrictModeException;
 import graphgen.observer.Observer;
-import graphgen.util.JavaUtils;
 import modsched.Edge;
 import modsched.Node;
 
@@ -36,7 +35,6 @@ import java.util.List;
 import java.util.Map;
 import java.util.Queue;
 import java.util.Set;
-import java.util.TreeMap;
 
 /**
  * The layer structure is an essential component of the graph generator. It contains a list of layers, which are
@@ -66,9 +64,22 @@ public final class LayerStructure implements Serializable, Iterable<Integer>, Ob
 	 * Constructs a new layer structure based on the provided list of layers and the ID depths map.
 	 *
 	 * @param depthsByID the depths of the IDs contained in the given list of layers
+	 * @throws IllegalArgumentException if any depth is negative
 	 */
 	public LayerStructure(Map<Integer, Integer> depthsByID) {
-		this.layers = new ArrayList<>(new TreeMap<>(JavaUtils.invertMap(depthsByID)).values());
+		this.layers = new ArrayList<>();
+
+		depthsByID.forEach((id, depth) -> {
+			if (depth < 0) {
+				throw new IllegalArgumentException("Depth of ID " + id + " cannot be negative.");
+			} else {
+				while (layers.size() <= depth) {
+					layers.add(new HashSet<>());
+				}
+			}
+			layers.get(depth).add(id);
+		});
+
 		this.height = layers.size();
 		this.depthsByID = new HashMap<>(depthsByID);
 		this.checker = new SwapChecker();
@@ -106,11 +117,13 @@ public final class LayerStructure implements Serializable, Iterable<Integer>, Ob
 			throw new IllegalSwapException(firstID, secondID);
 		}
 
-		int firstLevel = depthsByID.get(firstID);
-		int secondLevel = depthsByID.get(secondID);
+		int firstDepth = depthsByID.get(firstID);
+		int secondDepth = depthsByID.get(secondID);
 
-		moveNode(secondID, firstLevel);
-		moveNode(firstID, secondLevel);
+		if (firstDepth != secondDepth) {
+			moveNode(secondID, firstDepth);
+			moveNode(firstID, secondDepth);
+		}
 
 	}