imported patch tarjan-lowlink-index-array.patch

prism/src/explicit/SCCComputerTarjan.java

@@ -28,7 +28,7 @@
 package explicit;
 
 import java.util.ArrayDeque;
-import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.BitSet;
 import java.util.Deque;
 import java.util.function.IntPredicate;
@@ -50,10 +50,14 @@ public class SCCComputerTarjan extends SCCComputer
 	private int index = 0;
 	/* Stack of nodes */
 	private Deque<Integer> stack = new ArrayDeque<Integer>();
-	/* List of nodes in the graph. Invariant: {@code nodeList.get(i).id == i} */
-	private ArrayList<Node> nodeList;
 	/* Nodes currently on the stack. */
 	private BitSet onStack;
+
+	/** The lowlink information for the nodes (states) */
+	private int[] nodeLowlink;
+	/** The index information for the nodes (states) */
+	private int[] nodeIndex;
+
 	/** Should we filter trivial SCCs? */
 	private boolean filterTrivialSCCs;
 	private IntPredicate restrict;
@@ -66,10 +70,10 @@ public class SCCComputerTarjan extends SCCComputer
 		super(parent, consumer);
 		this.model = model;
 		this.numNodes = model.getNumStates();
-		this.nodeList = new ArrayList<Node>(numNodes);
-		for (int i = 0; i < numNodes; i++) {
-			nodeList.add(new Node(i));
-		}
+		nodeLowlink = new int[numNodes];
+		Arrays.fill(nodeLowlink, -1);
+		nodeIndex = new int[numNodes];
+		Arrays.fill(nodeIndex, -1);
 		onStack = new BitSet();
 	}
 
@@ -96,17 +100,16 @@ public class SCCComputerTarjan extends SCCComputer
 		for (int i = 0; i < numNodes; i++) {
 			if (restrict != null && !restrict.test(i))
 				continue; // skip state if not one of the relevant states
-			if (nodeList.get(i).lowlink == -1)
+			if (nodeLowlink[i] == -1)
 				tarjan(i);
 		}
 
 	}
 
-	private void tarjan(int i) throws PrismException
+	private void tarjan(final int i) throws PrismException
 	{
-		final Node v = nodeList.get(i);
-		v.index = index;
-		v.lowlink = index;
+		nodeIndex[i] = index;
+		nodeLowlink[i] = index;
 		index++;
 		stack.push(i);
 		onStack.set(i);
@@ -114,7 +117,7 @@ public class SCCComputerTarjan extends SCCComputer
 		boolean hadSelfloop = false;
 		SuccessorsIterator it = model.getSuccessors(i);
 		while (it.hasNext()) {
-			int e = it.nextInt();
+			final int e = it.nextInt();
 
 			if (e == i) {
 				hadSelfloop = true;
@@ -125,15 +128,14 @@ public class SCCComputerTarjan extends SCCComputer
 				continue; // ignore edge to state that is not relevant
 			}
 
-			Node n = nodeList.get(e);
-			if (n.index == -1) {
+			if (nodeIndex[e] == -1) {
 				tarjan(e);
-				v.lowlink = Math.min(v.lowlink, n.lowlink);
+				nodeLowlink[i] = Math.min(nodeLowlink[i], nodeLowlink[e]);
 			} else if (onStack.get(e)) {
-				v.lowlink = Math.min(v.lowlink, n.index);
+				nodeLowlink[i] = Math.min(nodeLowlink[i], nodeIndex[e]);
 			}
 		}
-		if (v.lowlink == v.index) {
+		if (nodeLowlink[i] == nodeIndex[i]) {
 			// this is a singleton SCC if the top of the stack equals i
 			boolean singletonSCC = (stack.peek() == i);
 			if (singletonSCC && filterTrivialSCCs) {
@@ -155,19 +157,4 @@ public class SCCComputerTarjan extends SCCComputer
 		}
 	}
 
-	/**
-	 * A small class wrapping a node.
-	 * It carries extra information necessary for Tarjan's algorithm.
-	 */
-	protected static class Node
-	{
-		public int lowlink = -1;
-		public int index = -1;
-		public int id;
-
-		public Node(int id)
-		{
-			this.id = id;
-		}
-	}
 }