prism-accumulation/prism/src/explicit/SCCComputerTarjan.java

//==============================================================================
//	
//	Copyright (c) 2002-
//	Authors:
//	* Christian von Essen <christian.vonessen@imag.fr> (Verimag, Grenoble)
//	* Dave Parker <d.a.parker@cs.bham.ac.uk> (University of Birmingham/Oxford)
//	
//------------------------------------------------------------------------------
//	
//	This file is part of PRISM.
//	
//	PRISM is free software; you can redistribute it and/or modify
//	it under the terms of the GNU General Public License as published by
//	the Free Software Foundation; either version 2 of the License, or
//	(at your option) any later version.
//	
//	PRISM is distributed in the hope that it will be useful,
//	but WITHOUT ANY WARRANTY; without even the implied warranty of
//	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//	GNU General Public License for more details.
//	
//	You should have received a copy of the GNU General Public License
//	along with PRISM; if not, write to the Free Software Foundation,
//	Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
//	
//==============================================================================

package explicit;

import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.BitSet;
import java.util.Deque;
import java.util.function.IntPredicate;

import prism.PrismComponent;
import prism.PrismException;

/**
 * Tarjan's SCC algorithm operating on a Model object.
 */
public class SCCComputerTarjan extends SCCComputer
{
	/* The model to compute (B)SCCs for */
	private Model model;
	/* Number of nodes (model states) */
	private int numNodes;

	/* Next index to give to a node */
	private int index = 0;
	/* Stack of nodes */
	private Deque<Integer> stack = new ArrayDeque<Integer>();
	/* 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;

	/**
	 * Build (B)SCC computer for a given model.
	 */
	public SCCComputerTarjan(PrismComponent parent, Model model, SCCConsumer consumer) throws PrismException
	{
		super(parent, consumer);
		this.model = model;
		this.numNodes = model.getNumStates();
		nodeLowlink = new int[numNodes];
		Arrays.fill(nodeLowlink, -1);
		nodeIndex = new int[numNodes];
		Arrays.fill(nodeIndex, -1);
		onStack = new BitSet();
	}

	// Methods for SCCComputer interface

	@Override
	public void computeSCCs(boolean filterTrivialSCCs, IntPredicate restrict) throws PrismException
	{
		this.filterTrivialSCCs = filterTrivialSCCs;
		consumer.notifyStart(model);
		this.restrict = restrict;
		tarjan();
		consumer.notifyDone();
	}

	// SCC Computation

	/**
	 * Execute Tarjan's algorithm. Determine maximal strongly connected components
	 * (SCCS) for the graph of the model and stored in {@code sccs}.
	 */
	public void tarjan() throws PrismException
	{
		for (int i = 0; i < numNodes; i++) {
			if (restrict != null && !restrict.test(i))
				continue; // skip state if not one of the relevant states
			if (nodeLowlink[i] == -1)
				tarjan(i);
		}

	}

	private void tarjan(final int i) throws PrismException
	{
		nodeIndex[i] = index;
		nodeLowlink[i] = index;
		index++;
		stack.push(i);
		onStack.set(i);

		boolean hadSelfloop = false;
		SuccessorsIterator it = model.getSuccessors(i);
		while (it.hasNext()) {
			final int e = it.nextInt();

			if (e == i) {
				hadSelfloop = true;
				continue;
			}

			if (restrict != null && !restrict.test(e)) {
				continue; // ignore edge to state that is not relevant
			}

			if (nodeIndex[e] == -1) {
				tarjan(e);
				nodeLowlink[i] = Math.min(nodeLowlink[i], nodeLowlink[e]);
			} else if (onStack.get(e)) {
				nodeLowlink[i] = Math.min(nodeLowlink[i], nodeIndex[e]);
			}
		}
		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) {
				if (!hadSelfloop) { // singleton SCC & no selfloop -> trivial
					stack.pop();
					onStack.set(i, false);
					return;
				}
			}

			int n;
			consumer.notifyStartSCC();
			do {
				n = stack.pop();
				onStack.set(n, false);
				consumer.notifyStateInSCC(n);
			} while (n != i);
			consumer.notifyEndSCC();
		}
	}

}