/*
 * Bytecode Analysis Framework
 * Copyright (C) 2003,2004 University of Maryland
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library 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
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

package edu.umd.cs.findbugs.ba;

import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Set;

import org.apache.bcel.generic.MethodGen;

import edu.umd.cs.findbugs.SystemProperties;
import edu.umd.cs.findbugs.ba.ch.Subtypes2;
import edu.umd.cs.findbugs.ba.type.ExceptionSet;
import edu.umd.cs.findbugs.ba.type.TypeDataflow;

/**
 * Prune a CFG to remove infeasible exception edges. In order to determine what
 * kinds of exceptions can be thrown by explicit ATHROW instructions, type
 * analysis must first be performed on the unpruned CFG.
 * 
 * @author David Hovemeyer
 * @see CFG
 * @see edu.umd.cs.findbugs.ba.type.TypeAnalysis
 */
public class PruneInfeasibleExceptionEdges implements EdgeTypes {
    private static final boolean DEBUG = SystemProperties.getBoolean("cfg.prune.debug");

    private static final boolean STATS = SystemProperties.getBoolean("cfg.prune.stats");

    private static int numEdgesPruned = 0;

    static {
        if (STATS) {
            Runtime.getRuntime().addShutdownHook(new Thread() {
                @Override
                public void run() {
                    System.err.println("Exception edges pruned: " + numEdgesPruned);
                }
            });
        }
    }

    /**
     * A momento to remind us of how we classified a particular exception edge.
     * If pruning and classifying succeeds, then these momentos can be applied
     * to actually change the state of the edges. The issue is that the entire
     * pruning/classifying operation must either fail or succeed as a whole.
     * Thus, we don't commit any CFG changes until we know everything was
     * successful.
     */
    private static class MarkedEdge {
        private Edge edge;

        private int flag;

        public MarkedEdge(Edge edge, int flag) {
            this.edge = edge;
            this.flag = flag;
        }

        public void apply() {
            int flags = edge.getFlags();
            flags |= this.flag;
            edge.setFlags(flags);
        }
    }

    private CFG cfg;

    private TypeDataflow typeDataflow;

    private boolean cfgModified;

    /**
     * Constructor.
     * 
     * @param cfg
     *            the CFG to prune
     * @param methodGen
     *            the method
     * @param typeDataflow
     *            initialized TypeDataflow object for the CFG, indicating the
     *            types of all stack locations
     */
    public PruneInfeasibleExceptionEdges(CFG cfg, MethodGen methodGen, TypeDataflow typeDataflow) {
        this.cfg = cfg;
        this.typeDataflow = typeDataflow;
    }

    /**
     * Prune infeasible exception edges from the CFG. If the method returns
     * normally, then the operation was successful, and the CFG should no longer
     * contain infeasible exception edges. If ClassNotFoundException or
     * DataflowAnalysisException are thrown, then the operation was
     * unsuccessful,
     * <em>but the CFG is still valid because it was not modified</em>. If a
     * runtime exception is thrown, then the CFG may be partially modified and
     * should be considered invalid.
     */
    public void execute() throws ClassNotFoundException {
        Set<Edge> deletedEdgeSet = new HashSet<Edge>();
        List<MarkedEdge> markedEdgeList = new LinkedList<MarkedEdge>();

        // Mark edges to delete,
        // mark edges to set properties of
        for (Iterator<Edge> i = cfg.edgeIterator(); i.hasNext();) {
            Edge edge = i.next();
            if (!edge.isExceptionEdge())
                continue;

            ExceptionSet exceptionSet = typeDataflow.getEdgeExceptionSet(edge);
            if (exceptionSet.isEmpty()) {
                // No exceptions are actually thrown on this edge,
                // so we can delete the edge.
                deletedEdgeSet.add(edge);
            } else {
                if (exceptionSet.isSingleton("java.lang.CloneNotSupportedException") && cfg.getMethodName().endsWith(".clone()")) {
                    String className = cfg.getMethodGen().getClassName();
                    if (Subtypes2.instanceOf(className, "java.lang.Cloneable")) {
                        deletedEdgeSet.add(edge);
                        continue;
                    }
                }
                // Some exceptions appear to be thrown on the edge.
                // Mark to indicate if any of the exceptions are checked,
                // and if any are explicit (checked or explicitly declared
                // or thrown unchecked).
                boolean someChecked = exceptionSet.containsCheckedExceptions();
                boolean someExplicit = exceptionSet.containsExplicitExceptions();

                int flags = 0;
                if (someChecked)
                    flags |= CHECKED_EXCEPTIONS_FLAG;
                if (someExplicit)
                    flags |= EXPLICIT_EXCEPTIONS_FLAG;

                markedEdgeList.add(new MarkedEdge(edge, flags));
            }
        }

        // Remove deleted edges
        for (Edge edge : deletedEdgeSet) {
            cfg.removeEdge(edge);
            if (STATS)
                ++numEdgesPruned;
            cfgModified = true;
        }

        // Mark edges
        for (MarkedEdge markedEdge : markedEdgeList) {
            markedEdge.apply();
        }
    }

    /**
     * @return true if modified
     */
    public boolean wasCFGModified() {
        return cfgModified;
    }
}

// vim:ts=4