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package org.apache.pig.newplan;
A walker to walk graphs in dependency order. It is guaranteed that a node will not be visited until all of its predecessors have been visited. This is equivalent to doing a topilogical sort on the graph and then visiting the nodes in order.
public class DependencyOrderWalker extends PlanWalker {


plan for this walker to traverse.
    public DependencyOrderWalker(OperatorPlan plan) {
    public PlanWalker spawnChildWalker(OperatorPlan plan) {
        return new DependencyOrderWalker(plan);

Begin traversing the graph.

visitor Visitor this walker is being used by.
org.apache.pig.impl.plan.VisitorException if an error is encountered while walking.
    public void walk(PlanVisitor visitorthrows FrontendException {
        // This is highly inefficient, but our graphs are small so it should be okay.
        // The algorithm works by starting at any node in the graph, finding it's
        // predecessors and calling itself for each of those predecessors.  When it
        // finds a node that has no unfinished predecessors it puts that node in the
        // list.  It then unwinds itself putting each of the other nodes in the list.
        // It keeps track of what nodes it's seen as it goes so it doesn't put any
        // nodes in the graph twice.
        List<Operatorfifo = new ArrayList<Operator>();
        Set<Operatorseen = new HashSet<Operator>();
        List<Operatorleaves = .getSinks();
        if (leaves == nullreturn;
        for (Operator op : leaves) {
        for (Operator opfifo) {
    protected void doAllPredecessors(Operator node,
                                   Collection<Operatorfifothrows FrontendException {
        if (!seen.contains(node)) {
            // We haven't seen this one before.
            Collection<Operatorpreds = Utils.mergeCollection(.getPredecessors(node), .getSoftLinkPredecessors(node));
            if (preds != null && preds.size() > 0) {
                // Do all our predecessors before ourself
                for (Operator op : preds) {
            // Now do ourself
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