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package org.apache.pig.impl.plan;
DependencyOrderWalker traverses the graph in such a way that no node is visited before all the nodes it depends on have been visited. Beyond this, it does not guarantee any particular order. So, you have a graph with node 1 2 3 4, and edges 1->3, 2->3, and 3->4, this walker guarnatees that 1 and 2 will be visited before 3 and 3 before 4, but it does not guarantee whether 1 or 2 will be visited first.
public class DependencyOrderWalker <O extends Operator, P extends OperatorPlan<O>>
    extends PlanWalker<O, P> {


plan Plan for this walker to traverse.
    public DependencyOrderWalker(P plan) {

Begin traversing the graph.

visitor Visitor this walker is being used by.
VisitorException if an error is encountered while walking.
    public void walk(PlanVisitor<O, P> visitorthrows VisitorException {
        // 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<O> fifo = new ArrayList<O>();
        Set<O> seen = new HashSet<O>();
        List<O> leaves = .getLeaves();
        if (leaves == nullreturn;
        for (O op : leaves) {
        for (O opfifo) {
    public PlanWalker<O, P> spawnChildWalker(P plan) { 
        return new DependencyOrderWalker<O, P>(plan);
    protected void doAllPredecessors(O node,
                                   Set<O> seen,
                                   Collection<O> fifothrows VisitorException {
        if (!seen.contains(node)) {
            // We haven't seen this one before.
            Collection<O> preds = Utils.mergeCollection(.getPredecessors(node), .getSoftLinkPredecessors(node));
            if (preds != null && preds.size() > 0) {
                // Do all our predecessors before ourself
                for (O op : preds) {
            // Now do ourself
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