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Basic functions
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package org.glassfish.pfl.basic.algorithm ;

import java.util.List ;
import java.util.Collection ;
import java.util.Collections ;
import java.util.Set ;
import java.util.HashSet ;
import java.util.ArrayList ;
import org.glassfish.pfl.basic.func.UnaryFunction;

public class Graph {
    /** A Finder finds the immediate successors of an element of the graph.
     */
    public interface Finder extends UnaryFunction> {} 

    private Set roots ;
    private List preorderTraversal = null ;
    private List postorderTraversal = null ;

    private void traverse( final E node, final Set visited, final Finder finder ) {
	if (!visited.contains( node )) {
	    visited.add( node ) ;

	    preorderTraversal.add( node ) ;

	    for (E child : finder.evaluate(node)) {
		traverse( child, visited, finder ) ;
	    }

	    postorderTraversal.add( node ) ;
	}
    }

    private void init( final Collection roots, final Finder finder ) {
	this.roots = new HashSet( roots ) ;
	this.roots = Collections.unmodifiableSet( this.roots ) ;
	this.preorderTraversal = new ArrayList() ;
	this.postorderTraversal = new ArrayList() ;
	final Set visited = new HashSet() ;
	for (E node : this.roots) {
	    traverse( node, visited, finder ) ;
	}
	this.preorderTraversal = Collections.unmodifiableList( this.preorderTraversal ) ;
	this.postorderTraversal = Collections.unmodifiableList( this.postorderTraversal ) ;
    }

    public Graph( final Collection roots, final Finder finder ) {
	init( roots, finder ) ;
    }

    public Graph( final E root, final Finder finder )   {
	final Set roots = new HashSet() ;
	roots.add( root ) ;
	init( roots, finder ) ;
    }

    public Set getRoots() {
	return roots ;
    }

    public List getPreorderList() {
	return preorderTraversal ;
    }

    public List getPostorderList() {
	return postorderTraversal ;
    }
}