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package com.sun.tools.xjc.reader.gbind;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;
/**
* {@link Expression} that represents an alphabet of a regular language.
*
* <p>
* Since this package is about a regular expression over element declarations,
* this represents an XML element declaration (hence the name.)
*
* Element needs to be interned, meaning one {@link Element} per one tag name.
*
* <p>
* Implements {@link ElementSet} to represent a self.
*
* @author Kohsuke Kawaguchi
*/
public abstract class Element extends Expression implements ElementSet {
/**
* Once we build a graph from {@link Expression},
* we represent an edge e1 -> e2 by {@code e1.foreEdges.contains(e2)}
* and {@code e2.backEdges.contains(e1)}.
*/
final Set<Element> foreEdges = new LinkedHashSet<Element>();
final Set<Element> backEdges = new LinkedHashSet<Element>();
/**
* Previous element in the DFS post-order traveral
* of the element graph.
*
* <p>
* We use {@code prevPostOrder==null} as a check if the element is visted in DFS,
* so this chain terminates by a self-reference, not by having null.
*
* Set in {@link #assignDfsPostOrder(Element)}
*/
/*package*/ Element prevPostOrder;
/**
* {@link ConnectedComponent} to which this element belongs.
*
* Set in {@link #buildStronglyConnectedComponents(List<ConnectedComponent>)}
*/
private ConnectedComponent cc;
protected Element() {
}
ElementSet lastSet() {
return this;
}
boolean isNullable() {
return false;
}
/**
* True if this {@link Element} is {@link SourceNode}.
*/
boolean isSource() {
return false;
}
/**
* True if this {@link Element} is {@link SinkNode}.
*/
boolean isSink() {
return false;
}
void buildDAG(ElementSet incoming) {
incoming.addNext(this);
}
public void addNext(Element element) {
foreEdges.add(element);
element.backEdges.add(this);
}
public boolean contains(ElementSet rhs) {
return this==rhs || rhs==ElementSet.EMPTY_SET;
}
/**
* Just to satisfy the {@link ElementSet} contract.
*
* @deprecated
* if you statically call this method, there's something wrong.
*/
public Iterator<Element> iterator() {
return Collections.singleton(this).iterator();
}
/**
* Traverses the {@link Element} graph with DFS
* and set {@link #prevPostOrder}.
*
* Should be first invoked on the source node of the graph.
*/
/*package*/ Element assignDfsPostOrder(Element prev) {
if(prevPostOrder!=null)
return prev; // already visited
prevPostOrder = this; // set a dummy value to prepare for cycles
for (Element next : foreEdges) {
prev = next.assignDfsPostOrder(prev);
}
this.prevPostOrder = prev; // set to the real value
return this;
}
/**
* Builds a set of strongly connected components and puts them
* all into the given set.
*/
public void buildStronglyConnectedComponents(List<ConnectedComponent> ccs) {
// store visited elements - loop detection
List<Element> visitedElements = new ArrayList<Element>();
for(Element cur=this; cur!=cur.prevPostOrder; cur=cur.prevPostOrder) {
if(visitedElements.contains(cur)) {
// if I've already processed cur element, I'm in a loop
break;
} else {
visitedElements.add(cur);
}
if(cur.belongsToSCC())
continue;
// start a new component
ConnectedComponent cc = new ConnectedComponent();
ccs.add(cc);
cur.formConnectedComponent(cc);
}
}
private boolean belongsToSCC() {
return cc!=null || isSource() || isSink();
}
/**
* Forms a strongly connected component by doing a reverse DFS.
*/
private void formConnectedComponent(ConnectedComponent group) {
if(belongsToSCC())
return;
this.cc=group;
group.add(this);
for (Element prev : backEdges)
prev.formConnectedComponent(group);
}
public boolean hasSelfLoop() {
// if foreEdges have a loop, backEdges must have one. Or vice versa
assert foreEdges.contains(this)==backEdges.contains(this);
return foreEdges.contains(this);
}
/**
* Checks if the given {@link ConnectedComponent} forms a cut-set
* of a graph.
*
* @param visited
* Used to keep track of visited nodes.
* @return
* true if it is indeed a cut-set. false if not.
*/
/*package*/ final boolean checkCutSet(ConnectedComponent cc, Set<Element> visited) {
assert belongsToSCC(); // SCC discomposition must be done first
if(isSink())
// the definition of the cut set is that without those nodes
// you can't reach from soruce to sink
return false;
if(!visited.add(this))
return true;
if(this.cc==cc)
return true;
for (Element next : foreEdges) {
if(!next.checkCutSet(cc,visited))
// we've found a path to the sink
return false;
}
return true;
}
}