package edu.stanford.nlp.parser.lexparser;
import edu.stanford.nlp.trees.HeadFinder;
import edu.stanford.nlp.trees.Tree;
public class BinaryHeadFinder implements HeadFinder {
private static final long serialVersionUID = 4794072338791804184L;
private final HeadFinder fallbackHF;
public BinaryHeadFinder() {
this(null);
}
public BinaryHeadFinder(HeadFinder fallbackHF) {
this.fallbackHF = fallbackHF;
}
/**
* Determine which daughter of the current parse tree is the head.
* It assumes that the daughters already have had their heads
* determined. Another method has to do the tree walking.
*
* @param t The parse tree to examine the daughters of
* @return The parse tree that is the head. The convention has been
* that this returns <code>null</code> if no head is found.
* But maybe it should throw an exception?
*/
public Tree determineHead(Tree t) {
Tree result = determineBinaryHead(t);
if (result == null && fallbackHF != null) {
result = fallbackHF.determineHead(t);
}
if (result != null) {
return result;
}
throw new IllegalStateException("BinaryHeadFinder: unexpected tree: " + t);
}
public Tree determineHead(Tree t, Tree parent){
Tree result = determineBinaryHead(t);
if (result == null && fallbackHF != null) {
result = fallbackHF.determineHead(t, parent);
}
if (result != null) {
return result;
}
throw new IllegalStateException("BinaryHeadFinder: unexpected tree: " + t);
}
private Tree determineBinaryHead(Tree t) {
if (t.numChildren() == 1) {
return t.firstChild();
} else {
String lval = t.firstChild().label().value();
if (lval != null && lval.startsWith("@")) {
return t.firstChild();
} else {
String rval = t.lastChild().label().value();
if (rval.startsWith("@") || rval.equals(Lexicon.BOUNDARY_TAG)) {
return t.lastChild();
}
}
}
return null;
}
} // end static class BinaryHeadFinder