CollapseUnaryTransformer.java

import java.util.List;

import edu.stanford.nlp.ling.Label;
import edu.stanford.nlp.trees.Tree;
import edu.stanford.nlp.trees.TreeTransformer;
import edu.stanford.nlp.util.Generics;

/**
 * This transformer collapses chains of unary nodes so that the top
 * node is the only node left.  The Sentiment model does not handle
 * unary nodes, so this simplifies them to make a binary tree consist
 * entirely of binary nodes and preterminals.  A new tree with new
 * nodes and labels is returned; the original tree is unchanged.
 *
 * @author John Bauer
 */
public class CollapseUnaryTransformer implements TreeTransformer {
  public Tree transformTree(Tree tree) {
    if (tree.isPreTerminal() || tree.isLeaf()) {
      return tree.deepCopy();
    }

    Label label = tree.label().labelFactory().newLabel(tree.label());
    Tree[] children = tree.children();
    while (children.length == 1 && !children[0].isLeaf()) {
      children = children[0].children();
    }
    List<Tree> processedChildren = Generics.newArrayList();
    for (Tree child : children) {
      processedChildren.add(transformTree(child));
    }
    return tree.treeFactory().newTreeNode(label, processedChildren);
  }
}