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Stanford CoreNLP provides a set of natural language analysis tools which can take raw English language text input and give the base forms of words, their parts of speech, whether they are names of companies, people, etc., normalize dates, times, and numeric quantities, mark up the structure of sentences in terms of phrases and word dependencies, and indicate which noun phrases refer to the same entities. It provides the foundational building blocks for higher level text understanding applications.

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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 null 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