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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.shiftreduce;

import java.util.List;

import edu.stanford.nlp.ling.CoreLabel;
import edu.stanford.nlp.parser.common.ParserConstraint;
import edu.stanford.nlp.trees.LabeledScoredTreeNode;
import edu.stanford.nlp.trees.Tree;
import edu.stanford.nlp.trees.TreeCoreAnnotations;
import edu.stanford.nlp.util.TreeShapedStack;

/**
 * Transition that makes a unary parse node in a partially finished tree.
 */
public class UnaryTransition implements Transition {
  public final String label;

  /** root transitions are illegal in the middle of the tree, naturally */
  public final boolean isRoot;

  public UnaryTransition(String label, boolean isRoot) {
    this.label = label;
    this.isRoot = isRoot;
  }

  /**
   * Legal as long as there is at least one item on the state's stack.
   */
  public boolean isLegal(State state, List constraints) {
    if (state.finished) {
      return false;
    }
    if (state.stack.size() == 0) {
      return false;
    }
    Tree top = state.stack.peek();
    if (top.label().value().equals(label)) {
      // Disallow unary transitions where the label doesn't change
      return false;
    }
    if (top.label().value().startsWith("@") && !label.equals(top.label().value().substring(1))) {
      return false;
    }
    if (top.children().length == 1) {
      Tree child = top.children()[0];
      if (child.children().length == 1) {
        Tree grandChild = child.children()[0];
        if (grandChild.children().length == 1) {
          // Three consecutive unary trees.  Not legal to keep adding unaries.
          // TODO: do preterminals count in that equation?
          return false;
        }
      }
    }
    if (isRoot && (state.stack.size() > 1 || !state.endOfQueue())) {
      return false;
    }
    // UnaryTransition actually doesn't care about the constraints.
    // If the constraint winds up unsatisfied, we'll get stuck and
    // have to do an "emergency transition" to fix the situation.
    return true;
  }

  /**
   * Add a unary node to the existing node on top of the stack
   */
  public State apply(State state) {
    return apply(state, 0.0);
  }

  static Tree addUnaryNode(Tree top, String label) {
    if (!(top.label() instanceof CoreLabel)) {
      throw new IllegalArgumentException("Stack should have CoreLabel nodes");
    }
    Tree newTop = createNode(top, label, top);
    return newTop;
  }

  static Tree createNode(Tree top, String label, Tree ... children) {
    CoreLabel headLabel = (CoreLabel) top.label();
    CoreLabel production = new CoreLabel();
    production.setValue(label);
    production.set(TreeCoreAnnotations.HeadWordLabelAnnotation.class, headLabel.get(TreeCoreAnnotations.HeadWordLabelAnnotation.class));
    production.set(TreeCoreAnnotations.HeadTagLabelAnnotation.class, headLabel.get(TreeCoreAnnotations.HeadTagLabelAnnotation.class));
    Tree newTop = new LabeledScoredTreeNode(production);
    for (Tree child : children) {
      newTop.addChild(child);
    }
    return newTop;
  }

  /**
   * Add a unary node to the existing node on top of the stack
   */
  public State apply(State state, double scoreDelta) {
    Tree top = state.stack.peek();
    Tree newTop = addUnaryNode(top, label);

    TreeShapedStack stack = state.stack.pop();
    stack = stack.push(newTop);
    return new State(stack, state.transitions.push(this), state.separators, state.sentence, state.tokenPosition, state.score + scoreDelta, false);    
  }

  @Override
  public boolean equals(Object o) {
    if (o == this) {
      return true;
    }
    if (!(o instanceof UnaryTransition)) {
      return false;
    }
    String otherLabel = ((UnaryTransition) o).label;
    return label.equals(otherLabel);
  }

  @Override
  public int hashCode() {
    return 29467607 ^ label.hashCode();
  }

  @Override
  public String toString() {
    return "Unary" + (isRoot ? "*" : "") + "(" + label + ")";
  }

  private static final long serialVersionUID = 1;  
}