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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.Collections;
import java.util.List;
import java.util.Set;

import edu.stanford.nlp.ling.CoreLabel;
import edu.stanford.nlp.trees.Tree;
import edu.stanford.nlp.trees.TreeCoreAnnotations;
import edu.stanford.nlp.util.Generics;

public class CreateTransitionSequence {
  // static methods only.  
  // we could change this if we wanted to include options.
  private CreateTransitionSequence() {}

  public static List> createTransitionSequences(List binarizedTrees, boolean compoundUnary, Set rootStates, Set rootOnlyStates) {
    List> transitionLists = Generics.newArrayList();
    for (Tree tree : binarizedTrees) {
      List transitions = createTransitionSequence(tree, compoundUnary, rootStates, rootOnlyStates);
      transitionLists.add(transitions);
    }
    return transitionLists;
  }

  public static List createTransitionSequence(Tree tree) {
    return createTransitionSequence(tree, true, Collections.singleton("ROOT"), Collections.singleton("ROOT"));
  }

  public static List createTransitionSequence(Tree tree, boolean compoundUnary, Set rootStates, Set rootOnlyStates) {
    List transitions = Generics.newArrayList();

    createTransitionSequenceHelper(transitions, tree, compoundUnary, rootOnlyStates);
    transitions.add(new FinalizeTransition(rootStates));
    transitions.add(new IdleTransition());

    return transitions;
  }

  private static void createTransitionSequenceHelper(List transitions, Tree tree, boolean compoundUnary, Set rootOnlyStates) {
    if (tree.isLeaf()) {
      // do nothing
    } else if (tree.isPreTerminal()) {
      transitions.add(new ShiftTransition());
    } else if (tree.children().length == 1) {
      boolean isRoot = rootOnlyStates.contains(tree.label().value());
      if (compoundUnary) {
        List labels = Generics.newArrayList();
        while (tree.children().length == 1 && !tree.isPreTerminal()) {
          labels.add(tree.label().value());
          tree = tree.children()[0];
        }
        createTransitionSequenceHelper(transitions, tree, compoundUnary, rootOnlyStates);
        transitions.add(new CompoundUnaryTransition(labels, isRoot));
      } else {
        createTransitionSequenceHelper(transitions, tree.children()[0], compoundUnary, rootOnlyStates);
        transitions.add(new UnaryTransition(tree.label().value(), isRoot));
      }
    } else if (tree.children().length == 2) {
      createTransitionSequenceHelper(transitions, tree.children()[0], compoundUnary, rootOnlyStates);
      createTransitionSequenceHelper(transitions, tree.children()[1], compoundUnary, rootOnlyStates);

      // This is the tricky part... need to decide if the binary
      // transition is a left or right transition.  This is done by
      // looking at the existing heads of this node and its two
      // children.  The expectation is that the tree already has heads
      // assigned; otherwise, exception is thrown
      if (!(tree.label() instanceof CoreLabel) || 
          !(tree.children()[0].label() instanceof CoreLabel) ||
          !(tree.children()[1].label() instanceof CoreLabel)) {
        throw new IllegalArgumentException("Expected tree labels to be CoreLabel");
      }
      CoreLabel label = (CoreLabel) tree.label();
      CoreLabel leftLabel = (CoreLabel) tree.children()[0].label();
      CoreLabel rightLabel = (CoreLabel) tree.children()[1].label();
      CoreLabel head = label.get(TreeCoreAnnotations.HeadWordLabelAnnotation.class);
      CoreLabel leftHead = leftLabel.get(TreeCoreAnnotations.HeadWordLabelAnnotation.class);
      CoreLabel rightHead = rightLabel.get(TreeCoreAnnotations.HeadWordLabelAnnotation.class);
      if (head == null || leftHead == null || rightHead == null) {
        throw new IllegalArgumentException("Expected tree labels to have their heads assigned");
      }
      if (head == leftHead) {
        transitions.add(new BinaryTransition(tree.label().value(), BinaryTransition.Side.LEFT));
      } else if (head == rightHead) {
        transitions.add(new BinaryTransition(tree.label().value(), BinaryTransition.Side.RIGHT));
      } else {
        throw new IllegalArgumentException("Heads were incorrectly assigned: tree's head is not matched to either the right or left head");
      }
    } else {
      throw new IllegalArgumentException("Expected a binarized tree");
    }
  }
}