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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.ie.crf; 
import edu.stanford.nlp.util.logging.Redwood;

import java.io.Serializable;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

import edu.stanford.nlp.stats.ClassicCounter;
import edu.stanford.nlp.stats.Counter;
import edu.stanford.nlp.stats.Counters;
import edu.stanford.nlp.util.Generics;
import edu.stanford.nlp.util.HashIndex;
import edu.stanford.nlp.util.Index;

/**
 * Constrains test-time inference to labels observed in training.
 *
 * @author Spence Green
 *
 */
public class LabelDictionary implements Serializable  {

  /** A logger for this class */
  private static Redwood.RedwoodChannels log = Redwood.channels(LabelDictionary.class);

  private static final long serialVersionUID = 6790400453922524056L;

  private final boolean DEBUG = false;

  /**
   * Initial capacity of the bookkeeping data structures.
   */
  private final int DEFAULT_CAPACITY = 30000;

  // Bookkeeping
  private Counter observationCounts;
  private Map> observedLabels;

  // Final data structure
  private Index observationIndex;
  private int[][] labelDictionary;

  /**
   * Constructor.
   */
  public LabelDictionary() {
    this.observationCounts = new ClassicCounter<>(DEFAULT_CAPACITY);
    this.observedLabels = Generics.newHashMap(DEFAULT_CAPACITY);
  }

  /**
   * Increment counts for an observation/label pair.
   *
   * @param observation
   * @param label
   */
  public void increment(String observation, String label) {
    if (labelDictionary != null) {
      throw new RuntimeException("Label dictionary is already locked.");
    }
    observationCounts.incrementCount(observation);
    if ( ! observedLabels.containsKey(observation)) {
      observedLabels.put(observation, new HashSet<>());
    }
    observedLabels.get(observation).add(label.intern());
  }

  /**
   * True if this observation is constrained, and false otherwise.
   */
  public boolean isConstrained(String observation) {
    return observationIndex.indexOf(observation) >= 0;
  }

  /**
   * Get the allowed label set for an observation.
   *
   * @param observation
   * @return The allowed label set, or null if the observation is unconstrained.
   */
  public int[] getConstrainedSet(String observation) {
    int i = observationIndex.indexOf(observation);
    return i >= 0 ? labelDictionary[i] : null;
  }

  /**
   * Setup the constrained label sets and free bookkeeping resources.
   *
   * @param threshold
   * @param labelIndex
   */
  public void lock(int threshold, Index labelIndex) {
    if (labelDictionary != null) throw new RuntimeException("Label dictionary is already locked");
    log.info("Label dictionary enabled");
    System.err.printf("#observations: %d%n", (int) observationCounts.totalCount());
    Counters.retainAbove(observationCounts, threshold);
    Set constrainedObservations = observationCounts.keySet();
    labelDictionary = new int[constrainedObservations.size()][];
    observationIndex = new HashIndex<>(constrainedObservations.size());
    for (String observation : constrainedObservations) {
      int i = observationIndex.addToIndex(observation);
      assert i < labelDictionary.length;
      Set allowedLabels = observedLabels.get(observation);
      labelDictionary[i] = new int[allowedLabels.size()];
      int j = 0;
      for (String label : allowedLabels) {
        labelDictionary[i][j++] = labelIndex.indexOf(label);
      }
      if (DEBUG) {
        System.err.printf("%s : %s%n", observation, allowedLabels.toString());
      }
    }
    observationIndex.lock();
    System.err.printf("#constraints: %d%n", labelDictionary.length);

    // Free bookkeeping data structures
    observationCounts = null;
    observedLabels = null;
  }
}