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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.util.concurrent;

import java.io.IOException;
import java.io.PrintWriter;
import java.io.Writer;
import java.util.AbstractCollection;
import java.util.AbstractList;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.RandomAccess;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.ReentrantLock;

import edu.stanford.nlp.io.IOUtils;
import edu.stanford.nlp.util.Generics;
import edu.stanford.nlp.util.Index;

/**
 * A fast threadsafe index that supports constant-time lookup in both directions. This
 * index is tuned for circumstances in which readers significantly outnumber writers.
 *
 * @author Spence Green
 *
 * @param 
 */
public class ConcurrentHashIndex extends AbstractCollection implements Index, RandomAccess {

  private static final long serialVersionUID = 6465313844985269109L;

  public static final int UNKNOWN_ID = -1;
  private static final int DEFAULT_INITIAL_CAPACITY = 100;

  private final ConcurrentHashMap item2Index;
  private int indexSize;
  private final ReentrantLock lock;
  private final AtomicReference index2Item;

  /**
   * Constructor.
   */
  public ConcurrentHashIndex() {
    this(DEFAULT_INITIAL_CAPACITY);
  }

  /**
   * Constructor.
   *
   * @param initialCapacity
   */
  public ConcurrentHashIndex(int initialCapacity) {
    item2Index = new ConcurrentHashMap<>(initialCapacity);
    indexSize = 0;
    lock = new ReentrantLock();
    Object[] arr = new Object[initialCapacity];
    index2Item = new AtomicReference<>(arr);
  }

  @SuppressWarnings("unchecked")
  @Override
  public E get(int i) {
    Object[] arr = index2Item.get();
    if (i < indexSize) {
      // arr.length guaranteed to be == to size() given the
      // implementation of indexOf below.
      return (E) arr[i];
    }
    throw new ArrayIndexOutOfBoundsException(String.format("Out of bounds: %d >= %d", i, indexSize));
  }

  @Override
  public int indexOf(E o) {
    Integer id = item2Index.get(o);
    return id == null ? UNKNOWN_ID : id;
  }

  @Override
  public int addToIndex(E o) {
    Integer index = item2Index.get(o);
    if (index != null) {
      return index;
    }

    lock.lock();
    try {
      // Recheck state
      if (item2Index.containsKey(o)) {
        return item2Index.get(o);

      } else {
        final int newIndex = indexSize++;
        Object[] arr = index2Item.get();
        assert newIndex <= arr.length;
        if (newIndex == arr.length) {
          // Increase size of array if necessary
          Object[] newArr = new Object[2*newIndex];
          System.arraycopy(arr, 0, newArr, 0, arr.length);
          arr = newArr;
        }
        arr[newIndex] = o;
        index2Item.set(arr);
        item2Index.put(o, newIndex);
        return newIndex;
      }
    } finally {
      lock.unlock();
    }
  }

  @Override
  @Deprecated
  public int indexOf(E o, boolean add) {
    if (add) {
      return addToIndex(o);
    } else {
      return indexOf(o);
    }
  }

  @Override
  public boolean add(E o) {
    return addToIndex(o) != UNKNOWN_ID;
  }

  @Override
  public boolean addAll(Collection c) {
    boolean changed = false;
    for (E element: c) {
      changed |= add(element);
    }
    return changed;
  }

  @Override
  public List objectsList() {
    return Generics.newArrayList(item2Index.keySet());
  }

  @Override
  public Collection objects(final int[] indices) {
    return new AbstractList() {
      @Override
      public E get(int index) {
        return ConcurrentHashIndex.this.get(indices[index]);
      }
      @Override
      public int size() {
        return indices.length;
      }
    };
  }

  @Override
  public boolean isLocked() {
    return false;
  }

  @Override
  public void lock() {
    throw new UnsupportedOperationException();
  }

  @Override
  public void unlock() {
    throw new UnsupportedOperationException();
  }

  @Override
  public void saveToWriter(Writer out) throws IOException {
    final String nl = System.getProperty("line.separator");
    for (int i = 0, sz = indexSize; i < sz; i++) {
      E o = get(i);
      if (o != null) {
        out.write(i + "=" + get(i) + nl);
      }
    }
  }

  @Override
  public void saveToFilename(String s) {
    PrintWriter bw = null;
    try {
      bw = IOUtils.getPrintWriter(s);
      for (int i = 0, size = indexSize; i < size; i++) {
        E o = get(i);
        if (o != null) {
          bw.printf("%d=%s%n", i, o.toString());
        }
      }
      bw.close();
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      if (bw != null) {
        bw.close();
      }
    }
  }

  @Override
  public Iterator iterator() {
    return new Iterator() {
      private int index = 0;
      private int size = ConcurrentHashIndex.this.size();
      @Override
      public boolean hasNext() {
        return index < size;
      }
      @Override
      public E next() {
        return ConcurrentHashIndex.this.get(index++);
      }
      @Override
      public void remove() {
        throw new UnsupportedOperationException();
      }
    };
  }

  @Override
  public int size() {
    return indexSize;
  }

  @Override
  public String toString() {
    StringBuilder buff = new StringBuilder("[");
    int i;
    final int size = size();
    for (i = 0; i < size; i++) {
      E e = get(i);
      if (e != null) {
        buff.append(i).append('=').append(e);
        if (i < (size-1)) buff.append(',');
      }
    }
    if (i < size()) buff.append("...");
    buff.append(']');
    return buff.toString();
  }

  @SuppressWarnings("unchecked")
  @Override
  public boolean contains(Object o) {
    return indexOf((E) o) != UNKNOWN_ID;
  }

  @Override
  public void clear() {
    lock.lock();
    try {
      item2Index.clear();
      indexSize = 0;
      Object[] arr = new Object[DEFAULT_INITIAL_CAPACITY];
      index2Item.set(arr);
    } finally {
      lock.unlock();
    }
  }
}