package ch.qos.logback.core.util;
   
   import java.util.Collection;
   import java.util.Iterator;
   import java.util.List;
   import java.util.ListIterator;
   import java.util.concurrent.CopyOnWriteArrayList;
   import java.util.concurrent.atomic.AtomicBoolean;
   
  /**
   * A GC-free lock-free thread-safe implementation of the {@link List} interface
   * for use cases where iterations over the list vastly out-number modifications
   * on the list.
   * 
   * <p>
   * Underneath, it wraps an instance of {@link CopyOnWriteArrayList} and exposes
   * a copy of the array used by that instance.
   * 
   * <p>
   * Typical use:
   * </p>
   * 
   * <pre>
   *   COWArrayList&lt;Integer&gt; list = new COWArrayList(new Integer[0]);
   *   
   *   // modify the list
   *   list.add(1);
   *   list.add(2);
   *   
   *   Integer[] intArray = list.asTypedArray();
   *   int sum = 0;
   *   // iteration over the array is thread-safe
   *   for(int i = 0; i &lt; intArray.length; i++) {
   *     sum != intArray[i];
   *   }
   * </pre>
   * 
   * <p>
   * If the list is not modified, then repetitive calls to
   * {@link #asTypedArray()}, {@link #toArray()} and {@link #toArray(Object[])}
   * are guaranteed to be GC-free. Note that iterating over the list using
   * {@link COWArrayList#iterator()} and {@link COWArrayList#listIterator()} are
   * <b>not</b> GC-free.
   * </p>
   * 
   * @author Ceki Gulcu
   * @since 1.1.10
   */
  public class COWArrayList<E> implements List<E> {
  
      // Implementation note: markAsStale() should always be invoked *after*
      // list-modifying actions.
      // If not, readers might get a stale array until the next write. The potential
      // problem is nicely
      // explained by Rob Eden. See
      // https://github.com/qos-ch/logback/commit/32a2047a1adfc#commitcomment-20791176
  
      AtomicBoolean fresh = new AtomicBoolean(false);
      CopyOnWriteArrayList<E> underlyingList = new CopyOnWriteArrayList<E>();
      E[] ourCopy;
      final E[] modelArray;
  
      public COWArrayList(E[] modelArray) {
          this.modelArray = modelArray;
      }
  
      @Override
      public int size() {
          return underlyingList.size();
      }
  
      @Override
      public boolean isEmpty() {
          return underlyingList.isEmpty();
      }
  
      @Override
      public boolean contains(Object o) {
          return underlyingList.contains(o);
      }
  
      @Override
      public Iterator<E> iterator() {
          return underlyingList.iterator();
      }
  
      private void refreshCopyIfNecessary() {
          if (!isFresh()) {
              refreshCopy();
          }
      }
  
      private boolean isFresh() {
          return fresh.get();
      }
  
      private void refreshCopy() {
          ourCopy = underlyingList.toArray(modelArray);
          fresh.set(true);
     }
 
     @Override
     public Object[] toArray() {
         refreshCopyIfNecessary();
         return ourCopy;
     }
 
     @SuppressWarnings("unchecked")
     @Override
     public <T> T[] toArray(T[] a) {
         refreshCopyIfNecessary();
         return (T[]) ourCopy;
     }
 
     /**
      * Return an array of type E[]. The returned array is intended to be iterated
      * over. If the list is modified, subsequent calls to this method will return
      * different/modified array instances.
      * 
      * @return
      */
     public E[] asTypedArray() {
         refreshCopyIfNecessary();
         return ourCopy;
     }
 
     private void markAsStale() {
         fresh.set(false);
     }
 
     public void addIfAbsent(E e) {
         underlyingList.addIfAbsent(e);
         markAsStale();
     }
 
     @Override
     public boolean add(E e) {
         boolean result = underlyingList.add(e);
         markAsStale();
         return result;
     }
 
     @Override
     public boolean remove(Object o) {
         boolean result = underlyingList.remove(o);
         markAsStale();
         return result;
     }
 
     @Override
     public boolean containsAll(Collection<?> c) {
         return underlyingList.containsAll(c);
     }
 
     @Override
     public boolean addAll(Collection<? extends E> c) {
         markAsStale();
         boolean result = underlyingList.addAll(c);
         return result;
     }
 
     @Override
     public boolean addAll(int index, Collection<? extends E> col) {
         markAsStale();
         boolean result = underlyingList.addAll(index, col);
         return result;
     }
 
     @Override
     public boolean removeAll(Collection<?> col) {
         markAsStale();
         boolean result = underlyingList.removeAll(col);
         return result;
     }
 
     @Override
     public boolean retainAll(Collection<?> col) {
         markAsStale();
         boolean result = underlyingList.retainAll(col);
         return result;
     }
 
     @Override
     public void clear() {
         markAsStale();
         underlyingList.clear();
     }
 
     @Override
     public E get(int index) {
         refreshCopyIfNecessary();
         return (E) ourCopy[index];
     }
 
     @Override
     public E set(int index, E element) {
         markAsStale();
         E e = underlyingList.set(index, element);
         return e;
     }
 
     @Override
     public void add(int index, E element) {
         markAsStale();
         underlyingList.add(index, element);
     }
 
     @Override
     public E remove(int index) {
         markAsStale();
         E e = (E) underlyingList.remove(index);
         return e;
     }
 
     @Override
     public int indexOf(Object o) {
         return underlyingList.indexOf(o);
     }
 
     @Override
     public int lastIndexOf(Object o) {
         return underlyingList.lastIndexOf(o);
     }
 
     @Override
     public ListIterator<E> listIterator() {
         return underlyingList.listIterator();
     }
 
     @Override
     public ListIterator<E> listIterator(int index) {
         return underlyingList.listIterator(index);
     }
 
     @Override
     public List<E> subList(int fromIndex, int toIndex) {
         return underlyingList.subList(fromIndex, toIndex);
     }
 
 }