Copyright (c) 2008, 2016 IBM Corporation and others.
This program and the accompanying materials
are made available under the terms of the Eclipse Public License 2.0
which accompanies this distribution, and is available at
https://www.eclipse.org/legal/epl-2.0/
SPDX-License-Identifier: EPL-2.0
Contributors:
    IBM Corporation - initial API and implementation
/*******************************************************************************
 * Copyright (c) 2008, 2016 IBM Corporation and others.
 *
 * This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License 2.0
 * which accompanies this distribution, and is available at
 * https://www.eclipse.org/legal/epl-2.0/
 *
 * SPDX-License-Identifier: EPL-2.0
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/

package org.eclipse.osgi.internal.serviceregistry;

import java.util.*;

A Shrinkable Collection. This class provides a wrapper for a list of collections
that allows items to be removed from the wrapped collections (shrinking) but
does not allow items to be added to the wrapped collections. 

The collections must act as sets in that each collection in the list
must not have two entries which are equal.

All the optional Collection operations except
add and addAll are supported. Attempting to add to the
collection will result in an UnsupportedOperationException.
/**
 * A Shrinkable Collection. This class provides a wrapper for a list of collections
 * that allows items to be removed from the wrapped collections (shrinking) but
 * does not allow items to be added to the wrapped collections. 
 * 
 * <p>
 * The collections must act as sets in that each collection in the list
 * must not have two entries which are equal.
 * 
 * <p>
 * All the optional <code>Collection</code> operations except
 * <code>add</code> and <code>addAll</code> are supported. Attempting to add to the
 * collection will result in an <code>UnsupportedOperationException</code>.
 *
 */

public class ShrinkableCollection<E> implements Collection<E> {
	private final Collection<? extends E> collection;
	private final List<Collection<? extends E>> list;

	public ShrinkableCollection(Collection<? extends E> c) {
		if (c == null) {
			throw new NullPointerException();
		}
		List<Collection<? extends E>> empty = Collections.<Collection<? extends E>> emptyList();
		list = empty;
		collection = c;
	}

	public ShrinkableCollection(Collection<? extends E> c1, Collection<? extends E> c2) {
		list = new ArrayList<>(2);
		list.add(c1);
		list.add(c2);
		collection = initComposite(list);
	}

	public ShrinkableCollection(List<Collection<? extends E>> l) {
		list = new ArrayList<>(l);
		collection = initComposite(list);
	}

	private static <E> Collection<? extends E> initComposite(List<Collection<? extends E>> collections) {
		int size = 0;
		for (Collection<? extends E> c : collections) {
			assert verifyNoDuplicates(c);
			size += c.size();
		}
		Collection<E> result = new ArrayList<>(size);
		for (Collection<? extends E> c : collections) {
			for (E e : c) {
				if (!result.contains(e)) {
					result.add(e);
				}
			}
		}
		return result;
	}

	private static <E> boolean verifyNoDuplicates(Collection<? extends E> c) {
		for (E e : c) {
			int count = 0;
			for (E f : c) {
				if (e == null) {
					if (f == null) {
						count++;
					}
				} else {
					if (e.equals(f)) {
						count++;
					}
				}
			}
			if (count != 1) {
				return false;
			}
		}
		return true;
	}

	@Override
	public boolean add(E e) {
		throw new UnsupportedOperationException();
	}

	@Override
	public boolean addAll(Collection<? extends E> c) {
		throw new UnsupportedOperationException();
	}

	@Override
	public void clear() {
		collection.clear();
		for (Collection<? extends E> c : list) {
			c.clear();
		}
	}

	@Override
	public boolean contains(Object o) {
		return collection.contains(o);
	}

	@Override
	public boolean containsAll(Collection<?> c) {
		return collection.containsAll(c);
	}

	@Override
	public boolean isEmpty() {
		return collection.isEmpty();
	}

	@Override
	public Iterator<E> iterator() {
		@SuppressWarnings("unchecked")
		final Iterator<E> iter = (Iterator<E>) collection.iterator();
		final List<Collection<? extends E>> collections = list;
		if (collections.isEmpty()) {
			return iter;
		}
		return new Iterator<E>() {
			private E last;

		@Override
			public boolean hasNext() {
				return iter.hasNext();
			}

		@Override
			public E next() {
				last = iter.next();
				return last;
			}

		@Override
			public void remove() {
				iter.remove();
				for (Collection<? extends E> c : collections) {
					c.remove(last);
				}
			}
		};
	}

	@Override
	public boolean remove(Object o) {
		final boolean result = collection.remove(o);
		if (result) {
			for (Collection<? extends E> c : list) {
				c.remove(o);
			}
		}
		return result;
	}

	@Override
	public boolean removeAll(Collection<?> c) {
		final boolean result = collection.removeAll(c);
		if (result) {
			for (Collection<? extends E> cc : list) {
				cc.removeAll(c);
			}
		}
		return result;
	}

	@Override
	public boolean retainAll(Collection<?> c) {
		final boolean result = collection.retainAll(c);
		if (result) {
			for (Collection<? extends E> cc : list) {
				cc.retainAll(c);
			}
		}
		return result;
	}

	@Override
	public int size() {
		return collection.size();
	}

	@Override
	public Object[] toArray() {
		return collection.toArray();
	}

	@Override
	public <T> T[] toArray(T[] var0) {
		return collection.toArray(var0);
	}

	@Override
	public String toString() {
		return collection.toString();
	}
}