/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import java.util.Set;

import javax.annotation.Nullable;

import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Preconditions;

/**
 * Implementation of {@link ImmutableSet} with exactly one element.
 *
 * @author Kevin Bourrillion
 * @author Nick Kralevich
 */
@GwtCompatible(serializable = true, emulated = true)
@SuppressWarnings("serial") // uses writeReplace(), not default serialization
final class SingletonImmutableSet<E> extends ImmutableSet<E> {

	final transient E element;
	// This is transient because it will be recalculated on the first
	// call to hashCode().
	//
	// A race condition is avoided since threads will either see that the value
	// is zero and recalculate it themselves, or two threads will see it at
	// the same time, and both recalculate it. If the cachedHashCode is 0,
	// it will always be recalculated, unfortunately.
	private transient int cachedHashCode;

	SingletonImmutableSet(E element) {
		this.element = Preconditions.checkNotNull(element);
	}

	SingletonImmutableSet(E element, int hashCode) {
		// Guaranteed to be non-null by the presence of the pre-computed hash code.
		this.element = element;
		cachedHashCode = hashCode;
	}

	@Override
	public int size() {
		return 1;
	}

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

	@Override
	public boolean contains(Object target) {
		return element.equals(target);
	}

	@Override
	public UnmodifiableIterator<E> iterator() {
		return Iterators.singletonIterator(element);
	}

	@Override
	boolean isPartialView() {
		return false;
	}

	@Override
	int copyIntoArray(Object[] dst, int offset) {
		dst[offset] = element;
		return offset + 1;
	}

	@Override
	public boolean equals(@Nullable Object object) {
		if (object == this) {
			return true;
		}
		if (object instanceof Set) {
			Set<?> that = (Set<?>) object;
			return that.size() == 1 && element.equals(that.iterator().next());
		}
		return false;
	}

	@Override
	public final int hashCode() {
		// Racy single-check.
		int code = cachedHashCode;
		if (code == 0) {
			cachedHashCode = code = element.hashCode();
		}
		return code;
	}

	@Override
	boolean isHashCodeFast() {
		return cachedHashCode != 0;
	}

	@Override
	public String toString() {
		String elementToString = element.toString();
		return new StringBuilder(elementToString.length() + 2).append('[').append(elementToString).append(']')
				.toString();
	}
}
