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/*
* Copyright 2011 Google Inc. All Rights Reserved.
*
* 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.googlecode.concurrentlinkedhashmap;
import java.util.AbstractCollection;
import java.util.Collection;
import java.util.Deque;
import java.util.Iterator;
import java.util.NoSuchElementException;
import javax.annotation.concurrent.NotThreadSafe;
/**
* Linked list implementation of the {@link Deque} interface where the link
* pointers are tightly integrated with the element. Linked deques have no
* capacity restrictions; they grow as necessary to support usage. They are not
* thread-safe; in the absence of external synchronization, they do not support
* concurrent access by multiple threads. Null elements are prohibited.
*
* Most LinkedDeque operations run in constant time by assuming that
* the {@link Linked} parameter is associated with the deque instance. Any usage
* that violates this assumption will result in non-deterministic behavior.
*
* The iterators returned by this class are not fail-fast: If
* the deque is modified at any time after the iterator is created, the iterator
* will be in an unknown state. Thus, in the face of concurrent modification,
* the iterator risks arbitrary, non-deterministic behavior at an undetermined
* time in the future.
*
* @author [email protected] (Ben Manes)
* @param the type of elements held in this collection
* @see
* http://code.google.com/p/concurrentlinkedhashmap/
*/
@NotThreadSafe
final class LinkedDeque> extends AbstractCollection implements Deque {
// This class provides a doubly-linked list that is optimized for the virtual
// machine. The first and last elements are manipulated instead of a slightly
// more convenient sentinel element to avoid the insertion of null checks with
// NullPointerException throws in the byte code. The links to a removed
// element are cleared to help a generational garbage collector if the
// discarded elements inhabit more than one generation.
/**
* Pointer to first node.
* Invariant: (first == null && last == null) ||
* (first.prev == null)
*/
E first;
/**
* Pointer to last node.
* Invariant: (first == null && last == null) ||
* (last.next == null)
*/
E last;
/**
* Links the element to the front of the deque so that it becomes the first
* element.
*
* @param e the unlinked element
*/
void linkFirst(final E e) {
final E f = first;
first = e;
if (f == null) {
last = e;
} else {
f.setPrevious(e);
e.setNext(f);
}
}
/**
* Links the element to the back of the deque so that it becomes the last
* element.
*
* @param e the unlinked element
*/
void linkLast(final E e) {
final E l = last;
last = e;
if (l == null) {
first = e;
} else {
l.setNext(e);
e.setPrevious(l);
}
}
/** Unlinks the non-null first element. */
E unlinkFirst() {
final E f = first;
final E next = f.getNext();
f.setNext(null);
first = next;
if (next == null) {
last = null;
} else {
next.setPrevious(null);
}
return f;
}
/** Unlinks the non-null last element. */
E unlinkLast() {
final E l = last;
final E prev = l.getPrevious();
l.setPrevious(null);
last = prev;
if (prev == null) {
first = null;
} else {
prev.setNext(null);
}
return l;
}
/** Unlinks the non-null element. */
void unlink(E e) {
final E prev = e.getPrevious();
final E next = e.getNext();
if (prev == null) {
first = next;
} else {
prev.setNext(next);
e.setPrevious(null);
}
if (next == null) {
last = prev;
} else {
next.setPrevious(prev);
e.setNext(null);
}
}
@Override
public boolean isEmpty() {
return (first == null);
}
void checkNotEmpty() {
if (isEmpty()) {
throw new NoSuchElementException();
}
}
/**
* {@inheritDoc}
*
* Beware that, unlike in most collections, this method is NOT a
* constant-time operation.
*/
@Override
public int size() {
int size = 0;
for (E e = first; e != null; e = e.getNext()) {
size++;
}
return size;
}
@Override
public void clear() {
for (E e = first; e != null;) {
E next = e.getNext();
e.setPrevious(null);
e.setNext(null);
e = next;
}
first = last = null;
}
@Override
public boolean contains(Object o) {
return (o instanceof Linked) && contains((Linked) o);
}
// A fast-path containment check
boolean contains(Linked e) {
return (e.getPrevious() != null)
|| (e.getNext() != null)
|| (e == first);
}
/**
* Moves the element to the front of the deque so that it becomes the first
* element.
*
* @param e the linked element
*/
public void moveToFront(E e) {
if (e != first) {
unlink(e);
linkFirst(e);
}
}
/**
* Moves the element to the back of the deque so that it becomes the last
* element.
*
* @param e the linked element
*/
public void moveToBack(E e) {
if (e != last) {
unlink(e);
linkLast(e);
}
}
@Override
public E peek() {
return peekFirst();
}
@Override
public E peekFirst() {
return first;
}
@Override
public E peekLast() {
return last;
}
@Override
public E getFirst() {
checkNotEmpty();
return peekFirst();
}
@Override
public E getLast() {
checkNotEmpty();
return peekLast();
}
@Override
public E element() {
return getFirst();
}
@Override
public boolean offer(E e) {
return offerLast(e);
}
@Override
public boolean offerFirst(E e) {
if (contains(e)) {
return false;
}
linkFirst(e);
return true;
}
@Override
public boolean offerLast(E e) {
if (contains(e)) {
return false;
}
linkLast(e);
return true;
}
@Override
public boolean add(E e) {
return offerLast(e);
}
@Override
public void addFirst(E e) {
if (!offerFirst(e)) {
throw new IllegalArgumentException();
}
}
@Override
public void addLast(E e) {
if (!offerLast(e)) {
throw new IllegalArgumentException();
}
}
@Override
public E poll() {
return pollFirst();
}
@Override
public E pollFirst() {
return isEmpty() ? null : unlinkFirst();
}
@Override
public E pollLast() {
return isEmpty() ? null : unlinkLast();
}
@Override
public E remove() {
return removeFirst();
}
@Override
@SuppressWarnings("unchecked")
public boolean remove(Object o) {
return (o instanceof Linked) && remove((E) o);
}
// A fast-path removal
boolean remove(E e) {
if (contains(e)) {
unlink(e);
return true;
}
return false;
}
@Override
public E removeFirst() {
checkNotEmpty();
return pollFirst();
}
@Override
public boolean removeFirstOccurrence(Object o) {
return remove(o);
}
@Override
public E removeLast() {
checkNotEmpty();
return pollLast();
}
@Override
public boolean removeLastOccurrence(Object o) {
return remove(o);
}
@Override
public boolean removeAll(Collection c) {
boolean modified = false;
for (Object o : c) {
modified |= remove(o);
}
return modified;
}
@Override
public void push(E e) {
addFirst(e);
}
@Override
public E pop() {
return removeFirst();
}
@Override
public Iterator iterator() {
return new AbstractLinkedIterator(first) {
@Override E computeNext() {
return cursor.getNext();
}
};
}
@Override
public Iterator descendingIterator() {
return new AbstractLinkedIterator(last) {
@Override E computeNext() {
return cursor.getPrevious();
}
};
}
abstract class AbstractLinkedIterator implements Iterator {
E cursor;
/**
* Creates an iterator that can can traverse the deque.
*
* @param start the initial element to begin traversal from
*/
AbstractLinkedIterator(E start) {
cursor = start;
}
@Override
public boolean hasNext() {
return (cursor != null);
}
@Override
public E next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
E e = cursor;
cursor = computeNext();
return e;
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
/**
* Retrieves the next element to traverse to or null if there are
* no more elements.
*/
abstract E computeNext();
}
}
/**
* An element that is linked on the {@link Deque}.
*/
interface Linked> {
/**
* Retrieves the previous element or null if either the element is
* unlinked or the first element on the deque.
*/
T getPrevious();
/** Sets the previous element or null if there is no link. */
void setPrevious(T prev);
/**
* Retrieves the next element or null if either the element is
* unlinked or the last element on the deque.
*/
T getNext();
/** Sets the next element or null if there is no link. */
void setNext(T next);
}