A circular queue from mina
//package com.google.code.yanf4j.util;
/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you 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.
*
*/
import java.io.Serializable;
import java.util.AbstractList;
import java.util.Arrays;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Queue;
/**
* A circular queue from mina
*
* @author dennis
*
* @param <E>
*/
public class CircularQueue<E> extends AbstractList<E> implements List<E>,
Queue<E>, Serializable {
private static final long serialVersionUID = 3993421269224511264L;
private static final int DEFAULT_CAPACITY = 4;
private final int initialCapacity;
// XXX: This volatile keyword here is a workaround for SUN Java Compiler
// bug,
// which produces buggy byte code. I don't event know why adding a volatile
// fixes the problem. Eclipse Java Compiler seems to produce correct byte
// code.
private volatile Object[] items;
private int mask;
private int first = 0;
private int last = 0;
private boolean full;
private int shrinkThreshold;
/**
* Construct a new, empty queue.
*/
public CircularQueue() {
this(DEFAULT_CAPACITY);
}
public CircularQueue(int initialCapacity) {
int actualCapacity = normalizeCapacity(initialCapacity);
this.items = new Object[actualCapacity];
this.mask = actualCapacity - 1;
this.initialCapacity = actualCapacity;
this.shrinkThreshold = 0;
}
private static int normalizeCapacity(int initialCapacity) {
int actualCapacity = 1;
while (actualCapacity < initialCapacity) {
actualCapacity <<= 1;
if (actualCapacity < 0) {
actualCapacity = 1 << 30;
break;
}
}
return actualCapacity;
}
/**
* Returns the capacity of this queue.
*/
public int capacity() {
return this.items.length;
}
@Override
public void clear() {
if (!isEmpty()) {
Arrays.fill(this.items, null);
this.first = 0;
this.last = 0;
this.full = false;
shrinkIfNeeded();
}
}
@SuppressWarnings("unchecked")
public E poll() {
if (isEmpty()) {
return null;
}
Object ret = this.items[this.first];
this.items[this.first] = null;
decreaseSize();
if (this.first == this.last) {
this.first = this.last = 0;
}
shrinkIfNeeded();
return (E) ret;
}
public boolean offer(E item) {
if (item == null) {
throw new NullPointerException("item");
}
expandIfNeeded();
this.items[this.last] = item;
increaseSize();
return true;
}
@SuppressWarnings("unchecked")
public E peek() {
if (isEmpty()) {
return null;
}
return (E) this.items[this.first];
}
@SuppressWarnings("unchecked")
@Override
public E get(int idx) {
checkIndex(idx);
return (E) this.items[getRealIndex(idx)];
}
@Override
public boolean isEmpty() {
return (this.first == this.last) && !this.full;
}
@Override
public int size() {
if (this.full) {
return capacity();
}
if (this.last >= this.first) {
return this.last - this.first;
} else {
return this.last - this.first + capacity();
}
}
@Override
public String toString() {
return "first=" + this.first + ", last=" + this.last + ", size="
+ size() + ", mask = " + this.mask;
}
private void checkIndex(int idx) {
if (idx < 0 || idx >= size()) {
throw new IndexOutOfBoundsException(String.valueOf(idx));
}
}
private int getRealIndex(int idx) {
return (this.first + idx) & this.mask;
}
private void increaseSize() {
this.last = (this.last + 1) & this.mask;
this.full = this.first == this.last;
}
private void decreaseSize() {
this.first = (this.first + 1) & this.mask;
this.full = false;
}
private void expandIfNeeded() {
if (this.full) {
// expand queue
final int oldLen = this.items.length;
final int newLen = oldLen << 1;
Object[] tmp = new Object[newLen];
if (this.first < this.last) {
System.arraycopy(this.items, this.first, tmp, 0, this.last
- this.first);
} else {
System.arraycopy(this.items, this.first, tmp, 0, oldLen
- this.first);
System.arraycopy(this.items, 0, tmp, oldLen - this.first,
this.last);
}
this.first = 0;
this.last = oldLen;
this.items = tmp;
this.mask = tmp.length - 1;
if (newLen >>> 3 > this.initialCapacity) {
this.shrinkThreshold = newLen >>> 3;
}
}
}
private void shrinkIfNeeded() {
int size = size();
if (size <= this.shrinkThreshold) {
// shrink queue
final int oldLen = this.items.length;
int newLen = normalizeCapacity(size);
if (size == newLen) {
newLen <<= 1;
}
if (newLen >= oldLen) {
return;
}
if (newLen < this.initialCapacity) {
if (oldLen == this.initialCapacity) {
return;
} else {
newLen = this.initialCapacity;
}
}
Object[] tmp = new Object[newLen];
// Copy only when there's something to copy.
if (size > 0) {
if (this.first < this.last) {
System.arraycopy(this.items, this.first, tmp, 0, this.last
- this.first);
} else {
System.arraycopy(this.items, this.first, tmp, 0, oldLen
- this.first);
System.arraycopy(this.items, 0, tmp, oldLen - this.first,
this.last);
}
}
this.first = 0;
this.last = size;
this.items = tmp;
this.mask = tmp.length - 1;
this.shrinkThreshold = 0;
}
}
@Override
public boolean add(E o) {
return offer(o);
}
@SuppressWarnings("unchecked")
@Override
public E set(int idx, E o) {
checkIndex(idx);
int realIdx = getRealIndex(idx);
Object old = this.items[realIdx];
this.items[realIdx] = o;
return (E) old;
}
@Override
public void add(int idx, E o) {
if (idx == size()) {
offer(o);
return;
}
checkIndex(idx);
expandIfNeeded();
int realIdx = getRealIndex(idx);
// Make a room for a new element.
if (this.first < this.last) {
System.arraycopy(this.items, realIdx, this.items, realIdx + 1,
this.last - realIdx);
} else {
if (realIdx >= this.first) {
System.arraycopy(this.items, 0, this.items, 1, this.last);
this.items[0] = this.items[this.items.length - 1];
System.arraycopy(this.items, realIdx, this.items, realIdx + 1,
this.items.length - realIdx - 1);
} else {
System.arraycopy(this.items, realIdx, this.items, realIdx + 1,
this.last - realIdx);
}
}
this.items[realIdx] = o;
increaseSize();
}
@SuppressWarnings("unchecked")
@Override
public E remove(int idx) {
if (idx == 0) {
return poll();
}
checkIndex(idx);
int realIdx = getRealIndex(idx);
Object removed = this.items[realIdx];
// Remove a room for the removed element.
if (this.first < this.last) {
System.arraycopy(this.items, this.first, this.items,
this.first + 1, realIdx - this.first);
} else {
if (realIdx >= this.first) {
System.arraycopy(this.items, this.first, this.items,
this.first + 1, realIdx - this.first);
} else {
System.arraycopy(this.items, 0, this.items, 1, realIdx);
this.items[0] = this.items[this.items.length - 1];
System.arraycopy(this.items, this.first, this.items,
this.first + 1, this.items.length - this.first - 1);
}
}
this.items[this.first] = null;
decreaseSize();
shrinkIfNeeded();
return (E) removed;
}
public E remove() {
if (isEmpty()) {
throw new NoSuchElementException();
}
return poll();
}
public E element() {
if (isEmpty()) {
throw new NoSuchElementException();
}
return peek();
}
}
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