org.apache.syncope.common.lib.collections.CircularFifoQueue Maven / Gradle / Ivy
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/*
* 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.
*/
package org.apache.syncope.common.lib.collections;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.AbstractCollection;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Queue;
/**
* CircularFifoQueue is a first-in first-out queue with a fixed size that
* replaces its oldest element if full.
*
* The removal order of a {@link CircularFifoQueue} is based on the
* insertion order; elements are removed in the same order in which they
* were added. The iteration order is the same as the removal order.
*
* The {@link #add(Object)}, {@link #remove()}, {@link #peek()}, {@link #poll},
* {@link #offer(Object)} operations all perform in constant time.
* All other operations perform in linear time or worse.
*
* This queue prevents null objects from being added.
*
* @param the type of elements held in this collection
*/
public class CircularFifoQueue extends AbstractCollection implements Queue, Serializable {
private static final long serialVersionUID = -8423413834657610406L;
/** Underlying storage array. */
private transient E[] elements;
/** Array index of first (oldest) queue element. */
private transient int start = 0;
/**
* Index mod maxElements of the array position following the last queue
* element. Queue elements start at elements[start] and "wrap around"
* elements[maxElements-1], ending at elements[decrement(end)].
* For example, elements = {c,a,b}, start=1, end=1 corresponds to
* the queue [a,b,c].
*/
private transient int end = 0;
/** Flag to indicate if the queue is currently full. */
private transient boolean full = false;
/** Capacity of the queue. */
private final int maxElements;
/**
* Constructor that creates a queue with the default size of 32.
*/
public CircularFifoQueue() {
this(32);
}
/**
* Constructor that creates a queue with the specified size.
*
* @param size the size of the queue (cannot be changed)
* @throws IllegalArgumentException if the size is < 1
*/
@SuppressWarnings("unchecked")
public CircularFifoQueue(final int size) {
if (size <= 0) {
throw new IllegalArgumentException("The size must be greater than 0");
}
elements = (E[]) new Object[size];
maxElements = elements.length;
}
/**
* Constructor that creates a queue from the specified collection.
* The collection size also sets the queue size.
*
* @param coll the collection to copy into the queue, may not be null
* @throws NullPointerException if the collection is null
*/
public CircularFifoQueue(final Collection coll) {
this(coll.size());
addAll(coll);
}
//-----------------------------------------------------------------------
/**
* Write the queue out using a custom routine.
*
* @param out the output stream
* @throws IOException if an I/O error occurs while writing to the output stream
*/
private void writeObject(final ObjectOutputStream out) throws IOException {
out.defaultWriteObject();
out.writeInt(size());
for (final E e : this) {
out.writeObject(e);
}
}
/**
* Read the queue in using a custom routine.
*
* @param in the input stream
* @throws IOException if an I/O error occurs while writing to the output stream
* @throws ClassNotFoundException if the class of a serialized object can not be found
*/
@SuppressWarnings("unchecked")
private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
in.defaultReadObject();
elements = (E[]) new Object[maxElements];
final int size = in.readInt();
for (int i = 0; i < size; i++) {
elements[i] = (E) in.readObject();
}
start = 0;
full = size == maxElements;
if (full) {
end = 0;
} else {
end = size;
}
}
//-----------------------------------------------------------------------
/**
* Returns the number of elements stored in the queue.
*
* @return this queue's size
*/
@Override
public int size() {
int size;
if (end < start) {
size = maxElements - start + end;
} else if (end == start) {
size = full ? maxElements : 0;
} else {
size = end - start;
}
return size;
}
/**
* Returns true if this queue is empty; false otherwise.
*
* @return true if this queue is empty
*/
@Override
public boolean isEmpty() {
return size() == 0;
}
/**
*
*
* A {@code CircularFifoQueue} can never be full, thus this returns always
* {@code false}.
*
* @return always returns {@code false}
*/
public static boolean isFull() {
return false;
}
/**
* Returns {@code true} if the capacity limit of this queue has been reached,
* i.e. the number of elements stored in the queue equals its maximum size.
*
* @return {@code true} if the capacity limit has been reached, {@code false} otherwise
* @since 4.1
*/
public boolean isAtFullCapacity() {
return size() == maxElements;
}
/**
* Gets the maximum size of the collection (the bound).
*
* @return the maximum number of elements the collection can hold
*/
public int maxSize() {
return maxElements;
}
/**
* Clears this queue.
*/
@Override
public void clear() {
full = false;
start = 0;
end = 0;
Arrays.fill(elements, null);
}
/**
* Adds the given element to this queue. If the queue is full, the least recently added
* element is discarded so that a new element can be inserted.
*
* @param element the element to add
* @return true, always
* @throws NullPointerException if the given element is null
*/
@Override
public boolean add(final E element) {
if (null == element) {
throw new NullPointerException("Attempted to add null object to queue");
}
if (isAtFullCapacity()) {
remove();
}
elements[end++] = element;
if (end >= maxElements) {
end = 0;
}
if (end == start) {
full = true;
}
return true;
}
/**
* Returns the element at the specified position in this queue.
*
* @param index the position of the element in the queue
* @return the element at position {@code index}
* @throws NoSuchElementException if the requested position is outside the range [0, size)
*/
public E get(final int index) {
final int sz = size();
if (index < 0 || index >= sz) {
throw new NoSuchElementException(
String.format("The specified index (%1$d) is outside the available range [0, %2$d)",
index, Integer.valueOf(sz)));
}
final int idx = (start + index) % maxElements;
return elements[idx];
}
//-----------------------------------------------------------------------
/**
* Adds the given element to this queue. If the queue is full, the least recently added
* element is discarded so that a new element can be inserted.
*
* @param element the element to add
* @return true, always
* @throws NullPointerException if the given element is null
*/
@Override
public boolean offer(final E element) {
return add(element);
}
@Override
public E poll() {
if (isEmpty()) {
return null;
}
return remove();
}
@Override
public E element() {
if (isEmpty()) {
throw new NoSuchElementException("queue is empty");
}
return peek();
}
@Override
public E peek() {
if (isEmpty()) {
return null;
}
return elements[start];
}
@Override
public E remove() {
if (isEmpty()) {
throw new NoSuchElementException("queue is empty");
}
final E element = elements[start];
if (null != element) {
elements[start++] = null;
if (start >= maxElements) {
start = 0;
}
full = false;
}
return element;
}
//-----------------------------------------------------------------------
/**
* Increments the internal index.
*
* @param index the index to increment
* @return the updated index
*/
private int increment(final int index) {
int result = index;
result++;
if (result >= maxElements) {
result = 0;
}
return result;
}
/**
* Decrements the internal index.
*
* @param index the index to decrement
* @return the updated index
*/
private int decrement(final int index) {
int result = index;
result--;
if (result < 0) {
result = maxElements - 1;
}
return result;
}
/**
* Returns an iterator over this queue's elements.
*
* @return an iterator over this queue's elements
*/
@Override
public Iterator iterator() {
return new Iterator<>() {
private int index = start;
private int lastReturnedIndex = -1;
private boolean isFirst = full;
@Override
public boolean hasNext() {
return isFirst || index != end;
}
@Override
public E next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
isFirst = false;
lastReturnedIndex = index;
index = increment(index);
return elements[lastReturnedIndex];
}
@Override
public void remove() {
if (lastReturnedIndex == -1) {
throw new IllegalStateException();
}
// First element can be removed quickly
if (lastReturnedIndex == start) {
CircularFifoQueue.this.remove();
lastReturnedIndex = -1;
return;
}
int pos = lastReturnedIndex + 1;
if (start < lastReturnedIndex && pos < end) {
// shift in one part
System.arraycopy(elements, pos, elements, lastReturnedIndex, end - pos);
} else {
// Other elements require us to shift the subsequent elements
while (pos != end) {
if (pos >= maxElements) {
elements[pos - 1] = elements[0];
pos = 0;
} else {
elements[decrement(pos)] = elements[pos];
pos = increment(pos);
}
}
}
lastReturnedIndex = -1;
end = decrement(end);
elements[end] = null;
full = false;
index = decrement(index);
}
};
}
}