/**
 * 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.camel.processor.resequencer;

import java.util.Timer;

import org.apache.camel.util.concurrent.ExecutorServiceHelper;

/**
 * Resequences elements based on a given {@link SequenceElementComparator}.
 * This resequencer is designed for resequencing element streams. Stream-based
 * resequencing has the advantage that the number of elements to be resequenced
 * need not be known in advance. Resequenced elements are delivered via a
 * {@link SequenceSender}.
 * <p>
 * The resequencer's behaviour for a given comparator is controlled by the
 * <code>timeout</code> property. This is the timeout (in milliseconds) for a
 * given element managed by this resequencer. An out-of-sequence element can
 * only be marked as <i>ready-for-delivery</i> if it either times out or if it
 * has an immediate predecessor (in that case it is in-sequence). If an
 * immediate predecessor of a waiting element arrives the timeout task for the
 * waiting element will be cancelled (which marks it as <i>ready-for-delivery</i>).
 * <p>
 * If the maximum out-of-sequence time difference between elements within a
 * stream is known, the <code>timeout</code> value should be set to this
 * value. In this case it is guaranteed that all elements of a stream will be
 * delivered in sequence via the {@link SequenceSender}. The lower the
 * <code>timeout</code> value is compared to the out-of-sequence time
 * difference between elements within a stream the higher the probability is for
 * out-of-sequence elements delivered by this resequencer. Delivery of elements
 * must be explicitly triggered by applications using the {@link #deliver()} or
 * {@link #deliverNext()} methods. Only elements that are <i>ready-for-delivery</i>
 * are delivered by these methods. The longer an application waits to trigger a
 * delivery the more elements may become <i>ready-for-delivery</i>.
 * <p>
 * The resequencer remembers the last-delivered element. If an element arrives
 * which is the immediate successor of the last-delivered element it is
 * <i>ready-for-delivery</i> immediately. After delivery the last-delivered
 * element is adjusted accordingly. If the last-delivered element is
 * <code>null</code> i.e. the resequencer was newly created the first arriving
 * element needs <code>timeout</code> milliseconds in any case for becoming
 * <i>ready-for-delivery</i>.
 * <p>
 *
 * @version 
 */
public class ResequencerEngine<E> {

    /**
     * The element that most recently hash been delivered or <code>null</code>
     * if no element has been delivered yet.
     */
    private Element<E> lastDelivered;

    /**
     * Minimum amount of time to wait for out-of-sequence elements.
     */
    private long timeout;

    /**
     * A sequence of elements for sorting purposes.
     */
    private Sequence<Element<E>> sequence;

    /**
     * A timer for scheduling timeout notifications.
     */
    private Timer timer;

    /**
     * A strategy for sending sequence elements.
     */
    private SequenceSender<E> sequenceSender;

    /**
     * Creates a new resequencer instance with a default timeout of 2000
     * milliseconds.
     *
     * @param comparator a sequence element comparator.
     */
    public ResequencerEngine(SequenceElementComparator<E> comparator) {
        this.sequence = createSequence(comparator);
        this.timeout = 2000L;
        this.lastDelivered = null;
    }

    public void start() {
        timer = new Timer(ExecutorServiceHelper.getThreadName("Camel Thread ${counter} - ${name}", "Stream Resequencer Timer"), true);
    }

    /**
     * Stops this resequencer (i.e. this resequencer's {@link Timer} instance).
     */
    public void stop() {
        timer.cancel();
    }

    /**
     * Returns the number of elements currently maintained by this resequencer.
     *
     * @return the number of elements currently maintained by this resequencer.
     */
    public synchronized int size() {
        return sequence.size();
    }

    /**
     * Returns this resequencer's timeout value.
     *
     * @return the timeout in milliseconds.
     */
    public long getTimeout() {
        return timeout;
    }

    /**
     * Sets this sequencer's timeout value.
     *
     * @param timeout the timeout in milliseconds.
     */
    public void setTimeout(long timeout) {
        this.timeout = timeout;
    }

    /**
     * Returns the sequence sender.
     *
     * @return the sequence sender.
     */
    public SequenceSender<E> getSequenceSender() {
        return sequenceSender;
    }

    /**
     * Sets the sequence sender.
     *
     * @param sequenceSender a sequence element sender.
     */
    public void setSequenceSender(SequenceSender<E> sequenceSender) {
        this.sequenceSender = sequenceSender;
    }

    /**
     * Returns the last delivered element.
     *
     * @return the last delivered element or <code>null</code> if no delivery
     *         has been made yet.
     */
    E getLastDelivered() {
        if (lastDelivered == null) {
            return null;
        }
        return lastDelivered.getObject();
    }

    /**
     * Sets the last delivered element. This is for testing purposes only.
     *
     * @param o an element.
     */
    void setLastDelivered(E o) {
        lastDelivered = new Element<E>(o);
    }

    /**
     * Inserts the given element into this resequencer. If the element is not
     * ready for immediate delivery and has no immediate presecessor then it is
     * scheduled for timing out. After being timed out it is ready for delivery.
     *
     * @param o an element.
     */
    public synchronized void insert(E o) {
        // wrap object into internal element
        Element<E> element = new Element<E>(o);
        // add element to sequence in proper order
        sequence.add(element);

        Element<E> successor = sequence.successor(element);

        // check if there is an immediate successor and cancel
        // timer task (no need to wait any more for timeout)
        if (successor != null) {
            successor.cancel();
        }

        // start delivery if current element is successor of last delivered element
        if (successorOfLastDelivered(element)) {
            // nothing to schedule
        } else if (sequence.predecessor(element) != null) {
            // nothing to schedule
        } else {
            element.schedule(defineTimeout());
        }
    }

    /**
     * Delivers all elements which are currently ready to deliver.
     *
     * @throws Exception thrown by {@link SequenceSender#sendElement(Object)}.
     *
     * @see ResequencerEngine#deliverNext() 
     */
    public synchronized void deliver() throws Exception {
        while (deliverNext()) {
            // do nothing here
        }
    }

    /**
     * Attempts to deliver a single element from the head of the resequencer
     * queue (sequence). Only elements which have not been scheduled for timing
     * out or which already timed out can be delivered. Elements are delivered via
     * {@link SequenceSender#sendElement(Object)}.
     *
     * @return <code>true</code> if the element has been delivered
     *         <code>false</code> otherwise.
     *
     * @throws Exception thrown by {@link SequenceSender#sendElement(Object)}.
     *
     */
    public boolean deliverNext() throws Exception {
        if (sequence.size() == 0) {
            return false;
        }
        // inspect element with lowest sequence value
        Element<E> element = sequence.first();

        // if element is scheduled do not deliver and return
        if (element.scheduled()) {
            return false;
        }

        // remove deliverable element from sequence
        sequence.remove(element);

        // set the delivered element to last delivered element
        lastDelivered = element;

        // deliver the sequence element
        sequenceSender.sendElement(element.getObject());

        // element has been delivered
        return true;
    }

    /**
     * Returns <code>true</code> if the given element is the immediate
     * successor of the last delivered element.
     *
     * @param element an element.
     * @return <code>true</code> if the given element is the immediate
     *         successor of the last delivered element.
     */
    private boolean successorOfLastDelivered(Element<E> element) {
        if (lastDelivered == null) {
            return false;
        }
        if (sequence.comparator().successor(element, lastDelivered)) {
            return true;
        }
        return false;
    }

    /**
     * Creates a timeout task based on the timeout setting of this resequencer.
     *
     * @return a new timeout task.
     */
    private Timeout defineTimeout() {
        return new Timeout(timer, timeout);
    }

    private static <E> Sequence<Element<E>> createSequence(SequenceElementComparator<E> comparator) {
        return new Sequence<Element<E>>(new ElementComparator<E>(comparator));
    }

}
