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/*******************************************************************************
 * Copyright 2018 Viridian Software Limited
 *
 * 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.elefana.http;

import java.util.PriorityQueue;
import java.util.Queue;

import io.netty.channel.ChannelDuplexHandler;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelPromise;
import io.netty.handler.codec.http.LastHttpContent;

/**
 * Based on https://github.com/typesafehub/netty-http-pipelining and
 * https://github.com/spinscale/netty4-http-pipelining
 */
public class HttpPipeliningHandler extends ChannelDuplexHandler {
	public static final int INITIAL_EVENTS_HELD = 4;

	private final int maxEventsHeld;
	private final Queue holdingQueue;

	private int sequence = 0;
	private int nextRequiredSequence = 0;

	/**
	 * @param maxEventsHeld
	 *            the maximum number of channel events that will be retained
	 *            prior to aborting the channel connection. This is required as
	 *            events cannot queue up indefinitely; we would run out of
	 *            memory if this was the case.
	 */
	public HttpPipeliningHandler(final int maxEventsHeld) {
		this.maxEventsHeld = maxEventsHeld;
		this.holdingQueue = new PriorityQueue<>(INITIAL_EVENTS_HELD);
	}

	@Override
	public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
		if (msg instanceof LastHttpContent) {
			super.channelRead(ctx, new HttpPipelinedRequest((LastHttpContent) msg, sequence++));
		}
	}

	@Override
	public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
		if (msg instanceof HttpPipelinedResponse) {
			boolean channelShouldClose = false;

			synchronized (holdingQueue) {
				if (holdingQueue.size() < maxEventsHeld) {
					final HttpPipelinedResponse currentEvent = (HttpPipelinedResponse) msg;
					holdingQueue.add(currentEvent);

					while (!holdingQueue.isEmpty()) {
						final HttpPipelinedResponse queuedPipelinedResponse = holdingQueue.peek();

						if (queuedPipelinedResponse.getSequenceId() != nextRequiredSequence) {
							break;
						}
						holdingQueue.remove();
						super.write(ctx, queuedPipelinedResponse.getResponse(), queuedPipelinedResponse.getPromise());
						nextRequiredSequence++;
					}
				} else {
					channelShouldClose = true;
				}
			}

			if (channelShouldClose) {
				ctx.close();
			}
		} else {
			super.write(ctx, msg, promise);
		}
	}
}