/*
 * Copyright 2014 The Netty Project
 *
 * The Netty Project 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 io.netty.handler.codec.http.websocketx.extensions;

import io.netty.channel.ChannelDuplexHandler;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelFutureListener;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelPromise;
import io.netty.handler.codec.http.HttpHeaderNames;
import io.netty.handler.codec.http.HttpRequest;
import io.netty.handler.codec.http.HttpResponse;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Iterator;
import java.util.List;

This handler negotiates and initializes the WebSocket Extensions.
It negotiates the extensions based on the client desired order,
ensures that the successfully negotiated extensions are consistent between them,
and initializes the channel pipeline with the extension decoder and encoder.
Find a basic implementation for compression extensions at
io.netty.handler.codec.http.websocketx.extensions.compression.WebSocketServerCompressionHandler.
/**
 * This handler negotiates and initializes the WebSocket Extensions.
 *
 * It negotiates the extensions based on the client desired order,
 * ensures that the successfully negotiated extensions are consistent between them,
 * and initializes the channel pipeline with the extension decoder and encoder.
 *
 * Find a basic implementation for compression extensions at
 * <tt>io.netty.handler.codec.http.websocketx.extensions.compression.WebSocketServerCompressionHandler</tt>.
 */
public class WebSocketServerExtensionHandler extends ChannelDuplexHandler {

    private final List<WebSocketServerExtensionHandshaker> extensionHandshakers;

    private List<WebSocketServerExtension> validExtensions;

    
Constructor
Params:
extensionHandshakers – 
     The extension handshaker in priority order. A handshaker could be repeated many times
     with fallback configuration./**
     * Constructor
     *
     * @param extensionHandshakers
     *      The extension handshaker in priority order. A handshaker could be repeated many times
     *      with fallback configuration.
     */
    public WebSocketServerExtensionHandler(WebSocketServerExtensionHandshaker... extensionHandshakers) {
        if (extensionHandshakers == null) {
            throw new NullPointerException("extensionHandshakers");
        }
        if (extensionHandshakers.length == 0) {
            throw new IllegalArgumentException("extensionHandshakers must contains at least one handshaker");
        }
        this.extensionHandshakers = Arrays.asList(extensionHandshakers);
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg)
            throws Exception {
        if (msg instanceof HttpRequest) {
            HttpRequest request = (HttpRequest) msg;

            if (WebSocketExtensionUtil.isWebsocketUpgrade(request.headers())) {
                String extensionsHeader = request.headers().getAsString(HttpHeaderNames.SEC_WEBSOCKET_EXTENSIONS);

                if (extensionsHeader != null) {
                    List<WebSocketExtensionData> extensions =
                            WebSocketExtensionUtil.extractExtensions(extensionsHeader);
                    int rsv = 0;

                    for (WebSocketExtensionData extensionData : extensions) {
                        Iterator<WebSocketServerExtensionHandshaker> extensionHandshakersIterator =
                                extensionHandshakers.iterator();
                        WebSocketServerExtension validExtension = null;

                        while (validExtension == null && extensionHandshakersIterator.hasNext()) {
                            WebSocketServerExtensionHandshaker extensionHandshaker =
                                    extensionHandshakersIterator.next();
                            validExtension = extensionHandshaker.handshakeExtension(extensionData);
                        }

                        if (validExtension != null && ((validExtension.rsv() & rsv) == 0)) {
                            if (validExtensions == null) {
                                validExtensions = new ArrayList<WebSocketServerExtension>(1);
                            }
                            rsv = rsv | validExtension.rsv();
                            validExtensions.add(validExtension);
                        }
                    }
                }
            }
        }

        super.channelRead(ctx, msg);
    }

    @Override
    public void write(final ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        if (msg instanceof HttpResponse &&
                WebSocketExtensionUtil.isWebsocketUpgrade(((HttpResponse) msg).headers()) && validExtensions != null) {
            HttpResponse response = (HttpResponse) msg;
            String headerValue = response.headers().getAsString(HttpHeaderNames.SEC_WEBSOCKET_EXTENSIONS);

            for (WebSocketServerExtension extension : validExtensions) {
                WebSocketExtensionData extensionData = extension.newReponseData();
                headerValue = WebSocketExtensionUtil.appendExtension(headerValue,
                        extensionData.name(), extensionData.parameters());
            }

            promise.addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) throws Exception {
                    if (future.isSuccess()) {
                        for (WebSocketServerExtension extension : validExtensions) {
                            WebSocketExtensionDecoder decoder = extension.newExtensionDecoder();
                            WebSocketExtensionEncoder encoder = extension.newExtensionEncoder();
                            ctx.pipeline().addAfter(ctx.name(), decoder.getClass().getName(), decoder);
                            ctx.pipeline().addAfter(ctx.name(), encoder.getClass().getName(), encoder);
                        }
                    }

                    ctx.pipeline().remove(ctx.name());
                }
            });

            if (headerValue != null) {
                response.headers().set(HttpHeaderNames.SEC_WEBSOCKET_EXTENSIONS, headerValue);
            }
        }

        super.write(ctx, msg, promise);
    }
}