/*
 * Copyright 2017 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.http2;

import io.netty.buffer.ByteBuf;
import io.netty.buffer.ByteBufUtil;
import io.netty.channel.ChannelHandler;
import io.netty.channel.ChannelHandlerAdapter;
import io.netty.channel.ChannelHandlerContext;
import io.netty.handler.codec.ByteToMessageDecoder;
import io.netty.handler.codec.http.HttpServerCodec;
import io.netty.handler.codec.http.HttpServerUpgradeHandler;
import io.netty.util.internal.UnstableApi;

import java.util.List;

import static io.netty.buffer.Unpooled.unreleasableBuffer;
import static io.netty.handler.codec.http2.Http2CodecUtil.connectionPrefaceBuf;

import static io.netty.util.internal.ObjectUtil.checkNotNull;

Performing cleartext upgrade, by h2c HTTP upgrade or Prior Knowledge.
This handler config pipeline for h2c upgrade when handler added.
And will update pipeline once it detect the connection is starting HTTP/2 by
prior knowledge or not.
/**
 * Performing cleartext upgrade, by h2c HTTP upgrade or Prior Knowledge.
 * This handler config pipeline for h2c upgrade when handler added.
 * And will update pipeline once it detect the connection is starting HTTP/2 by
 * prior knowledge or not.
 */
@UnstableApi
public final class CleartextHttp2ServerUpgradeHandler extends ChannelHandlerAdapter {
    private static final ByteBuf CONNECTION_PREFACE = unreleasableBuffer(connectionPrefaceBuf());

    private final HttpServerCodec httpServerCodec;
    private final HttpServerUpgradeHandler httpServerUpgradeHandler;
    private final ChannelHandler http2ServerHandler;

    
Creates the channel handler provide cleartext HTTP/2 upgrade from HTTP
upgrade or prior knowledge
Params:
httpServerCodec – the http server codec
httpServerUpgradeHandler – the http server upgrade handler for HTTP/2
http2ServerHandler – the http2 server handler, will be added into pipeline
                          when starting HTTP/2 by prior knowledge/**
     * Creates the channel handler provide cleartext HTTP/2 upgrade from HTTP
     * upgrade or prior knowledge
     *
     * @param httpServerCodec the http server codec
     * @param httpServerUpgradeHandler the http server upgrade handler for HTTP/2
     * @param http2ServerHandler the http2 server handler, will be added into pipeline
     *                           when starting HTTP/2 by prior knowledge
     */
    public CleartextHttp2ServerUpgradeHandler(HttpServerCodec httpServerCodec,
                                              HttpServerUpgradeHandler httpServerUpgradeHandler,
                                              ChannelHandler http2ServerHandler) {
        this.httpServerCodec = checkNotNull(httpServerCodec, "httpServerCodec");
        this.httpServerUpgradeHandler = checkNotNull(httpServerUpgradeHandler, "httpServerUpgradeHandler");
        this.http2ServerHandler = checkNotNull(http2ServerHandler, "http2ServerHandler");
    }

    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        ctx.pipeline()
           .addBefore(ctx.name(), null, new PriorKnowledgeHandler())
           .addBefore(ctx.name(), null, httpServerCodec)
           .replace(this, null, httpServerUpgradeHandler);
    }

    
Peek inbound message to determine current connection wants to start HTTP/2
by HTTP upgrade or prior knowledge
/**
     * Peek inbound message to determine current connection wants to start HTTP/2
     * by HTTP upgrade or prior knowledge
     */
    private final class PriorKnowledgeHandler extends ByteToMessageDecoder {
        @Override
        protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
            int prefaceLength = CONNECTION_PREFACE.readableBytes();
            int bytesRead = Math.min(in.readableBytes(), prefaceLength);

            if (!ByteBufUtil.equals(CONNECTION_PREFACE, CONNECTION_PREFACE.readerIndex(),
                                    in, in.readerIndex(), bytesRead)) {
                ctx.pipeline().remove(this);
            } else if (bytesRead == prefaceLength) {
                // Full h2 preface match, removed source codec, using http2 codec to handle
                // following network traffic
                ctx.pipeline()
                   .remove(httpServerCodec)
                   .remove(httpServerUpgradeHandler);

                ctx.pipeline().addAfter(ctx.name(), null, http2ServerHandler);
                ctx.pipeline().remove(this);

                ctx.fireUserEventTriggered(PriorKnowledgeUpgradeEvent.INSTANCE);
            }
        }
    }

    
User event that is fired to notify about HTTP/2 protocol is started.
/**
     * User event that is fired to notify about HTTP/2 protocol is started.
     */
    public static final class PriorKnowledgeUpgradeEvent {
        private static final PriorKnowledgeUpgradeEvent INSTANCE = new PriorKnowledgeUpgradeEvent();

        private PriorKnowledgeUpgradeEvent() {
        }
    }
}