/*
 * Copyright 2012 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;

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.embedded.EmbeddedChannel;
import io.netty.handler.codec.CodecException;
import io.netty.handler.codec.MessageToMessageDecoder;
import io.netty.util.ReferenceCountUtil;

import java.util.List;

Decodes the content of the received HttpRequest and HttpContent. The original content is replaced with the new content decoded by the EmbeddedChannel, which is created by newContentDecoder(String). Once decoding is finished, the value of the 'Content-Encoding' header is set to the target content encoding, as returned by getTargetContentEncoding(String). Also, the 'Content-Length' header is updated to the length of the decoded content. If the content encoding of the original is not supported by the decoder, newContentDecoder(String) should return null so that no decoding occurs (i.e. pass-through). 
 Please note that this is an abstract class. You have to extend this class and implement newContentDecoder(String) properly to make this class functional. For example, refer to the source code of HttpContentDecompressor. 
 This handler must be placed after HttpObjectDecoder in the pipeline so that this handler can intercept HTTP requests after HttpObjectDecoder converts ByteBufs into HTTP requests. /**
 * Decodes the content of the received {@link HttpRequest} and {@link HttpContent}.
 * The original content is replaced with the new content decoded by the
 * {@link EmbeddedChannel}, which is created by {@link #newContentDecoder(String)}.
 * Once decoding is finished, the value of the <tt>'Content-Encoding'</tt>
 * header is set to the target content encoding, as returned by {@link #getTargetContentEncoding(String)}.
 * Also, the <tt>'Content-Length'</tt> header is updated to the length of the
 * decoded content.  If the content encoding of the original is not supported
 * by the decoder, {@link #newContentDecoder(String)} should return {@code null}
 * so that no decoding occurs (i.e. pass-through).
 * <p>
 * Please note that this is an abstract class.  You have to extend this class
 * and implement {@link #newContentDecoder(String)} properly to make this class
 * functional.  For example, refer to the source code of {@link HttpContentDecompressor}.
 * <p>
 * This handler must be placed after {@link HttpObjectDecoder} in the pipeline
 * so that this handler can intercept HTTP requests after {@link HttpObjectDecoder}
 * converts {@link ByteBuf}s into HTTP requests.
 */
public abstract class HttpContentDecoder extends MessageToMessageDecoder<HttpObject> {

    static final String IDENTITY = HttpHeaderValues.IDENTITY.toString();

    protected ChannelHandlerContext ctx;
    private EmbeddedChannel decoder;
    private boolean continueResponse;

    @Override
    protected void decode(ChannelHandlerContext ctx, HttpObject msg, List<Object> out) throws Exception {
        if (msg instanceof HttpResponse && ((HttpResponse) msg).status().code() == 100) {

            if (!(msg instanceof LastHttpContent)) {
                continueResponse = true;
            }
            // 100-continue response must be passed through.
            out.add(ReferenceCountUtil.retain(msg));
            return;
        }

        if (continueResponse) {
            if (msg instanceof LastHttpContent) {
                continueResponse = false;
            }
            // 100-continue response must be passed through.
            out.add(ReferenceCountUtil.retain(msg));
            return;
        }

        if (msg instanceof HttpMessage) {
            cleanup();
            final HttpMessage message = (HttpMessage) msg;
            final HttpHeaders headers = message.headers();

            // Determine the content encoding.
            String contentEncoding = headers.get(HttpHeaderNames.CONTENT_ENCODING);
            if (contentEncoding != null) {
                contentEncoding = contentEncoding.trim();
            } else {
                contentEncoding = IDENTITY;
            }
            decoder = newContentDecoder(contentEncoding);

            if (decoder == null) {
                if (message instanceof HttpContent) {
                    ((HttpContent) message).retain();
                }
                out.add(message);
                return;
            }

            // Remove content-length header:
            // the correct value can be set only after all chunks are processed/decoded.
            // If buffering is not an issue, add HttpObjectAggregator down the chain, it will set the header.
            // Otherwise, rely on LastHttpContent message.
            if (headers.contains(HttpHeaderNames.CONTENT_LENGTH)) {
                headers.remove(HttpHeaderNames.CONTENT_LENGTH);
                headers.set(HttpHeaderNames.TRANSFER_ENCODING, HttpHeaderValues.CHUNKED);
            }
            // Either it is already chunked or EOF terminated.
            // See https://github.com/netty/netty/issues/5892

            // set new content encoding,
            CharSequence targetContentEncoding = getTargetContentEncoding(contentEncoding);
            if (HttpHeaderValues.IDENTITY.contentEquals(targetContentEncoding)) {
                // Do NOT set the 'Content-Encoding' header if the target encoding is 'identity'
                // as per: http://tools.ietf.org/html/rfc2616#section-14.11
                headers.remove(HttpHeaderNames.CONTENT_ENCODING);
            } else {
                headers.set(HttpHeaderNames.CONTENT_ENCODING, targetContentEncoding);
            }

            if (message instanceof HttpContent) {
                // If message is a full request or response object (headers + data), don't copy data part into out.
                // Output headers only; data part will be decoded below.
                // Note: "copy" object must not be an instance of LastHttpContent class,
                // as this would (erroneously) indicate the end of the HttpMessage to other handlers.
                HttpMessage copy;
                if (message instanceof HttpRequest) {
                    HttpRequest r = (HttpRequest) message; // HttpRequest or FullHttpRequest
                    copy = new DefaultHttpRequest(r.protocolVersion(), r.method(), r.uri());
                } else if (message instanceof HttpResponse) {
                    HttpResponse r = (HttpResponse) message; // HttpResponse or FullHttpResponse
                    copy = new DefaultHttpResponse(r.protocolVersion(), r.status());
                } else {
                    throw new CodecException("Object of class " + message.getClass().getName() +
                                             " is not a HttpRequest or HttpResponse");
                }
                copy.headers().set(message.headers());
                copy.setDecoderResult(message.decoderResult());
                out.add(copy);
            } else {
                out.add(message);
            }
        }

        if (msg instanceof HttpContent) {
            final HttpContent c = (HttpContent) msg;
            if (decoder == null) {
                out.add(c.retain());
            } else {
                decodeContent(c, out);
            }
        }
    }

    private void decodeContent(HttpContent c, List<Object> out) {
        ByteBuf content = c.content();

        decode(content, out);

        if (c instanceof LastHttpContent) {
            finishDecode(out);

            LastHttpContent last = (LastHttpContent) c;
            // Generate an additional chunk if the decoder produced
            // the last product on closure,
            HttpHeaders headers = last.trailingHeaders();
            if (headers.isEmpty()) {
                out.add(LastHttpContent.EMPTY_LAST_CONTENT);
            } else {
                out.add(new ComposedLastHttpContent(headers));
            }
        }
    }

    
Returns a new EmbeddedChannel that decodes the HTTP message content encoded in the specified contentEncoding.
Params:
contentEncoding – the value of the "Content-Encoding" header
Returns:
a new EmbeddedChannel if the specified encoding is supported. null otherwise (alternatively, you can throw an exception to block unknown encoding)./**
     * Returns a new {@link EmbeddedChannel} that decodes the HTTP message
     * content encoded in the specified <tt>contentEncoding</tt>.
     *
     * @param contentEncoding the value of the {@code "Content-Encoding"} header
     * @return a new {@link EmbeddedChannel} if the specified encoding is supported.
     *         {@code null} otherwise (alternatively, you can throw an exception
     *         to block unknown encoding).
     */
    protected abstract EmbeddedChannel newContentDecoder(String contentEncoding) throws Exception;

    
Returns the expected content encoding of the decoded content. This getMethod returns "identity" by default, which is the case for most decoders. 
Params:
contentEncoding – the value of the "Content-Encoding" header
Returns:
the expected content encoding of the new content/**
     * Returns the expected content encoding of the decoded content.
     * This getMethod returns {@code "identity"} by default, which is the case for
     * most decoders.
     *
     * @param contentEncoding the value of the {@code "Content-Encoding"} header
     * @return the expected content encoding of the new content
     */
    protected String getTargetContentEncoding(
            @SuppressWarnings("UnusedParameters") String contentEncoding) throws Exception {
        return IDENTITY;
    }

    @Override
    public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
        cleanupSafely(ctx);
        super.handlerRemoved(ctx);
    }

    @Override
    public void channelInactive(ChannelHandlerContext ctx) throws Exception {
        cleanupSafely(ctx);
        super.channelInactive(ctx);
    }

    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        this.ctx = ctx;
        super.handlerAdded(ctx);
    }

    private void cleanup() {
        if (decoder != null) {
            // Clean-up the previous decoder if not cleaned up correctly.
            decoder.finishAndReleaseAll();
            decoder = null;
        }
    }

    private void cleanupSafely(ChannelHandlerContext ctx) {
        try {
            cleanup();
        } catch (Throwable cause) {
            // If cleanup throws any error we need to propagate it through the pipeline
            // so we don't fail to propagate pipeline events.
            ctx.fireExceptionCaught(cause);
        }
    }

    private void decode(ByteBuf in, List<Object> out) {
        // call retain here as it will call release after its written to the channel
        decoder.writeInbound(in.retain());
        fetchDecoderOutput(out);
    }

    private void finishDecode(List<Object> out) {
        if (decoder.finish()) {
            fetchDecoderOutput(out);
        }
        decoder = null;
    }

    private void fetchDecoderOutput(List<Object> out) {
        for (;;) {
            ByteBuf buf = decoder.readInbound();
            if (buf == null) {
                break;
            }
            if (!buf.isReadable()) {
                buf.release();
                continue;
            }
            out.add(new DefaultHttpContent(buf));
        }
    }
}