/*
 * 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.buffer.ByteBufHolder;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.embedded.EmbeddedChannel;
import io.netty.handler.codec.MessageToMessageCodec;
import io.netty.util.ReferenceCountUtil;

import java.util.ArrayDeque;
import java.util.List;
import java.util.Queue;

Encodes the content of the outbound HttpResponse and HttpContent. The original content is replaced with the new content encoded by the EmbeddedChannel, which is created by beginEncode(HttpResponse, String). Once encoding is finished, the value of the 'Content-Encoding' header is set to the target content encoding, as returned by beginEncode(HttpResponse, String). Also, the 'Content-Length' header is updated to the length of the encoded content. If there is no supported or allowed encoding in the corresponding HttpRequest's "Accept-Encoding" header, beginEncode(HttpResponse, String) should return null so that no encoding occurs (i.e. pass-through). 
 Please note that this is an abstract class. You have to extend this class and implement beginEncode(HttpResponse, String) properly to make this class functional. For example, refer to the source code of HttpContentCompressor. 
 This handler must be placed after HttpObjectEncoder in the pipeline so that this handler can intercept HTTP responses before HttpObjectEncoder converts them into ByteBufs. /**
 * Encodes the content of the outbound {@link HttpResponse} and {@link HttpContent}.
 * The original content is replaced with the new content encoded by the
 * {@link EmbeddedChannel}, which is created by {@link #beginEncode(HttpResponse, String)}.
 * Once encoding is finished, the value of the <tt>'Content-Encoding'</tt> header
 * is set to the target content encoding, as returned by
 * {@link #beginEncode(HttpResponse, String)}.
 * Also, the <tt>'Content-Length'</tt> header is updated to the length of the
 * encoded content.  If there is no supported or allowed encoding in the
 * corresponding {@link HttpRequest}'s {@code "Accept-Encoding"} header,
 * {@link #beginEncode(HttpResponse, String)} should return {@code null} so that
 * no encoding occurs (i.e. pass-through).
 * <p>
 * Please note that this is an abstract class.  You have to extend this class
 * and implement {@link #beginEncode(HttpResponse, String)} properly to make
 * this class functional.  For example, refer to the source code of
 * {@link HttpContentCompressor}.
 * <p>
 * This handler must be placed after {@link HttpObjectEncoder} in the pipeline
 * so that this handler can intercept HTTP responses before {@link HttpObjectEncoder}
 * converts them into {@link ByteBuf}s.
 */
public abstract class HttpContentEncoder extends MessageToMessageCodec<HttpRequest, HttpObject> {

    private enum State {
        PASS_THROUGH,
        AWAIT_HEADERS,
        AWAIT_CONTENT
    }

    private static final CharSequence ZERO_LENGTH_HEAD = "HEAD";
    private static final CharSequence ZERO_LENGTH_CONNECT = "CONNECT";
    private static final int CONTINUE_CODE = HttpResponseStatus.CONTINUE.code();

    private final Queue<CharSequence> acceptEncodingQueue = new ArrayDeque<CharSequence>();
    private EmbeddedChannel encoder;
    private State state = State.AWAIT_HEADERS;

    @Override
    public boolean acceptOutboundMessage(Object msg) throws Exception {
        return msg instanceof HttpContent || msg instanceof HttpResponse;
    }

    @Override
    protected void decode(ChannelHandlerContext ctx, HttpRequest msg, List<Object> out)
            throws Exception {
        CharSequence acceptedEncoding = msg.headers().get(HttpHeaderNames.ACCEPT_ENCODING);
        if (acceptedEncoding == null) {
            acceptedEncoding = HttpContentDecoder.IDENTITY;
        }

        HttpMethod meth = msg.method();
        if (meth == HttpMethod.HEAD) {
            acceptedEncoding = ZERO_LENGTH_HEAD;
        } else if (meth == HttpMethod.CONNECT) {
            acceptedEncoding = ZERO_LENGTH_CONNECT;
        }

        acceptEncodingQueue.add(acceptedEncoding);
        out.add(ReferenceCountUtil.retain(msg));
    }

    @Override
    protected void encode(ChannelHandlerContext ctx, HttpObject msg, List<Object> out) throws Exception {
        final boolean isFull = msg instanceof HttpResponse && msg instanceof LastHttpContent;
        switch (state) {
            case AWAIT_HEADERS: {
                ensureHeaders(msg);
                assert encoder == null;

                final HttpResponse res = (HttpResponse) msg;
                final int code = res.status().code();
                final CharSequence acceptEncoding;
                if (code == CONTINUE_CODE) {
                    // We need to not poll the encoding when response with CONTINUE as another response will follow
                    // for the issued request. See https://github.com/netty/netty/issues/4079
                    acceptEncoding = null;
                } else {
                    // Get the list of encodings accepted by the peer.
                    acceptEncoding = acceptEncodingQueue.poll();
                    if (acceptEncoding == null) {
                        throw new IllegalStateException("cannot send more responses than requests");
                    }
                }

                /*
                 * per rfc2616 4.3 Message Body
                 * All 1xx (informational), 204 (no content), and 304 (not modified) responses MUST NOT include a
                 * message-body. All other responses do include a message-body, although it MAY be of zero length.
                 *
                 * 9.4 HEAD
                 * The HEAD method is identical to GET except that the server MUST NOT return a message-body
                 * in the response.
                 *
                 * Also we should pass through HTTP/1.0 as transfer-encoding: chunked is not supported.
                 *
                 * See https://github.com/netty/netty/issues/5382
                 */
                if (isPassthru(res.protocolVersion(), code, acceptEncoding)) {
                    if (isFull) {
                        out.add(ReferenceCountUtil.retain(res));
                    } else {
                        out.add(res);
                        // Pass through all following contents.
                        state = State.PASS_THROUGH;
                    }
                    break;
                }

                if (isFull) {
                    // Pass through the full response with empty content and continue waiting for the next resp.
                    if (!((ByteBufHolder) res).content().isReadable()) {
                        out.add(ReferenceCountUtil.retain(res));
                        break;
                    }
                }

                // Prepare to encode the content.
                final Result result = beginEncode(res, acceptEncoding.toString());

                // If unable to encode, pass through.
                if (result == null) {
                    if (isFull) {
                        out.add(ReferenceCountUtil.retain(res));
                    } else {
                        out.add(res);
                        // Pass through all following contents.
                        state = State.PASS_THROUGH;
                    }
                    break;
                }

                encoder = result.contentEncoder();

                // Encode the content and remove or replace the existing headers
                // so that the message looks like a decoded message.
                res.headers().set(HttpHeaderNames.CONTENT_ENCODING, result.targetContentEncoding());

                // Output the rewritten response.
                if (isFull) {
                    // Convert full message into unfull one.
                    HttpResponse newRes = new DefaultHttpResponse(res.protocolVersion(), res.status());
                    newRes.headers().set(res.headers());
                    out.add(newRes);

                    ensureContent(res);
                    encodeFullResponse(newRes, (HttpContent) res, out);
                    break;
                } else {
                    // Make the response chunked to simplify content transformation.
                    res.headers().remove(HttpHeaderNames.CONTENT_LENGTH);
                    res.headers().set(HttpHeaderNames.TRANSFER_ENCODING, HttpHeaderValues.CHUNKED);

                    out.add(res);
                    state = State.AWAIT_CONTENT;
                    if (!(msg instanceof HttpContent)) {
                        // only break out the switch statement if we have not content to process
                        // See https://github.com/netty/netty/issues/2006
                        break;
                    }
                    // Fall through to encode the content
                }
            }
            case AWAIT_CONTENT: {
                ensureContent(msg);
                if (encodeContent((HttpContent) msg, out)) {
                    state = State.AWAIT_HEADERS;
                }
                break;
            }
            case PASS_THROUGH: {
                ensureContent(msg);
                out.add(ReferenceCountUtil.retain(msg));
                // Passed through all following contents of the current response.
                if (msg instanceof LastHttpContent) {
                    state = State.AWAIT_HEADERS;
                }
                break;
            }
        }
    }

    private void encodeFullResponse(HttpResponse newRes, HttpContent content, List<Object> out) {
        int existingMessages = out.size();
        encodeContent(content, out);

        if (HttpUtil.isContentLengthSet(newRes)) {
            // adjust the content-length header
            int messageSize = 0;
            for (int i = existingMessages; i < out.size(); i++) {
                Object item = out.get(i);
                if (item instanceof HttpContent) {
                    messageSize += ((HttpContent) item).content().readableBytes();
                }
            }
            HttpUtil.setContentLength(newRes, messageSize);
        } else {
            newRes.headers().set(HttpHeaderNames.TRANSFER_ENCODING, HttpHeaderValues.CHUNKED);
        }
    }

    private static boolean isPassthru(HttpVersion version, int code, CharSequence httpMethod) {
        return code < 200 || code == 204 || code == 304 ||
               (httpMethod == ZERO_LENGTH_HEAD || (httpMethod == ZERO_LENGTH_CONNECT && code == 200)) ||
                version == HttpVersion.HTTP_1_0;
    }

    private static void ensureHeaders(HttpObject msg) {
        if (!(msg instanceof HttpResponse)) {
            throw new IllegalStateException(
                    "unexpected message type: " +
                    msg.getClass().getName() + " (expected: " + HttpResponse.class.getSimpleName() + ')');
        }
    }

    private static void ensureContent(HttpObject msg) {
        if (!(msg instanceof HttpContent)) {
            throw new IllegalStateException(
                    "unexpected message type: " +
                    msg.getClass().getName() + " (expected: " + HttpContent.class.getSimpleName() + ')');
        }
    }

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

        encode(content, out);

        if (c instanceof LastHttpContent) {
            finishEncode(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));
            }
            return true;
        }
        return false;
    }

    
Prepare to encode the HTTP message content.
Params:
headers – 
       the headers
acceptEncoding –  the value of the "Accept-Encoding" header
Returns:
the result of preparation, which is composed of the determined target content encoding and a new EmbeddedChannel that encodes the content into the target content encoding. null if acceptEncoding is unsupported or rejected and thus the content should be handled as-is (i.e. no encoding)./**
     * Prepare to encode the HTTP message content.
     *
     * @param headers
     *        the headers
     * @param acceptEncoding
     *        the value of the {@code "Accept-Encoding"} header
     *
     * @return the result of preparation, which is composed of the determined
     *         target content encoding and a new {@link EmbeddedChannel} that
     *         encodes the content into the target content encoding.
     *         {@code null} if {@code acceptEncoding} is unsupported or rejected
     *         and thus the content should be handled as-is (i.e. no encoding).
     */
    protected abstract Result beginEncode(HttpResponse headers, String acceptEncoding) throws Exception;

    @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);
    }

    private void cleanup() {
        if (encoder != null) {
            // Clean-up the previous encoder if not cleaned up correctly.
            encoder.finishAndReleaseAll();
            encoder = 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 encode(ByteBuf in, List<Object> out) {
        // call retain here as it will call release after its written to the channel
        encoder.writeOutbound(in.retain());
        fetchEncoderOutput(out);
    }

    private void finishEncode(List<Object> out) {
        if (encoder.finish()) {
            fetchEncoderOutput(out);
        }
        encoder = null;
    }

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

    public static final class Result {
        private final String targetContentEncoding;
        private final EmbeddedChannel contentEncoder;

        public Result(String targetContentEncoding, EmbeddedChannel contentEncoder) {
            if (targetContentEncoding == null) {
                throw new NullPointerException("targetContentEncoding");
            }
            if (contentEncoder == null) {
                throw new NullPointerException("contentEncoder");
            }

            this.targetContentEncoding = targetContentEncoding;
            this.contentEncoder = contentEncoder;
        }

        public String targetContentEncoding() {
            return targetContentEncoding;
        }

        public EmbeddedChannel contentEncoder() {
            return contentEncoder;
        }
    }
}