/*
 * Copyright 2015 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.ChannelFuture;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelPromise;
import io.netty.util.ReferenceCountUtil;
import io.netty.util.internal.UnstableApi;

import java.util.ArrayDeque;
import java.util.Iterator;
import java.util.Map;
import java.util.Queue;
import java.util.TreeMap;

import static io.netty.handler.codec.http2.Http2CodecUtil.SMALLEST_MAX_CONCURRENT_STREAMS;
import static io.netty.handler.codec.http2.Http2Error.PROTOCOL_ERROR;
import static io.netty.handler.codec.http2.Http2Exception.connectionError;

Implementation of a Http2ConnectionEncoder that dispatches all method call to another Http2ConnectionEncoder, until SETTINGS_MAX_CONCURRENT_STREAMS is reached. 

When this limit is hit, instead of rejecting any new streams this implementation buffers newly
created streams and their corresponding frames. Once an active stream gets closed or the maximum
number of concurrent streams is increased, this encoder will automatically try to empty its
buffer and create as many new streams as possible.

 If a GOAWAY frame is received from the remote endpoint, all buffered writes for streams with an ID less than the specified lastStreamId will immediately fail with a Http2GoAwayException. 

 If the channel/encoder gets closed, all new and buffered writes will immediately fail with a Http2ChannelClosedException. 
This implementation makes the buffering mostly transparent and is expected to be used as a drop-in decorator of DefaultHttp2ConnectionEncoder. 
/**
 * Implementation of a {@link Http2ConnectionEncoder} that dispatches all method call to another
 * {@link Http2ConnectionEncoder}, until {@code SETTINGS_MAX_CONCURRENT_STREAMS} is reached.
 * <p/>
 * <p>When this limit is hit, instead of rejecting any new streams this implementation buffers newly
 * created streams and their corresponding frames. Once an active stream gets closed or the maximum
 * number of concurrent streams is increased, this encoder will automatically try to empty its
 * buffer and create as many new streams as possible.
 * <p/>
 * <p>
 * If a {@code GOAWAY} frame is received from the remote endpoint, all buffered writes for streams
 * with an ID less than the specified {@code lastStreamId} will immediately fail with a
 * {@link Http2GoAwayException}.
 * <p/>
 * <p>
 * If the channel/encoder gets closed, all new and buffered writes will immediately fail with a
 * {@link Http2ChannelClosedException}.
 * </p>
 * <p>This implementation makes the buffering mostly transparent and is expected to be used as a
 * drop-in decorator of {@link DefaultHttp2ConnectionEncoder}.
 * </p>
 */
@UnstableApi
public class StreamBufferingEncoder extends DecoratingHttp2ConnectionEncoder {

    
Thrown if buffered streams are terminated due to this encoder being closed.
/**
     * Thrown if buffered streams are terminated due to this encoder being closed.
     */
    public static final class Http2ChannelClosedException extends Http2Exception {
        private static final long serialVersionUID = 4768543442094476971L;

        public Http2ChannelClosedException() {
            super(Http2Error.REFUSED_STREAM, "Connection closed");
        }
    }

    
Thrown by StreamBufferingEncoder if buffered streams are terminated due to receipt of a GOAWAY. /**
     * Thrown by {@link StreamBufferingEncoder} if buffered streams are terminated due to
     * receipt of a {@code GOAWAY}.
     */
    public static final class Http2GoAwayException extends Http2Exception {
        private static final long serialVersionUID = 1326785622777291198L;
        private final int lastStreamId;
        private final long errorCode;
        private final byte[] debugData;

        public Http2GoAwayException(int lastStreamId, long errorCode, byte[] debugData) {
            super(Http2Error.STREAM_CLOSED);
            this.lastStreamId = lastStreamId;
            this.errorCode = errorCode;
            this.debugData = debugData;
        }

        public int lastStreamId() {
            return lastStreamId;
        }

        public long errorCode() {
            return errorCode;
        }

        public byte[] debugData() {
            return debugData;
        }
    }

    
Buffer for any streams and corresponding frames that could not be created due to the maximum
concurrent stream limit being hit.
/**
     * Buffer for any streams and corresponding frames that could not be created due to the maximum
     * concurrent stream limit being hit.
     */
    private final TreeMap<Integer, PendingStream> pendingStreams = new TreeMap<Integer, PendingStream>();
    private int maxConcurrentStreams;
    private boolean closed;

    public StreamBufferingEncoder(Http2ConnectionEncoder delegate) {
        this(delegate, SMALLEST_MAX_CONCURRENT_STREAMS);
    }

    public StreamBufferingEncoder(Http2ConnectionEncoder delegate, int initialMaxConcurrentStreams) {
        super(delegate);
        this.maxConcurrentStreams = initialMaxConcurrentStreams;
        connection().addListener(new Http2ConnectionAdapter() {

            @Override
            public void onGoAwayReceived(int lastStreamId, long errorCode, ByteBuf debugData) {
                cancelGoAwayStreams(lastStreamId, errorCode, debugData);
            }

            @Override
            public void onStreamClosed(Http2Stream stream) {
                tryCreatePendingStreams();
            }
        });
    }

    
Indicates the number of streams that are currently buffered, awaiting creation.
/**
     * Indicates the number of streams that are currently buffered, awaiting creation.
     */
    public int numBufferedStreams() {
        return pendingStreams.size();
    }

    @Override
    public ChannelFuture writeHeaders(ChannelHandlerContext ctx, int streamId, Http2Headers headers,
                                      int padding, boolean endStream, ChannelPromise promise) {
        return writeHeaders(ctx, streamId, headers, 0, Http2CodecUtil.DEFAULT_PRIORITY_WEIGHT,
                false, padding, endStream, promise);
    }

    @Override
    public ChannelFuture writeHeaders(ChannelHandlerContext ctx, int streamId, Http2Headers headers,
                                      int streamDependency, short weight, boolean exclusive,
                                      int padding, boolean endOfStream, ChannelPromise promise) {
        if (closed) {
            return promise.setFailure(new Http2ChannelClosedException());
        }
        if (isExistingStream(streamId) || connection().goAwayReceived()) {
            return super.writeHeaders(ctx, streamId, headers, streamDependency, weight,
                    exclusive, padding, endOfStream, promise);
        }
        if (canCreateStream()) {
            return super.writeHeaders(ctx, streamId, headers, streamDependency, weight,
                    exclusive, padding, endOfStream, promise);
        }
        PendingStream pendingStream = pendingStreams.get(streamId);
        if (pendingStream == null) {
            pendingStream = new PendingStream(ctx, streamId);
            pendingStreams.put(streamId, pendingStream);
        }
        pendingStream.frames.add(new HeadersFrame(headers, streamDependency, weight, exclusive,
                padding, endOfStream, promise));
        return promise;
    }

    @Override
    public ChannelFuture writeRstStream(ChannelHandlerContext ctx, int streamId, long errorCode,
                                        ChannelPromise promise) {
        if (isExistingStream(streamId)) {
            return super.writeRstStream(ctx, streamId, errorCode, promise);
        }
        // Since the delegate doesn't know about any buffered streams we have to handle cancellation
        // of the promises and releasing of the ByteBufs here.
        PendingStream stream = pendingStreams.remove(streamId);
        if (stream != null) {
            // Sending a RST_STREAM to a buffered stream will succeed the promise of all frames
            // associated with the stream, as sending a RST_STREAM means that someone "doesn't care"
            // about the stream anymore and thus there is not point in failing the promises and invoking
            // error handling routines.
            stream.close(null);
            promise.setSuccess();
        } else {
            promise.setFailure(connectionError(PROTOCOL_ERROR, "Stream does not exist %d", streamId));
        }
        return promise;
    }

    @Override
    public ChannelFuture writeData(ChannelHandlerContext ctx, int streamId, ByteBuf data,
                                   int padding, boolean endOfStream, ChannelPromise promise) {
        if (isExistingStream(streamId)) {
            return super.writeData(ctx, streamId, data, padding, endOfStream, promise);
        }
        PendingStream pendingStream = pendingStreams.get(streamId);
        if (pendingStream != null) {
            pendingStream.frames.add(new DataFrame(data, padding, endOfStream, promise));
        } else {
            ReferenceCountUtil.safeRelease(data);
            promise.setFailure(connectionError(PROTOCOL_ERROR, "Stream does not exist %d", streamId));
        }
        return promise;
    }

    @Override
    public void remoteSettings(Http2Settings settings) throws Http2Exception {
        // Need to let the delegate decoder handle the settings first, so that it sees the
        // new setting before we attempt to create any new streams.
        super.remoteSettings(settings);

        // Get the updated value for SETTINGS_MAX_CONCURRENT_STREAMS.
        maxConcurrentStreams = connection().local().maxActiveStreams();

        // Try to create new streams up to the new threshold.
        tryCreatePendingStreams();
    }

    @Override
    public void close() {
        try {
            if (!closed) {
                closed = true;

                // Fail all buffered streams.
                Http2ChannelClosedException e = new Http2ChannelClosedException();
                while (!pendingStreams.isEmpty()) {
                    PendingStream stream = pendingStreams.pollFirstEntry().getValue();
                    stream.close(e);
                }
            }
        } finally {
            super.close();
        }
    }

    private void tryCreatePendingStreams() {
        while (!pendingStreams.isEmpty() && canCreateStream()) {
            Map.Entry<Integer, PendingStream> entry = pendingStreams.pollFirstEntry();
            PendingStream pendingStream = entry.getValue();
            try {
                pendingStream.sendFrames();
            } catch (Throwable t) {
                pendingStream.close(t);
            }
        }
    }

    private void cancelGoAwayStreams(int lastStreamId, long errorCode, ByteBuf debugData) {
        Iterator<PendingStream> iter = pendingStreams.values().iterator();
        Exception e = new Http2GoAwayException(lastStreamId, errorCode, ByteBufUtil.getBytes(debugData));
        while (iter.hasNext()) {
            PendingStream stream = iter.next();
            if (stream.streamId > lastStreamId) {
                iter.remove();
                stream.close(e);
            }
        }
    }

    
Determines whether or not we're allowed to create a new stream right now.
/**
     * Determines whether or not we're allowed to create a new stream right now.
     */
    private boolean canCreateStream() {
        return connection().local().numActiveStreams() < maxConcurrentStreams;
    }

    private boolean isExistingStream(int streamId) {
        return streamId <= connection().local().lastStreamCreated();
    }

    private static final class PendingStream {
        final ChannelHandlerContext ctx;
        final int streamId;
        final Queue<Frame> frames = new ArrayDeque<Frame>(2);

        PendingStream(ChannelHandlerContext ctx, int streamId) {
            this.ctx = ctx;
            this.streamId = streamId;
        }

        void sendFrames() {
            for (Frame frame : frames) {
                frame.send(ctx, streamId);
            }
        }

        void close(Throwable t) {
            for (Frame frame : frames) {
                frame.release(t);
            }
        }
    }

    private abstract static class Frame {
        final ChannelPromise promise;

        Frame(ChannelPromise promise) {
            this.promise = promise;
        }

        
Release any resources (features, buffers, ...) associated with the frame.
/**
         * Release any resources (features, buffers, ...) associated with the frame.
         */
        void release(Throwable t) {
            if (t == null) {
                promise.setSuccess();
            } else {
                promise.setFailure(t);
            }
        }

        abstract void send(ChannelHandlerContext ctx, int streamId);
    }

    private final class HeadersFrame extends Frame {
        final Http2Headers headers;
        final int streamDependency;
        final short weight;
        final boolean exclusive;
        final int padding;
        final boolean endOfStream;

        HeadersFrame(Http2Headers headers, int streamDependency, short weight, boolean exclusive,
                     int padding, boolean endOfStream, ChannelPromise promise) {
            super(promise);
            this.headers = headers;
            this.streamDependency = streamDependency;
            this.weight = weight;
            this.exclusive = exclusive;
            this.padding = padding;
            this.endOfStream = endOfStream;
        }

        @Override
        void send(ChannelHandlerContext ctx, int streamId) {
            writeHeaders(ctx, streamId, headers, streamDependency, weight, exclusive, padding, endOfStream, promise);
        }
    }

    private final class DataFrame extends Frame {
        final ByteBuf data;
        final int padding;
        final boolean endOfStream;

        DataFrame(ByteBuf data, int padding, boolean endOfStream, ChannelPromise promise) {
            super(promise);
            this.data = data;
            this.padding = padding;
            this.endOfStream = endOfStream;
        }

        @Override
        void release(Throwable t) {
            super.release(t);
            ReferenceCountUtil.safeRelease(data);
        }

        @Override
        void send(ChannelHandlerContext ctx, int streamId) {
            writeData(ctx, streamId, data, padding, endOfStream, promise);
        }
    }
}