/*
 * Copyright 2016 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.flush;

import io.netty.channel.Channel;
import io.netty.channel.ChannelHandler;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelOutboundInvoker;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.ChannelPromise;
import io.netty.util.internal.ObjectUtil;

import java.util.concurrent.Future;

/**
 * {@link ChannelHandler} which consolidates {@link Channel#flush()} / {@link ChannelHandlerContext#flush()}
 * operations (which also includes
 * {@link Channel#writeAndFlush(Object)} / {@link Channel#writeAndFlush(Object, ChannelPromise)} and
 * {@link ChannelOutboundInvoker#writeAndFlush(Object)} /
 * {@link ChannelOutboundInvoker#writeAndFlush(Object, ChannelPromise)}).
 * <p>
 * Flush operations are generally speaking expensive as these may trigger a syscall on the transport level. Thus it is
 * in most cases (where write latency can be traded with throughput) a good idea to try to minimize flush operations
 * as much as possible.
 * <p>
 * If a read loop is currently ongoing, {@link #flush(ChannelHandlerContext)} will not be passed on to the next
 * {@link ChannelHandler} in the {@link ChannelPipeline}, as it will pick up any pending flushes when
 * {@link #channelReadComplete(ChannelHandlerContext)} is triggered.
 * If no read loop is ongoing, the behavior depends on the {@code consolidateWhenNoReadInProgress} constructor argument:
 * <ul>
 *     <li>if {@code false}, flushes are passed on to the next handler directly;</li>
 *     <li>if {@code true}, the invocation of the next handler is submitted as a separate task on the event loop. Under
 *     high throughput, this gives the opportunity to process other flushes before the task gets executed, thus
 *     batching multiple flushes into one.</li>
 * </ul>
 * If {@code explicitFlushAfterFlushes} is reached the flush will be forwarded as well (whether while in a read loop, or
 * while batching outside of a read loop).
 * <p>
 * If the {@link Channel} becomes non-writable it will also try to execute any pending flush operations.
 * <p>
 * The {@link FlushConsolidationHandler} should be put as first {@link ChannelHandler} in the
 * {@link ChannelPipeline} to have the best effect.
 */
public class FlushConsolidationHandler implements ChannelHandler {
    private final int explicitFlushAfterFlushes;
    private final boolean consolidateWhenNoReadInProgress;
    private final Runnable flushTask;
    private int flushPendingCount;
    private boolean readInProgress;
    private ChannelHandlerContext ctx;
    private Future<?> nextScheduledFlush;

    /**
     * The default number of flushes after which a flush will be forwarded to downstream handlers (whether while in a
     * read loop, or while batching outside of a read loop).
     */
    public static final int DEFAULT_EXPLICIT_FLUSH_AFTER_FLUSHES = 256;

    /**
     * Create new instance which explicit flush after {@value DEFAULT_EXPLICIT_FLUSH_AFTER_FLUSHES} pending flush
     * operations at the latest.
     */
    public FlushConsolidationHandler() {
        this(DEFAULT_EXPLICIT_FLUSH_AFTER_FLUSHES, false);
    }

    /**
     * Create new instance which doesn't consolidate flushes when no read is in progress.
     *
     * @param explicitFlushAfterFlushes the number of flushes after which an explicit flush will be done.
     */
    public FlushConsolidationHandler(int explicitFlushAfterFlushes) {
        this(explicitFlushAfterFlushes, false);
    }

    /**
     * Create new instance.
     *
     * @param explicitFlushAfterFlushes the number of flushes after which an explicit flush will be done.
     * @param consolidateWhenNoReadInProgress whether to consolidate flushes even when no read loop is currently
     *                                        ongoing.
     */
    public FlushConsolidationHandler(int explicitFlushAfterFlushes, boolean consolidateWhenNoReadInProgress) {
        this.explicitFlushAfterFlushes =
                ObjectUtil.checkPositive(explicitFlushAfterFlushes, "explicitFlushAfterFlushes");
        this.consolidateWhenNoReadInProgress = consolidateWhenNoReadInProgress;
        flushTask = consolidateWhenNoReadInProgress ?
                () -> {
                    if (flushPendingCount > 0 && !readInProgress) {
                        flushPendingCount = 0;
                        nextScheduledFlush = null;
                        ctx.flush();
                    } // else we'll flush when the read completes
                }
                : null;
    }

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

    @Override
    public void flush(ChannelHandlerContext ctx) throws Exception {
        if (readInProgress) {
            // If there is still a read in progress we are sure we will see a channelReadComplete(...) call. Thus
            // we only need to flush if we reach the explicitFlushAfterFlushes limit.
            if (++flushPendingCount == explicitFlushAfterFlushes) {
                flushNow(ctx);
            }
        } else if (consolidateWhenNoReadInProgress) {
            // Flush immediately if we reach the threshold, otherwise schedule
            if (++flushPendingCount == explicitFlushAfterFlushes) {
                flushNow(ctx);
            } else {
                scheduleFlush(ctx);
            }
        } else {
            // Always flush directly
            flushNow(ctx);
        }
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        // This may be the last event in the read loop, so flush now!
        resetReadAndFlushIfNeeded(ctx);
        ctx.fireChannelReadComplete();
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        readInProgress = true;
        ctx.fireChannelRead(msg);
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        // To ensure we not miss to flush anything, do it now.
        resetReadAndFlushIfNeeded(ctx);
        ctx.fireExceptionCaught(cause);
    }

    @Override
    public void disconnect(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception {
        // Try to flush one last time if flushes are pending before disconnect the channel.
        resetReadAndFlushIfNeeded(ctx);
        ctx.disconnect(promise);
    }

    @Override
    public void close(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception {
        // Try to flush one last time if flushes are pending before close the channel.
        resetReadAndFlushIfNeeded(ctx);
        ctx.close(promise);
    }

    @Override
    public void channelWritabilityChanged(ChannelHandlerContext ctx) throws Exception {
        if (!ctx.channel().isWritable()) {
            // The writability of the channel changed to false, so flush all consolidated flushes now to free up memory.
            flushIfNeeded(ctx);
        }
        ctx.fireChannelWritabilityChanged();
    }

    @Override
    public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
        flushIfNeeded(ctx);
    }

    private void resetReadAndFlushIfNeeded(ChannelHandlerContext ctx) {
        readInProgress = false;
        flushIfNeeded(ctx);
    }

    private void flushIfNeeded(ChannelHandlerContext ctx) {
        if (flushPendingCount > 0) {
            flushNow(ctx);
        }
    }

    private void flushNow(ChannelHandlerContext ctx) {
        cancelScheduledFlush();
        flushPendingCount = 0;
        ctx.flush();
    }

    private void scheduleFlush(final ChannelHandlerContext ctx) {
        if (nextScheduledFlush == null) {
            // Run as soon as possible, but still yield to give a chance for additional writes to enqueue.
            nextScheduledFlush = ctx.channel().eventLoop().submit(flushTask);
        }
    }

    private void cancelScheduledFlush() {
        if (nextScheduledFlush != null) {
            nextScheduledFlush.cancel(false);
            nextScheduledFlush = null;
        }
    }
}
