/*
    * 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:
    *
    *   https://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.netty5.handler.flush;
  
  import io.netty5.channel.Channel;
  import io.netty5.channel.ChannelHandler;
  import io.netty5.channel.ChannelHandlerContext;
  import io.netty5.channel.ChannelOutboundInvoker;
  import io.netty5.channel.ChannelPipeline;
  import io.netty5.util.concurrent.Future;
  import io.netty5.util.internal.ObjectUtil;
  
  /**
   * {@link ChannelHandler} which consolidates {@link Channel#flush()} / {@link ChannelHandlerContext#flush()}
   * operations (which also includes
   * {@link Channel#writeAndFlush(Object)} and
   * {@link ChannelOutboundInvoker#writeAndFlush(Object)}.
   * <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 ChannelHandler#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) {
         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 channelExceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
         // To ensure we not miss to flush anything, do it now.
         resetReadAndFlushIfNeeded(ctx);
         ctx.fireChannelExceptionCaught(cause);
     }
 
     @Override
     public Future<Void> disconnect(ChannelHandlerContext ctx) {
         // Try to flush one last time if flushes are pending before disconnect the channel.
         resetReadAndFlushIfNeeded(ctx);
         return ctx.disconnect();
     }
 
     @Override
     public Future<Void> close(ChannelHandlerContext ctx) {
         // Try to flush one last time if flushes are pending before close the channel.
         resetReadAndFlushIfNeeded(ctx);
         return ctx.close();
     }
 
     @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().executor().submit(flushTask);
         }
     }
 
     private void cancelScheduledFlush() {
         if (nextScheduledFlush != null) {
             nextScheduledFlush.cancel();
             nextScheduledFlush = null;
         }
     }
 }