/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF 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 org.apache.flink.runtime.io.disk.iomanager;

import org.apache.flink.core.memory.MemorySegment;

import java.io.IOException;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.TimeUnit;

/**
 * A reader that reads data in blocks from a file channel. The reader reads the blocks into a 
 * {@link org.apache.flink.core.memory.MemorySegment} in an asynchronous fashion. That is, a read
 * request is not processed by the thread that issues it, but by an asynchronous reader thread. Once the read request
 * is done, the asynchronous reader adds the full MemorySegment to a <i>return queue</i> where it can be popped by the
 * worker thread, once it needs the data. The return queue is in this case a
 * {@link java.util.concurrent.LinkedBlockingQueue}, such that the working thread blocks until the request has been served,
 * if the request is still pending when the it requires the data. 
 * <p>
 * Typical pre-fetching reads are done by issuing the read requests early and popping the return queue once the data
 * is actually needed.
 * <p>
 * The reader has no notion whether the size of the memory segments is actually the size of the blocks on disk,
 * or even whether the file was written in blocks of the same size, or in blocks at all. Ensuring that the
 * writing and reading is consistent with each other (same blocks sizes) is up to the programmer.  
 */
public class AsynchronousBlockReader extends AsynchronousFileIOChannel<MemorySegment, ReadRequest> implements BlockChannelReader<MemorySegment> {
	
	private final LinkedBlockingQueue<MemorySegment> returnSegments;
	
	/**
	 * Creates a new block channel reader for the given channel.
	 *  
	 * @param channelID The ID of the channel to read.
	 * @param requestQueue The request queue of the asynchronous reader thread, to which the I/O requests
	 *                     are added.
	 * @param returnSegments The return queue, to which the full Memory Segments are added.
	 * @throws IOException Thrown, if the underlying file channel could not be opened.
	 */
	protected AsynchronousBlockReader(FileIOChannel.ID channelID, RequestQueue<ReadRequest> requestQueue,
			LinkedBlockingQueue<MemorySegment> returnSegments)
	throws IOException
	{
		super(channelID, requestQueue, new QueuingCallback<MemorySegment>(returnSegments), false);
		this.returnSegments = returnSegments;
	}	

	/**
	 * Issues a read request, which will asynchronously fill the given segment with the next block in the
	 * underlying file channel. Once the read request is fulfilled, the segment will be added to this reader's
	 * return queue.
	 *  
	 * @param segment The segment to read the block into.
	 * @throws IOException Thrown, when the reader encounters an I/O error. Due to the asynchronous nature of the
	 *                     reader, the exception thrown here may have been caused by an earlier read request. 
	 */
	@Override
	public void readBlock(MemorySegment segment) throws IOException {
		addRequest(new SegmentReadRequest(this, segment));
	}

	@Override
	public void seekToPosition(long position) throws IOException {
		requestQueue.add(new SeekRequest(this, position));
	}

	/**
	 * Gets the next memory segment that has been filled with data by the reader. This method blocks until
	 * such a segment is available, or until an error occurs in the reader, or the reader is closed.
	 * <p>
	 * WARNING: If this method is invoked without any segment ever returning (for example, because the
	 * {@link #readBlock(MemorySegment)} method has not been invoked appropriately), the method may block
	 * forever.
	 * 
	 * @return The next memory segment from the reader's return queue.
	 * @throws IOException Thrown, if an I/O error occurs in the reader while waiting for the request to return.
	 */
	@Override
	public MemorySegment getNextReturnedBlock() throws IOException {
		try {
			while (true) {
				final MemorySegment next = this.returnSegments.poll(1000, TimeUnit.MILLISECONDS);
				if (next != null) {
					return next;
				} else {
					if (this.closed) {
						throw new IOException("The reader has been asynchronously closed.");
					}
					checkErroneous();
				}
			}
		} catch (InterruptedException iex) {
			throw new IOException("Reader was interrupted while waiting for the next returning segment.");
		}
	}
	
	/**
	 * Gets the queue in which the full memory segments are queued after the asynchronous read
	 * is complete.
	 * 
	 * @return The queue with the full memory segments.
	 */
	@Override
	public LinkedBlockingQueue<MemorySegment> getReturnQueue() {
		return this.returnSegments;
	}
}