/**
 * 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.hadoop.hdfs.server.datanode.fsdataset.impl;

import java.io.BufferedInputStream;
import java.io.Closeable;
import java.io.DataInputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.FileChannel.MapMode;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.fs.ChecksumException;
import org.apache.hadoop.hdfs.server.datanode.BlockMetadataHeader;
import org.apache.hadoop.io.nativeio.NativeIO;
import org.apache.hadoop.util.DataChecksum;

import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;

/**
 * Represents an HDFS block that is mmapped by the DataNode.
 */
@InterfaceAudience.Private
@InterfaceStability.Unstable
public class MappableBlock implements Closeable {
  private MappedByteBuffer mmap;
  private final long length;

  MappableBlock(MappedByteBuffer mmap, long length) {
    this.mmap = mmap;
    this.length = length;
    assert length > 0;
  }

  public long getLength() {
    return length;
  }

  /**
   * Load the block.
   *
   * mmap and mlock the block, and then verify its checksum.
   *
   * @param length         The current length of the block.
   * @param blockIn        The block input stream.  Should be positioned at the
   *                       start.  The caller must close this.
   * @param metaIn         The meta file input stream.  Should be positioned at
   *                       the start.  The caller must close this.
   * @param blockFileName  The block file name, for logging purposes.
   *
   * @return               The Mappable block.
   */
  public static MappableBlock load(long length,
      FileInputStream blockIn, FileInputStream metaIn,
      String blockFileName) throws IOException {
    MappableBlock mappableBlock = null;
    MappedByteBuffer mmap = null;
    try {
      FileChannel blockChannel = blockIn.getChannel();
      if (blockChannel == null) {
        throw new IOException("Block InputStream has no FileChannel.");
      }
      mmap = blockChannel.map(MapMode.READ_ONLY, 0, length);
      NativeIO.POSIX.getCacheManipulator().mlock(blockFileName, mmap, length);
      verifyChecksum(length, metaIn, blockChannel, blockFileName);
      mappableBlock = new MappableBlock(mmap, length);
    } finally {
      if (mappableBlock == null) {
        if (mmap != null) {
          NativeIO.POSIX.munmap(mmap); // unmapping also unlocks
        }
      }
    }
    return mappableBlock;
  }

  /**
   * Verifies the block's checksum. This is an I/O intensive operation.
   * @return if the block was successfully checksummed.
   */
  private static void verifyChecksum(long length,
      FileInputStream metaIn, FileChannel blockChannel, String blockFileName)
          throws IOException, ChecksumException {
    // Verify the checksum from the block's meta file
    // Get the DataChecksum from the meta file header
    BlockMetadataHeader header =
        BlockMetadataHeader.readHeader(new DataInputStream(
            new BufferedInputStream(metaIn, BlockMetadataHeader
                .getHeaderSize())));
    FileChannel metaChannel = metaIn.getChannel();
    if (metaChannel == null) {
      throw new IOException("Block InputStream meta file has no FileChannel.");
    }
    DataChecksum checksum = header.getChecksum();
    final int bytesPerChecksum = checksum.getBytesPerChecksum();
    final int checksumSize = checksum.getChecksumSize();
    final int numChunks = (8*1024*1024) / bytesPerChecksum;
    ByteBuffer blockBuf = ByteBuffer.allocate(numChunks*bytesPerChecksum);
    ByteBuffer checksumBuf = ByteBuffer.allocate(numChunks*checksumSize);
    // Verify the checksum
    int bytesVerified = 0;
    while (bytesVerified < length) {
      Preconditions.checkState(bytesVerified % bytesPerChecksum == 0,
          "Unexpected partial chunk before EOF");
      assert bytesVerified % bytesPerChecksum == 0;
      int bytesRead = fillBuffer(blockChannel, blockBuf);
      if (bytesRead == -1) {
        throw new IOException("checksum verification failed: premature EOF");
      }
      blockBuf.flip();
      // Number of read chunks, including partial chunk at end
      int chunks = (bytesRead+bytesPerChecksum-1) / bytesPerChecksum;
      checksumBuf.limit(chunks*checksumSize);
      fillBuffer(metaChannel, checksumBuf);
      checksumBuf.flip();
      checksum.verifyChunkedSums(blockBuf, checksumBuf, blockFileName,
          bytesVerified);
      // Success
      bytesVerified += bytesRead;
      blockBuf.clear();
      checksumBuf.clear();
    }
  }

  /**
   * Reads bytes into a buffer until EOF or the buffer's limit is reached
   */
  private static int fillBuffer(FileChannel channel, ByteBuffer buf)
      throws IOException {
    int bytesRead = channel.read(buf);
    if (bytesRead < 0) {
      //EOF
      return bytesRead;
    }
    while (buf.remaining() > 0) {
      int n = channel.read(buf);
      if (n < 0) {
        //EOF
        return bytesRead;
      }
      bytesRead += n;
    }
    return bytesRead;
  }

  @Override
  public void close() {
    if (mmap != null) {
      NativeIO.POSIX.munmap(mmap);
      mmap = null;
    }
  }
}