/**
* 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.io;
import java.io.*;
import java.util.*;
import java.rmi.server.UID;
import java.security.MessageDigest;
import java.text.ParseException;
import org.apache.commons.logging.*;
import org.apache.hadoop.fs.*;
import org.apache.hadoop.fs.permission.FsPermission;
import org.apache.hadoop.io.compress.CodecPool;
import org.apache.hadoop.io.compress.CompressionCodec;
import org.apache.hadoop.io.compress.CompressionInputStream;
import org.apache.hadoop.io.compress.CompressionOutputStream;
import org.apache.hadoop.io.compress.Compressor;
import org.apache.hadoop.io.compress.Decompressor;
import org.apache.hadoop.io.compress.DefaultCodec;
import org.apache.hadoop.io.compress.GzipCodec;
import org.apache.hadoop.io.compress.zlib.ZlibFactory;
import org.apache.hadoop.io.serializer.Deserializer;
import org.apache.hadoop.io.serializer.SerializationFactory;
import org.apache.hadoop.io.serializer.Serializer;
import org.apache.hadoop.conf.*;
import org.apache.hadoop.util.Progressable;
import org.apache.hadoop.util.Progress;
import org.apache.hadoop.util.ReflectionUtils;
import org.apache.hadoop.util.NativeCodeLoader;
import org.apache.hadoop.util.MergeSort;
import org.apache.hadoop.util.PriorityQueue;
/**
* <code>SequenceFile</code>s are flat files consisting of binary key/value
* pairs.
*
* <p><code>SequenceFile</code> provides {@link Writer}, {@link Reader} and
* {@link Sorter} classes for writing, reading and sorting respectively.</p>
*
* There are three <code>SequenceFile</code> <code>Writer</code>s based on the
* {@link CompressionType} used to compress key/value pairs:
* <ol>
* <li>
* <code>Writer</code> : Uncompressed records.
* </li>
* <li>
* <code>RecordCompressWriter</code> : Record-compressed files, only compress
* values.
* </li>
* <li>
* <code>BlockCompressWriter</code> : Block-compressed files, both keys &
* values are collected in 'blocks'
* separately and compressed. The size of
* the 'block' is configurable.
* </ol>
*
* <p>The actual compression algorithm used to compress key and/or values can be
* specified by using the appropriate {@link CompressionCodec}.</p>
*
* <p>The recommended way is to use the static <tt>createWriter</tt> methods
* provided by the <code>SequenceFile</code> to chose the preferred format.</p>
*
* <p>The {@link Reader} acts as the bridge and can read any of the above
* <code>SequenceFile</code> formats.</p>
*
* <h4 id="Formats">SequenceFile Formats</h4>
*
* <p>Essentially there are 3 different formats for <code>SequenceFile</code>s
* depending on the <code>CompressionType</code> specified. All of them share a
* <a href="#Header">common header</a> described below.
*
* <h5 id="Header">SequenceFile Header</h5>
* <ul>
* <li>
* version - 3 bytes of magic header <b>SEQ</b>, followed by 1 byte of actual
* version number (e.g. SEQ4 or SEQ6)
* </li>
* <li>
* keyClassName -key class
* </li>
* <li>
* valueClassName - value class
* </li>
* <li>
* compression - A boolean which specifies if compression is turned on for
* keys/values in this file.
* </li>
* <li>
* blockCompression - A boolean which specifies if block-compression is
* turned on for keys/values in this file.
* </li>
* <li>
* compression codec - <code>CompressionCodec</code> class which is used for
* compression of keys and/or values (if compression is
* enabled).
* </li>
* <li>
* metadata - {@link Metadata} for this file.
* </li>
* <li>
* sync - A sync marker to denote end of the header.
* </li>
* </ul>
*
* <h5 id="#UncompressedFormat">Uncompressed SequenceFile Format</h5>
* <ul>
* <li>
* <a href="#Header">Header</a>
* </li>
* <li>
* Record
* <ul>
* <li>Record length</li>
* <li>Key length</li>
* <li>Key</li>
* <li>Value</li>
* </ul>
* </li>
* <li>
* A sync-marker every few <code>100</code> bytes or so.
* </li>
* </ul>
*
* <h5 id="#RecordCompressedFormat">Record-Compressed SequenceFile Format</h5>
* <ul>
* <li>
* <a href="#Header">Header</a>
* </li>
* <li>
* Record
* <ul>
* <li>Record length</li>
* <li>Key length</li>
* <li>Key</li>
* <li><i>Compressed</i> Value& 