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Common functionality for HBase
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/*
 * 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.hbase.io.encoding;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.hbase.Cell;
import org.apache.hadoop.hbase.CellComparator;
import org.apache.hadoop.hbase.io.hfile.HFileContext;
import org.apache.hadoop.hbase.nio.ByteBuff;
import org.apache.yetus.audience.InterfaceAudience;

/**
 * Encoding of KeyValue. It aims to be fast and efficient using assumptions:
 * 
 * the KeyValues are stored sorted by key
 * we know the structure of KeyValue
 * the values are always iterated forward from beginning of block
 * knowledge of Key Value format
 * 
 * It is designed to work fast enough to be feasible as in memory compression.
 */
@InterfaceAudience.Private
public interface DataBlockEncoder {
  // TODO: This Interface should be deprecated and replaced. It presumes hfile and carnal knowledge
  // of
  // Cell internals. It was done for a different time. Remove. Purge.
  /**
   * Starts encoding for a block of KeyValues. Call
   * {@link #endBlockEncoding(HFileBlockEncodingContext, DataOutputStream, byte[])} to finish
   * encoding of a block.
   */
  void startBlockEncoding(HFileBlockEncodingContext encodingCtx, DataOutputStream out)
    throws IOException;

  /**
   * Encodes a KeyValue. After the encode, {@link EncodingState#postCellEncode(int, int)} needs to
   * be called to keep track of the encoded and unencoded data size
   */
  void encode(Cell cell, HFileBlockEncodingContext encodingCtx, DataOutputStream out)
    throws IOException;

  /**
   * Ends encoding for a block of KeyValues. Gives a chance for the encoder to do the finishing
   * stuff for the encoded block. It must be called at the end of block encoding.
   */
  void endBlockEncoding(HFileBlockEncodingContext encodingCtx, DataOutputStream out,
    byte[] uncompressedBytesWithHeader) throws IOException;

  /**
   * Decode.
   * @param source Compressed stream of KeyValues.
   * @return Uncompressed block of KeyValues.
   * @throws IOException If there is an error in source.
   */
  ByteBuffer decodeKeyValues(DataInputStream source, HFileBlockDecodingContext decodingCtx)
    throws IOException;

  /**
   * Return first key in block as a cell. Useful for indexing. Typically does not make a deep copy
   * but returns a buffer wrapping a segment of the actual block's byte array. This is because the
   * first key in block is usually stored unencoded.
   * @param block encoded block we want index, the position will not change
   * @return First key in block as a cell.
   */
  Cell getFirstKeyCellInBlock(ByteBuff block);

  /**
   * Create a HFileBlock seeker which find KeyValues within a block.
   * @return A newly created seeker.
   */
  EncodedSeeker createSeeker(HFileBlockDecodingContext decodingCtx);

  /**
   * Creates a encoder specific encoding context store configuration encoding strategy used header
   * bytes to be written, put a dummy header here if the header is unknown HFile meta data
   * @return a newly created encoding context
   */
  HFileBlockEncodingContext newDataBlockEncodingContext(Configuration conf,
    DataBlockEncoding encoding, byte[] headerBytes, HFileContext meta);

  /**
   * Creates an encoder specific decoding context, which will prepare the data before actual
   * decoding store configuration HFile meta data
   * @return a newly created decoding context
   */
  HFileBlockDecodingContext newDataBlockDecodingContext(Configuration conf, HFileContext meta);

  /**
   * An interface which enable to seek while underlying data is encoded. It works on one HFileBlock,
   * but it is reusable. See {@link #setCurrentBuffer(ByteBuff)}.
   */
  interface EncodedSeeker {
    /**
     * Set on which buffer there will be done seeking.
     * @param buffer Used for seeking.
     */
    void setCurrentBuffer(ByteBuff buffer);

    /**
     * From the current position creates a cell using the key part of the current buffer
     * @return key at current position
     */
    Cell getKey();

    /**
     * Does a shallow copy of the value at the current position. A shallow copy is possible because
     * the returned buffer refers to the backing array of the original encoded buffer.
     * @return value at current position
     */
    ByteBuffer getValueShallowCopy();

    /** Returns the Cell at the current position. Includes memstore timestamp. */
    Cell getCell();

    /** Set position to beginning of given block */
    void rewind();

    /**
     * Move to next position
     * @return true on success, false if there is no more positions.
     */
    boolean next();

    /**
     * Moves the seeker position within the current block to:
     * 
     * the last key that that is less than or equal to the given key if seekBefore
     * is false
     * the last key that is strictly less than the given key if 
     * seekBefore is true. The caller is responsible for loading the previous block if the
     * requested key turns out to be the first key of the current block.
     * 
     * @param key        - Cell to which the seek should happen
     * @param seekBefore find the key strictly less than the given key in case of an exact match.
     *                   Does not matter in case of an inexact match.
     * @return 0 on exact match, 1 on inexact match.
     */
    int seekToKeyInBlock(Cell key, boolean seekBefore);

    /**
     * Compare the given key against the current key
     * @return -1 is the passed key is smaller than the current key, 0 if equal and 1 if greater
     */
    public int compareKey(CellComparator comparator, Cell key);
  }
}