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Lucene example source code file (MultiLevelSkipListReader.java)

This example Lucene source code file (MultiLevelSkipListReader.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Lucene tags/keywords

bufferedindexinput, indexinput, indexinput, io, ioexception, ioexception, multilevelskiplistreader, override, override, skipbuffer, skipbuffer, util

The Lucene MultiLevelSkipListReader.java source code

package org.apache.lucene.index;

/**
 * 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.
 */

import java.io.IOException;
import java.util.Arrays;

import org.apache.lucene.store.BufferedIndexInput;
import org.apache.lucene.store.IndexInput;

/**
 * This abstract class reads skip lists with multiple levels.
 * 
 * See {@link MultiLevelSkipListWriter} for the information about the encoding 
 * of the multi level skip lists. 
 * 
 * Subclasses must implement the abstract method {@link #readSkipData(int, IndexInput)}
 * which defines the actual format of the skip data.
 */
abstract class MultiLevelSkipListReader {
  // the maximum number of skip levels possible for this index
  private int maxNumberOfSkipLevels; 
  
  // number of levels in this skip list
  private int numberOfSkipLevels;
  
  // Expert: defines the number of top skip levels to buffer in memory.
  // Reducing this number results in less memory usage, but possibly
  // slower performance due to more random I/Os.
  // Please notice that the space each level occupies is limited by
  // the skipInterval. The top level can not contain more than
  // skipLevel entries, the second top level can not contain more
  // than skipLevel^2 entries and so forth.
  private int numberOfLevelsToBuffer = 1;
  
  private int docCount;
  private boolean haveSkipped;
  
  private IndexInput[] skipStream;    // skipStream for each level
  private long skipPointer[];         // the start pointer of each skip level
  private int skipInterval[];         // skipInterval of each level
  private int[] numSkipped;           // number of docs skipped per level
    
  private int[] skipDoc;              // doc id of current skip entry per level 
  private int lastDoc;                // doc id of last read skip entry with docId <= target
  private long[] childPointer;        // child pointer of current skip entry per level
  private long lastChildPointer;      // childPointer of last read skip entry with docId <= target
  
  private boolean inputIsBuffered;
  
  public MultiLevelSkipListReader(IndexInput skipStream, int maxSkipLevels, int skipInterval) {
    this.skipStream = new IndexInput[maxSkipLevels];
    this.skipPointer = new long[maxSkipLevels];
    this.childPointer = new long[maxSkipLevels];
    this.numSkipped = new int[maxSkipLevels];
    this.maxNumberOfSkipLevels = maxSkipLevels;
    this.skipInterval = new int[maxSkipLevels];
    this.skipStream [0]= skipStream;
    this.inputIsBuffered = (skipStream instanceof BufferedIndexInput);
    this.skipInterval[0] = skipInterval;
    for (int i = 1; i < maxSkipLevels; i++) {
      // cache skip intervals
      this.skipInterval[i] = this.skipInterval[i - 1] * skipInterval;
    }
    skipDoc = new int[maxSkipLevels];
  }

  
  /** Returns the id of the doc to which the last call of {@link #skipTo(int)}
   *  has skipped.  */
  int getDoc() {
    return lastDoc;
  }
  
  
  /** Skips entries to the first beyond the current whose document number is
   *  greater than or equal to <i>target. Returns the current doc count. 
   */
  int skipTo(int target) throws IOException {
    if (!haveSkipped) {
      // first time, load skip levels
      loadSkipLevels();
      haveSkipped = true;
    }
  
    // walk up the levels until highest level is found that has a skip
    // for this target
    int level = 0;
    while (level < numberOfSkipLevels - 1 && target > skipDoc[level + 1]) {
      level++;
    }    

    while (level >= 0) {
      if (target > skipDoc[level]) {
        if (!loadNextSkip(level)) {
          continue;
        }
      } else {
        // no more skips on this level, go down one level
        if (level > 0 && lastChildPointer > skipStream[level - 1].getFilePointer()) {
          seekChild(level - 1);
        } 
        level--;
      }
    }
    
    return numSkipped[0] - skipInterval[0] - 1;
  }
  
  private boolean loadNextSkip(int level) throws IOException {
    // we have to skip, the target document is greater than the current
    // skip list entry        
    setLastSkipData(level);
      
    numSkipped[level] += skipInterval[level];
      
    if (numSkipped[level] > docCount) {
      // this skip list is exhausted
      skipDoc[level] = Integer.MAX_VALUE;
      if (numberOfSkipLevels > level) numberOfSkipLevels = level; 
      return false;
    }

    // read next skip entry
    skipDoc[level] += readSkipData(level, skipStream[level]);
    
    if (level != 0) {
      // read the child pointer if we are not on the leaf level
      childPointer[level] = skipStream[level].readVLong() + skipPointer[level - 1];
    }
    
    return true;

  }
  
  /** Seeks the skip entry on the given level */
  protected void seekChild(int level) throws IOException {
    skipStream[level].seek(lastChildPointer);
    numSkipped[level] = numSkipped[level + 1] - skipInterval[level + 1];
    skipDoc[level] = lastDoc;
    if (level > 0) {
        childPointer[level] = skipStream[level].readVLong() + skipPointer[level - 1];
    }
  }
  
  void close() throws IOException {
    for (int i = 1; i < skipStream.length; i++) {
      if (skipStream[i] != null) {
        skipStream[i].close();
      }
    }
  }

  /** initializes the reader */
  void init(long skipPointer, int df) {
    this.skipPointer[0] = skipPointer;
    this.docCount = df;
    Arrays.fill(skipDoc, 0);
    Arrays.fill(numSkipped, 0);
    Arrays.fill(childPointer, 0);
    
    haveSkipped = false;
    for (int i = 1; i < numberOfSkipLevels; i++) {
      skipStream[i] = null;
    }
  }
  
  /** Loads the skip levels  */
  private void loadSkipLevels() throws IOException {
    numberOfSkipLevels = docCount == 0 ? 0 : (int) Math.floor(Math.log(docCount) / Math.log(skipInterval[0]));
    if (numberOfSkipLevels > maxNumberOfSkipLevels) {
      numberOfSkipLevels = maxNumberOfSkipLevels;
    }

    skipStream[0].seek(skipPointer[0]);
    
    int toBuffer = numberOfLevelsToBuffer;
    
    for (int i = numberOfSkipLevels - 1; i > 0; i--) {
      // the length of the current level
      long length = skipStream[0].readVLong();
      
      // the start pointer of the current level
      skipPointer[i] = skipStream[0].getFilePointer();
      if (toBuffer > 0) {
        // buffer this level
        skipStream[i] = new SkipBuffer(skipStream[0], (int) length);
        toBuffer--;
      } else {
        // clone this stream, it is already at the start of the current level
        skipStream[i] = (IndexInput) skipStream[0].clone();
        if (inputIsBuffered && length < BufferedIndexInput.BUFFER_SIZE) {
          ((BufferedIndexInput) skipStream[i]).setBufferSize((int) length);
        }
        
        // move base stream beyond the current level
        skipStream[0].seek(skipStream[0].getFilePointer() + length);
      }
    }
   
    // use base stream for the lowest level
    skipPointer[0] = skipStream[0].getFilePointer();
  }
  
  /**
   * Subclasses must implement the actual skip data encoding in this method.
   *  
   * @param level the level skip data shall be read from
   * @param skipStream the skip stream to read from
   */  
  protected abstract int readSkipData(int level, IndexInput skipStream) throws IOException;
  
  /** Copies the values of the last read skip entry on this level */
  protected void setLastSkipData(int level) {
    lastDoc = skipDoc[level];
    lastChildPointer = childPointer[level];
  }

  
  /** used to buffer the top skip levels */
  private final static class SkipBuffer extends IndexInput {
    private byte[] data;
    private long pointer;
    private int pos;
    
    SkipBuffer(IndexInput input, int length) throws IOException {
      data = new byte[length];
      pointer = input.getFilePointer();
      input.readBytes(data, 0, length);
    }
    
    @Override
    public void close() throws IOException {
      data = null;
    }

    @Override
    public long getFilePointer() {
      return pointer + pos;
    }

    @Override
    public long length() {
      return data.length;
    }

    @Override
    public byte readByte() throws IOException {
      return data[pos++];
    }

    @Override
    public void readBytes(byte[] b, int offset, int len) throws IOException {
      System.arraycopy(data, pos, b, offset, len);
      pos += len;
    }

    @Override
    public void seek(long pos) throws IOException {
      this.pos =  (int) (pos - pointer);
    }
    
  }
}

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