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 org.apache.lucene.store.IndexOutput;
import org.apache.lucene.store.RAMOutputStream;
/**
* This abstract class writes skip lists with multiple levels.
*
* Example for skipInterval = 3:
* c (skip level 2)
* c c c (skip level 1)
* x x x x x x x x x x (skip level 0)
* d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d d (posting list)
* 3 6 9 12 15 18 21 24 27 30 (df)
*
* d - document
* x - skip data
* c - skip data with child pointer
*
* Skip level i contains every skipInterval-th entry from skip level i-1.
* Therefore the number of entries on level i is: floor(df / ((skipInterval ^ (i + 1))).
*
* Each skip entry on a level i>0 contains a pointer to the corresponding skip entry in list i-1.
* This guarantees a logarithmic amount of skips to find the target document.
*
* While this class takes care of writing the different skip levels,
* subclasses must define the actual format of the skip data.
*
*/
abstract class MultiLevelSkipListWriter {
// number of levels in this skip list
private int numberOfSkipLevels;
// the skip interval in the list with level = 0
private int skipInterval;
// for every skip level a different buffer is used
private RAMOutputStream[] skipBuffer;
protected MultiLevelSkipListWriter(int skipInterval, int maxSkipLevels, int df) {
this.skipInterval = skipInterval;
// calculate the maximum number of skip levels for this document frequency
numberOfSkipLevels = df == 0 ? 0 : (int) Math.floor(Math.log(df) / Math.log(skipInterval));
// make sure it does not exceed maxSkipLevels
if (numberOfSkipLevels > maxSkipLevels) {
numberOfSkipLevels = maxSkipLevels;
}
}
protected void init() {
skipBuffer = new RAMOutputStream[numberOfSkipLevels];
for (int i = 0; i < numberOfSkipLevels; i++) {
skipBuffer[i] = new RAMOutputStream();
}
}
protected void resetSkip() {
// creates new buffers or empties the existing ones
if (skipBuffer == null) {
init();
} else {
for (int i = 0; i < skipBuffer.length; i++) {
skipBuffer[i].reset();
}
}
}
/**
* Subclasses must implement the actual skip data encoding in this method.
*
* @param level the level skip data shall be writing for
* @param skipBuffer the skip buffer to write to
*/
protected abstract void writeSkipData(int level, IndexOutput skipBuffer) throws IOException;
/**
* Writes the current skip data to the buffers. The current document frequency determines
* the max level is skip data is to be written to.
*
* @param df the current document frequency
* @throws IOException
*/
void bufferSkip(int df) throws IOException {
int numLevels;
// determine max level
for (numLevels = 0; (df % skipInterval) == 0 && numLevels < numberOfSkipLevels; df /= skipInterval) {
numLevels++;
}
long childPointer = 0;
for (int level = 0; level < numLevels; level++) {
writeSkipData(level, skipBuffer[level]);
long newChildPointer = skipBuffer[level].getFilePointer();
if (level != 0) {
// store child pointers for all levels except the lowest
skipBuffer[level].writeVLong(childPointer);
}
//remember the childPointer for the next level
childPointer = newChildPointer;
}
}
/**
* Writes the buffered skip lists to the given output.
*
* @param output the IndexOutput the skip lists shall be written to
* @return the pointer the skip list starts
*/
long writeSkip(IndexOutput output) throws IOException {
long skipPointer = output.getFilePointer();
if (skipBuffer == null || skipBuffer.length == 0) return skipPointer;
for (int level = numberOfSkipLevels - 1; level > 0; level--) {
long length = skipBuffer[level].getFilePointer();
if (length > 0) {
output.writeVLong(length);
skipBuffer[level].writeTo(output);
}
}
skipBuffer[0].writeTo(output);
return skipPointer;
}
}