package org.apache.lucene.index.codecs.standard;

/**
 * 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.IndexOutput;
import org.apache.lucene.index.codecs.MultiLevelSkipListWriter;


/**
 * Implements the skip list writer for the default posting list format
 * that stores positions and payloads.
 * @lucene.experimental
 */
public class DefaultSkipListWriter extends MultiLevelSkipListWriter {
  private int[] lastSkipDoc;
  private int[] lastSkipPayloadLength;
  private long[] lastSkipFreqPointer;
  private long[] lastSkipProxPointer;
  
  private IndexOutput freqOutput;
  private IndexOutput proxOutput;

  private int curDoc;
  private boolean curStorePayloads;
  private int curPayloadLength;
  private long curFreqPointer;
  private long curProxPointer;

  public DefaultSkipListWriter(int skipInterval, int numberOfSkipLevels, int docCount, IndexOutput freqOutput, IndexOutput proxOutput) {
    super(skipInterval, numberOfSkipLevels, docCount);
    this.freqOutput = freqOutput;
    this.proxOutput = proxOutput;
    
    lastSkipDoc = new int[numberOfSkipLevels];
    lastSkipPayloadLength = new int[numberOfSkipLevels];
    lastSkipFreqPointer = new long[numberOfSkipLevels];
    lastSkipProxPointer = new long[numberOfSkipLevels];
  }

  /**
   * Sets the values for the current skip data. 
   */
  public void setSkipData(int doc, boolean storePayloads, int payloadLength) {
    this.curDoc = doc;
    this.curStorePayloads = storePayloads;
    this.curPayloadLength = payloadLength;
    this.curFreqPointer = freqOutput.getFilePointer();
    if (proxOutput != null)
      this.curProxPointer = proxOutput.getFilePointer();
  }

  @Override
  public void resetSkip() {
    super.resetSkip();
    Arrays.fill(lastSkipDoc, 0);
    Arrays.fill(lastSkipPayloadLength, -1);  // we don't have to write the first length in the skip list
    Arrays.fill(lastSkipFreqPointer, freqOutput.getFilePointer());
    if (proxOutput != null)
      Arrays.fill(lastSkipProxPointer, proxOutput.getFilePointer());
  }
  
  @Override
  protected void writeSkipData(int level, IndexOutput skipBuffer) throws IOException {
    // To efficiently store payloads in the posting lists we do not store the length of
    // every payload. Instead we omit the length for a payload if the previous payload had
    // the same length.
    // However, in order to support skipping the payload length at every skip point must be known.
    // So we use the same length encoding that we use for the posting lists for the skip data as well:
    // Case 1: current field does not store payloads
    //           SkipDatum                 --> DocSkip, FreqSkip, ProxSkip
    //           DocSkip,FreqSkip,ProxSkip --> VInt
    //           DocSkip records the document number before every SkipInterval th  document in TermFreqs. 
    //           Document numbers are represented as differences from the previous value in the sequence.
    // Case 2: current field stores payloads
    //           SkipDatum                 --> DocSkip, PayloadLength?, FreqSkip,ProxSkip
    //           DocSkip,FreqSkip,ProxSkip --> VInt
    //           PayloadLength             --> VInt    
    //         In this case DocSkip/2 is the difference between
    //         the current and the previous value. If DocSkip
    //         is odd, then a PayloadLength encoded as VInt follows,
    //         if DocSkip is even, then it is assumed that the
    //         current payload length equals the length at the previous
    //         skip point
    if (curStorePayloads) {
      int delta = curDoc - lastSkipDoc[level];
      if (curPayloadLength == lastSkipPayloadLength[level]) {
        // the current payload length equals the length at the previous skip point,
        // so we don't store the length again
        skipBuffer.writeVInt(delta * 2);
      } else {
        // the payload length is different from the previous one. We shift the DocSkip, 
        // set the lowest bit and store the current payload length as VInt.
        skipBuffer.writeVInt(delta * 2 + 1);
        skipBuffer.writeVInt(curPayloadLength);
        lastSkipPayloadLength[level] = curPayloadLength;
      }
    } else {
      // current field does not store payloads
      skipBuffer.writeVInt(curDoc - lastSkipDoc[level]);
    }
    skipBuffer.writeVInt((int) (curFreqPointer - lastSkipFreqPointer[level]));
    skipBuffer.writeVInt((int) (curProxPointer - lastSkipProxPointer[level]));

    lastSkipDoc[level] = curDoc;
    
    lastSkipFreqPointer[level] = curFreqPointer;
    lastSkipProxPointer[level] = curProxPointer;
  }

}