/*
 * 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.lucene.facet.taxonomy;

import java.io.IOException;

import org.apache.lucene.facet.FacetsConfig;
import org.apache.lucene.index.BinaryDocValues;
import org.apache.lucene.index.DocValues;
import org.apache.lucene.index.LeafReaderContext;
import org.apache.lucene.util.ArrayUtil;
import org.apache.lucene.util.BytesRef;
import org.apache.lucene.util.IntsRef;

Decodes ordinals previously indexed into a BinaryDocValues field /** Decodes ordinals previously indexed into a BinaryDocValues field */
public class DocValuesOrdinalsReader extends OrdinalsReader {
  private final String field;

  
Default constructor. /** Default constructor. */
  public DocValuesOrdinalsReader() {
    this(FacetsConfig.DEFAULT_INDEX_FIELD_NAME);
  }

  
Create this, with the specified indexed field name. /** Create this, with the specified indexed field name. */
  public DocValuesOrdinalsReader(String field) {
    this.field = field;
  }

  @Override
  public OrdinalsSegmentReader getReader(LeafReaderContext context) throws IOException {
    BinaryDocValues values0 = context.reader().getBinaryDocValues(field);
    if (values0 == null) {
      values0 = DocValues.emptyBinary();
    }

    final BinaryDocValues values = values0;

    return new OrdinalsSegmentReader() {

      private int lastDocID;
      
      @Override
      public void get(int docID, IntsRef ordinals) throws IOException {
        if (docID < lastDocID) {
          throw new AssertionError("docs out of order: lastDocID=" + lastDocID + " vs docID=" + docID);
        }
        lastDocID = docID;
        if (docID > values.docID()) {
          values.advance(docID);
        }
        final BytesRef bytes;
        if (values.docID() == docID) {
          bytes = values.binaryValue();
        } else {
          bytes = new BytesRef(BytesRef.EMPTY_BYTES);
        }
        decode(bytes, ordinals);
      }
    };
  }

  @Override
  public String getIndexFieldName() {
    return field;
  }

 
Subclass and override if you change the encoding. The method is marked 'public' to allow decoding of binary payload containing ordinals without instantiating an OrdinalsSegmentReader. This takes care of use cases where an application instantiates BinaryDocValues reader for a facet field outside this class, reads the binary payload for a document and decodes the ordinals in the payload. 
Params:
buf – binary payload containing encoded ordinals
ordinals – buffer for decoded ordinals/** 
   * Subclass and override if you change the encoding. 
   * The method is marked 'public' to allow decoding of binary payload containing ordinals
   * without instantiating an {@link org.apache.lucene.facet.taxonomy.OrdinalsReader.OrdinalsSegmentReader}.
   *  
   * This takes care of use cases where an application instantiates {@link org.apache.lucene.index.BinaryDocValues} 
   * reader for a facet field outside this class, reads the binary payload for a document and decodes the ordinals
   * in the payload.
   *
   * @param buf binary payload containing encoded ordinals
   * @param ordinals buffer for decoded ordinals
   */
  public void decode(BytesRef buf, IntsRef ordinals) {

    // grow the buffer up front, even if by a large number of values (buf.length)
    // that saves the need to check inside the loop for every decoded value if
    // the buffer needs to grow.
    if (ordinals.ints.length < buf.length) {
      ordinals.ints = ArrayUtil.grow(ordinals.ints, buf.length);
    }

    ordinals.offset = 0;
    ordinals.length = 0;

    // it is better if the decoding is inlined like so, and not e.g.
    // in a utility method
    int upto = buf.offset + buf.length;
    int value = 0;
    int offset = buf.offset;
    int prev = 0;
    while (offset < upto) {
      byte b = buf.bytes[offset++];
      if (b >= 0) {
        ordinals.ints[ordinals.length] = ((value << 7) | b) + prev;
        value = 0;
        prev = ordinals.ints[ordinals.length];
        ordinals.length++;
      } else {
        value = (value << 7) | (b & 0x7F);
      }
    }
  }
}