lucene/facet/src/java/org/apache/lucene/facet/sortedset/SortedSetDocValuesReaderState.java

/*
 * 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.sortedset;

import java.io.IOException;
import java.util.Map;
import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.SortedSetDocValues;
import org.apache.lucene.util.Accountable;

/**
 * Wraps a {@link IndexReader} and resolves ords using existing {@link SortedSetDocValues} APIs
 * without a separate taxonomy index. This only supports flat facets (dimension + label), and it
 * makes faceting a bit slower, adds some cost at reopen time, but avoids managing the separate
 * taxonomy index. It also requires less RAM than the taxonomy index, as it manages the flat
 * (2-level) hierarchy more efficiently. In addition, the tie-break during faceting is now
 * meaningful (in label sorted order).
 *
 * <p><b>NOTE</b>: creating an instance of this class is somewhat costly, as it computes per-segment
 * ordinal maps, so you should create it once and re-use that one instance for a given {@link
 * IndexReader}.
 */
public abstract class SortedSetDocValuesReaderState implements Accountable {

  /**
   * Holds start/end range of ords, which maps to one dimension (someday we may generalize it to map
   * to hierarchies within one dimension).
   */
  public static final class OrdRange {
    /** Start of range, inclusive: */
    public final int start;
    /** End of range, inclusive: */
    public final int end;

    /** Start and end are inclusive. */
    public OrdRange(int start, int end) {
      this.start = start;
      this.end = end;
    }
  }

  /** Sole constructor. */
  protected SortedSetDocValuesReaderState() {}

  /** Return top-level doc values. */
  public abstract SortedSetDocValues getDocValues() throws IOException;

  /** Indexed field we are reading. */
  public abstract String getField();

  /** Returns the {@link OrdRange} for this dimension. */
  public abstract OrdRange getOrdRange(String dim);

  /** Returns mapping from prefix to {@link OrdRange}. */
  public abstract Map<String, OrdRange> getPrefixToOrdRange();

  /** Returns top-level index reader. */
  public abstract IndexReader getReader();

  /** Number of unique labels. */
  public abstract int getSize();
}