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Apache Lucene (module: codecs)
There is a newer version: 9.11.1
/*
 * 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.codecs.blockterms;


import java.io.Closeable;
import java.io.IOException;

import org.apache.lucene.index.FieldInfo;
import org.apache.lucene.util.Accountable;
import org.apache.lucene.util.BytesRef;


// TODO
//   - allow for non-regular index intervals?  eg with a
//     long string of rare terms, you don't need such
//     frequent indexing

/**
 * {@link BlockTermsReader} interacts with an instance of this class
 * to manage its terms index.  The writer must accept
 * indexed terms (many pairs of BytesRef text + long
 * fileOffset), and then this reader must be able to
 * retrieve the nearest index term to a provided term
 * text. 
 * @lucene.experimental */

public abstract class TermsIndexReaderBase implements Closeable, Accountable {

  public abstract FieldIndexEnum getFieldEnum(FieldInfo fieldInfo);

  @Override
  public abstract void close() throws IOException;

  public abstract boolean supportsOrd();

  /** 
   * Similar to TermsEnum, except, the only "metadata" it
   * reports for a given indexed term is the long fileOffset
   * into the main terms dictionary file.
   */
  public static abstract class FieldIndexEnum {

    /** Seeks to "largest" indexed term that's <=
     *  term; returns file pointer index (into the main
     *  terms index file) for that term */
    public abstract long seek(BytesRef term) throws IOException;

    /** Returns -1 at end */
    public abstract long next() throws IOException;

    public abstract BytesRef term();

    /** Only implemented if {@link TermsIndexReaderBase#supportsOrd()} returns true. */
    public abstract long seek(long ord) throws IOException;
    
    /** Only implemented if {@link TermsIndexReaderBase#supportsOrd()} returns true. */
    public abstract long ord();
  }

}