content: package org.apache.lucene.search.function;
  
  /**
   * 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 org.apache.lucene.index.IndexReader;
  import org.apache.lucene.search.function.DocValues;
  
  import java.io.IOException;
  import java.io.Serializable;
  
  /**
   * Expert: source of values for basic function queries.
   * <P>At its default/simplest form, values - one per doc - are used as the score of that doc.
   * <P>Values are instantiated as 
   * {@link org.apache.lucene.search.function.DocValues DocValues} for a particular reader.
   * <P>ValueSource implementations differ in RAM requirements: it would always be a factor
   * of the number of documents, but for each document the number of bytes can be 1, 2, 4, or 8. 
   *
   * @lucene.experimental
   *
   *
   */
  public abstract class ValueSource implements Serializable {
  
    /**
     * Return the DocValues used by the function query.
     * @param reader the IndexReader used to read these values.
     * If any caching is involved, that caching would also be IndexReader based.  
     * @throws IOException for any error.
     */
    public abstract DocValues getValues(IndexReader reader) throws IOException;
  
    /** 
     * description of field, used in explain() 
     */
    public abstract String description();
  
    /* (non-Javadoc) @see java.lang.Object#toString() */
    @Override
    public String toString() {
      return description();
    }
  
    /**
     * Needed for possible caching of query results - used by {@link ValueSourceQuery#equals(Object)}.
     * @see Object#equals(Object)
     */
    @Override
    public abstract boolean equals(Object o);
  
    /**
     * Needed for possible caching of query results - used by {@link ValueSourceQuery#hashCode()}.
     * @see Object#hashCode()
     */
    @Override
    public abstract int hashCode();
    
  }
  