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Lucene Queries Module
There is a newer version: 9.11.1
package org.apache.lucene.queries.function.valuesource;

/*
 * 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.Map;

import org.apache.lucene.index.AtomicReaderContext;
import org.apache.lucene.index.BinaryDocValues;
import org.apache.lucene.index.FieldInfo;
import org.apache.lucene.index.FieldInfo.DocValuesType;
import org.apache.lucene.queries.function.FunctionValues;
import org.apache.lucene.queries.function.docvalues.DocTermsIndexDocValues;
import org.apache.lucene.search.FieldCache;
import org.apache.lucene.util.Bits;
import org.apache.lucene.util.BytesRef;

/**
 * An implementation for retrieving {@link FunctionValues} instances for string based fields.
 */
public class BytesRefFieldSource extends FieldCacheSource {

  public BytesRefFieldSource(String field) {
    super(field);
  }

  @Override
  public FunctionValues getValues(Map context, AtomicReaderContext readerContext) throws IOException {
    final FieldInfo fieldInfo = readerContext.reader().getFieldInfos().fieldInfo(field);
    // To be sorted or not to be sorted, that is the question
    // TODO: do it cleaner?
    if (fieldInfo != null && fieldInfo.getDocValuesType() == DocValuesType.BINARY) {
      final BinaryDocValues binaryValues = FieldCache.DEFAULT.getTerms(readerContext.reader(), field);
      return new FunctionValues() {

        @Override
        public boolean exists(int doc) {
          return true; // doc values are dense
        }

        @Override
        public boolean bytesVal(int doc, BytesRef target) {
          binaryValues.get(doc, target);
          return target.length > 0;
        }

        public String strVal(int doc) {
          final BytesRef bytes = new BytesRef();
          return bytesVal(doc, bytes)
              ? bytes.utf8ToString()
              : null;
        }

        @Override
        public Object objectVal(int doc) {
          return strVal(doc);
        }

        @Override
        public String toString(int doc) {
          return description() + '=' + strVal(doc);
        }
      };
    } else {
      return new DocTermsIndexDocValues(this, readerContext, field) {

        @Override
        protected String toTerm(String readableValue) {
          return readableValue;
        }

        @Override
        public Object objectVal(int doc) {
          return strVal(doc);
        }

        @Override
        public String toString(int doc) {
          return description() + '=' + strVal(doc);
        }
      };
    }
  }
}