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Apache Solr (module: core)
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/*
 * 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.solr.update.processor;

import static org.apache.solr.update.processor.FieldMutatingUpdateProcessor.SELECT_NO_FIELDS;

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import org.apache.solr.core.SolrCore;
import org.apache.solr.update.processor.FieldMutatingUpdateProcessor.FieldNameSelector;

/**
 * Removes duplicate values found in fields matching the specified conditions. The existing field
 * values are iterated in order, and values are removed when they are equal to a value that has
 * already been seen for this field.
 *
 * By default this processor matches no fields.
 *
 * 
In the example configuration below, if a document initially contains the values 
 * "Steve","Lucy","Jim",Steve","Alice","Bob","Alice" in a field named foo_uniq
 * then using this processor will result in the final list of field values being 
 * "Steve","Lucy","Jim","Alice","Bob"
 *
 * 
 *  <processor class="solr.UniqFieldsUpdateProcessorFactory">
 *    <str name="fieldRegex">.*_uniq</str>
 *  </processor>
 * 
 *
 * @since 3.4.0
 */
public class UniqFieldsUpdateProcessorFactory extends FieldValueSubsetUpdateProcessorFactory {

  @Override
  public FieldNameSelector getDefaultSelector(SolrCore core) {
    return SELECT_NO_FIELDS;
  }

  @Override
  public  Collection pickSubset(Collection values) {
    // The trivial implementation return new HashSet<>(values) would not preserve order
    // So we use a secondary set to track which values are new
    Set uniqs = new HashSet<>();
    List result = new ArrayList<>(values.size());
    for (T t : values) {
      if (uniqs.add(t)) {
        result.add(t);
      }
    }
    return result;
  }
}