package org.wikimedia.search.extra.util;
   
   import java.io.IOException;
   import java.util.Collections;
   import java.util.List;
   import java.util.Map;
   import java.util.stream.Collectors;
   
   import org.apache.lucene.index.IndexReader;
  import org.elasticsearch.common.bytes.BytesReference;
  import org.elasticsearch.common.xcontent.XContentHelper;
  import org.elasticsearch.common.xcontent.support.XContentMapValues;
  import org.elasticsearch.index.fieldvisitor.CustomFieldsVisitor;
  
  import com.google.common.collect.ImmutableSet;
  
  /**
   * Hub for fetching field values.
   */
  public abstract class FieldValues {
      /**
       * Loads field values.
       */
      public interface Loader {
          /**
           * Load the value of the string at path from reader for docId.
           */
          List<String> load(String path, IndexReader reader, int docId) throws IOException;
      }
  
      /**
       * Load field values from source. Note that values aren't cached between
       * calls so providing the same arguments over and over again would call down
       * into Lucene every time.
       */
      public static FieldValues.Loader loadFromSource() {
          return Source.INSTANCE;
      }
  
      /**
       * Load field values from a stored field. Note that values aren't cached
       * between calls so providing the same arguments over and over again would
       * call down into Lucene every time.
       */
      public static FieldValues.Loader loadFromStoredField() {
          return Stored.INSTANCE;
      }
  
      private FieldValues() {
          // Util class
      }
  
      private static final class Source implements FieldValues.Loader {
          private static final FieldValues.Loader INSTANCE = new Source();
          @Override
          public List<String> load(String path, IndexReader reader, int docId) throws IOException {
              CustomFieldsVisitor visitor = new CustomFieldsVisitor(Collections.emptySet(), true);
              reader.document(docId, visitor);
              BytesReference source = visitor.source();
              // deprecated but still in use in core
              // Monitor how it evolves in core and FetchSubPhase.java
              // https://github.com/elastic/elasticsearch/blob/master/server/src/main/java/org/elasticsearch/search/fetch/FetchPhase.java#L290
              Map<String, Object> map = XContentHelper.convertToMap(source, false).v2();
              return XContentMapValues.extractRawValues(path, map).stream()
                      .map(Object::toString)
                      .collect(Collectors.toList());
          }
      }
  
      private static final class Stored implements FieldValues.Loader {
          private static final FieldValues.Loader INSTANCE = new Stored();
          @Override
          public List<String> load(String path, IndexReader reader, int docId) throws IOException {
              CustomFieldsVisitor visitor = new CustomFieldsVisitor(ImmutableSet.of(path), false);
              reader.document(docId, visitor);
              return visitor.fields().get(path).stream().map(Object::toString).collect(Collectors.toList());
          }
      }
  }