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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.lucene.index;


import java.io.IOException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import org.apache.lucene.util.MergedIterator;

/**
 * Provides a single {@link Fields} term index view over an
 * {@link IndexReader}.
 * This is useful when you're interacting with an {@link
 * IndexReader} implementation that consists of sequential
 * sub-readers (eg {@link DirectoryReader} or {@link
 * MultiReader}) and you must treat it as a {@link LeafReader}.
 *
 * NOTE: for composite readers, you'll get better
 * performance by gathering the sub readers using
 * {@link IndexReader#getContext()} to get the
 * atomic leaves and then operate per-LeafReader,
 * instead of using this class.
 *
 * @lucene.internal
 */
public final class MultiFields extends Fields {
  private final Fields[] subs;
  private final ReaderSlice[] subSlices;
  private final Map terms = new ConcurrentHashMap<>();

  /**
   * Sole constructor.
   */
  public MultiFields(Fields[] subs, ReaderSlice[] subSlices) {
    this.subs = subs;
    this.subSlices = subSlices;
  }

  @SuppressWarnings({"unchecked","rawtypes"})
  @Override
  public Iterator iterator() {
    Iterator subIterators[] = new Iterator[subs.length];
    for(int i=0;i(subIterators);
  }

  @Override
  public Terms terms(String field) throws IOException {
    Terms result = terms.get(field);
    if (result != null)
      return result;


    // Lazy init: first time this field is requested, we
    // create & add to terms:
    final List subs2 = new ArrayList<>();
    final List slices2 = new ArrayList<>();

    // Gather all sub-readers that share this field
    for(int i=0;i