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Lucene Join Module
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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.search.join;

import java.io.IOException;
import org.apache.lucene.index.LeafReaderContext;
import org.apache.lucene.index.ReaderUtil;
import org.apache.lucene.search.DocIdSetIterator;
import org.apache.lucene.search.Explanation;
import org.apache.lucene.search.IndexSearcher;
import org.apache.lucene.search.Query;
import org.apache.lucene.search.QueryVisitor;
import org.apache.lucene.search.Scorer;
import org.apache.lucene.search.Weight;
import org.apache.lucene.util.BitSet;

/**
 * A query that returns all the matching child documents for a specific parent document indexed
 * together in the same block. The provided child query determines which matching child doc is being
 * returned.
 *
 * @lucene.experimental
 */
public class ParentChildrenBlockJoinQuery extends Query {

  private final BitSetProducer parentFilter;
  private final Query childQuery;
  private final int parentDocId;

  /**
   * Creates a ParentChildrenBlockJoinQuery instance
   *
   * @param parentFilter A filter identifying parent documents.
   * @param childQuery A child query that determines which child docs are matching
   * @param parentDocId The top level doc id of that parent to return children documents for
   */
  public ParentChildrenBlockJoinQuery(
      BitSetProducer parentFilter, Query childQuery, int parentDocId) {
    this.parentFilter = parentFilter;
    this.childQuery = childQuery;
    this.parentDocId = parentDocId;
  }

  @Override
  public boolean equals(Object obj) {
    if (sameClassAs(obj) == false) {
      return false;
    }
    ParentChildrenBlockJoinQuery other = (ParentChildrenBlockJoinQuery) obj;
    return parentFilter.equals(other.parentFilter)
        && childQuery.equals(other.childQuery)
        && parentDocId == other.parentDocId;
  }

  @Override
  public int hashCode() {
    int hash = classHash();
    hash = 31 * hash + parentFilter.hashCode();
    hash = 31 * hash + childQuery.hashCode();
    hash = 31 * hash + parentDocId;
    return hash;
  }

  @Override
  public String toString(String field) {
    return "ParentChildrenBlockJoinQuery (" + childQuery + ")";
  }

  @Override
  public void visit(QueryVisitor visitor) {
    visitor.visitLeaf(this);
  }

  @Override
  public Query rewrite(IndexSearcher indexSearcher) throws IOException {
    final Query childRewrite = childQuery.rewrite(indexSearcher);
    if (childRewrite != childQuery) {
      return new ParentChildrenBlockJoinQuery(parentFilter, childRewrite, parentDocId);
    } else {
      return super.rewrite(indexSearcher);
    }
  }

  @Override
  public Weight createWeight(
      IndexSearcher searcher, org.apache.lucene.search.ScoreMode scoreMode, float boost)
      throws IOException {
    final Weight childWeight = childQuery.createWeight(searcher, scoreMode, boost);
    final int readerIndex = ReaderUtil.subIndex(parentDocId, searcher.getIndexReader().leaves());
    return new Weight(this) {

      @Override
      public Explanation explain(LeafReaderContext context, int doc) throws IOException {
        return Explanation.noMatch(
            "Not implemented, use ToParentBlockJoinQuery explain why a document matched");
      }

      @Override
      public Scorer scorer(LeafReaderContext context) throws IOException {
        // Childs docs only reside in a single segment, so no need to evaluate all segments
        if (context.ord != readerIndex) {
          return null;
        }

        final int localParentDocId = parentDocId - context.docBase;
        // If parentDocId == 0 then a parent doc doesn't have child docs, because child docs are
        // stored
        // before the parent doc and because parent doc is 0 we can safely assume that there are no
        // child docs.
        if (localParentDocId == 0) {
          return null;
        }

        final BitSet parents = parentFilter.getBitSet(context);
        final int firstChildDocId = parents.prevSetBit(localParentDocId - 1) + 1;
        // A parent doc doesn't have child docs, so we can early exit here:
        if (firstChildDocId == localParentDocId) {
          return null;
        }

        final Scorer childrenScorer = childWeight.scorer(context);
        if (childrenScorer == null) {
          return null;
        }
        DocIdSetIterator childrenIterator = childrenScorer.iterator();
        final DocIdSetIterator it =
            new DocIdSetIterator() {

              int doc = -1;

              @Override
              public int docID() {
                return doc;
              }

              @Override
              public int nextDoc() throws IOException {
                return advance(doc + 1);
              }

              @Override
              public int advance(int target) throws IOException {
                target = Math.max(firstChildDocId, target);
                if (target >= localParentDocId) {
                  // We're outside the child nested scope, so it is done
                  return doc = NO_MORE_DOCS;
                } else {
                  int advanced = childrenIterator.advance(target);
                  if (advanced >= localParentDocId) {
                    // We're outside the child nested scope, so it is done
                    return doc = NO_MORE_DOCS;
                  } else {
                    return doc = advanced;
                  }
                }
              }

              @Override
              public long cost() {
                return Math.min(childrenIterator.cost(), localParentDocId - firstChildDocId);
              }
            };
        return new Scorer(this) {
          @Override
          public int docID() {
            return it.docID();
          }

          @Override
          public float score() throws IOException {
            return childrenScorer.score();
          }

          @Override
          public float getMaxScore(int upTo) throws IOException {
            return Float.POSITIVE_INFINITY;
          }

          @Override
          public DocIdSetIterator iterator() {
            return it;
          }
        };
      }

      @Override
      public boolean isCacheable(LeafReaderContext ctx) {
        return false; // TODO delegate to BitSetProducer?
      }
    };
  }
}