Github user jpountz commented on a diff in the pull request:

    https://github.com/apache/lucene-solr/pull/345#discussion_r179439888
  
    --- Diff: lucene/core/src/java/org/apache/lucene/search/Matches.java ---
    @@ -0,0 +1,161 @@
    +/*
    + * 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;
    +
    +import java.io.IOException;
    +import java.util.ArrayList;
    +import java.util.HashMap;
    +import java.util.HashSet;
    +import java.util.List;
    +import java.util.Map;
    +import java.util.Set;
    +
    +import org.apache.lucene.index.LeafReaderContext;
    +import org.apache.lucene.util.BytesRef;
    +
    +/**
    + * Reports the positions and optionally offsets of all matching terms in a 
query
    + * for a single document
    + *
    + * To find all fields that have matches, call {@link #getMatchFields()}
    + *
    + * To obtain a {@link MatchesIterator} for a particular field, call {@link 
#getMatches(String)}
    + */
    +public class Matches {
    +
    +  private final Map<String, MatchesIterator> matches;
    +
    +  /**
    +   * Create a simple {@link Matches} for a single field
    +   */
    +  public static Matches fromField(String field, MatchesIterator it) {
    +    if (it == null) {
    +      return null;
    +    }
    +    return new Matches(field, it);
    +  }
    +
    +  /**
    +   * Create an empty {@link Matches} for a Weight
    +   *
    +   * If the Weight's parent query does not match this document, returns 
{@code null},
    +   * otherwise returns a {@link Matches} document with an empty iterator 
on the given
    +   * fields
    +   */
    +  public static Matches emptyMatches(LeafReaderContext context, int doc, 
Weight weight, String... fields) throws IOException {
    +    Scorer scorer = weight.scorer(context);
    +    if (scorer == null || scorer.iterator().advance(doc) != doc) {
    +      return null;
    +    }
    +    List<Matches> matches = new ArrayList<>();
    +    for (String field : fields) {
    +      matches.add(Matches.fromField(field, EMPTY));
    +    }
    +    return Matches.fromSubMatches(matches);
    +  }
    +
    +  /**
    +   * Amalgamate a collection of {@link Matches} into a single object
    +   */
    +  public static Matches fromSubMatches(List<Matches> subMatches) throws 
IOException {
    +    if (subMatches == null || subMatches.size() == 0) {
    +      return null;
    +    }
    +    if (subMatches.size() == 1) {
    +      return subMatches.get(0);
    +    }
    +    Map<String, MatchesIterator> matches = new HashMap<>();
    +    Set<String> allFields = new HashSet<>();
    +    for (Matches m : subMatches) {
    +      allFields.addAll(m.getMatchFields());
    +    }
    +    for (String field : allFields) {
    +      List<MatchesIterator> mis = new ArrayList<>();
    +      for (Matches m : subMatches) {
    +        MatchesIterator mi = m.getMatches(field);
    +        if (mi != null) {
    +          mis.add(mi);
    +        }
    +      }
    +      matches.put(field, DisjunctionMatchesIterator.fromSubIterators(mis));
    +    }
    +    return new Matches(matches);
    +  }
    +
    +  /**
    +   * Create a {@link Matches} from a map of fields to iterators
    +   */
    +  protected Matches(Map<String, MatchesIterator> matches) {
    +    this.matches = matches;
    +  }
    +
    +  private Matches(String field, MatchesIterator iterator) {
    +    this.matches = new HashMap<>();
    +    this.matches.put(field, iterator);
    +  }
    +
    +  /**
    +   * Returns a {@link MatchesIterator} over the matches for a single field,
    +   * or {@code null} if there are no matches in that field
    +   */
    +  public MatchesIterator getMatches(String field) {
    +    return matches.get(field);
    +  }
    +
    +  /**
    +   * Returns the fields with matches for this document
    +   */
    +  public Set<String> getMatchFields() {
    +    return matches.keySet();
    +  }
    +
    +  private static final BytesRef EMPTY_BYTES = new BytesRef();
    --- End diff --
    
    let's make it an instance member of EMPTY? It doesn't look used elsewhere.


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