@@ -262,9 +262,9 @@ public final class DisjunctionMaxQuery extends Query implements Iterable<Query>
     return buffer.toString();
   }
 
-  /** Return true iff we represent the same query as o
+  /** Return true if we represent the same query as other
    * @param other another object
-   * @return true iff o is a DisjunctionMaxQuery with the same boost and the same subqueries, in the same order, as us
+   * @return true if other is a DisjunctionMaxQuery with the same boost and the same subqueries, in the same order, as us
    */
   @Override
   public boolean equals(Object other) {
/*
 * 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.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Objects;

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.LeafReaderContext;

/**
 * A query that generates the union of documents produced by its subqueries, and that scores each document with the maximum
 * score for that document as produced by any subquery, plus a tie breaking increment for any additional matching subqueries.
 * This is useful when searching for a word in multiple fields with different boost factors (so that the fields cannot be
 * combined equivalently into a single search field).  We want the primary score to be the one associated with the highest boost,
 * not the sum of the field scores (as BooleanQuery would give).
 * If the query is "albino elephant" this ensures that "albino" matching one field and "elephant" matching
 * another gets a higher score than "albino" matching both fields.
 * To get this result, use both BooleanQuery and DisjunctionMaxQuery:  for each term a DisjunctionMaxQuery searches for it in
 * each field, while the set of these DisjunctionMaxQuery's is combined into a BooleanQuery.
 * The tie breaker capability allows results that include the same term in multiple fields to be judged better than results that
 * include this term in only the best of those multiple fields, without confusing this with the better case of two different terms
 * in the multiple fields.
 */
public final class DisjunctionMaxQuery extends Query implements Iterable<Query> {

  /* The subqueries */
  private final Query[] disjuncts;

  /* Multiple of the non-max disjunct scores added into our final score.  Non-zero values support tie-breaking. */
  private final float tieBreakerMultiplier;

  /**
   * Creates a new DisjunctionMaxQuery
   * @param disjuncts a {@code Collection<Query>} of all the disjuncts to add
   * @param tieBreakerMultiplier  the score of each non-maximum disjunct for a document is multiplied by this weight
   *        and added into the final score.  If non-zero, the value should be small, on the order of 0.1, which says that
   *        10 occurrences of word in a lower-scored field that is also in a higher scored field is just as good as a unique
   *        word in the lower scored field (i.e., one that is not in any higher scored field.
   */
  public DisjunctionMaxQuery(Collection<Query> disjuncts, float tieBreakerMultiplier) {
    Objects.requireNonNull(disjuncts, "Collection of Querys must not be null");
    if (tieBreakerMultiplier < 0 || tieBreakerMultiplier > 1) {
      throw new IllegalArgumentException("tieBreakerMultiplier must be in [0, 1]");
    }
    this.tieBreakerMultiplier = tieBreakerMultiplier;
    this.disjuncts = disjuncts.toArray(new Query[disjuncts.size()]);
  }

  /** @return An {@code Iterator<Query>} over the disjuncts */
  @Override
  public Iterator<Query> iterator() {
    return getDisjuncts().iterator();
  }
  
  /**
   * @return the disjuncts.
   */
  public List<Query> getDisjuncts() {
    return Collections.unmodifiableList(Arrays.asList(disjuncts));
  }

  /**
   * @return tie breaker value for multiple matches.
   */
  public float getTieBreakerMultiplier() {
    return tieBreakerMultiplier;
  }

  /**
   * Expert: the Weight for DisjunctionMaxQuery, used to
   * normalize, score and explain these queries.
   *
   * <p>NOTE: this API and implementation is subject to
   * change suddenly in the next release.</p>
   */
  protected class DisjunctionMaxWeight extends Weight {

    /** The Weights for our subqueries, in 1-1 correspondence with disjuncts */
    protected final ArrayList<Weight> weights = new ArrayList<>();  // The Weight's for our subqueries, in 1-1 correspondence with disjuncts
    private final ScoreMode scoreMode;

    /** Construct the Weight for this Query searched by searcher.  Recursively construct subquery weights. */
    public DisjunctionMaxWeight(IndexSearcher searcher, ScoreMode scoreMode, float boost) throws IOException {
      super(DisjunctionMaxQuery.this);
      for (Query disjunctQuery : disjuncts) {
        weights.add(searcher.createWeight(disjunctQuery, scoreMode, boost));
      }
      this.scoreMode = scoreMode;
    }

    @Override
    public Matches matches(LeafReaderContext context, int doc) throws IOException {
      List<Matches> mis = new ArrayList<>();
      for (Weight weight : weights) {
        Matches mi = weight.matches(context, doc);
        if (mi != null) {
          mis.add(mi);
        }
      }
      return MatchesUtils.fromSubMatches(mis);
    }

    /** Create the scorer used to score our associated DisjunctionMaxQuery */
    @Override
    public Scorer scorer(LeafReaderContext context) throws IOException {
      List<Scorer> scorers = new ArrayList<>();
      for (Weight w : weights) {
        // we will advance() subscorers
        Scorer subScorer = w.scorer(context);
        if (subScorer != null) {
          scorers.add(subScorer);
        }
      }
      if (scorers.isEmpty()) {
        // no sub-scorers had any documents
        return null;
      } else if (scorers.size() == 1) {
        // only one sub-scorer in this segment
        return scorers.get(0);
      } else {
        return new DisjunctionMaxScorer(this, tieBreakerMultiplier, scorers, scoreMode);
      }
    }

    @Override
    public boolean isCacheable(LeafReaderContext ctx) {
      if (weights.size() > TermInSetQuery.BOOLEAN_REWRITE_TERM_COUNT_THRESHOLD) {
        // Disallow caching large dismax queries to not encourage users
        // to build large dismax queries as a workaround to the fact that
        // we disallow caching large TermInSetQueries.
        return false;
      }
      for (Weight w : weights) {
        if (w.isCacheable(ctx) == false)
          return false;
      }
      return true;
    }

    /** Explain the score we computed for doc */
    @Override
    public Explanation explain(LeafReaderContext context, int doc) throws IOException {
      boolean match = false;
      double max = 0;
      double otherSum = 0;
      List<Explanation> subs = new ArrayList<>();
      for (Weight wt : weights) {
        Explanation e = wt.explain(context, doc);
        if (e.isMatch()) {
          match = true;
          subs.add(e);
          double score = e.getValue().doubleValue();
          if (score >= max) {
            otherSum += max;
            max = score;
          } else {
            otherSum += score;
          }
        }
      }
      if (match) {
        final float score = (float) (max + otherSum * tieBreakerMultiplier);
        final String desc = tieBreakerMultiplier == 0.0f ? "max of:" : "max plus " + tieBreakerMultiplier + " times others of:";
        return Explanation.match(score, desc, subs);
      } else {
        return Explanation.noMatch("No matching clause");
      }
    }
    
  }  // end of DisjunctionMaxWeight inner class

  /** Create the Weight used to score us */
  @Override
  public Weight createWeight(IndexSearcher searcher, ScoreMode scoreMode, float boost) throws IOException {
    return new DisjunctionMaxWeight(searcher, scoreMode, boost);
  }

  /** Optimize our representation and our subqueries representations
   * @param reader the IndexReader we query
   * @return an optimized copy of us (which may not be a copy if there is nothing to optimize) */
  @Override
  public Query rewrite(IndexReader reader) throws IOException {
    if (disjuncts.length == 1) {
      return disjuncts[0];
    }

    if (tieBreakerMultiplier == 1.0f) {
      BooleanQuery.Builder builder = new BooleanQuery.Builder();
      for (Query sub : disjuncts) {
        builder.add(sub, BooleanClause.Occur.SHOULD);
      }
      return builder.build();
    }

    boolean actuallyRewritten = false;
    List<Query> rewrittenDisjuncts = new ArrayList<>();
    for (Query sub : disjuncts) {
      Query rewrittenSub = sub.rewrite(reader);
      actuallyRewritten |= rewrittenSub != sub;
      rewrittenDisjuncts.add(rewrittenSub);
    }

    if (actuallyRewritten) {
      return new DisjunctionMaxQuery(rewrittenDisjuncts, tieBreakerMultiplier);
    }

    return super.rewrite(reader);
  }

  @Override
  public void visit(QueryVisitor visitor) {
    QueryVisitor v = visitor.getSubVisitor(BooleanClause.Occur.SHOULD, this);
    for (Query q : disjuncts) {
      q.visit(v);
    }
  }

  /** Prettyprint us.
   * @param field the field to which we are applied
   * @return a string that shows what we do, of the form "(disjunct1 | disjunct2 | ... | disjunctn)^boost"
   */
  @Override
  public String toString(String field) {
    StringBuilder buffer = new StringBuilder();
    buffer.append("(");
    for (int i = 0 ; i < disjuncts.length; i++) {
      Query subquery = disjuncts[i];
      if (subquery instanceof BooleanQuery) {   // wrap sub-bools in parens
        buffer.append("(");
        buffer.append(subquery.toString(field));
        buffer.append(")");
      }
      else buffer.append(subquery.toString(field));
      if (i != disjuncts.length-1) buffer.append(" | ");
    }
    buffer.append(")");
    if (tieBreakerMultiplier != 0.0f) {
      buffer.append("~");
      buffer.append(tieBreakerMultiplier);
    }
    return buffer.toString();
  }

  /** Return true iff we represent the same query as o
   * @param other another object
   * @return true iff o is a DisjunctionMaxQuery with the same boost and the same subqueries, in the same order, as us
   */
  @Override
  public boolean equals(Object other) {
    return sameClassAs(other) &&
           equalsTo(getClass().cast(other));
  }
  
  private boolean equalsTo(DisjunctionMaxQuery other) {
    return tieBreakerMultiplier == other.tieBreakerMultiplier && 
           Arrays.equals(disjuncts, other.disjuncts);
  }

  /** Compute a hash code for hashing us
   * @return the hash code
   */
  @Override
  public int hashCode() {
    int h = classHash();
    h = 31 * h + Float.floatToIntBits(tieBreakerMultiplier);
    h = 31 * h + Arrays.hashCode(disjuncts);
    return h;
  }


}

I've seen "iff" used before as shorthand for "if and only if". So maybe that's what the original author here was going for? But the docs also work without it, so it's hard to say for sure.