lucene/core/src/java/org/apache/lucene/search/Sort.java

/*
 * 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.Arrays;


/**
 * Encapsulates sort criteria for returned hits.
 *
 * <p>The fields used to determine sort order must be carefully chosen.
 * Documents must contain a single term in such a field,
 * and the value of the term should indicate the document's relative position in
 * a given sort order.  The field must be indexed, but should not be tokenized,
 * and does not need to be stored (unless you happen to want it back with the
 * rest of your document data).  In other words:
 *
 * <p><code>document.add (new Field ("byNumber", Integer.toString(x), Field.Store.NO, Field.Index.NOT_ANALYZED));</code></p>
 * 
 *
 * <h3>Valid Types of Values</h3>
 *
 * <p>There are four possible kinds of term values which may be put into
 * sorting fields: Integers, Longs, Floats, or Strings.  Unless
 * {@link SortField SortField} objects are specified, the type of value
 * in the field is determined by parsing the first term in the field.
 *
 * <p>Integer term values should contain only digits and an optional
 * preceding negative sign.  Values must be base 10 and in the range
 * <code>Integer.MIN_VALUE</code> and <code>Integer.MAX_VALUE</code> inclusive.
 * Documents which should appear first in the sort
 * should have low value integers, later documents high values
 * (i.e. the documents should be numbered <code>1..n</code> where
 * <code>1</code> is the first and <code>n</code> the last).
 *
 * <p>Long term values should contain only digits and an optional
 * preceding negative sign.  Values must be base 10 and in the range
 * <code>Long.MIN_VALUE</code> and <code>Long.MAX_VALUE</code> inclusive.
 * Documents which should appear first in the sort
 * should have low value integers, later documents high values.
 * 
 * <p>Float term values should conform to values accepted by
 * {@link Float Float.valueOf(String)} (except that <code>NaN</code>
 * and <code>Infinity</code> are not supported).
 * Documents which should appear first in the sort
 * should have low values, later documents high values.
 *
 * <p>String term values can contain any valid String, but should
 * not be tokenized.  The values are sorted according to their
 * {@link Comparable natural order}.  Note that using this type
 * of term value has higher memory requirements than the other
 * two types.
 *
 * <h3>Object Reuse</h3>
 *
 * <p>One of these objects can be
 * used multiple times and the sort order changed between usages.
 *
 * <p>This class is thread safe.
 *
 * <h3>Memory Usage</h3>
 *
 * <p>Sorting uses of caches of term values maintained by the
 * internal HitQueue(s).  The cache is static and contains an integer
 * or float array of length <code>IndexReader.maxDoc()</code> for each field
 * name for which a sort is performed.  In other words, the size of the
 * cache in bytes is:
 *
 * <p><code>4 * IndexReader.maxDoc() * (# of different fields actually used to sort)</code>
 *
 * <p>For String fields, the cache is larger: in addition to the
 * above array, the value of every term in the field is kept in memory.
 * If there are many unique terms in the field, this could
 * be quite large.
 *
 * <p>Note that the size of the cache is not affected by how many
 * fields are in the index and <i>might</i> be used to sort - only by
 * the ones actually used to sort a result set.
 *
 * <p>Created: Feb 12, 2004 10:53:57 AM
 *
 * @since   lucene 1.4
 */
public class Sort {

  /**
   * Represents sorting by computed relevance. Using this sort criteria returns
   * the same results as calling
   * {@link IndexSearcher#search(Query,int) IndexSearcher#search()}without a sort criteria,
   * only with slightly more overhead.
   */
  public static final Sort RELEVANCE = new Sort();

  /** Represents sorting by index order. */
  public static final Sort INDEXORDER = new Sort(SortField.FIELD_DOC);

  // internal representation of the sort criteria
  SortField[] fields;

  /**
   * Sorts by computed relevance. This is the same sort criteria as calling
   * {@link IndexSearcher#search(Query,int) IndexSearcher#search()}without a sort criteria,
   * only with slightly more overhead.
   */
  public Sort() {
    this(SortField.FIELD_SCORE);
  }

  /** Sorts by the criteria in the given SortField. */
  public Sort(SortField field) {
    setSort(field);
  }

  /** Sets the sort to the given criteria in succession: the
   *  first SortField is checked first, but if it produces a
   *  tie, then the second SortField is used to break the tie,
   *  etc.  Finally, if there is still a tie after all SortFields
   *  are checked, the internal Lucene docid is used to break it. */
  public Sort(SortField... fields) {
    setSort(fields);
  }

  /** Sets the sort to the given criteria. */
  public void setSort(SortField field) {
    this.fields = new SortField[] { field };
  }

  /** Sets the sort to the given criteria in succession: the
   *  first SortField is checked first, but if it produces a
   *  tie, then the second SortField is used to break the tie,
   *  etc.  Finally, if there is still a tie after all SortFields
   *  are checked, the internal Lucene docid is used to break it. */
  public void setSort(SortField... fields) {
    if (fields.length == 0) {
      throw new IllegalArgumentException("There must be at least 1 sort field");
    }
    this.fields = fields;
  }
  
  /**
   * Representation of the sort criteria.
   * @return Array of SortField objects used in this sort criteria
   */
  public SortField[] getSort() {
    return fields;
  }

  /**
   * Rewrites the SortFields in this Sort, returning a new Sort if any of the fields
   * changes during their rewriting.
   *
   * @param searcher IndexSearcher to use in the rewriting
   * @return {@code this} if the Sort/Fields have not changed, or a new Sort if there
   *        is a change
   * @throws IOException Can be thrown by the rewriting
   * @lucene.experimental
   */
  public Sort rewrite(IndexSearcher searcher) throws IOException {
    boolean changed = false;
    
    SortField[] rewrittenSortFields = new SortField[fields.length];
    for (int i = 0; i < fields.length; i++) {
      rewrittenSortFields[i] = fields[i].rewrite(searcher);
      if (fields[i] != rewrittenSortFields[i]) {
        changed = true;
      }
    }

    return (changed) ? new Sort(rewrittenSortFields) : this;
  }

  @Override
  public String toString() {
    StringBuilder buffer = new StringBuilder();

    for (int i = 0; i < fields.length; i++) {
      buffer.append(fields[i].toString());
      if ((i+1) < fields.length)
        buffer.append(',');
    }

    return buffer.toString();
  }

  /** Returns true if <code>o</code> is equal to this. */
  @Override
  public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof Sort)) return false;
    final Sort other = (Sort)o;
    return Arrays.equals(this.fields, other.fields);
  }

  /** Returns a hash code value for this object. */
  @Override
  public int hashCode() {
    return 0x45aaf665 + Arrays.hashCode(fields);
  }

  /** Returns true if the relevance score is needed to sort documents. */
  public boolean needsScores() {
    for (SortField sortField : fields) {
      if (sortField.needsScores()) {
        return true;
      }
    }
    return false;
  }

}