content: /*
   * 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.calcite.rel;
  
  import org.apache.calcite.plan.RelMultipleTrait;
  import org.apache.calcite.util.ImmutableIntList;
  
  import java.util.List;
  
  /**
   * Description of the physical ordering of a relational expression.
   *
   * <p>An ordering consists of a list of one or more column ordinals and the
   * direction of the ordering.
   */
  public interface RelCollation extends RelMultipleTrait {
    //~ Methods ----------------------------------------------------------------
  
    /**
     * Returns the ordinals and directions of the columns in this ordering.
     */
    List<RelFieldCollation> getFieldCollations();
  
    /**
     * Returns the ordinals of the key columns.
     */
    default ImmutableIntList getKeys() {
      final List<RelFieldCollation> collations = getFieldCollations();
      final int size = collations.size();
      final int[] keys = new int[size];
      for (int i = 0; i < size; i++) {
        keys[i] = collations.get(i).getFieldIndex();
      }
      return ImmutableIntList.of(keys);
    }
  }
  