content: /**
   * Licensed 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.aurora.scheduler.updater.strategy;
  
  import java.util.Objects;
  import java.util.Set;
  import java.util.stream.Collectors;
  
  import com.google.common.base.Preconditions;
  import com.google.common.collect.Ordering;
  
  /**
   * A strategy that limits the number of instances selected by the subclass.
   *
   * @param <T> Instance type.
   */
  abstract class ActiveLimitedStrategy<T extends Comparable<T>> implements UpdateStrategy<T> {
    private final Ordering<T> ordering;
    protected final int maxActive;
  
    /**
     * Creates an active-limited strategy that applies an upper bound to all results.
     *
     * @param maxActive Maximum number of values to return from
     * {@link #getNextGroup(java.util.Set, java.util.Set)}.
     */
    protected ActiveLimitedStrategy(Ordering<T> ordering, int maxActive) {
      this.ordering = Objects.requireNonNull(ordering);
      Preconditions.checkArgument(maxActive > 0);
      this.maxActive = maxActive;
    }
  
    @Override
    public final Set<T> getNextGroup(Set<T> idle, Set<T> active) {
      return ordering.sortedCopy(doGetNextGroup(idle, active)).stream()
          .limit(Math.max(0, maxActive - active.size()))
          .collect(Collectors.toSet());
    }
  
    /**
     * Delegate function for the subclass. The implementation may return as many or few results
     * as they wish. If the result is larger than {@link #maxActive}, it will be truncated.
     *
     * @param idle Idle instances, candidate for being updated.
     * @param active Instances currently being updated.
     * @return A subset of {@code idle}, instances to start updating.
     */
    abstract Set<T> doGetNextGroup(Set<T> idle, Set<T> active);
  }
  