/*
 * 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.beam.sdk.transforms.windowing;

import static com.google.common.base.Preconditions.checkArgument;

import com.google.common.base.Joiner;
import java.util.Arrays;
import java.util.List;
import org.apache.beam.sdk.annotations.Experimental;
import org.joda.time.Instant;

/**
 * A composite {@link Trigger} that executes its sub-triggers in order.
 * Only one sub-trigger is executing at a time,
 * and any time it fires the {@code AfterEach} fires. When the currently executing
 * sub-trigger finishes, the {@code AfterEach} starts executing the next sub-trigger.
 *
 * <p>{@code AfterEach.inOrder(t1, t2, ...)} finishes when all of the sub-triggers have finished.
 *
 * <p>The following properties hold:
 * <ul>
 *   <li> {@code AfterEach.inOrder(AfterEach.inOrder(a, b), c)} behaves the same as
 *   {@code AfterEach.inOrder(a, b, c)} and {@code AfterEach.inOrder(a, AfterEach.inOrder(b, c)}.
 *   <li> {@code AfterEach.inOrder(Repeatedly.forever(a), b)} behaves the same as
 *   {@code Repeatedly.forever(a)}, since the repeated trigger never finishes.
 * </ul>
 */
@Experimental(Experimental.Kind.TRIGGER)
public class AfterEach extends Trigger {

  private AfterEach(List<Trigger> subTriggers) {
    super(subTriggers);
    checkArgument(subTriggers.size() > 1);
  }

  /**
   * Returns an {@code AfterEach} {@code Trigger} with the given subtriggers.
   */
  @SafeVarargs
  public static AfterEach inOrder(Trigger... triggers) {
    return new AfterEach(Arrays.<Trigger>asList(triggers));
  }

  /**
   * Returns an {@code AfterEach} {@code Trigger} with the given subtriggers.
   */
  public static AfterEach inOrder(List<Trigger> triggers) {
    return new AfterEach(triggers);
  }

  @Override
  public Instant getWatermarkThatGuaranteesFiring(BoundedWindow window) {
    // This trigger will fire at least once when the first trigger in the sequence
    // fires at least once.
    return subTriggers.get(0).getWatermarkThatGuaranteesFiring(window);
  }

  @Override
  protected Trigger getContinuationTrigger(List<Trigger> continuationTriggers) {
    return Repeatedly.forever(new AfterFirst(continuationTriggers));
  }

  @Override
  public String toString() {
    StringBuilder builder = new StringBuilder("AfterEach.inOrder(");
    Joiner.on(", ").appendTo(builder, subTriggers);
    builder.append(")");

    return builder.toString();
  }
}