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/*
 * 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.hudi.common.table.timeline;

import org.apache.hudi.common.table.timeline.HoodieInstant.State;
import org.apache.hudi.common.util.CollectionUtils;
import org.apache.hudi.common.util.Option;
import org.apache.hudi.common.util.StringUtils;
import org.apache.hudi.common.util.ValidationUtils;
import org.apache.hudi.exception.HoodieException;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.Serializable;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.Set;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import static org.apache.hudi.common.table.timeline.InstantComparison.GREATER_THAN;
import static org.apache.hudi.common.table.timeline.InstantComparison.GREATER_THAN_OR_EQUALS;
import static org.apache.hudi.common.table.timeline.InstantComparison.LESSER_THAN;
import static org.apache.hudi.common.table.timeline.InstantComparison.LESSER_THAN_OR_EQUALS;
import static org.apache.hudi.common.table.timeline.InstantComparison.compareTimestamps;
import static org.apache.hudi.common.util.StringUtils.getUTF8Bytes;

/**
 * HoodieDefaultTimeline is a default implementation of the HoodieTimeline. It provides methods to inspect a
 * List[HoodieInstant]. Function to get the details of the instant is passed in as a lambda.
 *
 * @see HoodieTimeline
 */
public abstract class BaseHoodieTimeline implements HoodieTimeline {

  private static final Logger LOG = LoggerFactory.getLogger(BaseHoodieTimeline.class);

  private static final long serialVersionUID = 1L;

  private static final String HASHING_ALGORITHM = "SHA-256";

  protected transient Function> details;
  private List instants;
  // for efficient #contains queries.
  private transient volatile Set instantTimeSet;
  // for efficient #isPendingClusteringInstant queries
  protected transient volatile Set pendingClusteringInstants;
  // for efficient #isBeforeTimelineStarts check.
  private transient volatile Option firstNonSavepointCommit;
  // for efficient #isBeforeTimelineStartsByCompletionTime
  private transient volatile Option firstNonSavepointCommitByCompletionTime;
  private String timelineHash;

  protected TimelineFactory factory;
  protected InstantComparator instantComparator;
  protected InstantGenerator instantGenerator;

  public BaseHoodieTimeline(TimelineLayout layout) {
    this.factory = layout.getTimelineFactory();
    this.instantComparator = layout.getInstantComparator();
    this.instantGenerator = layout.getInstantGenerator();
  }

  public BaseHoodieTimeline(Stream instants, Function> details,
                            TimelineFactory factory, InstantComparator instantComparator, InstantGenerator instantGenerator) {
    this.details = details;
    this.factory = factory;
    this.instantComparator = instantComparator;
    this.instantGenerator = instantGenerator;
    setInstants(instants.collect(Collectors.toList()));
  }

  @Override
  public void setInstants(List instants) {
    this.instants = instants;
    this.timelineHash = computeTimelineHash(this.instants);
    clearState();
  }

  protected void appendInstants(List newInstants) {
    if (newInstants.isEmpty()) {
      // the new instants is empty, nothing to do.
      return;
    }
    if (this.instants.isEmpty()) {
      // the existing instants is empty, set up the new ones directly.
      setInstants(newInstants);
      return;
    }
    this.instants = mergeInstants(newInstants, this.instants);
    this.timelineHash = computeTimelineHash(this.instants);
    clearState();
  }

  @Override
  public HoodieTimeline filterInflights() {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(HoodieInstant::isInflight), details);
  }

  @Override
  public HoodieTimeline filterInflightsAndRequested() {
    return factory.createDefaultTimeline(
        getInstantsAsStream().filter(i -> i.getState().equals(State.REQUESTED) || i.getState().equals(State.INFLIGHT)),
        details);
  }

  @Override
  public HoodieTimeline filterPendingExcludingCompaction() {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(instant -> (!instant.isCompleted())
        && (!instant.getAction().equals(HoodieTimeline.COMPACTION_ACTION))), details);
  }

  @Override
  public HoodieTimeline filterPendingExcludingLogCompaction() {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(instant -> (!instant.isCompleted())
        && (!instant.getAction().equals(HoodieTimeline.LOG_COMPACTION_ACTION))), details);
  }

  @Override
  public HoodieTimeline filterPendingExcludingCompactionAndLogCompaction() {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(instant -> (!instant.isCompleted())
        && (!instant.getAction().equals(HoodieTimeline.COMPACTION_ACTION)
        || !instant.getAction().equals(HoodieTimeline.LOG_COMPACTION_ACTION))), details);
  }

  @Override
  public HoodieTimeline filterCompletedInstants() {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(HoodieInstant::isCompleted), details);
  }

  @Override
  public HoodieTimeline filterCompletedAndCompactionInstants() {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(s -> s.isCompleted()
        || s.getAction().equals(HoodieTimeline.COMPACTION_ACTION)), details);
  }

  @Override
  public HoodieTimeline filterCompletedOrMajorOrMinorCompactionInstants() {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(s -> s.isCompleted()
        || s.getAction().equals(HoodieTimeline.COMPACTION_ACTION) || s.getAction().equals(HoodieTimeline.LOG_COMPACTION_ACTION)), details);
  }

  @Override
  public HoodieTimeline filterCompletedInstantsOrRewriteTimeline() {
    Set validActions = CollectionUtils.createSet(COMPACTION_ACTION, LOG_COMPACTION_ACTION, REPLACE_COMMIT_ACTION);
    return factory.createDefaultTimeline(getInstantsAsStream().filter(s -> s.isCompleted() || validActions.contains(s.getAction())), details);
  }

  @Override
  public HoodieTimeline getWriteTimeline() {
    Set validActions = CollectionUtils.createSet(COMMIT_ACTION, DELTA_COMMIT_ACTION, COMPACTION_ACTION, LOG_COMPACTION_ACTION, REPLACE_COMMIT_ACTION, CLUSTERING_ACTION);
    return factory.createDefaultTimeline(getInstantsAsStream().filter(s -> validActions.contains(s.getAction())), details);
  }

  @Override
  public HoodieTimeline getContiguousCompletedWriteTimeline() {
    Option earliestPending = getWriteTimeline().filterInflightsAndRequested().firstInstant();
    if (earliestPending.isPresent()) {
      return getWriteTimeline().filterCompletedInstants()
          .filter(instant -> compareTimestamps(instant.requestedTime(), LESSER_THAN, earliestPending.get().requestedTime()));
    }
    return getWriteTimeline().filterCompletedInstants();
  }

  @Override
  public HoodieTimeline getCompletedReplaceTimeline() {
    return factory.createDefaultTimeline(
        getInstantsAsStream().filter(s -> s.getAction().equals(REPLACE_COMMIT_ACTION)).filter(HoodieInstant::isCompleted), details);
  }

  @Override
  public HoodieTimeline filterPendingReplaceTimeline() {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(
        s -> s.getAction().equals(HoodieTimeline.REPLACE_COMMIT_ACTION) && !s.isCompleted()), details);
  }

  @Override
  public abstract HoodieTimeline filterPendingClusteringTimeline();

  @Override
  public abstract HoodieTimeline filterPendingReplaceOrClusteringTimeline();

  @Override
  public abstract HoodieTimeline filterPendingReplaceClusteringAndCompactionTimeline();

  @Override
  public HoodieTimeline filterPendingRollbackTimeline() {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(
        s -> s.getAction().equals(HoodieTimeline.ROLLBACK_ACTION) && !s.isCompleted()), details);
  }

  @Override
  public HoodieTimeline filterRequestedRollbackTimeline() {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(
        s -> s.getAction().equals(HoodieTimeline.ROLLBACK_ACTION) && s.isRequested()), details);
  }

  @Override
  public HoodieTimeline filterPendingCompactionTimeline() {
    return factory.createDefaultTimeline(
        getInstantsAsStream().filter(s -> s.getAction().equals(HoodieTimeline.COMPACTION_ACTION) && !s.isCompleted()), details);
  }

  @Override
  public HoodieTimeline filterPendingLogCompactionTimeline() {
    return factory.createDefaultTimeline(
        getInstantsAsStream().filter(s -> s.getAction().equals(HoodieTimeline.LOG_COMPACTION_ACTION) && !s.isCompleted()), details);
  }

  /**
   * Compaction and logcompaction operation on MOR table is called major and minor compaction respectively.
   */
  @Override
  public HoodieTimeline filterPendingMajorOrMinorCompactionTimeline() {
    return factory.createDefaultTimeline(
        getInstantsAsStream().filter(s -> s.getAction().equals(HoodieTimeline.COMPACTION_ACTION)
            || s.getAction().equals(HoodieTimeline.LOG_COMPACTION_ACTION)
            && !s.isCompleted()), details);
  }

  @Override
  public HoodieTimeline findInstantsInRange(String startTs, String endTs) {
    return factory.createDefaultTimeline(
        getInstantsAsStream().filter(s -> InstantComparison.isInRange(s.requestedTime(), startTs, endTs)), details);
  }

  @Override
  public HoodieTimeline findInstantsInClosedRange(String startTs, String endTs) {
    return factory.createDefaultTimeline(
        instants.stream().filter(instant -> InstantComparison.isInClosedRange(instant.requestedTime(), startTs, endTs)), details);
  }

  @Override
  public HoodieTimeline findInstantsInRangeByCompletionTime(String startTs, String endTs) {
    return factory.createDefaultTimeline(
        getInstantsAsStream().filter(s -> s.getCompletionTime() != null && InstantComparison.isInClosedRange(s.getCompletionTime(), startTs, endTs)),
        details);
  }

  @Override
  public HoodieTimeline findInstantsModifiedAfterByCompletionTime(String instantTime) {
    return factory.createDefaultTimeline(instants.stream()
        // either pending or completionTime greater than instantTime
        .filter(s -> (s.getCompletionTime() == null && compareTimestamps(s.requestedTime(), GREATER_THAN, instantTime))
            || (s.getCompletionTime() != null && compareTimestamps(s.getCompletionTime(), GREATER_THAN, instantTime) && !s.requestedTime().equals(instantTime))),
        details);
  }

  @Override
  public HoodieTimeline findInstantsAfter(String instantTime, int numCommits) {
    return factory.createDefaultTimeline(getInstantsAsStream()
        .filter(s -> compareTimestamps(s.requestedTime(), GREATER_THAN, instantTime)).limit(numCommits),
        details);
  }

  @Override
  public HoodieTimeline findInstantsAfter(String instantTime) {
    return factory.createDefaultTimeline(getInstantsAsStream()
        .filter(s -> compareTimestamps(s.requestedTime(), GREATER_THAN, instantTime)), details);
  }

  @Override
  public HoodieTimeline findInstantsAfterOrEquals(String commitTime, int numCommits) {
    return factory.createDefaultTimeline(getInstantsAsStream()
        .filter(s -> compareTimestamps(s.requestedTime(), GREATER_THAN_OR_EQUALS, commitTime))
        .limit(numCommits), details);
  }

  @Override
  public HoodieTimeline findInstantsAfterOrEquals(String commitTime) {
    return factory.createDefaultTimeline(getInstantsAsStream()
        .filter(s -> compareTimestamps(s.requestedTime(), GREATER_THAN_OR_EQUALS, commitTime)), details);
  }

  @Override
  public HoodieTimeline findInstantsBefore(String instantTime) {
    return factory.createDefaultTimeline(getInstantsAsStream()
        .filter(s -> compareTimestamps(s.requestedTime(), LESSER_THAN, instantTime)),
        details);
  }

  @Override
  public Option findInstantBefore(String instantTime) {
    return Option.fromJavaOptional(instants.stream()
        .filter(instant -> compareTimestamps(instant.requestedTime(), LESSER_THAN, instantTime))
        .max(Comparator.comparing(HoodieInstant::requestedTime)));
  }

  @Override
  public HoodieTimeline findInstantsBeforeOrEquals(String instantTime) {
    return factory.createDefaultTimeline(getInstantsAsStream()
        .filter(s -> compareTimestamps(s.requestedTime(), LESSER_THAN_OR_EQUALS, instantTime)),
        details);
  }

  @Override
  public HoodieTimeline filter(Predicate filter) {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(filter), details);
  }

  @Override
  public HoodieTimeline filterPendingIndexTimeline() {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(s -> s.getAction().equals(INDEXING_ACTION) && !s.isCompleted()), details);
  }

  @Override
  public HoodieTimeline filterCompletedIndexTimeline() {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(s -> s.getAction().equals(INDEXING_ACTION) && s.isCompleted()), details);
  }

  @Override
  public HoodieTimeline getCommitsAndCompactionTimeline() {
    return getTimelineOfActions(CollectionUtils.createSet(COMMIT_ACTION, DELTA_COMMIT_ACTION, REPLACE_COMMIT_ACTION, CLUSTERING_ACTION, COMPACTION_ACTION));
  }

  @Override
  public HoodieTimeline getAllCommitsTimeline() {
    return getTimelineOfActions(CollectionUtils.createSet(COMMIT_ACTION, DELTA_COMMIT_ACTION,
        CLEAN_ACTION, COMPACTION_ACTION, SAVEPOINT_ACTION, ROLLBACK_ACTION, REPLACE_COMMIT_ACTION, CLUSTERING_ACTION,
        INDEXING_ACTION, LOG_COMPACTION_ACTION));
  }

  @Override
  public HoodieTimeline getCommitAndReplaceTimeline() {
    //TODO: Make sure this change does not break existing functionality.
    return getTimelineOfActions(CollectionUtils.createSet(COMMIT_ACTION, REPLACE_COMMIT_ACTION, CLUSTERING_ACTION));
  }

  @Override
  public HoodieTimeline getCommitTimeline() {
    return getTimelineOfActions(CollectionUtils.createSet(COMMIT_ACTION));
  }

  @Override
  public HoodieTimeline getDeltaCommitTimeline() {
    return factory.createDefaultTimeline(filterInstantsByAction(DELTA_COMMIT_ACTION),
        (Function> & Serializable) this::getInstantDetails);
  }

  @Override
  public HoodieTimeline getTimelineOfActions(Set actions) {
    return factory.createDefaultTimeline(getInstantsAsStream().filter(s -> actions.contains(s.getAction())),
        (Function> & Serializable) this::getInstantDetails);
  }

  @Override
  public HoodieTimeline getCleanerTimeline() {
    return factory.createDefaultTimeline(filterInstantsByAction(CLEAN_ACTION),
        (Function> & Serializable) this::getInstantDetails);
  }

  @Override
  public HoodieTimeline getRollbackTimeline() {
    return factory.createDefaultTimeline(filterInstantsByAction(ROLLBACK_ACTION),
        (Function> & Serializable) this::getInstantDetails);
  }

  @Override
  public HoodieTimeline getRollbackAndRestoreTimeline() {
    return  getTimelineOfActions(CollectionUtils.createSet(ROLLBACK_ACTION, RESTORE_ACTION));
  }

  @Override
  public HoodieTimeline getSavePointTimeline() {
    return factory.createDefaultTimeline(filterInstantsByAction(SAVEPOINT_ACTION),
        (Function> & Serializable) this::getInstantDetails);
  }

  @Override
  public HoodieTimeline getRestoreTimeline() {
    return factory.createDefaultTimeline(filterInstantsByAction(RESTORE_ACTION),
        (Function> & Serializable) this::getInstantDetails);
  }

  protected Stream filterInstantsByAction(String action) {
    return getInstantsAsStream().filter(s -> s.getAction().equals(action));
  }

  @Override
  public boolean empty() {
    return instants.isEmpty();
  }

  @Override
  public int countInstants() {
    return instants.size();
  }

  @Override
  public Option firstInstant() {
    return Option.fromJavaOptional(getInstantsAsStream().findFirst());
  }

  @Override
  public Option firstInstant(String action, State state) {
    return Option.fromJavaOptional(getInstantsAsStream()
        .filter(s -> action.equals(s.getAction()) && state.equals(s.getState())).findFirst());
  }

  @Override
  public Option nthInstant(int n) {
    if (empty() || n >= countInstants()) {
      return Option.empty();
    }
    return Option.of(getInstants().get(n));
  }

  @Override
  public Option lastInstant() {
    return empty() ? Option.empty() : nthInstant(countInstants() - 1);
  }

  @Override
  public Option nthFromLastInstant(int n) {
    if (countInstants() < n + 1) {
      return Option.empty();
    }
    return nthInstant(countInstants() - 1 - n);
  }

  @Override
  public boolean containsInstant(HoodieInstant instant) {
    return getInstantsAsStream().anyMatch(s -> s.equals(instant));
  }

  @Override
  public boolean containsInstant(String ts) {
    // Check for 0.10.0+ timestamps which have msec granularity
    if (getOrCreateInstantSet().contains(ts)) {
      return true;
    }

    // Check for older timestamp which have sec granularity and an extension of DEFAULT_MILLIS_EXT may have been added via Timeline operations
    if (ts.length() == HoodieInstantTimeGenerator.MILLIS_INSTANT_TIMESTAMP_FORMAT_LENGTH && ts.endsWith(HoodieInstantTimeGenerator.DEFAULT_MILLIS_EXT)) {
      String actualOlderFormatTs = ts.substring(0, ts.length() - HoodieInstantTimeGenerator.DEFAULT_MILLIS_EXT.length());
      return containsInstant(actualOlderFormatTs);
    }

    return false;
  }

  @Override
  public boolean containsOrBeforeTimelineStarts(String instant) {
    return containsInstant(instant) || isBeforeTimelineStarts(instant);
  }

  @Override
  public String getTimelineHash() {
    return timelineHash;
  }

  @Override
  public Stream getInstantsAsStream() {
    return instants.stream();
  }

  @Override
  public List getInstants() {
    return new ArrayList<>(instants);
  }

  @Override
  public Stream getReverseOrderedInstants() {
    return getInstantsAsStream().sorted(instantComparator.requestedTimeOrderedComparator().reversed());
  }

  @Override
  public Option getLatestCompletionTime() {
    return Option.fromJavaOptional(getInstantsAsStream().filter(s -> s.getCompletionTime() != null)
        .max(instantComparator.completionTimeOrderedComparator())
        .map(HoodieInstant::getCompletionTime));
  }

  @Override
  public Stream getInstantsOrderedByCompletionTime() {
    return getInstantsAsStream().filter(s -> s.getCompletionTime() != null)
        .sorted(instantComparator.completionTimeOrderedComparator());
  }

  @Override
  public boolean isBeforeTimelineStarts(String instant) {
    Option firstNonSavepointCommit = getFirstNonSavepointCommit();
    return firstNonSavepointCommit.isPresent()
        && compareTimestamps(instant, LESSER_THAN, firstNonSavepointCommit.get().requestedTime());
  }

  @Override
  public boolean isBeforeTimelineStartsByCompletionTime(String completionTime) {
    Option firstNonSavepointCommit = getFirstNonSavepointCommitByCompletionTime();
    return firstNonSavepointCommit.isPresent()
        && compareTimestamps(completionTime, LESSER_THAN, firstNonSavepointCommit.get().getCompletionTime());
  }

  @Override
  public Option getFirstNonSavepointCommit() {
    if (this.firstNonSavepointCommit == null) {
      synchronized (this) {
        if (this.firstNonSavepointCommit == null) {
          this.firstNonSavepointCommit =
              findFirstNonSavepointCommit(this.instants, instantComparator.requestedTimeOrderedComparator());
        }
      }
    }
    return this.firstNonSavepointCommit;
  }

  public Option getFirstNonSavepointCommitByCompletionTime() {
    if (this.firstNonSavepointCommitByCompletionTime == null) {
      synchronized (this) {
        if (this.firstNonSavepointCommitByCompletionTime == null) {
          // only consider completed instants that have completion time.
          this.firstNonSavepointCommitByCompletionTime =
              findFirstNonSavepointCommit(
                  this.instants.stream().filter(HoodieInstant::isCompleted).collect(Collectors.toList()),
                  instantComparator.completionTimeOrderedComparator());
        }
      }
    }
    return this.firstNonSavepointCommitByCompletionTime;
  }

  @Override
  public abstract Option getLastClusteringInstant();

  @Override
  public abstract Option getFirstPendingClusterInstant();

  @Override
  public abstract Option getLastPendingClusterInstant();

  @Override
  public abstract boolean isPendingClusteringInstant(String instantTime);

  @Override
  public Option getInstantDetails(HoodieInstant instant) {
    return details.apply(instant);
  }

  @Override
  public boolean isEmpty(HoodieInstant instant) {
    return getInstantDetails(instant).get().length == 0;
  }

  @Override
  public String toString() {
    return this.getClass().getName() + ": " + getInstantsAsStream().map(Object::toString).collect(Collectors.joining(","));
  }

  private Set getOrCreateInstantSet() {
    if (this.instantTimeSet == null) {
      synchronized (this) {
        if (this.instantTimeSet == null) {
          this.instantTimeSet = this.instants.stream().map(HoodieInstant::requestedTime).collect(Collectors.toSet());
        }
      }
    }
    return this.instantTimeSet;
  }

  /**
   * Returns the first non savepoint commit on the timeline.
   *
   * @param instants           list of instants to consider
   * @param instantComparator  the comparator to order instants
   * @return the first non savepoint commit on the timeline
   */
  private static Option findFirstNonSavepointCommit(
      List instants,
      Comparator instantComparator) {
    Set savepointTimestamps = instants.stream()
        .filter(entry -> entry.getAction().equals(HoodieTimeline.SAVEPOINT_ACTION))
        .map(HoodieInstant::requestedTime)
        .collect(Collectors.toSet());
    // There are chances that there could be holes in the timeline due to archival and savepoint interplay.
    // So, the first non-savepoint commit is considered as beginning of the active timeline.
    return Option.fromJavaOptional(instants.stream()
        .filter(entry -> !savepointTimestamps.contains(entry.requestedTime()))
        .min(instantComparator));
  }

  private void clearState() {
    instantTimeSet = null;
    firstNonSavepointCommit = null;
  }

  @Override
  public HoodieTimeline mergeTimeline(HoodieTimeline timeline) {
    Stream instantStream = Stream.concat(getInstantsAsStream(), timeline.getInstantsAsStream()).sorted();
    Function> details = instant -> {
      if (getInstantsAsStream().anyMatch(i -> i.equals(instant))) {
        return this.getInstantDetails(instant);
      } else {
        return timeline.getInstantDetails(instant);
      }
    };
    return factory.createDefaultTimeline(instantStream, details);
  }

  /**
   * Computes the timeline hash and returns.
   */
  private String computeTimelineHash(List instants) {
    MessageDigest md;
    try {
      md = MessageDigest.getInstance(HASHING_ALGORITHM);
      instants.forEach(i -> md
          .update(getUTF8Bytes(StringUtils.joinUsingDelim("_", i.requestedTime(), i.getAction(), i.getState().name()))));
    } catch (NoSuchAlgorithmException nse) {
      throw new HoodieException(nse);
    }
    return StringUtils.toHexString(md.digest());
  }

  /**
   * Merges the given instant list into one and keep the sequence.
   */
  private static List mergeInstants(List instants1, List instants2) {
    ValidationUtils.checkArgument(!instants1.isEmpty() && !instants2.isEmpty(), "The instants to merge can not be empty");
    // some optimizations are based on the assumption all the instant lists are already sorted.
    // skip when one list contains all the instants of the other one.
    List merged;
    if (InstantComparison.compareTimestamps(instants1.get(instants1.size() - 1).requestedTime(), LESSER_THAN_OR_EQUALS, instants2.get(0).requestedTime())) {
      merged = new ArrayList<>(instants1);
      merged.addAll(instants2);
    } else if (InstantComparison.compareTimestamps(instants2.get(instants2.size() - 1).requestedTime(), LESSER_THAN_OR_EQUALS, instants1.get(0).requestedTime())) {
      merged = new ArrayList<>(instants2);
      merged.addAll(instants1);
    } else {
      merged = new ArrayList<>(instants1);
      merged.addAll(instants2);
      // sort the instants explicitly
      Collections.sort(merged);
    }
    return merged;
  }
}