gt:     if (trackGroups == null) {
context: /*
   * Copyright (C) 2016 The Android Open Source Project
   *
   * 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 com.google.android.exoplayer.com.google.android.exoplayer2.source;
  
  import com.google.android.exoplayer2.C;
  import com.google.android.exoplayer2.trackselection.TrackSelection;
  import com.google.android.exoplayer2.util.Assertions;
  import java.io.IOException;
  import java.util.ArrayList;
  import java.util.IdentityHashMap;
  
  /**
   * Merges multiple {@link MediaPeriod}s.
   */
  /* package */ final class MergingMediaPeriod implements MediaPeriod, MediaPeriod.Callback {
  
    public final MediaPeriod[] periods;
  
    private final IdentityHashMap<SampleStream, Integer> streamPeriodIndices;
  
    private Callback callback;
    private int pendingChildPrepareCount;
    private TrackGroupArray trackGroups;
  
    private MediaPeriod[] enabledPeriods;
    private SequenceableLoader sequenceableLoader;
  
    public MergingMediaPeriod(MediaPeriod... periods) {
      this.periods = periods;
      streamPeriodIndices = new IdentityHashMap<>();
    }
  
    @Override
    public void prepare(Callback callback, long positionUs) {
      this.callback = callback;
      pendingChildPrepareCount = periods.length;
      for (MediaPeriod period : periods) {
        period.prepare(this, positionUs);
      }
    }
  
    @Override
    public void maybeThrowPrepareError() throws IOException {
      for (MediaPeriod period : periods) {
        period.maybeThrowPrepareError();
      }
    }
  
    @Override
    public TrackGroupArray getTrackGroups() {
      return trackGroups;
    }
  
    @Override
    public long selectTracks(TrackSelection[] selections, boolean[] mayRetainStreamFlags,
        SampleStream[] streams, boolean[] streamResetFlags, long positionUs) {
      // Map each selection and stream onto a child period index.
      int[] streamChildIndices = new int[selections.length];
      int[] selectionChildIndices = new int[selections.length];
      for (int i = 0; i < selections.length; i++) {
        streamChildIndices[i] = streams[i] == null ? C.INDEX_UNSET
            : streamPeriodIndices.get(streams[i]);
        selectionChildIndices[i] = C.INDEX_UNSET;
        if (selections[i] != null) {
          TrackGroup trackGroup = selections[i].getTrackGroup();
          for (int j = 0; j < periods.length; j++) {
            if (periods[j].getTrackGroups().indexOf(trackGroup) != C.INDEX_UNSET) {
              selectionChildIndices[i] = j;
              break;
            }
          }
        }
      }
      streamPeriodIndices.clear();
      // Select tracks for each child, copying the resulting streams back into a new streams array.
      SampleStream[] newStreams = new SampleStream[selections.length];
      SampleStream[] childStreams = new SampleStream[selections.length];
      TrackSelection[] childSelections = new TrackSelection[selections.length];
      ArrayList<MediaPeriod> enabledPeriodsList = new ArrayList<>(periods.length);
      for (int i = 0; i < periods.length; i++) {
        for (int j = 0; j < selections.length; j++) {
          childStreams[j] = streamChildIndices[j] == i ? streams[j] : null;
          childSelections[j] = selectionChildIndices[j] == i ? selections[j] : null;
        }
        long selectPositionUs = periods[i].selectTracks(childSelections, mayRetainStreamFlags,
            childStreams, streamResetFlags, positionUs);
        if (i == 0) {
          positionUs = selectPositionUs;
        } else if (selectPositionUs != positionUs) {
          throw new IllegalStateException("Children enabled at different positions");
        }
        boolean periodEnabled = false;
        for (int j = 0; j < selections.length; j++) {
          if (selectionChildIndices[j] == i) {
            // Assert that the child provided a stream for the selection.
            Assertions.checkState(childStreams[j] != null);
            newStreams[j] = childStreams[j];
            periodEnabled = true;
            streamPeriodIndices.put(childStreams[j], i);
          } else if (streamChildIndices[j] == i) {
            // Assert that the child cleared any previous stream.
            Assertions.checkState(childStreams[j] == null);
          }
        }
        if (periodEnabled) {
          enabledPeriodsList.add(periods[i]);
        }
      }
      // Copy the new streams back into the streams array.
      System.arraycopy(newStreams, 0, streams, 0, newStreams.length);
      // Update the local state.
      enabledPeriods = new MediaPeriod[enabledPeriodsList.size()];
      enabledPeriodsList.toArray(enabledPeriods);
      sequenceableLoader = new CompositeSequenceableLoader(enabledPeriods);
      return positionUs;
    }
  
    @Override
    public void discardBuffer(long positionUs) {
      for (MediaPeriod period : enabledPeriods) {
        period.discardBuffer(positionUs);
      }
    }
  
    @Override
    public boolean continueLoading(long positionUs) {
      return sequenceableLoader.continueLoading(positionUs);
    }
  
    @Override
    public long getNextLoadPositionUs() {
      return sequenceableLoader.getNextLoadPositionUs();
    }
  
    @Override
    public long readDiscontinuity() {
      long positionUs = periods[0].readDiscontinuity();
      // Periods other than the first one are not allowed to report discontinuities.
      for (int i = 1; i < periods.length; i++) {
        if (periods[i].readDiscontinuity() != C.TIME_UNSET) {
          throw new IllegalStateException("Child reported discontinuity");
        }
      }
      // It must be possible to seek enabled periods to the new position, if there is one.
      if (positionUs != C.TIME_UNSET) {
        for (MediaPeriod enabledPeriod : enabledPeriods) {
          if (enabledPeriod != periods[0]
              && enabledPeriod.seekToUs(positionUs) != positionUs) {
            throw new IllegalStateException("Children seeked to different positions");
          }
        }
      }
      return positionUs;
    }
  
    @Override
    public long getBufferedPositionUs() {
      return sequenceableLoader.getBufferedPositionUs();
    }
  
    @Override
    public long seekToUs(long positionUs) {
      positionUs = enabledPeriods[0].seekToUs(positionUs);
      // Additional periods must seek to the same position.
      for (int i = 1; i < enabledPeriods.length; i++) {
        if (enabledPeriods[i].seekToUs(positionUs) != positionUs) {
          throw new IllegalStateException("Children seeked to different positions");
        }
      }
      return positionUs;
    }
  
    // MediaPeriod.Callback implementation
  
    @Override
    public void onPrepared(MediaPeriod ignored) {
      if (--pendingChildPrepareCount > 0) {
        return;
      }
      int totalTrackGroupCount = 0;
      for (MediaPeriod period : periods) {
        totalTrackGroupCount += period.getTrackGroups().length;
      }
      TrackGroup[] trackGroupArray = new TrackGroup[totalTrackGroupCount];
      int trackGroupIndex = 0;
      for (MediaPeriod period : periods) {
        TrackGroupArray periodTrackGroups = period.getTrackGroups();
        int periodTrackGroupCount = periodTrackGroups.length;
        for (int j = 0; j < periodTrackGroupCount; j++) {
          trackGroupArray[trackGroupIndex++] = periodTrackGroups.get(j);
        }
      }
      trackGroups = new TrackGroupArray(trackGroupArray);
      callback.onPrepared(this);
    }
  
    @Override
    public void onContinueLoadingRequested(MediaPeriod ignored) {
  