Keep window sequence number of playing period if queue is cleared.

This allows to keep the same window sequence number if playback fails and
is then retried without resetting the position.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188339509

library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java

@@ -691,7 +691,7 @@ import java.util.Collections;
       resetRendererPosition(periodPositionUs);
       maybeContinueLoading();
     } else {
-      queue.clear();
+      queue.clear(/* keepFrontPeriodUid= */ true);
       resetRendererPosition(periodPositionUs);
     }
 
@@ -776,7 +776,7 @@ import java.util.Collections;
       }
     }
     enabledRenderers = new Renderer[0];
-    queue.clear();
+    queue.clear(/* keepFrontPeriodUid= */ !resetPosition);
     setIsLoading(false);
     if (resetPosition) {
       pendingInitialSeekPosition = null;

library/core/src/main/java/com/google/android/exoplayer2/MediaPeriodQueue.java

@@ -52,6 +52,8 @@ import com.google.android.exoplayer2.util.Assertions;
   private MediaPeriodHolder reading;
   private MediaPeriodHolder loading;
   private int length;
+  private Object oldFrontPeriodUid;
+  private long oldFrontPeriodWindowSequenceNumber;
 
   /** Creates a new media period queue. */
   public MediaPeriodQueue() {
@@ -162,6 +164,7 @@ import com.google.android.exoplayer2.util.Assertions;
       Assertions.checkState(hasPlayingPeriod());
       loading.next = newPeriodHolder;
     }
+    oldFrontPeriodUid = null;
     loading = newPeriodHolder;
     length++;
     return newPeriodHolder.mediaPeriod;
@@ -276,12 +279,21 @@ import com.google.android.exoplayer2.util.Assertions;
     return removedReading;
   }
 
-  /** Clears the queue. */
-  public void clear() {
+  /**
+   * Clears the queue.
+   *
+   * @param keepFrontPeriodUid Whether the queue should keep the id of the media period in the front
+   *     of queue (typically the playing one) for later reuse.
+   */
+  public void clear(boolean keepFrontPeriodUid) {
     MediaPeriodHolder front = getFrontPeriod();
     if (front != null) {
+      oldFrontPeriodUid = keepFrontPeriodUid ? front.uid : null;
+      oldFrontPeriodWindowSequenceNumber = front.info.id.windowSequenceNumber;
       front.release();
       removeAfter(front);
+    } else if (!keepFrontPeriodUid) {
+      oldFrontPeriodUid = null;
     }
     playing = null;
     loading = null;
@@ -408,6 +420,17 @@ import com.google.android.exoplayer2.util.Assertions;
    */
   private long resolvePeriodIndexToWindowSequenceNumber(int periodIndex) {
     Object periodUid = timeline.getPeriod(periodIndex, period, /* setIds= */ true).uid;
+    int windowIndex = period.windowIndex;
+    if (oldFrontPeriodUid != null) {
+      int oldFrontPeriodIndex = timeline.getIndexOfPeriod(oldFrontPeriodUid);
+      if (oldFrontPeriodIndex != C.INDEX_UNSET) {
+        int oldFrontWindowIndex = timeline.getPeriod(oldFrontPeriodIndex, period).windowIndex;
+        if (oldFrontWindowIndex == windowIndex) {
+          // Try to match old front uid after the queue has been cleared.
+          return oldFrontPeriodWindowSequenceNumber;
+        }
+      }
+    }
     MediaPeriodHolder mediaPeriodHolder = getFrontPeriod();
     while (mediaPeriodHolder != null) {
       if (mediaPeriodHolder.uid.equals(periodUid)) {
@@ -416,7 +439,6 @@ import com.google.android.exoplayer2.util.Assertions;
       }
       mediaPeriodHolder = mediaPeriodHolder.next;
     }
-    int windowIndex = period.windowIndex;
     mediaPeriodHolder = getFrontPeriod();
     while (mediaPeriodHolder != null) {
       int indexOfHolderInTimeline = timeline.getIndexOfPeriod(mediaPeriodHolder.uid);