Count buffer ahead periods rather than using their indices.

This is in preparation for allowing MediaSources to skip over periods when one
period ends, which is needed for starting to play a multi-period live stream
that is concatenated on to another period at the live edge.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=128674659

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

@@ -612,6 +612,8 @@ import java.util.ArrayList;
     public boolean isReady;
     public boolean isEnded;
 
+    private int bufferAheadPeriodCount;
+
     private Period playingPeriod;
     private Period readingPeriod;
     private Period loadingPeriod;
@@ -693,6 +695,7 @@ import java.util.ArrayList;
 
         Period previousPeriod = playingPeriod;
         boolean seenReadingPeriod = false;
+        bufferAheadPeriodCount = 0;
         while (previousPeriod.nextPeriod != null) {
           Period period = previousPeriod.nextPeriod;
           index++;
@@ -716,6 +719,7 @@ import java.util.ArrayList;
             break;
           }
 
+          bufferAheadPeriodCount++;
           period.index = index;
           period.isLast = timeline.isFinal() && index == periodCount - 1;
           if (period == readingPeriod) {
@@ -729,6 +733,7 @@ import java.util.ArrayList;
         if (index == Timeline.NO_PERIOD_INDEX) {
           loadingPeriod.release();
           loadingPeriod = null;
+          bufferAheadPeriodCount = 0;
         } else {
           int periodCount = timeline.getPeriodCount();
           loadingPeriod.index = index;
@@ -758,8 +763,7 @@ import java.util.ArrayList;
 
       // Update the loading period.
       if (loadingPeriod == null || (loadingPeriod.isFullyBuffered() && !loadingPeriod.isLast
-          && (playingPeriod == null || loadingPeriod.index - playingPeriod.index
-              < MAXIMUM_BUFFER_AHEAD_PERIODS))) {
+          && bufferAheadPeriodCount < MAXIMUM_BUFFER_AHEAD_PERIODS)) {
         // Try to obtain the next period to start loading.
         int periodIndex = loadingPeriod == null ? playbackInfo.periodIndex
             : loadingPeriod.index + 1;
@@ -772,6 +776,7 @@ import java.util.ArrayList;
           if (loadingPeriod != null) {
             loadingPeriod.setNextPeriod(newPeriod);
           }
+          bufferAheadPeriodCount++;
           loadingPeriod = newPeriod;
           long startPositionUs = playingPeriod == null ? playbackInfo.positionUs : 0;
           setIsLoading(true);
@@ -802,6 +807,7 @@ import java.util.ArrayList;
         // reached the end of the playing period, so advance playback to the next period.
         playingPeriod.release();
         setPlayingPeriod(playingPeriod.nextPeriod);
+        bufferAheadPeriodCount--;
         playbackInfo = new PlaybackInfo(playingPeriod.index);
         updatePlaybackPositions();
         eventHandler.obtainMessage(MSG_PERIOD_CHANGED, playbackInfo).sendToTarget();
@@ -903,6 +909,7 @@ import java.util.ArrayList;
         period = period.nextPeriod;
       }
 
+      bufferAheadPeriodCount = 0;
       if (newPlayingPeriod != null) {
         newPlayingPeriod.nextPeriod = null;
         setPlayingPeriod(newPlayingPeriod);
@@ -957,6 +964,7 @@ import java.util.ArrayList;
         readingPeriod = playingPeriod;
         loadingPeriod = playingPeriod;
         playingPeriodEndPositionUs = C.UNSET_TIME_US;
+        bufferAheadPeriodCount = 0;
 
         // Update streams for the new selection, recreating all streams if reading ahead.
         boolean recreateStreams = readingPeriod != playingPeriod;
@@ -1000,6 +1008,7 @@ import java.util.ArrayList;
         while (period != null) {
           period.release();
           period = period.nextPeriod;
+          bufferAheadPeriodCount--;
         }
         loadingPeriod.nextPeriod = null;
         long positionUs = Math.max(0, internalPositionUs - loadingPeriod.offsetUs);
@@ -1017,6 +1026,7 @@ import java.util.ArrayList;
       readingPeriod = null;
       loadingPeriod = null;
       timeline = null;
+      bufferAheadPeriodCount = 0;
     }
 
     private void releasePeriodsFrom(Period period) {