Account for missing preroll when converting adPodIndex to adGroupIndex

IMA always starts midrolls at index 1. So if there is no preroll ad,
the ad group index in AdPlaybackState is off by 1 all the time, and
may also lead to ArrayIndexOutOfBoundsExceptions when trying to access
the last midroll ad

Issue: #1741
PiperOrigin-RevId: 682324368

RELEASENOTES.md

@@ -82,6 +82,9 @@
 *   Effect:
 *   Muxers:
 *   IMA extension:
+    *   Fix bug where server-side inserted DAI streams without a preroll can
+        result in an `ArrayIndexOutOfBoundsException` when playing past the last
+        midroll ([#1741](https://github.com/androidx/media/issues/1741)).
 *   Session:
     *   Fix bug that caused custom commands sent from a `MediaBrowser` being
         dispatched to the `MediaSessionCompat.Callback` instead of the

libraries/exoplayer_ima/src/main/java/androidx/media3/exoplayer/ima/ImaServerSideAdInsertionMediaSource.java

@@ -897,9 +897,7 @@ private static long getAdDuration(double startTimeSeconds, double endTimeSeconds
 
   private static AdPlaybackState setVodAdInPlaceholder(Ad ad, AdPlaybackState adPlaybackState) {
     AdPodInfo adPodInfo = ad.getAdPodInfo();
-    // Handle post rolls that have a podIndex of -1.
-    int adGroupIndex =
-        adPodInfo.getPodIndex() == -1 ? adPlaybackState.adGroupCount - 1 : adPodInfo.getPodIndex();
+    int adGroupIndex = getAdGroupIndexFromAdPodInfo(adPodInfo, adPlaybackState);
     AdPlaybackState.AdGroup adGroup = adPlaybackState.getAdGroup(adGroupIndex);
     int adIndexInAdGroup = adPodInfo.getAdPosition() - 1;
     if (adGroup.count < adPodInfo.getTotalAds()) {
@@ -924,12 +922,27 @@ private static AdPlaybackState setVodAdInPlaceholder(Ad ad, AdPlaybackState adPl
 
   private static AdPlaybackState skipAd(Ad ad, AdPlaybackState adPlaybackState) {
     AdPodInfo adPodInfo = ad.getAdPodInfo();
-    int adGroupIndex = adPodInfo.getPodIndex();
+    int adGroupIndex = getAdGroupIndexFromAdPodInfo(adPodInfo, adPlaybackState);
     // IMA SDK always returns index starting at 1.
     int adIndexInAdGroup = adPodInfo.getAdPosition() - 1;
     return adPlaybackState.withSkippedAd(adGroupIndex, adIndexInAdGroup);
   }
 
+  private static int getAdGroupIndexFromAdPodInfo(
+      AdPodInfo adPodInfo, AdPlaybackState adPlaybackState) {
+    int adPodIndex = adPodInfo.getPodIndex();
+    if (adPodIndex == -1) {
+      // Post-roll
+      return adPlaybackState.adGroupCount - 1;
+    }
+    if (adPlaybackState.getAdGroup(0).timeUs == 0) {
+      // When a pre-roll exists, the index starts at zero.
+      return adPodIndex;
+    }
+    // Mid-rolls always start at 1.
+    return adPodIndex - 1;
+  }
+
   private final class ComponentListener
       implements AdEvent.AdEventListener, Player.Listener, AdPlaybackStateUpdater {