Search for TrueHD syncframes

In MatroskaExtractor TrueHD audio samples are joined into larger chunks. For
some streams the resulting chunked samples seem never to start with a syncframe.
This could result in playback of TrueHD streams getting stuck after seeking
because we could never read a syncframe at the start of a sample to determine
the sample size.

Instead of expecting to find a syncframe at the start of a sample, search for it
within the sample, to fix this issue.

Note: this means that we may search a few thousand bytes to find the sample
size, but the cost is only incurred for the first audio sample read.

Issue: #3845

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191775779

RELEASENOTES.md

@@ -36,6 +36,9 @@
     advancing ([#3841](https://github.com/google/ExoPlayer/issues/3841)).
   * Add an option to skip silent audio in `PlaybackParameters`
     ((#2635)[https://github.com/google/ExoPlayer/issues/2635]).
+  * Fix an issue where playback of TrueHD streams would get stuck after seeking
+    due to not finding a syncframe
+    ((#3845)[https://github.com/google/ExoPlayer/issues/3845]).
 * Caching:
   * Add release method to Cache interface.
   * Prevent multiple instances of SimpleCache in the same folder.

library/core/src/main/java/com/google/android/exoplayer2/audio/Ac3Util.java

@@ -99,7 +99,7 @@ public final class Ac3Util {
   /**
    * The number of bytes that must be parsed from a TrueHD syncframe to calculate the sample count.
    */
-  public static final int TRUEHD_SYNCFRAME_PREFIX_LENGTH = 12;
+  public static final int TRUEHD_SYNCFRAME_PREFIX_LENGTH = 10;
 
   /**
    * The number of new samples per (E-)AC-3 audio block.
@@ -463,6 +463,26 @@ public final class Ac3Util {
         : BLOCKS_PER_SYNCFRAME_BY_NUMBLKSCOD[(buffer.get(buffer.position() + 4) & 0x30) >> 4]);
   }
 
+  /**
+   * Returns the offset relative to the buffer's position of the start of a TrueHD syncframe, or
+   * {@link C#INDEX_UNSET} if no syncframe was found. The buffer's position is not modified.
+   *
+   * @param buffer The {@link ByteBuffer} within which to find a syncframe.
+   * @return The offset relative to the buffer's position of the start of a TrueHD syncframe, or
+   *     {@link C#INDEX_UNSET} if no syncframe was found.
+   */
+  public static int findTrueHdSyncframeOffset(ByteBuffer buffer) {
+    int startIndex = buffer.position();
+    int endIndex = buffer.limit() - TRUEHD_SYNCFRAME_PREFIX_LENGTH;
+    for (int i = startIndex; i <= endIndex; i++) {
+      // The syncword ends 0xBA for TrueHD or 0xBB for MLP.
+      if ((buffer.getInt(i + 4) & 0xFEFFFFFF) == 0xBA6F72F8) {
+        return i - startIndex;
+      }
+    }
+    return C.INDEX_UNSET;
+  }
+
   /**
    * Returns the number of audio samples represented by the given TrueHD syncframe, or 0 if the
    * buffer is not the start of a syncframe.
@@ -481,25 +501,22 @@ public final class Ac3Util {
         || (syncframe[7] & 0xFE) != 0xBA) {
       return 0;
     }
-    return 40 << (syncframe[8] & 7);
+    boolean isMlp = (syncframe[7] & 0xFF) == 0xBB;
+    return 40 << ((syncframe[isMlp ? 9 : 8] >> 4) & 0x07);
   }
 
   /**
-   * Reads the number of audio samples represented by the given TrueHD syncframe, or 0 if the buffer
-   * is not the start of a syncframe. The buffer's position is not modified.
+   * Reads the number of audio samples represented by a TrueHD syncframe. The buffer's position is
+   * not modified.
    *
-   * @param buffer The {@link ByteBuffer} from which to read the syncframe. Must have at least
-   *     {@link #TRUEHD_SYNCFRAME_PREFIX_LENGTH} bytes remaining.
-   * @return The number of audio samples represented by the syncframe, or 0 if the buffer is not the
-   *     start of a syncframe.
+   * @param buffer The {@link ByteBuffer} from which to read the syncframe.
+   * @param offset The offset of the start of the syncframe relative to the buffer's position.
+   * @return The number of audio samples represented by the syncframe.
    */
-  public static int parseTrueHdSyncframeAudioSampleCount(ByteBuffer buffer) {
+  public static int parseTrueHdSyncframeAudioSampleCount(ByteBuffer buffer, int offset) {
     // TODO: Link to specification if available.
-    // The syncword ends 0xBA for TrueHD or 0xBB for MLP.
-    if ((buffer.getInt(buffer.position() + 4) & 0xFEFFFFFF) != 0xBA6F72F8) {
-      return 0;
-    }
-    return 40 << (buffer.get(buffer.position() + 8) & 0x07);
+    boolean isMlp = (buffer.get(buffer.position() + offset + 7) & 0xFF) == 0xBB;
+    return 40 << ((buffer.get(buffer.position() + offset + (isMlp ? 9 : 8)) >> 4) & 0x07);
   }
 
   private static int getAc3SyncframeSize(int fscod, int frmsizecod) {

library/core/src/main/java/com/google/android/exoplayer2/audio/DefaultAudioSink.java

@@ -1076,8 +1076,11 @@ public final class DefaultAudioSink implements AudioSink {
     } else if (encoding == C.ENCODING_E_AC3) {
       return Ac3Util.parseEAc3SyncframeAudioSampleCount(buffer);
     } else if (encoding == C.ENCODING_DOLBY_TRUEHD) {
-      return Ac3Util.parseTrueHdSyncframeAudioSampleCount(buffer)
-          * Ac3Util.TRUEHD_RECHUNK_SAMPLE_COUNT;
+      int syncframeOffset = Ac3Util.findTrueHdSyncframeOffset(buffer);
+      return syncframeOffset == C.INDEX_UNSET
+          ? 0
+          : (Ac3Util.parseTrueHdSyncframeAudioSampleCount(buffer, syncframeOffset)
+              * Ac3Util.TRUEHD_RECHUNK_SAMPLE_COUNT);
     } else {
       throw new IllegalStateException("Unexpected audio encoding: " + encoding);
     }