Parse opus gain correctly as a signed value

Issue: #7046
PiperOrigin-RevId: 303354941

RELEASENOTES.md

@@ -24,6 +24,8 @@
   in spherical playbacks
   ([#6761](https://github.com/google/ExoPlayer/issues/6761)).
 * FFmpeg extension: Add support for x86_64.
+* Opus extension: Fix parsing of negative gain values
+  ([#7046](https://github.com/google/ExoPlayer/issues/7046)).
 
 ### 2.11.3 (2020-02-19) ###
 

extensions/opus/src/androidTest/java/com/google/android/exoplayer2/ext/opus/OpusPlaybackTest.java

@@ -37,7 +37,9 @@ import org.junit.runner.RunWith;
 @RunWith(AndroidJUnit4.class)
 public class OpusPlaybackTest {
 
-  private static final String BEAR_OPUS_URI = "asset:///bear-opus.webm";
+  private static final String BEAR_OPUS_URI = "asset:///bear-opus.mka";
+  private static final String BEAR_OPUS_NEGATIVE_GAIN_URI =
+      "asset:///bear-opus-negative-gain.mka";
 
   @Before
   public void setUp() {
@@ -51,6 +53,11 @@ public class OpusPlaybackTest {
     playUri(BEAR_OPUS_URI);
   }
 
+  @Test
+  public void basicPlaybackNegativeGain() throws Exception {
+    playUri(BEAR_OPUS_NEGATIVE_GAIN_URI);
+  }
+
   private void playUri(String uri) throws Exception {
     TestPlaybackRunnable testPlaybackRunnable =
         new TestPlaybackRunnable(Uri.parse(uri), ApplicationProvider.getApplicationContext());

extensions/opus/src/main/java/com/google/android/exoplayer2/ext/opus/OpusDecoder.java

@@ -90,8 +90,8 @@ import java.util.List;
     if (channelCount > 8) {
       throw new OpusDecoderException("Invalid channel count: " + channelCount);
     }
-    int preskip = readLittleEndian16(headerBytes, 10);
-    int gain = readLittleEndian16(headerBytes, 16);
+    int preskip = readUnsignedLittleEndian16(headerBytes, 10);
+    int gain = readSignedLittleEndian16(headerBytes, 16);
 
     byte[] streamMap = new byte[8];
     int numStreams;
@@ -228,12 +228,16 @@ import java.util.List;
     return (int) (ns * SAMPLE_RATE / 1000000000);
   }
 
-  private static int readLittleEndian16(byte[] input, int offset) {
+  private static int readUnsignedLittleEndian16(byte[] input, int offset) {
     int value = input[offset] & 0xFF;
     value |= (input[offset + 1] & 0xFF) << 8;
     return value;
   }
 
+  private static int readSignedLittleEndian16(byte[] input, int offset) {
+    return (short) readUnsignedLittleEndian16(input, offset);
+  }
+
   private native long opusInit(int sampleRate, int channelCount, int numStreams, int numCoupled,
       int gain, byte[] streamMap);
   private native int opusDecode(long decoder, long timeUs, ByteBuffer inputBuffer, int inputSize,