Fix sequence number calculation logic.

Issue: google/ExoPlayer#9697

Before, the `MAX_SEQUENCE_NUMBER` is 65535, such that the logic to get the next
sequence number:

`previousSeqNumber + 1 % MAX_SEQUENCE_NUMBER`

yields 0 when `previousSeqNumber` is 65534. However, the next sequence number
should be 65535.

PiperOrigin-RevId: 410530098

libraries/exoplayer_rtsp/src/main/java/androidx/media3/exoplayer/rtsp/RtpPacket.java

@@ -24,6 +24,7 @@ import androidx.media3.common.C;
 import androidx.media3.common.util.ParsableByteArray;
 import androidx.media3.common.util.UnstableApi;
 import androidx.media3.common.util.Util;
+import com.google.common.math.IntMath;
 import java.nio.ByteBuffer;
 
 /**
@@ -134,6 +135,16 @@ public final class RtpPacket {
   public static final int MAX_SEQUENCE_NUMBER = 0xFFFF;
   public static final int CSRC_SIZE = 4;
 
+  /** Returns the next sequence number of the {@code sequenceNumber}. */
+  public static int getNextSequenceNumber(int sequenceNumber) {
+    return IntMath.mod(sequenceNumber + 1, MAX_SEQUENCE_NUMBER + 1);
+  }
+
+  /** Returns the previous sequence number from the {@code sequenceNumber}. */
+  public static int getPreviousSequenceNumber(int sequenceNumber) {
+    return IntMath.mod(sequenceNumber - 1, MAX_SEQUENCE_NUMBER + 1);
+  }
+
   private static final byte[] EMPTY = new byte[0];
 
   /** The RTP version field (Word 0, bits 0-1), should always be 2. */

libraries/exoplayer_rtsp/src/main/java/androidx/media3/exoplayer/rtsp/RtpPacketReorderingQueue.java

@@ -34,8 +34,6 @@ import java.util.TreeSet;
   /** The maximum sequence number discontinuity allowed without resetting the re-ordering buffer. */
   @VisibleForTesting /* package */ static final int MAX_SEQUENCE_LEAP_ALLOWED = 1000;
 
-  private static final int MAX_SEQUENCE_NUMBER = RtpPacket.MAX_SEQUENCE_NUMBER;
-
   /** Queue size threshold for resetting the queue. 5000 packets equate about 7MB in buffer size. */
   private static final int QUEUE_SIZE_THRESHOLD_FOR_RESET = 5000;
 
@@ -96,13 +94,13 @@ import java.util.TreeSet;
     int packetSequenceNumber = packet.sequenceNumber;
     if (!started) {
       reset();
-      lastDequeuedSequenceNumber = prevSequenceNumber(packetSequenceNumber);
+      lastDequeuedSequenceNumber = RtpPacket.getPreviousSequenceNumber(packetSequenceNumber);
       started = true;
       addToQueue(new RtpPacketContainer(packet, receivedTimestampMs));
       return true;
     }
 
-    int expectedSequenceNumber = nextSequenceNumber(lastReceivedSequenceNumber);
+    int expectedSequenceNumber = RtpPacket.getNextSequenceNumber(lastReceivedSequenceNumber);
     // A positive shift means the packet succeeds the last received packet.
     int sequenceNumberShift =
         calculateSequenceNumberShift(packetSequenceNumber, expectedSequenceNumber);
@@ -114,7 +112,7 @@ import java.util.TreeSet;
       }
     } else {
       // Discard all previous received packets and start subsequent receiving from here.
-      lastDequeuedSequenceNumber = prevSequenceNumber(packetSequenceNumber);
+      lastDequeuedSequenceNumber = RtpPacket.getPreviousSequenceNumber(packetSequenceNumber);
       packetQueue.clear();
       addToQueue(new RtpPacketContainer(packet, receivedTimestampMs));
       return true;
@@ -140,7 +138,7 @@ import java.util.TreeSet;
     RtpPacketContainer packetContainer = packetQueue.first();
     int packetSequenceNumber = packetContainer.packet.sequenceNumber;
 
-    if (packetSequenceNumber == nextSequenceNumber(lastDequeuedSequenceNumber)
+    if (packetSequenceNumber == RtpPacket.getNextSequenceNumber(lastDequeuedSequenceNumber)
         || cutoffTimestampMs >= packetContainer.receivedTimestampMs) {
       packetQueue.pollFirst();
       lastDequeuedSequenceNumber = packetSequenceNumber;
@@ -168,16 +166,6 @@ import java.util.TreeSet;
     }
   }
 
-  private static int nextSequenceNumber(int sequenceNumber) {
-    return (sequenceNumber + 1) % MAX_SEQUENCE_NUMBER;
-  }
-
-  private static int prevSequenceNumber(int sequenceNumber) {
-    return sequenceNumber == 0
-        ? MAX_SEQUENCE_NUMBER - 1
-        : (sequenceNumber - 1) % MAX_SEQUENCE_NUMBER;
-  }
-
   /**
    * Calculates the sequence number shift, accounting for wrapping around.
    *
@@ -193,7 +181,7 @@ import java.util.TreeSet;
       int shift =
           min(sequenceNumber, previousSequenceNumber)
               - max(sequenceNumber, previousSequenceNumber)
-              + MAX_SEQUENCE_NUMBER;
+              + RtpPacket.MAX_SEQUENCE_NUMBER;
       // Check whether this is actually an wrap-over. For example, it is a wrap around if receiving
       // 65500 (prevSequenceNumber) after 1 (sequenceNumber); but it is not when prevSequenceNumber
       // is 30000.

libraries/exoplayer_rtsp/src/main/java/androidx/media3/exoplayer/rtsp/reader/RtpH264Reader.java

@@ -259,7 +259,7 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
       fuScratchBuffer.setPosition(1);
     } else {
       // Check that this packet is in the sequence of the previous packet.
-      int expectedSequenceNumber = (previousSequenceNumber + 1) % RtpPacket.MAX_SEQUENCE_NUMBER;
+      int expectedSequenceNumber = RtpPacket.getNextSequenceNumber(previousSequenceNumber);
       if (packetSequenceNumber != expectedSequenceNumber) {
         Log.w(
             TAG,

libraries/exoplayer_rtsp/src/test/java/androidx/media3/exoplayer/rtsp/RtpPacketTest.java

@@ -18,6 +18,8 @@ package androidx.media3.exoplayer.rtsp;
 
 import static androidx.media3.common.util.Assertions.checkNotNull;
 import static androidx.media3.common.util.Util.getBytesFromHexString;
+import static androidx.media3.exoplayer.rtsp.RtpPacket.getNextSequenceNumber;
+import static androidx.media3.exoplayer.rtsp.RtpPacket.getPreviousSequenceNumber;
 import static com.google.common.truth.Truth.assertThat;
 
 import androidx.media3.common.C;
@@ -184,4 +186,26 @@ public final class RtpPacketTest {
     builtPacket.writeToBuffer(builtPacketBytes, /* offset= */ 0, packetSize);
     assertThat(builtPacketBytes).isEqualTo(rtpDataWithLargeTimestamp);
   }
+
+  @Test
+  public void getNextSequenceNumber_invokingAtWrapOver() {
+    assertThat(getNextSequenceNumber(65534)).isEqualTo(65535);
+    assertThat(getNextSequenceNumber(65535)).isEqualTo(0);
+    assertThat(getNextSequenceNumber(0)).isEqualTo(1);
+  }
+
+  @Test
+  public void getPreviousSequenceNumber_invokingAtWrapOver() {
+    assertThat(getPreviousSequenceNumber(1)).isEqualTo(0);
+    assertThat(getPreviousSequenceNumber(0)).isEqualTo(65535);
+    assertThat(getPreviousSequenceNumber(65535)).isEqualTo(65534);
+  }
+
+  @Test
+  public void getSequenceNumber_isSymmetric() {
+    for (int i = 0; i < RtpPacket.MAX_SEQUENCE_NUMBER; i++) {
+      assertThat(getPreviousSequenceNumber(getNextSequenceNumber(i))).isEqualTo(i);
+      assertThat(getNextSequenceNumber(getPreviousSequenceNumber(i))).isEqualTo(i);
+    }
+  }
 }