Use 4 Byte size field in fMP4 mdat boxes

Since mdat box can be huge so there is a provision to use 64 bit size field.
In case of fragmented MP4, individual fragments should not have large mdat box
so a 32 bit size field should be sufficient.

PiperOrigin-RevId: 599219041

libraries/muxer/src/main/java/androidx/media3/muxer/FragmentedMp4Writer.java

@@ -17,6 +17,7 @@ package androidx.media3.muxer;
 
 import static androidx.media3.common.util.Assertions.checkArgument;
 import static androidx.media3.common.util.Assertions.checkNotNull;
+import static androidx.media3.muxer.Mp4Utils.UNSIGNED_INT_MAX_VALUE;
 import static java.lang.Math.max;
 import static java.lang.Math.min;
 
@@ -174,13 +175,10 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
 
   private void writeMdatBox() throws IOException {
     long mdatStartPosition = output.position();
-    // 4 bytes (indicating a 64-bit length field) + 4 bytes (box name) + 8 bytes (the actual length)
-    ByteBuffer header = ByteBuffer.allocate(16);
-    // This 32-bit integer in general contains the total length of the box. Here value 1 indicates
-    // that the actual length is stored as 64-bit integer after the box name.
-    header.putInt(1);
+    int mdatHeaderSize = 8; // 4 bytes (box size) + 4 bytes (box name)
+    ByteBuffer header = ByteBuffer.allocate(mdatHeaderSize);
+    header.putInt(mdatHeaderSize); // The total box size so far.
     header.put(Util.getUtf8Bytes("mdat"));
-    header.putLong(16); // The total box length so far.
     header.flip();
     output.write(header);
 
@@ -201,13 +199,15 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
 
     long currentPosition = output.position();
 
-    // Skip 4 bytes (64-bit length indication) + 4 bytes (box name).
-    output.position(mdatStartPosition + 8);
-    ByteBuffer mdatSize = ByteBuffer.allocate(8); // 64-bit length.
-    // Additional 4 bytes (64-bit length indication) + 4 bytes (box name) + 8 bytes (actual length).
-    mdatSize.putLong(bytesWritten + 16);
-    mdatSize.flip();
-    output.write(mdatSize);
+    output.position(mdatStartPosition);
+    ByteBuffer mdatSizeByteBuffer = ByteBuffer.allocate(4);
+    long mdatSize = bytesWritten + mdatHeaderSize;
+    checkArgument(
+        mdatSize <= UNSIGNED_INT_MAX_VALUE,
+        "Only 32-bit long mdat size supported in the fragmented MP4");
+    mdatSizeByteBuffer.putInt((int) mdatSize);
+    mdatSizeByteBuffer.flip();
+    output.write(mdatSizeByteBuffer);
     output.position(currentPosition);
   }
 

libraries/muxer/src/main/java/androidx/media3/muxer/MetadataCollector.java

@@ -17,6 +17,7 @@ package androidx.media3.muxer;
 
 import static androidx.media3.common.util.Assertions.checkArgument;
 import static androidx.media3.common.util.Assertions.checkState;
+import static androidx.media3.muxer.Mp4Utils.UNSIGNED_INT_MAX_VALUE;
 
 import androidx.media3.container.Mp4Util;
 import java.nio.ByteBuffer;
@@ -61,9 +62,9 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
   }
 
   public void setModificationTime(long unixTimestampMs) {
-    long maxUnsignedInt = 4294967295L;
     long timestampSeconds = Mp4Util.unixTimeToMp4TimeSeconds(unixTimestampMs);
-    checkArgument(timestampSeconds <= maxUnsignedInt, "Only 32-bit long timestamp supported");
+    checkArgument(
+        timestampSeconds <= UNSIGNED_INT_MAX_VALUE, "Only 32-bit long timestamp supported");
     this.modificationTimestampSeconds = (int) timestampSeconds;
   }
 }

libraries/muxer/src/main/java/androidx/media3/muxer/Mp4Utils.java

@@ -25,6 +25,9 @@ package androidx.media3.muxer;
    */
   public static final int MAX_FIXED_LEAF_BOX_SIZE = 200;
 
+  /** The maximum value of a 32-bit unsigned int. */
+  public static final long UNSIGNED_INT_MAX_VALUE = 4_294_967_295L;
+
   /**
    * The per-video timebase, used for durations in MVHD and TKHD even if the per-track timebase is
    * different (e.g. typically the sample rate for audio).

libraries/test_data/src/test/assets/muxerdumps/hdr10-720p.mp4_fragmented_box_structure.dump

@@ -71,7 +71,7 @@ moof (2852 bytes):
       Data = length 8, hash 94446F03
     trun (1684 bytes):
       Data = length 1676, hash 46E974DC
-mdat (5712395 bytes):
+mdat (5712387 bytes):
   Data = length 5712379, hash 86B2819D
 moof (1220 bytes):
   mfhd (16 bytes):
@@ -86,5 +86,5 @@ moof (1220 bytes):
       Data = length 8, hash 94446F03
     trun (688 bytes):
       Data = length 680, hash 4E3D2F16
-mdat (2364929 bytes):
+mdat (2364921 bytes):
   Data = length 2364913, hash D363A845