MP3: Assume an Info header indicates CBR for seeking purposes

The seek table in a Xing/Info header is very imprecise (max resolution
of 255 to describe each of 100 byte positions in the file). Seeking
using a constant bitrate assumption is more accurate, especially for
longer files (which exacerbates the imprecision of the Info header).

VBR files should contain an Xing header, while an Info header is
identical but indicates the file is CBR.

Issue: androidx/media#878
PiperOrigin-RevId: 597827891

RELEASENOTES.md

@@ -55,6 +55,10 @@
         `ColorInfo.colorSpace`, `ColorInfo.colorTransfer`, and
         `ColorInfo.colorRange` values
         ([#692](https://github.com/androidx/media/pull/692)).
+    *   MP3: Use constant bitrate (CBR) seeking for files with an `Info` header
+        (the CBR equivalent of the `Xing` header). Previously we used the seek
+        table from the `Info` header, but this results in less precise seeking
+        than if we ignore it and assume the file is CBR.
 *   Audio:
 *   Video:
     *   Change the `MediaCodecVideoRenderer` constructor that takes a

libraries/extractor/src/main/java/androidx/media3/extractor/mp3/Mp3Extractor.java

@@ -524,11 +524,23 @@ public final class Mp3Extractor implements Extractor {
           gaplessInfoHolder.encoderDelay = xingFrame.encoderDelay;
           gaplessInfoHolder.encoderPadding = xingFrame.encoderPadding;
         }
-        seeker = XingSeeker.create(input.getLength(), xingFrame, input.getPosition());
+        long startPosition = input.getPosition();
         input.skipFully(synchronizedHeader.frameSize);
-        if (seeker != null && !seeker.isSeekable() && seekHeader == SEEK_HEADER_INFO) {
+        // An Xing frame indicates the file is VBR (so we have to use the seek header for seeking)
-          // Fall back to constant bitrate seeking for Info headers missing a table of contents.
+        // while an Info header indicates the file is CBR, in which case ConstantBitrateSeeker will
-          return getConstantBitrateSeeker(input, /* allowSeeksIfLengthUnknown= */ false);
+        // give more accurate seeking than the low-resolution seek table in the Info header. We can
+        // still use the length from the Info frame if we don't know the stream length directly.
+        if (seekHeader == SEEK_HEADER_XING) {
+          seeker = XingSeeker.create(input.getLength(), xingFrame, startPosition);
+        } else { // seekHeader == SEEK_HEADER_INFO
+          long streamLength =
+              xingFrame.dataSize != C.LENGTH_UNSET
+                  ? startPosition + xingFrame.dataSize
+                  : C.LENGTH_UNSET;
+          // TODO: b/319235116 - Consider using the duration derived from the Xing/Info frame when
+          // it considers encoding delay and padding.
+          seeker =
+              getConstantBitrateSeeker(input, streamLength, /* allowSeeksIfLengthUnknown= */ false);
         }
         break;
       case SEEK_HEADER_VBRI:
@@ -548,11 +560,26 @@ public final class Mp3Extractor implements Extractor {
   /** Peeks the next frame and returns a {@link ConstantBitrateSeeker} based on its bitrate. */
   private Seeker getConstantBitrateSeeker(ExtractorInput input, boolean allowSeeksIfLengthUnknown)
       throws IOException {
+    return getConstantBitrateSeeker(input, C.LENGTH_UNSET, allowSeeksIfLengthUnknown);
+  }
+
+  /**
+   * Peeks the next frame and returns a {@link ConstantBitrateSeeker} based on its bitrate. {@code
+   * streamLengthFallback} is used if {@link ExtractorInput#getLength() input.getLength()} is {@link
+   * C#LENGTH_UNSET}. {@code streamLengthFallback} may also be {@link C#LENGTH_UNSET} to indicate
+   * the length is unknown.
+   */
+  private Seeker getConstantBitrateSeeker(
+      ExtractorInput input, long streamLengthFallback, boolean allowSeeksIfLengthUnknown)
+      throws IOException {
     input.peekFully(scratch.getData(), 0, 4);
     scratch.setPosition(0);
     synchronizedHeader.setForHeaderData(scratch.readInt());
     return new ConstantBitrateSeeker(
-        input.getLength(), input.getPosition(), synchronizedHeader, allowSeeksIfLengthUnknown);
+        input.getLength() != C.LENGTH_UNSET ? input.getLength() : streamLengthFallback,
+        input.getPosition(),
+        synchronizedHeader,
+        allowSeeksIfLengthUnknown);
   }
 
   @EnsuresNonNull({"extractorOutput", "realTrackOutput"})

libraries/test_data/src/test/assets/extractordumps/mp3/test-cbr-info-header.mp3.0.dump

@@ -2,9 +2,9 @@ seekMap:
   isSeekable = true
   duration = 1044875
   getPosition(0) = [[timeUs=0, position=227]]
-  getPosition(1) = [[timeUs=1, position=227]]
+  getPosition(1) = [[timeUs=0, position=227], [timeUs=26000, position=435]]
-  getPosition(522437) = [[timeUs=522437, position=4582]]
+  getPosition(522437) = [[timeUs=520000, position=4387], [timeUs=546000, position=4595]]
-  getPosition(1044875) = [[timeUs=1044875, position=8585]]
+  getPosition(1044875) = [[timeUs=1018875, position=8378]]
 numberOfTracks = 1
 track 0:
   total output bytes = 8359

libraries/test_data/src/test/assets/extractordumps/mp3/test-cbr-info-header.mp3.1.dump

@@ -2,13 +2,13 @@ seekMap:
   isSeekable = true
   duration = 1044875
   getPosition(0) = [[timeUs=0, position=227]]
-  getPosition(1) = [[timeUs=1, position=227]]
+  getPosition(1) = [[timeUs=0, position=227], [timeUs=26000, position=435]]
-  getPosition(522437) = [[timeUs=522437, position=4582]]
+  getPosition(522437) = [[timeUs=520000, position=4387], [timeUs=546000, position=4595]]
-  getPosition(1044875) = [[timeUs=1044875, position=8585]]
+  getPosition(1044875) = [[timeUs=1018875, position=8378]]
 numberOfTracks = 1
 track 0:
-  total output bytes = 5434
+  total output bytes = 5643
-  sample count = 26
+  sample count = 27
   format 0:
     sampleMimeType = audio/mpeg
     maxInputSize = 4096
@@ -18,107 +18,111 @@ track 0:
     encoderPadding = 1404
     metadata = entries=[TSSE: description=null: values=[Lavf58.45.100]]
   sample 0:
-    time = 344808
+    time = 339500
+    flags = 1
+    data = length 209, hash 6CCBBB3B
+  sample 1:
+    time = 365622
     flags = 1
     data = length 209, hash 34191E1
-  sample 1:
+  sample 2:
-    time = 370930
+    time = 391744
     flags = 1
     data = length 209, hash 57323ED7
-  sample 2:
+  sample 3:
-    time = 397052
+    time = 417867
     flags = 1
     data = length 209, hash 75618CF3
-  sample 3:
+  sample 4:
-    time = 423175
+    time = 443989
     flags = 1
     data = length 209, hash 784C973B
-  sample 4:
+  sample 5:
-    time = 449297
+    time = 470112
     flags = 1
     data = length 209, hash 49106390
-  sample 5:
+  sample 6:
-    time = 475420
+    time = 496234
     flags = 1
     data = length 209, hash 70F6A563
-  sample 6:
+  sample 7:
-    time = 501542
+    time = 522357
     flags = 1
     data = length 209, hash 721882B0
-  sample 7:
+  sample 8:
-    time = 527665
+    time = 548479
     flags = 1
     data = length 209, hash 81C62AEE
-  sample 8:
+  sample 9:
-    time = 553787
+    time = 574602
     flags = 1
     data = length 209, hash 16D22463
-  sample 9:
+  sample 10:
-    time = 579910
+    time = 600724
     flags = 1
     data = length 209, hash 47033534
-  sample 10:
+  sample 11:
-    time = 606032
+    time = 626846
     flags = 1
     data = length 209, hash CECB37A6
-  sample 11:
+  sample 12:
-    time = 632154
+    time = 652969
     flags = 1
     data = length 209, hash 6C9C307B
-  sample 12:
+  sample 13:
-    time = 658277
+    time = 679091
     flags = 1
     data = length 209, hash 3EB1A364
-  sample 13:
+  sample 14:
-    time = 684399
+    time = 705214
     flags = 1
     data = length 209, hash 30962500
-  sample 14:
+  sample 15:
-    time = 710522
+    time = 731336
     flags = 1
     data = length 209, hash 2C5CCBB7
-  sample 15:
+  sample 16:
-    time = 736644
+    time = 757459
     flags = 1
     data = length 209, hash F9CB9E37
-  sample 16:
+  sample 17:
-    time = 762767
+    time = 783581
     flags = 1
     data = length 209, hash F75BC8C0
-  sample 17:
+  sample 18:
-    time = 788889
+    time = 809704
     flags = 1
     data = length 209, hash D00ED607
-  sample 18:
+  sample 19:
-    time = 815012
+    time = 835826
     flags = 1
     data = length 209, hash B4338395
-  sample 19:
+  sample 20:
-    time = 841134
+    time = 861948
     flags = 1
     data = length 209, hash E3E838A0
-  sample 20:
+  sample 21:
-    time = 867256
+    time = 888071
     flags = 1
     data = length 209, hash 2B0CF78
-  sample 21:
+  sample 22:
-    time = 893379
+    time = 914193
     flags = 1
     data = length 209, hash 31906FA9
-  sample 22:
+  sample 23:
-    time = 919501
+    time = 940316
     flags = 1
     data = length 209, hash C92FC08F
-  sample 23:
+  sample 24:
-    time = 945624
+    time = 966438
     flags = 1
     data = length 209, hash 7C89994
-  sample 24:
+  sample 25:
-    time = 971746
+    time = 992561
     flags = 1
     data = length 209, hash EC37743B
-  sample 25:
+  sample 26:
-    time = 997869
+    time = 1018683
     flags = 1
     data = length 209, hash C974F6FB
 tracksEnded = true

libraries/test_data/src/test/assets/extractordumps/mp3/test-cbr-info-header.mp3.2.dump

@@ -2,13 +2,13 @@ seekMap:
   isSeekable = true
   duration = 1044875
   getPosition(0) = [[timeUs=0, position=227]]
-  getPosition(1) = [[timeUs=1, position=227]]
+  getPosition(1) = [[timeUs=0, position=227], [timeUs=26000, position=435]]
-  getPosition(522437) = [[timeUs=522437, position=4582]]
+  getPosition(522437) = [[timeUs=520000, position=4387], [timeUs=546000, position=4595]]
-  getPosition(1044875) = [[timeUs=1044875, position=8585]]
+  getPosition(1044875) = [[timeUs=1018875, position=8378]]
 numberOfTracks = 1
 track 0:
-  total output bytes = 2717
+  total output bytes = 2926
-  sample count = 13
+  sample count = 14
   format 0:
     sampleMimeType = audio/mpeg
     maxInputSize = 4096
@@ -18,55 +18,59 @@ track 0:
     encoderPadding = 1404
     metadata = entries=[TSSE: description=null: values=[Lavf58.45.100]]
   sample 0:
-    time = 679168
+    time = 679125
+    flags = 1
+    data = length 209, hash 3EB1A364
+  sample 1:
+    time = 705247
     flags = 1
     data = length 209, hash 30962500
-  sample 1:
+  sample 2:
-    time = 705290
+    time = 731369
     flags = 1
     data = length 209, hash 2C5CCBB7
-  sample 2:
+  sample 3:
-    time = 731412
+    time = 757492
     flags = 1
     data = length 209, hash F9CB9E37
-  sample 3:
+  sample 4:
-    time = 757535
+    time = 783614
     flags = 1
     data = length 209, hash F75BC8C0
-  sample 4:
+  sample 5:
-    time = 783657
+    time = 809737
     flags = 1
     data = length 209, hash D00ED607
-  sample 5:
+  sample 6:
-    time = 809780
+    time = 835859
     flags = 1
     data = length 209, hash B4338395
-  sample 6:
+  sample 7:
-    time = 835902
+    time = 861982
     flags = 1
     data = length 209, hash E3E838A0
-  sample 7:
+  sample 8:
-    time = 862025
+    time = 888104
     flags = 1
     data = length 209, hash 2B0CF78
-  sample 8:
+  sample 9:
-    time = 888147
+    time = 914227
     flags = 1
     data = length 209, hash 31906FA9
-  sample 9:
+  sample 10:
-    time = 914270
+    time = 940349
     flags = 1
     data = length 209, hash C92FC08F
-  sample 10:
+  sample 11:
-    time = 940392
+    time = 966471
     flags = 1
     data = length 209, hash 7C89994
-  sample 11:
+  sample 12:
-    time = 966514
+    time = 992594
     flags = 1
     data = length 209, hash EC37743B
-  sample 12:
+  sample 13:
-    time = 992637
+    time = 1018716
     flags = 1
     data = length 209, hash C974F6FB
 tracksEnded = true

libraries/test_data/src/test/assets/extractordumps/mp3/test-cbr-info-header.mp3.3.dump

@@ -2,9 +2,9 @@ seekMap:
   isSeekable = true
   duration = 1044875
   getPosition(0) = [[timeUs=0, position=227]]
-  getPosition(1) = [[timeUs=1, position=227]]
+  getPosition(1) = [[timeUs=0, position=227], [timeUs=26000, position=435]]
-  getPosition(522437) = [[timeUs=522437, position=4582]]
+  getPosition(522437) = [[timeUs=520000, position=4387], [timeUs=546000, position=4595]]
-  getPosition(1044875) = [[timeUs=1044875, position=8585]]
+  getPosition(1044875) = [[timeUs=1018875, position=8378]]
 numberOfTracks = 1
 track 0:
   total output bytes = 0

libraries/test_data/src/test/assets/extractordumps/mp3/test-cbr-info-header.mp3.unknown_length.dump

@@ -2,9 +2,9 @@ seekMap:
   isSeekable = true
   duration = 1044875
   getPosition(0) = [[timeUs=0, position=227]]
-  getPosition(1) = [[timeUs=1, position=227]]
+  getPosition(1) = [[timeUs=0, position=227], [timeUs=26000, position=435]]
-  getPosition(522437) = [[timeUs=522437, position=4582]]
+  getPosition(522437) = [[timeUs=520000, position=4387], [timeUs=546000, position=4595]]
-  getPosition(1044875) = [[timeUs=1044875, position=8585]]
+  getPosition(1044875) = [[timeUs=1018875, position=8378]]
 numberOfTracks = 1
 track 0:
   total output bytes = 8359