MP3: Extract Xing/Info parsing code from XingSeeker

This means in a later change we can still use some of the info for CBR
files, even if we want to ignore the imprecise table of contents and
seek based on a constant bitrate assumption instead.

PiperOrigin-RevId: 597193997

libraries/extractor/src/main/java/androidx/media3/extractor/GaplessInfoHolder.java

@@ -52,23 +52,6 @@ public final class GaplessInfoHolder {
     encoderPadding = Format.NO_VALUE;
   }
 
-  /**
-   * Populates the holder with data from an MP3 Xing header, if valid and non-zero.
-   *
-   * @param value The 24-bit value to decode.
-   * @return Whether the holder was populated.
-   */
-  public boolean setFromXingHeaderValue(int value) {
-    int encoderDelay = value >> 12;
-    int encoderPadding = value & 0x0FFF;
-    if (encoderDelay > 0 || encoderPadding > 0) {
-      this.encoderDelay = encoderDelay;
-      this.encoderPadding = encoderPadding;
-      return true;
-    }
-    return false;
-  }
-
   /**
    * Populates the holder with data parsed from ID3 {@link Metadata}.
    *

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

@@ -517,16 +517,14 @@ public final class Mp3Extractor implements Extractor {
     switch (seekHeader) {
       case SEEK_HEADER_XING:
       case SEEK_HEADER_INFO:
-        seeker =
-            XingSeeker.create(input.getLength(), input.getPosition(), synchronizedHeader, frame);
-        if (seeker != null && !gaplessInfoHolder.hasGaplessInfo()) {
-          // If there is a Xing header, read gapless playback metadata at a fixed offset.
-          input.resetPeekPosition();
-          input.advancePeekPosition(xingBase + 141);
-          input.peekFully(scratch.getData(), 0, 3);
-          scratch.setPosition(0);
-          gaplessInfoHolder.setFromXingHeaderValue(scratch.readUnsignedInt24());
+        XingFrame xingFrame = XingFrame.parse(synchronizedHeader, frame);
+        if (!gaplessInfoHolder.hasGaplessInfo()
+            && xingFrame.encoderDelay != C.LENGTH_UNSET
+            && xingFrame.encoderPadding != C.LENGTH_UNSET) {
+          gaplessInfoHolder.encoderDelay = xingFrame.encoderDelay;
+          gaplessInfoHolder.encoderPadding = xingFrame.encoderPadding;
         }
+        seeker = XingSeeker.create(input.getLength(), xingFrame, input.getPosition());
         input.skipFully(synchronizedHeader.frameSize);
         if (seeker != null && !seeker.isSeekable() && seekHeader == SEEK_HEADER_INFO) {
           // Fall back to constant bitrate seeking for Info headers missing a table of contents.

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

@@ -0,0 +1,123 @@
+/*
+ * Copyright 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package androidx.media3.extractor.mp3;
+
+import androidx.annotation.Nullable;
+import androidx.media3.common.C;
+import androidx.media3.common.util.ParsableByteArray;
+import androidx.media3.extractor.MpegAudioUtil;
+
+/** Representation of a LAME Xing or Info frame. */
+/* package */ final class XingFrame {
+
+  private static final String TAG = "XingHeader";
+
+  /** The header of the Xing or Info frame. */
+  public final MpegAudioUtil.Header header;
+
+  /** The frame count, or {@link C#LENGTH_UNSET} if not present in the header. */
+  public final long frameCount;
+
+  /**
+   * Data size, including the XING frame, or {@link C#LENGTH_UNSET} if not present in the header.
+   */
+  public final long dataSize;
+
+  /**
+   * The number of samples to skip at the start of the stream, or {@link C#LENGTH_UNSET} if not
+   * present in the header.
+   */
+  public final int encoderDelay;
+
+  /**
+   * The number of samples to skip at the end of the stream, or {@link C#LENGTH_UNSET} if not
+   * present in the header.
+   */
+  public final int encoderPadding;
+
+  /**
+   * Entries are in the range [0, 255], but are stored as long integers for convenience. Null if the
+   * table of contents was missing from the header, in which case seeking is not be supported.
+   */
+  @Nullable public final long[] tableOfContents;
+
+  private XingFrame(
+      MpegAudioUtil.Header header,
+      long frameCount,
+      long dataSize,
+      @Nullable long[] tableOfContents,
+      int encoderDelay,
+      int encoderPadding) {
+    this.header = header;
+    this.frameCount = frameCount;
+    this.dataSize = dataSize;
+    this.tableOfContents = tableOfContents;
+    this.encoderDelay = encoderDelay;
+    this.encoderPadding = encoderPadding;
+  }
+
+  /**
+   * Returns a {@link XingFrame} containing the info parsed from a LAME Xing (VBR) or Info (CBR)
+   * frame.
+   *
+   * <p>The {@link ParsableByteArray#getPosition()} in {@code frame} when this method exits is
+   * undefined.
+   *
+   * @param mpegAudioHeader The MPEG audio header associated with the frame.
+   * @param frame The data in this audio frame, with its position set to immediately after the
+   *     'Xing' or 'Info' tag.
+   */
+  public static XingFrame parse(MpegAudioUtil.Header mpegAudioHeader, ParsableByteArray frame) {
+    int samplesPerFrame = mpegAudioHeader.samplesPerFrame;
+    int sampleRate = mpegAudioHeader.sampleRate;
+
+    int flags = frame.readInt();
+    int frameCount = (flags & 0x01) != 0 ? frame.readUnsignedIntToInt() : C.LENGTH_UNSET;
+    long dataSize = (flags & 0x02) != 0 ? frame.readUnsignedInt() : C.LENGTH_UNSET;
+
+    long[] tableOfContents;
+    if ((flags & 0x04) == 0x04) {
+      tableOfContents = new long[100];
+      for (int i = 0; i < 100; i++) {
+        tableOfContents[i] = frame.readUnsignedByte();
+      }
+    } else {
+      tableOfContents = null;
+    }
+
+    if ((flags & 0x8) != 0) {
+      frame.skipBytes(4); // Quality indicator
+    }
+
+    int encoderDelay;
+    int encoderPadding;
+    // Skip: version string (9), revision & VBR method (1), lowpass filter (1), replay gain (8),
+    //       encoding flags & ATH type (1), bitrate (1).
+    int bytesToSkipBeforeEncoderDelayAndPadding = 9 + 1 + 1 + 8 + 1 + 1;
+    if (frame.bytesLeft() >= bytesToSkipBeforeEncoderDelayAndPadding + 3) {
+      frame.skipBytes(bytesToSkipBeforeEncoderDelayAndPadding);
+      int encoderDelayAndPadding = frame.readUnsignedInt24();
+      encoderDelay = (encoderDelayAndPadding & 0xFFF000) >> 12;
+      encoderPadding = (encoderDelayAndPadding & 0xFFF);
+    } else {
+      encoderDelay = C.LENGTH_UNSET;
+      encoderPadding = C.LENGTH_UNSET;
+    }
+
+    return new XingFrame(
+        mpegAudioHeader, frameCount, dataSize, tableOfContents, encoderDelay, encoderPadding);
+  }
+}

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

@@ -19,9 +19,7 @@ import androidx.annotation.Nullable;
 import androidx.media3.common.C;
 import androidx.media3.common.util.Assertions;
 import androidx.media3.common.util.Log;
-import androidx.media3.common.util.ParsableByteArray;
 import androidx.media3.common.util.Util;
-import androidx.media3.extractor.MpegAudioUtil;
 import androidx.media3.extractor.SeekPoint;
 
 /** MP3 seeker that uses metadata from a Xing header. */
@@ -43,43 +41,33 @@ import androidx.media3.extractor.SeekPoint;
    *     information is not present.
    */
   @Nullable
-  public static XingSeeker create(
-      long inputLength,
-      long position,
-      MpegAudioUtil.Header mpegAudioHeader,
-      ParsableByteArray frame) {
-    int samplesPerFrame = mpegAudioHeader.samplesPerFrame;
-    int sampleRate = mpegAudioHeader.sampleRate;
-
-    int flags = frame.readInt();
-    int frameCount;
-    if ((flags & 0x01) != 0x01 || (frameCount = frame.readUnsignedIntToInt()) == 0) {
+  public static XingSeeker create(long inputLength, XingFrame xingFrame, long position) {
+    if (xingFrame.frameCount == C.LENGTH_UNSET && xingFrame.frameCount == 0) {
       // If the frame count is missing/invalid, the header can't be used to determine the duration.
       return null;
     }
+    // TODO: b/319235116 - Handle encoder delay and padding when calculating duration.
     // Audio requires both a start and end PCM sample, so subtract one from the sample count before
     // calculating the duration.
-    long durationUs = Util.sampleCountToDurationUs((frameCount * samplesPerFrame) - 1, sampleRate);
-    if ((flags & 0x06) != 0x06) {
+    long durationUs =
+        Util.sampleCountToDurationUs(
+            (xingFrame.frameCount * xingFrame.header.samplesPerFrame) - 1,
+            xingFrame.header.sampleRate);
+    if (xingFrame.dataSize == C.LENGTH_UNSET || xingFrame.tableOfContents == null) {
       // If the size in bytes or table of contents is missing, the stream is not seekable.
-      return new XingSeeker(position, mpegAudioHeader.frameSize, durationUs);
+      return new XingSeeker(position, xingFrame.header.frameSize, durationUs);
     }
 
-    long dataSize = frame.readUnsignedInt();
-    long[] tableOfContents = new long[100];
-    for (int i = 0; i < 100; i++) {
-      tableOfContents[i] = frame.readUnsignedByte();
-    }
-
-    // TODO: Handle encoder delay and padding in 3 bytes offset by xingBase + 213 bytes:
-    // delay = (frame.readUnsignedByte() << 4) + (frame.readUnsignedByte() >> 4);
-    // padding = ((frame.readUnsignedByte() & 0x0F) << 8) + frame.readUnsignedByte();
-
-    if (inputLength != C.LENGTH_UNSET && inputLength != position + dataSize) {
-      Log.w(TAG, "XING data size mismatch: " + inputLength + ", " + (position + dataSize));
+    if (inputLength != C.LENGTH_UNSET && inputLength != position + xingFrame.dataSize) {
+      Log.w(
+          TAG, "XING data size mismatch: " + inputLength + ", " + (position + xingFrame.dataSize));
     }
     return new XingSeeker(
-        position, mpegAudioHeader.frameSize, durationUs, dataSize, tableOfContents);
+        position,
+        xingFrame.header.frameSize,
+        durationUs,
+        xingFrame.dataSize,
+        xingFrame.tableOfContents);
   }
 
   private final long dataStartPosition;

libraries/extractor/src/test/java/androidx/media3/extractor/mp3/XingSeekerTest.java

@@ -64,15 +64,13 @@ public final class XingSeekerTest {
     seeker =
         XingSeeker.create(
             C.LENGTH_UNSET,
-            XING_FRAME_POSITION,
-            xingFrameHeader,
-            new ParsableByteArray(XING_FRAME_PAYLOAD));
+            XingFrame.parse(xingFrameHeader, new ParsableByteArray(XING_FRAME_PAYLOAD)),
+            XING_FRAME_POSITION);
     seekerWithInputLength =
         XingSeeker.create(
-            STREAM_LENGTH,
-            XING_FRAME_POSITION,
-            xingFrameHeader,
-            new ParsableByteArray(XING_FRAME_PAYLOAD));
+            C.LENGTH_UNSET,
+            XingFrame.parse(xingFrameHeader, new ParsableByteArray(XING_FRAME_PAYLOAD)),
+            XING_FRAME_POSITION);
     xingFrameSize = xingFrameHeader.frameSize;
   }