music: add opus base gain support

OPUS has another volume adjustment field on top of the existing R128
adjustments. I was under the impression this was handled by the android
system, but apparently not. This commit applies the base gain to files
by just adding them onto the existing ReplayGain values.

Resolves #521.

CHANGELOG.md

@@ -2,6 +2,9 @@
 
 ## dev
 
+#### What's Improved
+- The OPUS base volume adjustment field is now parsed and used in as a ReplayGain adjustment
+
 ## 3.3.0
 
 #### What's New

app/src/main/java/org/oxycblt/auxio/music/metadata/TagWorker.kt

@@ -20,6 +20,7 @@ package org.oxycblt.auxio.music.metadata
 
 import androidx.core.text.isDigitsOnly
 import androidx.media3.common.MediaItem
+import androidx.media3.common.MimeTypes
 import androidx.media3.exoplayer.MetadataRetriever
 import androidx.media3.exoplayer.source.MediaSource
 import androidx.media3.exoplayer.source.TrackGroupArray
@@ -97,6 +98,21 @@ private class TagWorkerImpl(
             val textTags = TextTags(metadata)
             populateWithId3v2(textTags.id3v2)
             populateWithVorbis(textTags.vorbis)
+
+            // If this metadata is of a vorbis file, we actually need to extract it's base gain
+            // and divide it by 256 to get the gain in decibels.
+            if (format.sampleMimeType == MimeTypes.AUDIO_OPUS
+                && format.initializationData.isNotEmpty()
+                && format.initializationData[0].size >= 18) {
+                val header = format.initializationData[0]
+                val gain = header[1].toInt() or ((header[0].toInt() shl 8) and 0xFF)
+                logD("Obtained opus base gain: ${gain / 256f} dB")
+                rawSong.replayGainTrackAdjustment =
+                    rawSong.replayGainTrackAdjustment?.plus(gain / 256f)
+                rawSong.replayGainAlbumAdjustment =
+                    rawSong.replayGainAlbumAdjustment?.plus(gain / 256f)
+            }
+
         } else {
             logD("No metadata could be extracted for ${rawSong.name}")
         }