playback: add support for mp4 replaygain

Add support for MP4 ReplayGain tags. These are usually under a `----`
atom with an iTunes domain and ReplayGain description. These are
mapped to an ID3v2 internal frame within ExoPlayer, which is why
Auxio did not support them, as it only expected Vorbis comments and
ID3v2 TXXX frames.

Resolves #292.

CHANGELOG.md

@@ -16,6 +16,7 @@
 - Music loader now caches parsed metadata for faster load times
 - Redesigned icon
 	- Added animated splash screen on Android 12+
+- Added support for MP4 ReplayGain (`----`) atoms
 
 #### What's Improved
 - Sorting now takes accented characters into account

README.md

@@ -51,7 +51,7 @@ I primarily built Auxio for myself, but you can use it too, I guess.
   precise/original dates, sort tags, and more
 - SD Card-aware folder management
 - Reliable playback state persistence
-- Full ReplayGain support (On MP3, FLAC, OGG, OPUS, and (some) MP4 files)
+- Full ReplayGain support (On MP3, FLAC, OGG, OPUS, and MP4 files)
 - External equalizer support (ex. Wavelet)
 - Edge-to-edge
 - Embedded covers support

app/src/main/java/org/oxycblt/auxio/playback/replaygain/ReplayGainAudioProcessor.kt

@@ -23,6 +23,7 @@ import com.google.android.exoplayer2.audio.AudioProcessor
 import com.google.android.exoplayer2.audio.BaseAudioProcessor
 import com.google.android.exoplayer2.metadata.Metadata
 import com.google.android.exoplayer2.metadata.id3.TextInformationFrame
+import com.google.android.exoplayer2.metadata.id3.InternalFrame
 import com.google.android.exoplayer2.metadata.vorbis.VorbisComment
 import java.nio.ByteBuffer
 import kotlin.math.pow
@@ -129,9 +130,15 @@ class ReplayGainAudioProcessor(context: Context) : BaseAudioProcessor() {
                 // (like "replaygain_track_gain"), but can also be uppercase. Make sure that
                 // capitalization is consistent before continuing.
                 is TextInformationFrame -> {
-                    key = entry.description?.uppercase()
+                    key = entry.description
                     value = entry.values[0]
                 }
+                // Internal Frame. This is actually MP4's "----" atom, but mapped to an ID3v2
+                // frame by ExoPlayer (presumably to reduce duplication).
+                is InternalFrame -> {
+                    key = entry.description
+                    value = entry.text
+                }
                 // Vorbis comment. These are nearly always uppercase, so a check for such is
                 // skipped.
                 is VorbisComment -> {
@@ -148,14 +155,14 @@ class ReplayGainAudioProcessor(context: Context) : BaseAudioProcessor() {
 
         // Case 1: Normal ReplayGain, most commonly found on MPEG files.
         tags
-            .findLast { tag -> tag.key == RG_TRACK }
+            .findLast { tag -> tag.key.equals(RG_TRACK, ignoreCase = true) }
             ?.let { tag ->
                 trackGain = tag.value
                 found = true
             }
 
         tags
-            .findLast { tag -> tag.key == RG_ALBUM }
+            .findLast { tag -> tag.key.equals(RG_ALBUM, ignoreCase = true) }
             ?.let { tag ->
                 albumGain = tag.value
                 found = true
@@ -168,14 +175,14 @@ class ReplayGainAudioProcessor(context: Context) : BaseAudioProcessor() {
         // want to read it is to zero previous ReplayGain values for being invalid, however there
         // is no demand to fix that edge case right now.
         tags
-            .findLast { tag -> tag.key == R128_TRACK }
+            .findLast { tag -> tag.key.equals(R128_TRACK, ignoreCase = true) }
             ?.let { tag ->
                 trackGain += tag.value / 256f
                 found = true
             }
 
         tags
-            .findLast { tag -> tag.key == R128_ALBUM }
+            .findLast { tag -> tag.key.equals(R128_ALBUM, ignoreCase = true) }
             ?.let { tag ->
                 albumGain += tag.value / 256f
                 found = true
@@ -253,10 +260,10 @@ class ReplayGainAudioProcessor(context: Context) : BaseAudioProcessor() {
     }
 
     companion object {
-        private const val RG_TRACK = "REPLAYGAIN_TRACK_GAIN"
-        private const val RG_ALBUM = "REPLAYGAIN_ALBUM_GAIN"
-        private const val R128_TRACK = "R128_TRACK_GAIN"
-        private const val R128_ALBUM = "R128_ALBUM_GAIN"
+        private const val RG_TRACK = "replaygain_track_gain"
+        private const val RG_ALBUM = "replaygain_album_gain"
+        private const val R128_TRACK = "r128_track_gain"
+        private const val R128_ALBUM = "r128_album_gain"
 
         private val REPLAY_GAIN_TAGS = arrayOf(RG_TRACK, RG_ALBUM, R128_ALBUM, R128_TRACK)
     }