Add AcoustID and R128 loudness tag support

- Add AcoustIdId and AcoustIdFingerprint properties for audio fingerprinting
- Add R128TrackGain and R128AlbumGain for EBU R 128 loudness normalization
- Implement in Tag base class, Id3v2Tag, and VorbisComment
- Update CopyTo method to include new fields
- Add comprehensive tests for all new properties

Author: decriptor

src/TagLibSharp2/Core/Tag.cs

@@ -526,6 +526,48 @@ public virtual string[] Composers {
 	/// </remarks>
 	public virtual string? ReplayGainAlbumPeak { get => null; set { } }
 
+	// R128 Loudness (EBU R 128)
+
+	/// <summary>
+	/// Gets or sets the R128 track gain value.
+	/// </summary>
+	/// <remarks>
+	/// <para>
+	/// EBU R 128 is a loudness normalization recommendation from the European Broadcasting Union.
+	/// The track gain is stored as an integer representing gain in 1/256th of a dB (Q7.8 format).
+	/// </para>
+	/// <para>
+	/// For example, a value of "256" represents +1 dB, "-512" represents -2 dB.
+	/// Some implementations store this as a plain dB string like "-14.00 dB".
+	/// </para>
+	/// <para>
+	/// In ID3v2, this is stored in a TXXX frame with description "R128_TRACK_GAIN".
+	/// In Vorbis Comments, this is stored as R128_TRACK_GAIN.
+	/// Not all tag formats support this field. Default implementation returns null.
+	/// </para>
+	/// </remarks>
+	public virtual string? R128TrackGain { get => null; set { } }
+
+	/// <summary>
+	/// Gets or sets the R128 album gain value.
+	/// </summary>
+	/// <remarks>
+	/// <para>
+	/// EBU R 128 is a loudness normalization recommendation from the European Broadcasting Union.
+	/// The album gain is stored as an integer representing gain in 1/256th of a dB (Q7.8 format).
+	/// </para>
+	/// <para>
+	/// For example, a value of "256" represents +1 dB, "-512" represents -2 dB.
+	/// Some implementations store this as a plain dB string like "-14.00 dB".
+	/// </para>
+	/// <para>
+	/// In ID3v2, this is stored in a TXXX frame with description "R128_ALBUM_GAIN".
+	/// In Vorbis Comments, this is stored as R128_ALBUM_GAIN.
+	/// Not all tag formats support this field. Default implementation returns null.
+	/// </para>
+	/// </remarks>
+	public virtual string? R128AlbumGain { get => null; set { } }
+
 	// MusicBrainz IDs
 
 	/// <summary>
@@ -652,6 +694,39 @@ public virtual string? MusicBrainzReleaseArtistId {
 	/// </remarks>
 	public virtual string? MusicBrainzReleaseCountry { get => null; set { } }
 
+	/// <summary>
+	/// Gets or sets the AcoustID identifier.
+	/// </summary>
+	/// <remarks>
+	/// <para>
+	/// A UUID identifying this audio track in the AcoustID database.
+	/// AcoustID uses audio fingerprints to identify tracks regardless of metadata.
+	/// </para>
+	/// <para>
+	/// In ID3v2, this is stored in a TXXX frame with description "ACOUSTID_ID".
+	/// In Vorbis Comments, this is stored as ACOUSTID_ID.
+	/// Not all tag formats support this field. Default implementation returns null.
+	/// </para>
+	/// </remarks>
+	public virtual string? AcoustIdId { get => null; set { } }
+
+	/// <summary>
+	/// Gets or sets the AcoustID audio fingerprint.
+	/// </summary>
+	/// <remarks>
+	/// <para>
+	/// The chromaprint audio fingerprint used by AcoustID for audio identification.
+	/// This is a compressed representation of the audio's acoustic characteristics.
+	/// </para>
+	/// <para>
+	/// In ID3v2, this is stored in a TXXX frame with description "ACOUSTID_FINGERPRINT".
+	/// In Vorbis Comments, this is stored as ACOUSTID_FINGERPRINT.
+	/// Fingerprints are typically generated by the chromaprint/fpcalc utility.
+	/// Not all tag formats support this field. Default implementation returns null.
+	/// </para>
+	/// </remarks>
+	public virtual string? AcoustIdFingerprint { get => null; set { } }
+
 	/// <summary>
 	/// Gets or sets the sort order for the composer name.
 	/// </summary>
@@ -950,7 +1025,7 @@ public void CopyTo (Tag target, TagCopyOptions options = TagCopyOptions.All)
 				target.ComposersSort = ComposersSort;
 		}
 
-		// ReplayGain
+		// ReplayGain and R128
 		if ((options & TagCopyOptions.ReplayGain) != 0) {
 			if (ReplayGainTrackGain is not null)
 				target.ReplayGainTrackGain = ReplayGainTrackGain;
@@ -960,6 +1035,10 @@ public void CopyTo (Tag target, TagCopyOptions options = TagCopyOptions.All)
 				target.ReplayGainAlbumGain = ReplayGainAlbumGain;
 			if (ReplayGainAlbumPeak is not null)
 				target.ReplayGainAlbumPeak = ReplayGainAlbumPeak;
+			if (R128TrackGain is not null)
+				target.R128TrackGain = R128TrackGain;
+			if (R128AlbumGain is not null)
+				target.R128AlbumGain = R128AlbumGain;
 		}
 
 		// MusicBrainz IDs
@@ -986,6 +1065,10 @@ public void CopyTo (Tag target, TagCopyOptions options = TagCopyOptions.All)
 				target.MusicBrainzReleaseType = MusicBrainzReleaseType;
 			if (MusicBrainzReleaseCountry is not null)
 				target.MusicBrainzReleaseCountry = MusicBrainzReleaseCountry;
+			if (AcoustIdId is not null)
+				target.AcoustIdId = AcoustIdId;
+			if (AcoustIdFingerprint is not null)
+				target.AcoustIdFingerprint = AcoustIdFingerprint;
 		}
 
 		// Pictures

src/TagLibSharp2/Id3/Id3v2/Id3v2Tag.cs

@@ -708,6 +708,18 @@ public override string? ReplayGainAlbumPeak {
 		set => SetUserText ("REPLAYGAIN_ALBUM_PEAK", value);
 	}
 
+	/// <inheritdoc/>
+	public override string? R128TrackGain {
+		get => GetUserText ("R128_TRACK_GAIN");
+		set => SetUserText ("R128_TRACK_GAIN", value);
+	}
+
+	/// <inheritdoc/>
+	public override string? R128AlbumGain {
+		get => GetUserText ("R128_ALBUM_GAIN");
+		set => SetUserText ("R128_ALBUM_GAIN", value);
+	}
+
 	/// <inheritdoc/>
 	public override string? MusicBrainzTrackId {
 		get => GetUserText ("MUSICBRAINZ_TRACKID");
@@ -802,6 +814,18 @@ public override string? MusicIpId {
 		set => SetUserText ("MusicIP PUID", value);
 	}
 
+	/// <inheritdoc/>
+	public override string? AcoustIdId {
+		get => GetUserText ("ACOUSTID_ID");
+		set => SetUserText ("ACOUSTID_ID", value);
+	}
+
+	/// <inheritdoc/>
+	public override string? AcoustIdFingerprint {
+		get => GetUserText ("ACOUSTID_FINGERPRINT");
+		set => SetUserText ("ACOUSTID_FINGERPRINT", value);
+	}
+
 	/// <summary>
 	/// Gets or sets the roles of the performers.
 	/// </summary>

src/TagLibSharp2/Xiph/VorbisComment.cs

@@ -487,6 +487,18 @@ public override string? ReplayGainAlbumPeak {
 		set => SetValue ("REPLAYGAIN_ALBUM_PEAK", value);
 	}
 
+	/// <inheritdoc/>
+	public override string? R128TrackGain {
+		get => GetValue ("R128_TRACK_GAIN");
+		set => SetValue ("R128_TRACK_GAIN", value);
+	}
+
+	/// <inheritdoc/>
+	public override string? R128AlbumGain {
+		get => GetValue ("R128_ALBUM_GAIN");
+		set => SetValue ("R128_ALBUM_GAIN", value);
+	}
+
 	/// <inheritdoc/>
 	public override string? MusicBrainzTrackId {
 		get => GetValue ("MUSICBRAINZ_TRACKID");
@@ -588,6 +600,18 @@ public override string? MusicIpId {
 		set => SetValue ("MUSICIP_PUID", value);
 	}
 
+	/// <inheritdoc/>
+	public override string? AcoustIdId {
+		get => GetValue ("ACOUSTID_ID");
+		set => SetValue ("ACOUSTID_ID", value);
+	}
+
+	/// <inheritdoc/>
+	public override string? AcoustIdFingerprint {
+		get => GetValue ("ACOUSTID_FINGERPRINT");
+		set => SetValue ("ACOUSTID_FINGERPRINT", value);
+	}
+
 	/// <inheritdoc/>
 #pragma warning disable CA1819 // Properties should not return arrays - TagLib# API compatibility
 	public override string[]? PerformersRole {

tests/TagLibSharp2.Tests/Id3/Id3v2/Id3v2TagExtendedTests.cs

@@ -188,6 +188,104 @@ public void MusicBrainz_AllFields_RenderAndReadBack ()
 
 	#endregion
 
+	#region R128 Loudness Tests
+
+	[TestMethod]
+	public void R128TrackGain_GetSet_RoundTrips ()
+	{
+		var tag = new Id3v2Tag ();
+		var gain = "-512"; // -2 dB in Q7.8 format
+
+		tag.R128TrackGain = gain;
+
+		Assert.AreEqual (gain, tag.R128TrackGain);
+		Assert.AreEqual (gain, tag.GetUserText ("R128_TRACK_GAIN"));
+	}
+
+	[TestMethod]
+	public void R128AlbumGain_GetSet_RoundTrips ()
+	{
+		var tag = new Id3v2Tag ();
+		var gain = "256"; // +1 dB in Q7.8 format
+
+		tag.R128AlbumGain = gain;
+
+		Assert.AreEqual (gain, tag.R128AlbumGain);
+		Assert.AreEqual (gain, tag.GetUserText ("R128_ALBUM_GAIN"));
+	}
+
+	[TestMethod]
+	public void R128_AllFields_RenderAndReadBack ()
+	{
+		var tag = new Id3v2Tag ();
+		tag.R128TrackGain = "-512";
+		tag.R128AlbumGain = "256";
+
+		var rendered = tag.Render ();
+		var readResult = Id3v2Tag.Read (rendered.Span);
+
+		Assert.IsTrue (readResult.IsSuccess);
+		Assert.AreEqual ("-512", readResult.Tag!.R128TrackGain);
+		Assert.AreEqual ("256", readResult.Tag.R128AlbumGain);
+	}
+
+	#endregion
+
+	#region AcoustID Tests
+
+	[TestMethod]
+	public void AcoustIdId_GetSet_RoundTrips ()
+	{
+		var tag = new Id3v2Tag ();
+		var acoustId = "f4e7c9d8-1234-5678-9abc-def012345678";
+
+		tag.AcoustIdId = acoustId;
+
+		Assert.AreEqual (acoustId, tag.AcoustIdId);
+		Assert.AreEqual (acoustId, tag.GetUserText ("ACOUSTID_ID"));
+	}
+
+	[TestMethod]
+	public void AcoustIdFingerprint_GetSet_RoundTrips ()
+	{
+		var tag = new Id3v2Tag ();
+		var fingerprint = "AQADtNQyRUkSRZEGAAAAAA";
+
+		tag.AcoustIdFingerprint = fingerprint;
+
+		Assert.AreEqual (fingerprint, tag.AcoustIdFingerprint);
+		Assert.AreEqual (fingerprint, tag.GetUserText ("ACOUSTID_FINGERPRINT"));
+	}
+
+	[TestMethod]
+	public void AcoustId_SetNull_RemovesField ()
+	{
+		var tag = new Id3v2Tag ();
+		tag.AcoustIdId = "f4e7c9d8-1234-5678-9abc-def012345678";
+
+		tag.AcoustIdId = null;
+
+		Assert.IsNull (tag.AcoustIdId);
+		Assert.IsNull (tag.GetUserText ("ACOUSTID_ID"));
+	}
+
+	[TestMethod]
+	public void AcoustId_AllFields_RenderAndReadBack ()
+	{
+		var tag = new Id3v2Tag ();
+		tag.AcoustIdId = "f4e7c9d8-1234-5678-9abc-def012345678";
+		tag.AcoustIdFingerprint = "AQADtNQyRUkSRZEGAAAAAA";
+
+		var rendered = tag.Render ();
+		var readResult = Id3v2Tag.Read (rendered.Span);
+
+		Assert.IsTrue (readResult.IsSuccess);
+		Assert.AreEqual ("f4e7c9d8-1234-5678-9abc-def012345678", readResult.Tag!.AcoustIdId);
+		Assert.AreEqual ("AQADtNQyRUkSRZEGAAAAAA", readResult.Tag.AcoustIdFingerprint);
+	}
+
+	#endregion
+
 	#region Combined Tests
 
 	[TestMethod]