[patch] Spell fully and half-diminished sevenths as vii°7 and viiø7

Semantics.Music/Key.cs

@@ -145,7 +145,7 @@ public string RomanNumeralOf(Chord chord)
 
 	/// <summary>Parses a roman-numeral function relative to this key back into a concrete chord.</summary>
 	/// <param name="numeral">
-	/// A roman numeral such as "Imaj7", "ii7", "V7", "bII", or "vii°7": an optional accidental prefix
+	/// A roman numeral such as "Imaj7", "ii7", "V7", "bII", "vii°7", or "viiø7": an optional accidental prefix
 	/// (b/♭ or #/♯), the degree numeral (upper-case major, lower-case minor), then a quality suffix.
 	/// </param>
 	/// <returns>The chord rooted at the resolved scale degree.</returns>
@@ -179,11 +179,16 @@ public Chord ChordFromRomanNumeral(string numeral)
 
 		PitchClass root = PitchClass.Create(pitchClasses[degree - 1].Value + alteration);
 
-		// Normalise the suffix into a chord-symbol body, mapping °/+ onto dim/aug and
-		// supplying a leading "m" for lower-case (minor) numerals.
+		// Normalise the suffix into a chord-symbol body, mapping °/+ onto dim/aug, passing ø
+		// through as half-diminished, and supplying a leading "m" for lower-case (minor) numerals.
 		string suffix = text[index..];
 		string quality;
-		if (suffix.StartsWith('°'))
+		if (suffix.StartsWith('ø'))
+		{
+			quality = "ø";
+			suffix = suffix[1..];
+		}
+		else if (suffix.StartsWith('°'))
 		{
 			quality = "dim";
 			suffix = suffix[1..];
@@ -234,14 +239,18 @@ private static string QualitySuffix(Chord chord)
 		string seventh = chord.Seventh switch
 		{
 			SeventhType.Major => "maj7",
-			SeventhType.Dominant => "7",
-			SeventhType.Diminished => chord.Quality == ChordQuality.Diminished ? "" : "7",
+			SeventhType.Dominant or SeventhType.Diminished => "7",
 			_ => "",
 		};
 
+		// A diminished triad under a minor seventh is half-diminished (viiø7), which has to be told
+		// apart from the fully diminished seventh (vii°7): ChordFromRomanNumeral reads ° as dim, so
+		// spelling both with ° would turn a half-diminished chord into a fully diminished one.
+		bool halfDiminished = chord.Quality == ChordQuality.Diminished && chord.Seventh == SeventhType.Dominant;
+
 		string quality = chord.Quality switch
 		{
-			ChordQuality.Diminished => "°",
+			ChordQuality.Diminished => halfDiminished ? "ø" : "°",
 			ChordQuality.Augmented => "+",
 			ChordQuality.Sus2 => "sus2",
 			ChordQuality.Sus4 => "sus4",

Semantics.Test/Music/RomanNumeralParseTests.cs

@@ -45,6 +45,28 @@ public void Parse_LeadingToneDiminishedSeventh()
 		Assert.AreEqual(SeventhType.Diminished, vii.Seventh);
 	}
 
+	[TestMethod]
+	public void Parse_LeadingToneHalfDiminishedSeventh()
+	{
+		Chord vii = CMajor.ChordFromRomanNumeral("viiø7");
+		Assert.AreEqual(11, vii.Root.Value); // B
+		Assert.AreEqual(Chord.Parse("Bm7b5"), vii);
+	}
+
+	[TestMethod]
+	public void RomanNumeralOf_KeepsTheSeventhOfAFullyDiminishedSeventh()
+	{
+		Assert.AreEqual("vii°", CMajor.RomanNumeralOf(Chord.Parse("Bdim")));
+		Assert.AreEqual("vii°7", CMajor.RomanNumeralOf(Chord.Parse("Bdim7")));
+	}
+
+	[TestMethod]
+	public void RomanNumeralOf_SpellsAHalfDiminishedSeventhWithØ()
+	{
+		Assert.AreEqual("viiø7", CMajor.RomanNumeralOf(Chord.Parse("Bm7b5")));
+		Assert.AreEqual(Chord.Parse("Bm7b5"), CMajor.ChordFromRomanNumeral(CMajor.RomanNumeralOf(Chord.Parse("Bm7b5"))));
+	}
+
 	[TestMethod]
 	public void Parse_FlatSixInMinor()
 	{
@@ -56,7 +78,7 @@ public void Parse_FlatSixInMinor()
 	[TestMethod]
 	public void Parse_IsInverseOfRomanNumeralOf()
 	{
-		string[] numerals = ["Imaj7", "ii7", "iii", "IV", "V7", "vi", "vii°"];
+		string[] numerals = ["Imaj7", "ii7", "iii", "IV", "V7", "vi", "vii°", "vii°7", "viiø7"];
 		foreach (string numeral in numerals)
 		{
 			Chord chord = CMajor.ChordFromRomanNumeral(numeral);