Merge pull request #406 from ktsu-dev/fix/304-leading-tone-sharp-seven

[patch] Spell the raised seventh above a flat seventh as #7, not a lowered tonic

Author: matt-edmondson

Semantics.Music/Scale.cs

@@ -72,7 +72,9 @@ public ScaleDegree DegreeOf(PitchClass pitchClass)
 		}
 
 		// Chromatic: spell against the lower neighbour (raised/sharp) or the upper
-		// neighbour (lowered/flat), whichever is closer; tie prefers the flat.
+		// neighbour (lowered/flat), whichever is closer; tie prefers the flat, except
+		// above the last degree, where a flat would wrap to a lowered tonic and the
+		// raised last degree (the minor-key leading tone, #7) is the conventional reading.
 		int lowerIndex = -1;
 		for (int i = 0; i < offsets.Count; i++)
 		{
@@ -98,7 +100,12 @@ public ScaleDegree DegreeOf(PitchClass pitchClass)
 		int flatDegree = upperIndex >= 0 ? upperIndex + 1 : 1;
 		int flatAlteration = semitone - (upperIndex >= 0 ? offsets[upperIndex] : offsets[0] + 12);
 
-		return Math.Abs(flatAlteration) <= Math.Abs(sharpAlteration)
+		bool flatWraps = upperIndex < 0;
+		bool preferFlat = flatWraps
+			? Math.Abs(flatAlteration) < Math.Abs(sharpAlteration)
+			: Math.Abs(flatAlteration) <= Math.Abs(sharpAlteration);
+
+		return preferFlat
 			? new ScaleDegree(flatDegree, flatAlteration)
 			: new ScaleDegree(sharpDegree, sharpAlteration);
 	}

Semantics.Test/Music/KeyTests.cs

@@ -36,6 +36,23 @@ public void RomanNumeral_ChromaticGetsAccidental()
 		Assert.AreEqual("bII", CMajor.RomanNumeralOf(Chord.Parse("Db")));
 	}
 
+	[TestMethod]
+	public void RomanNumeral_MinorLeadingToneChordIsSharpSeven()
+	{
+		Key aMinor = Key.Parse("A aeolian");
+		Assert.AreEqual("#vii°", aMinor.RomanNumeralOf(Chord.Parse("G#dim")));
+		Assert.AreEqual("#vii°", Key.Parse("G mixolydian").RomanNumeralOf(Chord.Parse("F#dim")));
+	}
+
+	[TestMethod]
+	public void RomanNumeral_MinorLeadingToneChordRoundTrips()
+	{
+		Key aMinor = Key.Parse("A aeolian");
+		Chord leadingTone = Chord.Parse("G#dim");
+		Chord roundTripped = aMinor.ChordFromRomanNumeral(aMinor.RomanNumeralOf(leadingTone));
+		Assert.AreEqual(leadingTone, roundTripped);
+	}
+
 	[TestMethod]
 	public void RomanNumeral_DistinctDegreesNeverShareALabel()
 	{

Semantics.Test/Music/ScaleTests.cs

@@ -50,6 +50,17 @@ public void DegreeOf_SharpFourIsCloserToFourthRaised()
 		Assert.AreEqual(-1, tritone.Alteration);
 	}
 
+	[TestMethod]
+	public void DegreeOf_RaisedSeventhAboveAFlatSeventhIsSharpSeven()
+	{
+		// G# in A aeolian sits between the flat seventh (G) and the octave; it is the
+		// leading tone (#7), not a lowered tonic wrapped round the octave.
+		Scale aMinor = Scale.Create(PitchClass.Parse("A"), Mode.Aeolian);
+		ScaleDegree leadingTone = aMinor.DegreeOf(PitchClass.Parse("G#"));
+		Assert.AreEqual(7, leadingTone.Degree);
+		Assert.AreEqual(1, leadingTone.Alteration);
+	}
+
 	[TestMethod]
 	public void ToStringIsRootSpaceMode()
 	{