From d39a43de4ada870e3c4126fbd5e24b257ade5757 Mon Sep 17 00:00:00 2001
From: Matthew Edmondson <matthew.edmondson@gmail.com>
Date: Mon, 28 Sep 2026 04:25:03 +0000
Subject: [PATCH] Parse "ionian" as the major mode, so a parsed Ionian key
 equals C major [patch]

Mode.TryParse built a mode named "ionian", which is not equal to Mode.Ionian
(an alias of Major), so a key parsed from "C ionian" was analyzed with the
minor-key tables. Aliases now map to their canonical mode when parsed.

Fixes ktsu-dev/Semantics#290

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PHrrj7bWzA48Gf3akqi476
---
 Semantics.Music/Mode.cs                       | 14 +++++++++++++-
 .../Music/ChromaticAnalysisTests.cs           | 15 +++++++++++++++
 Semantics.Test/Music/KeyTests.cs              |  7 +++++++
 Semantics.Test/Music/ModeTests.cs             | 19 +++++++++++++++++++
 4 files changed, 54 insertions(+), 1 deletion(-)

diff --git a/Semantics.Music/Mode.cs b/Semantics.Music/Mode.cs
index 681db413..0ab05e6b 100644
--- a/Semantics.Music/Mode.cs
+++ b/Semantics.Music/Mode.cs
@@ -14,7 +14,6 @@ public sealed record Mode
 	{
 		// Diatonic modes.
 		["major"] = [0, 2, 4, 5, 7, 9, 11],
-		["ionian"] = [0, 2, 4, 5, 7, 9, 11],
 		["dorian"] = [0, 2, 3, 5, 7, 9, 10],
 		["phrygian"] = [0, 1, 3, 5, 7, 8, 10],
 		["lydian"] = [0, 2, 4, 6, 7, 9, 11],
@@ -55,6 +54,13 @@ public sealed record Mode
 		["blues_major"] = [0, 2, 3, 4, 7, 9],
 	};
 
+	// Alternative spellings that name the same mode as a shape above. Parsing maps them to the
+	// canonical name, so a parsed alias equals the static instance it aliases.
+	private static readonly Dictionary<string, string> Aliases = new(StringComparer.OrdinalIgnoreCase)
+	{
+		["ionian"] = "major",
+	};
+
 	/// <summary>Gets the canonical lower-case name of the mode.</summary>
 	public string Name { get; private init; } = "major";
 
@@ -168,11 +174,17 @@ public static Mode Parse(string name)
 	}
 
 	/// <summary>Tries to parse a mode by name, case-insensitively.</summary>
+	/// <remarks>An alias such as "ionian" parses to its canonical mode (<see cref="Major"/>).</remarks>
 	/// <param name="name">The mode name.</param>
 	/// <param name="result">The matching mode, or null on failure.</param>
 	/// <returns><see langword="true"/> when the name is a known mode.</returns>
 	public static bool TryParse(string? name, [System.Diagnostics.CodeAnalysis.NotNullWhen(true)] out Mode? result)
 	{
+		if (name is not null && Aliases.TryGetValue(name, out string? canonical))
+		{
+			name = canonical;
+		}
+
 		if (name is not null && Shapes.ContainsKey(name))
 		{
 			result = new() { Name = name.ToLowerInvariant() };
diff --git a/Semantics.Test/Music/ChromaticAnalysisTests.cs b/Semantics.Test/Music/ChromaticAnalysisTests.cs
index 05cedee3..50360e75 100644
--- a/Semantics.Test/Music/ChromaticAnalysisTests.cs
+++ b/Semantics.Test/Music/ChromaticAnalysisTests.cs
@@ -48,6 +48,21 @@ public void ChromaticChords_DetectsBorrowedMinorSubdominant()
 		Assert.AreEqual("from parallel minor", analyses[0].Detail);
 	}
 
+	[TestMethod]
+	public void ChromaticChords_ParsedIonianKey_IsAnalyzedAsMajor()
+	{
+		Key ionian = Key.Parse("C ionian");
+
+		System.Collections.Generic.IReadOnlyList<ChromaticAnalysis> secondary =
+			Progression.Parse("4/4  C | D7 | G7 | C").ChromaticChords(ionian);
+		Assert.AreEqual("V/V", secondary[0].Detail);
+
+		System.Collections.Generic.IReadOnlyList<ChromaticAnalysis> borrowed =
+			Progression.Parse("4/4  C | Fm | C").ChromaticChords(ionian);
+		Assert.AreEqual(ChromaticKind.BorrowedChord, borrowed[0].Kind);
+		Assert.AreEqual("from parallel minor", borrowed[0].Detail);
+	}
+
 	[TestMethod]
 	public void ChromaticAnalysis_Create_RejectsNegativeIndex() =>
 		_ = Assert.ThrowsExactly<ArgumentOutOfRangeException>(() => ChromaticAnalysis.Create(-1, ChromaticKind.Chromatic, null));
diff --git a/Semantics.Test/Music/KeyTests.cs b/Semantics.Test/Music/KeyTests.cs
index ed365e3f..39de6097 100644
--- a/Semantics.Test/Music/KeyTests.cs
+++ b/Semantics.Test/Music/KeyTests.cs
@@ -96,6 +96,13 @@ public void RoundTrip()
 		Assert.AreEqual(k, Key.Parse(k.ToString()));
 	}
 
+	[TestMethod]
+	public void Parse_IonianSpelling_IsTheSameKeyAsMajor()
+	{
+		Assert.AreEqual(Key.Parse("C major"), Key.Parse("C ionian"));
+		Assert.AreEqual(CMajor, Key.Parse("C ionian"));
+	}
+
 	[TestMethod]
 	public void TryParseFailsOnUnknownMode()
 	{
diff --git a/Semantics.Test/Music/ModeTests.cs b/Semantics.Test/Music/ModeTests.cs
index bf8ea911..4b0475c2 100644
--- a/Semantics.Test/Music/ModeTests.cs
+++ b/Semantics.Test/Music/ModeTests.cs
@@ -27,6 +27,25 @@ public void Parse_IsCaseInsensitive_AndEqualsStaticInstance()
 		Assert.AreEqual(Mode.Lydian, Mode.Parse("LYDIAN"));
 	}
 
+	[TestMethod]
+	[DataRow("ionian")]
+	[DataRow("Ionian")]
+	[DataRow("IONIAN")]
+	public void Parse_IonianAlias_EqualsMajor(string name)
+	{
+		Mode parsed = Mode.Parse(name);
+		Assert.AreEqual(Mode.Ionian, parsed);
+		Assert.AreEqual(Mode.Major, parsed);
+		Assert.AreEqual("major", parsed.ToString());
+	}
+
+	[TestMethod]
+	public void TryParse_IonianAlias_EqualsMajor()
+	{
+		Assert.IsTrue(Mode.TryParse("ionian", out Mode? result));
+		Assert.AreEqual(Mode.Major, result);
+	}
+
 	[TestMethod]
 	public void Parse_RejectsUnknown()
 	{