Break StandardOrder.Compare ties between case variants ordinally [patch]

Frontmatter.Test/StandardOrderCompareTotalOrderTests.cs

@@ -0,0 +1,66 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.Frontmatter.Test;
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+/// <summary>
+/// Names that differ only in case must not compare as equal, or a sorted collection
+/// built on <see cref="StandardOrder.Compare"/> throws or drops keys (#197).
+/// </summary>
+[TestClass]
+public class StandardOrderCompareTotalOrderTests
+{
+	private static readonly Comparer<string> Comparer = Comparer<string>.Create(StandardOrder.Compare);
+
+	[TestMethod]
+	[DataRow("Title", "title")]
+	[DataRow("MyKey", "mykey")]
+	[DataRow("TAGS", "Tags")]
+	public void Compare_CaseVariants_AreNotEqualAndAntisymmetric(string a, string b)
+	{
+		int forward = StandardOrder.Compare(a, b);
+		int backward = StandardOrder.Compare(b, a);
+
+		Assert.AreNotEqual(0, forward, $"'{a}' and '{b}' are different strings and must not compare equal");
+		Assert.AreEqual(-Math.Sign(forward), Math.Sign(backward), "Compare must be antisymmetric");
+	}
+
+	[TestMethod]
+	public void Compare_SameString_IsZero()
+	{
+		Assert.AreEqual(0, StandardOrder.Compare("Title", "Title"));
+		Assert.AreEqual(0, StandardOrder.Compare("mykey", "mykey"));
+	}
+
+	[TestMethod]
+	public void SortedDictionary_KeepsCaseVariantKeys()
+	{
+		SortedDictionary<string, object> dictionary = new(Comparer)
+		{
+			{ "title", "a" },
+			{ "Title", "b" },
+		};
+
+		Assert.HasCount(2, dictionary);
+		Assert.AreEqual("a", dictionary["title"]);
+		Assert.AreEqual("b", dictionary["Title"]);
+	}
+
+	[TestMethod]
+	public void SortedSet_KeepsCaseVariantKeys()
+	{
+		SortedSet<string> set = new(["tags", "Tags", "myKey", "MyKey"], Comparer);
+
+		Assert.HasCount(4, set);
+	}
+
+	[TestMethod]
+	public void Compare_OrderAcrossPositionsIsUnchanged()
+	{
+		Assert.IsLessThan(0, StandardOrder.Compare("Title", "author"), "Known keys keep their standard order regardless of case");
+		Assert.IsLessThan(0, StandardOrder.Compare("Tags", "zzz_unknown"), "Known keys come before unknown keys");
+		Assert.IsLessThan(0, StandardOrder.Compare("Alpha", "beta"), "Unknown keys still sort case-insensitively");
+		Assert.IsGreaterThan(0, StandardOrder.Compare("zzz_unknown", "TITLE"), "Unknown keys come after known keys");
+	}
+}

Frontmatter/StandardOrder.cs

@@ -203,9 +203,13 @@ public static class StandardOrder
 	/// <param name="b">The second property name.</param>
 	/// <returns>
 	/// A negative value if 'a' should appear before 'b',
-	/// zero if they are equal in order,
+	/// zero only if 'a' and 'b' are the same string,
 	/// a positive value if 'a' should appear after 'b'.
 	/// </returns>
+	/// <remarks>
+	/// Names that differ only in case share a position, so they are ordered against each other ordinally.
+	/// That keeps the comparison a total order, so it can back a sorted collection without losing keys.
+	/// </remarks>
 	public static int Compare(string a, string b)
 	{
 		Ensure.NotNull(a);
@@ -216,6 +220,12 @@ public static int Compare(string a, string b)
 			return 0;
 		}
 
+		int result = CompareByPosition(a, b);
+		return result != 0 ? result : string.CompareOrdinal(a, b);
+	}
+
+	private static int CompareByPosition(string a, string b)
+	{
 		// Get the indices from the standard property list
 		int indexA = Array.IndexOf(PropertyNames, a.ToLowerInvariant());
 		int indexB = Array.IndexOf(PropertyNames, b.ToLowerInvariant());