Keep a capitalised apostrophe name like "O'Neil" in one word [patch]

CaseConverter.Test/CaseConverterTests.cs

@@ -436,6 +436,14 @@ public void ToSnakeCaseShouldStillDropAstralCharactersThatAreNotLetters()
 	[DataRow("don\u2019t stop", "DontStop", "dontStop", "dont_stop", "dont-stop", "DONT_STOP")]
 	[DataRow("o'neil", "Oneil", "oneil", "oneil", "oneil", "ONEIL")]
 	[DataRow("o\u2019neil", "Oneil", "oneil", "oneil", "oneil", "ONEIL")]
+	[DataRow("O'Neil", "Oneil", "oneil", "oneil", "oneil", "ONEIL")]
+	[DataRow("O\u2019Neil", "Oneil", "oneil", "oneil", "oneil", "ONEIL")]
+	[DataRow("O'NEIL", "Oneil", "oneil", "oneil", "oneil", "ONEIL")]
+	[DataRow("D'Angelo", "Dangelo", "dangelo", "dangelo", "dangelo", "DANGELO")]
+	[DataRow("d'angelo", "Dangelo", "dangelo", "dangelo", "dangelo", "DANGELO")]
+	[DataRow("Don'T stop", "DontStop", "dontStop", "dont_stop", "dont-stop", "DONT_STOP")]
+	[DataRow("O'Neil's car", "OneilsCar", "oneilsCar", "oneils_car", "oneils-car", "ONEILS_CAR")]
+	[DataRow("McDonald's menu", "McDonaldsMenu", "mcDonaldsMenu", "mc_donalds_menu", "mc-donalds-menu", "MC_DONALDS_MENU")]
 	[DataRow("DON'T stop", "DontStop", "dontStop", "dont_stop", "dont-stop", "DONT_STOP")]
 	[DataRow("API's", "Apis", "apis", "apis", "apis", "APIS")]
 	[DataRow("CEO's office", "CeosOffice", "ceosOffice", "ceos_office", "ceos-office", "CEOS_OFFICE")]
@@ -461,6 +469,16 @@ public void ApostropheNotBetweenLettersShouldStillSeparateWords(string input, st
 		Assert.AreEqual(expected, input.ToSnakeCase());
 	}
 
+	[TestMethod]
+	[DataRow("O'Neil", "o'neil")]
+	[DataRow("D'Angelo", "d'angelo")]
+	[DataRow("Don'T", "don't")]
+	public void ACapitalisedApostropheNameShouldConvertLikeItsLowercaseSpelling(string input, string lowercase)
+	{
+		Assert.AreEqual(lowercase.ToSnakeCase(), input.ToSnakeCase());
+		Assert.AreEqual(input.ToSnakeCase(), input.ToTitleCase().ToSnakeCase());
+	}
+
 	// "ß" and "fi" are lowercase letters with no single-character uppercase form, so they survive
 	// ToMacroCase. They used to read as lowercase when that output was converted again, which split
 	// "STRAßE" into "STR Aß E" and kept IsAllCaps from recognizing it.

CaseConverter/CaseConverter.cs

@@ -69,12 +69,19 @@ public static partial class CaseConverter
 	/// capitalised word and splits into <c>"CE Os"</c>. <c>"CEOS"</c> is one all-caps word, which every
 	/// converter then normalizes the same way as <c>"CEO"</c>.
 	/// </para>
+	/// <para>
+	/// Otherwise, a capital straight after the apostrophe is lowercased when the letters around it are
+	/// lowercase, as in <c>"O'Neil"</c> or <c>"Don'T"</c>. Dropping the apostrophe alone would leave
+	/// <c>"ONeil"</c>, which <see cref="SplitOnCaseChange(string)"/> splits into <c>"O Neil"</c>, so
+	/// <c>"O'Neil"</c> would convert differently from <c>"o'neil"</c> and <c>"O'NEIL"</c>.
+	/// </para>
 	/// </remarks>
 	private static string ReplaceNonAlphaNumericWithSpace(string input)
 	{
 		StringBuilder builder = new(input.Length);
 		int previousStart = -1;
 		bool uppercaseSuffix = false;
+		bool lowercaseNextLetter = false;
 
 		for (int i = 0; i < input.Length;)
 		{
@@ -90,11 +97,14 @@ private static string ReplaceNonAlphaNumericWithSpace(string input)
 #else
 				string letter = input[i..nextStart];
 #endif
+				letter = LowercaseIf(letter, lowercaseNextLetter);
+				lowercaseNextLetter = false;
 				builder.Append(uppercaseSuffix ? ToUpperInvariantFull(letter) : letter);
 			}
 			else if (IsApostropheWithinWord(input, previousStart, i, nextStart))
 			{
 				uppercaseSuffix = char.IsUpper(input, previousStart) && AreLettersFromIndexLowercase(input, nextStart);
+				lowercaseNextLetter = !uppercaseSuffix && IsCapitalAfterApostropheInLowercaseContext(input, previousStart, nextStart);
 			}
 			else
 			{
@@ -109,6 +119,40 @@ private static string ReplaceNonAlphaNumericWithSpace(string input)
 		return builder.ToString();
 	}
 
+	/// <summary>
+	/// Returns <paramref name="letter"/> lowercased when <paramref name="lowercase"/> is set, otherwise unchanged.
+	/// </summary>
+	/// <param name="letter">The letter, as a string of one or two UTF-16 code units.</param>
+	/// <param name="lowercase">Whether to lowercase it.</param>
+	/// <returns>The letter, lowercased if asked.</returns>
+	private static string LowercaseIf(string letter, bool lowercase) => lowercase ? ToLowerInvariantFull(letter) : letter;
+
+	/// <summary>
+	/// Determines whether the letter after an in-word apostrophe is a capital that the letters around it
+	/// show should be lowercase.
+	/// </summary>
+	/// <param name="input">The string to inspect.</param>
+	/// <param name="previousStart">The index of the letter before the apostrophe.</param>
+	/// <param name="nextStart">The index of the letter after the apostrophe.</param>
+	/// <returns><c>true</c> if the letter after the apostrophe should be lowercased; otherwise, <c>false</c>.</returns>
+	/// <remarks>
+	/// The letter follows the case of the letter after it when there is one, so <c>"O'Neil"</c> lowers the
+	/// <c>"N"</c> while <c>"O'NEIL"</c> keeps it. At the end of the word it follows the letter before the
+	/// apostrophe instead, so <c>"Don'T"</c> lowers the <c>"T"</c> while <c>"DON'T"</c> keeps it.
+	/// </remarks>
+	private static bool IsCapitalAfterApostropheInLowercaseContext(string input, int previousStart, int nextStart)
+	{
+		if (!char.IsUpper(input, nextStart))
+		{
+			return false;
+		}
+
+		int afterNextStart = nextStart + CodePointLength(input, nextStart);
+		return afterNextStart < input.Length && char.IsLetter(input, afterNextStart)
+			? char.IsLower(input, afterNextStart)
+			: !char.IsUpper(input, previousStart);
+	}
+
 	/// <summary>
 	/// Determines whether every letter in the run of letters starting at <paramref name="start"/> is lowercase.
 	/// </summary>