Normalize line endings in one pass, so Mac keeps every break

Extensions.Test/StringExtensionsTests.cs

@@ -304,6 +304,34 @@ public void NormalizeLineEndingsToNone()
 		Assert.AreEqual(expected, result);
 	}
 
+	// A CRLF followed by an LF, and a CR followed by a CRLF, are two line breaks each. Normalizing in
+	// several passes used to merge the first into a new CRLF under Mac, losing a break and leaving a
+	// Windows ending in Mac output.
+
+	[TestMethod]
+	[DataRow("a\r\n\nb", LineEndingStyle.Unix, "a\n\nb")]
+	[DataRow("a\r\n\nb", LineEndingStyle.Windows, "a\r\n\r\nb")]
+	[DataRow("a\r\n\nb", LineEndingStyle.Mac, "a\r\rb")]
+	[DataRow("a\r\n\nb", LineEndingStyle.None, "ab")]
+	[DataRow("a\r\n\nb", LineEndingStyle.Mixed, "a\n\nb")]
+	[DataRow("a\r\r\nb", LineEndingStyle.Unix, "a\n\nb")]
+	[DataRow("a\r\r\nb", LineEndingStyle.Windows, "a\r\n\r\nb")]
+	[DataRow("a\r\r\nb", LineEndingStyle.Mac, "a\r\rb")]
+	[DataRow("a\r\r\nb", LineEndingStyle.None, "ab")]
+	[DataRow("a\r\r\nb", LineEndingStyle.Mixed, "a\n\nb")]
+	public void NormalizeLineEndingsKeepsEveryBreakOfAdjacentMixedEndings(string input, LineEndingStyle style, string expected) =>
+		Assert.AreEqual(expected, input.NormalizeLineEndings(style));
+
+	[TestMethod]
+	[DataRow(LineEndingStyle.Unix)]
+	[DataRow(LineEndingStyle.Windows)]
+	[DataRow(LineEndingStyle.Mac)]
+	public void NormalizeLineEndingsOfAdjacentMixedEndingsIsDetectedAsTheTargetStyle(LineEndingStyle style)
+	{
+		Assert.AreEqual(style, "a\r\n\nb".NormalizeLineEndings(style).DetermineLineEndings());
+		Assert.AreEqual(style, "a\r\r\nb".NormalizeLineEndings(style).DetermineLineEndings());
+	}
+
 	[TestMethod]
 	public void NormalizeLineEndingsToMixed()
 	{

Extensions/StringExtensions.cs

@@ -122,10 +122,14 @@
 #endif
 	}
 
 	private static Regex LineEndingRegexUnix { get; } = new(@"(?<!\r)\n", RegexOptions.Compiled);
 	private static Regex LineEndingRegexWindows { get; } = new(@"\r\n", RegexOptions.Compiled);
 	private static Regex LineEndingRegexMac { get; } = new(@"\r(?!\n)", RegexOptions.Compiled);
 
+	// One pass over every kind of line ending. Normalizing in several passes lets an earlier pass
+	// build a sequence the next one reads differently: under Mac, "\r\n\n" became "\r" + "\n", a new CRLF.
+	private static Regex AnyLineEndingRegex { get; } = new(@"\r\n|\r|\n", RegexOptions.Compiled, TimeSpan.FromSeconds(1));
+
 	/// <summary>
 	/// Determines the line ending style of the specified string.
 	/// </summary>
@@ -202,11 +206,11 @@
 
 		return style switch
 		{
-			LineEndingStyle.None => LineEndingRegexUnix.Replace(LineEndingRegexWindows.Replace(LineEndingRegexMac.Replace(s, ""), ""), ""),
-			LineEndingStyle.Unix => LineEndingRegexWindows.Replace(LineEndingRegexMac.Replace(s, "\n"), "\n"),
-			LineEndingStyle.Windows => LineEndingRegexUnix.Replace(LineEndingRegexMac.Replace(s, "\r\n"), "\r\n"),
-			LineEndingStyle.Mac => LineEndingRegexUnix.Replace(LineEndingRegexWindows.Replace(s, "\r"), "\r"),
-			LineEndingStyle.Mixed => LineEndingRegexWindows.Replace(LineEndingRegexMac.Replace(s, "\n"), "\n"),
+			LineEndingStyle.None => AnyLineEndingRegex.Replace(s, ""),
+			LineEndingStyle.Unix => AnyLineEndingRegex.Replace(s, "\n"),
+			LineEndingStyle.Windows => AnyLineEndingRegex.Replace(s, "\r\n"),
+			LineEndingStyle.Mac => AnyLineEndingRegex.Replace(s, "\r"),
+			LineEndingStyle.Mixed => AnyLineEndingRegex.Replace(s, "\n"),
 			_ => throw new NotImplementedException("Unknown line ending style."),
 		};
 	}
@@ -255,7 +259,7 @@
 	/// </summary>
 	private const char SoftHyphen = '­';
 
 	private static IEnumerable<string> NominalWordWrapIterator(string text, int maxCharsPerLine)
 	{
 		string normalized = text.NormalizeLineEndings(LineEndingStyle.Unix);
 		string[] segments = normalized.Split('\n');
@@ -288,7 +292,7 @@
 
 					yield return line.ToString();
 					line.Clear();
 					atLineStart = true;
 				}
 				else if (!atLineStart)
 				{