[patch] Search only the file name for FileExtension and FullFileExtension

Semantics.Paths/README.md

@@ -162,7 +162,7 @@ The primitive component types `FileName`, `FileExtension`, and `DirectoryName` a
 | `Exists` | `bool` | True if the path is an existing file or directory. |
 | `IsFile` / `IsDirectory` | `bool` | Filesystem-backed checks. |
 | `FileName` | `FileName` | Filename portion (file paths). |
-| `FileExtension` / `FullFileExtension` | `FileExtension` | Last extension / everything from the first dot. |
+| `FileExtension` / `FullFileExtension` | `FileExtension` | Last extension / everything from the first dot, searching the file name only. A leading dot marks a dotfile, not an extension (`.bashrc` has none). |
 | `DirectoryPath` | `DirectoryPath` | Directory portion of a file path. |
 | `Parent` / `Name` / `Depth` / `IsRoot` | directory members | Directory navigation. |
 | `AsAbsolute()` | typed absolute path | Resolve against the current working directory. |

Semantics.Paths/SemanticFilePath.cs

@@ -9,83 +9,64 @@ public abstract record SemanticFilePath<TDerived> : SemanticPath<TDerived>
 	where TDerived : SemanticFilePath<TDerived>
 {
 	/// <summary>
-	/// Gets the file extension including the leading period, or empty if no extension
+	/// Gets the file extension including the leading period, or empty if no extension.
 	/// </summary>
+	/// <remarks>
+	/// Only the file-name segment is searched, so a dot in a directory name (<c>./notes.txt</c>,
+	/// <c>/opt/app.v2/README</c>) is never mistaken for an extension. A leading dot in the file name
+	/// marks a dotfile rather than an extension: <c>.bashrc</c> has no extension, and
+	/// <c>.config.json</c> has <c>.json</c>.
+	/// </remarks>
 	public FileExtension FileExtension
 	{
 		get
 		{
-#if NETSTANDARD2_0
-			string span = WeakString;
+			string path = WeakString;
+			int nameStart = FileNameStart(path);
 
-			// Find the last dot
-			int lastDotIndex = span.LastIndexOf('.');
-			if (lastDotIndex == -1 || lastDotIndex == span.Length - 1)
+			// A dot at the start of the name, or before it, is not an extension; neither is a trailing dot
+			int lastDotIndex = path.LastIndexOf('.');
+			if (lastDotIndex <= nameStart || lastDotIndex == path.Length - 1)
 			{
-				// No extension or trailing dot
 				return FileExtension.Create<FileExtension>("");
 			}
 
-			// Return extension including the dot
-			string extension = span.Substring(lastDotIndex);
-			return FileExtension.Create<FileExtension>(extension);
-#else
-			ReadOnlySpan<char> span = WeakString.AsSpan();
-
-			// Find the last dot
-			int lastDotIndex = span.LastIndexOf('.');
-			if (lastDotIndex == -1 || lastDotIndex == span.Length - 1)
-			{
-				// No extension or trailing dot
-				return FileExtension.Create<FileExtension>("");
-			}
-
-			// Return extension including the dot
-			ReadOnlySpan<char> extension = span[lastDotIndex..];
-			return FileExtension.Create<FileExtension>(extension.ToString());
-#endif
+			return FileExtension.Create<FileExtension>(path.AsSpan(lastDotIndex));
 		}
 	}
 
 	/// <summary>
-	/// Gets all trailing period-delimited segments including the leading period, or empty if no extensions
+	/// Gets all trailing period-delimited segments including the leading period, or empty if no extensions.
 	/// </summary>
+	/// <remarks>
+	/// Follows the same rules as <see cref="FileExtension"/>: only the file-name segment is searched, and
+	/// a leading dot in the file name is not an extension, so <c>.bashrc</c> has none.
+	/// </remarks>
 	public FileExtension FullFileExtension
 	{
 		get
 		{
-#if NETSTANDARD2_0
-			string span = WeakString;
-
-			// Find the first dot
-			int firstDotIndex = span.IndexOf('.');
-			if (firstDotIndex == -1)
-			{
-				// No extension
-				return FileExtension.Create<FileExtension>("");
-			}
-
-			// Return everything from the first dot onward
-			string fullExtension = span.Substring(firstDotIndex);
-			return FileExtension.Create<FileExtension>(fullExtension);
-#else
-			ReadOnlySpan<char> span = WeakString.AsSpan();
+			string path = WeakString;
+			int nameStart = FileNameStart(path);
 
-			// Find the first dot
-			int firstDotIndex = span.IndexOf('.');
+			// Skip the name's first character, so a dotfile's leading dot is not taken as an extension
+			int firstDotIndex = nameStart + 1 < path.Length ? path.IndexOf('.', nameStart + 1) : -1;
 			if (firstDotIndex == -1)
 			{
-				// No extension
 				return FileExtension.Create<FileExtension>("");
 			}
 
-			// Return everything from the first dot onward
-			ReadOnlySpan<char> fullExtension = span[firstDotIndex..];
-			return FileExtension.Create<FileExtension>(fullExtension.ToString());
-#endif
+			return FileExtension.Create<FileExtension>(path.AsSpan(firstDotIndex));
 		}
 	}
 
+	/// <summary>
+	/// Returns the index of the first character of the file-name segment of <paramref name="path"/>.
+	/// </summary>
+	/// <param name="path">The path to inspect.</param>
+	/// <returns>The index just past the last directory separator, or 0 if there is none.</returns>
+	private static int FileNameStart(string path) => path.LastIndexOfAny(['/', '\\']) + 1;
+
 	/// <summary>
 	/// Gets the filename portion of the path
 	/// </summary>

Semantics.Test/SemanticPathTests.cs

@@ -94,6 +94,38 @@ public void SemanticFilePath_FullFileExtension_MultipleExtensions_ShouldReturnAl
 		Assert.AreEqual(".tar.gz", fullExtension.ToString());
 	}
 
+	[TestMethod]
+	[DataRow(new[] { ".", "notes.txt" }, ".txt", ".txt")]
+	[DataRow(new[] { "..", "data", "archive.tar.gz" }, ".gz", ".tar.gz")]
+	[DataRow(new[] { "app.v2", "README" }, "", "")]
+	[DataRow(new[] { "user", ".config", "settings.json" }, ".json", ".json")]
+	[DataRow(new[] { "user", ".bashrc" }, "", "")]
+	[DataRow(new[] { ".bashrc" }, "", "")]
+	[DataRow(new[] { "user", ".config.json" }, ".json", ".json")]
+	public void SemanticFilePath_Extensions_OnlySearchTheFileName(string[] segments, string expectedExtension, string expectedFullExtension)
+	{
+		FilePath filePath = FilePath.Create<FilePath>(TestPaths.Relative(segments));
+
+		Assert.AreEqual(expectedExtension, filePath.FileExtension.ToString());
+		Assert.AreEqual(expectedFullExtension, filePath.FullFileExtension.ToString());
+	}
+
+	[TestMethod]
+	public void SemanticFilePath_Extensions_IgnoreDotsInDirectoriesOfAbsoluteAndRelativePaths()
+	{
+		AbsoluteFilePath absolute = AbsoluteFilePath.Create<AbsoluteFilePath>(TestPaths.Absolute("home", "user", ".config", "settings.json"));
+		Assert.AreEqual(".json", absolute.FileExtension.ToString());
+		Assert.AreEqual(".json", absolute.FullFileExtension.ToString());
+
+		AbsoluteFilePath noExtension = AbsoluteFilePath.Create<AbsoluteFilePath>(TestPaths.Absolute("opt", "app.v2", "README"));
+		Assert.AreEqual("", noExtension.FileExtension.ToString());
+		Assert.AreEqual("", noExtension.FullFileExtension.ToString());
+
+		RelativeFilePath relative = RelativeFilePath.Create<RelativeFilePath>(TestPaths.Relative("..", "data", "archive.tar.gz"));
+		Assert.AreEqual(".gz", relative.FileExtension.ToString());
+		Assert.AreEqual(".tar.gz", relative.FullFileExtension.ToString());
+	}
+
 	[TestMethod]
 	public void SemanticFilePath_FileName_ShouldReturnCorrectFileName()
 	{