Merge pull request #422 from ktsu-dev/fix/388-empty-relative-combine

Keep a path relative when its left operand is empty [patch]

Author: matt-edmondson

Semantics.Paths/Utilities/PooledStringBuilder.cs

@@ -67,11 +67,12 @@ public static string CombinePaths(params string[] components)
 			sb.Append(components[0]);
 			for (int i = 1; i < components.Length; i++)
 			{
-
+				// An empty leading component contributes nothing, so a separator after it would root the
+				// result: "" combined with "x.txt" must be "x.txt", not "/x.txt".
 #if NETSTANDARD2_0
-				if (!SpanPathUtilities.EndsWithDirectorySeparator(sb.ToString()))
+				if (sb.Length > 0 && !SpanPathUtilities.EndsWithDirectorySeparator(sb.ToString()))
 #else
-				if (!SpanPathUtilities.EndsWithDirectorySeparator(sb.ToString().AsSpan()))
+				if (sb.Length > 0 && !SpanPathUtilities.EndsWithDirectorySeparator(sb.ToString().AsSpan()))
 #endif
 				{
 					sb.Append(Path.DirectorySeparatorChar);

Semantics.Test/Paths/PathOperatorTests.cs

@@ -216,4 +216,23 @@ public void DirectoryNameOperator_WithSpecialCharacters_HandlesCorrectly()
 		Assert.IsTrue(result.IsValid());
 		Assert.Contains("my-app_v2 (beta)", result.WeakString);
 	}
+
+	[TestMethod]
+	public void RelativeDirectoryOperators_EmptyLeftOperand_StayRelative()
+	{
+		// Parent of a single-segment path is the empty path, the usual source of an empty left operand.
+		RelativeDirectoryPath empty = RelativeDirectoryPath.Create<RelativeDirectoryPath>("a").Parent;
+		Assert.AreEqual(string.Empty, empty.WeakString);
+
+		RelativeFilePath file = empty / FileName.Create<FileName>("x.txt");
+		Assert.AreEqual("x.txt", file.WeakString);
+
+		RelativeDirectoryPath directory = empty / RelativeDirectoryPath.Create<RelativeDirectoryPath>("b");
+		Assert.AreEqual("b", directory.WeakString);
+
+		string nested = TestPaths.Relative("c", "d.txt");
+		RelativeFilePath nestedFile = empty / RelativeFilePath.Create<RelativeFilePath>(nested);
+		Assert.AreEqual(nested, nestedFile.WeakString);
+		Assert.IsFalse(Path.IsPathRooted(nestedFile.WeakString));
+	}
 }