C#: Implement support for path transformers

csharp/extractor/Semmle.Extraction.Tests/PathTransformer.cs

@@ -0,0 +1,45 @@
+using Semmle.Util;
+using Xunit;
+
+namespace Semmle.Extraction.Tests
+{
+    class PathCacheStub : IPathCache
+    {
+        public string GetCanonicalPath(string path) => path;
+    }
+
+    public class PathTransformerTests
+    {
+        [Fact]
+        public void TestTransformerFile()
+        {
+            var spec = new string[]
+            {
+                @"#D:\src",
+                @"C:\agent*\src//",
+                @"-C:\agent*\src\external",
+                @"",
+                @"#empty",
+                @"",
+                @"#src2",
+                @"/agent*//src",
+                @"",
+                @"#optsrc",
+                @"opt/src//"
+            };
+
+            var pathTransformer = new PathTransformer(new PathCacheStub(), spec);
+
+            // Windows-style matching
+            Assert.Equal(@"C:\bar.cs", pathTransformer.Transform(@"C:\bar.cs").Value);
+            Assert.Equal("D:/src/file.cs", pathTransformer.Transform(@"C:\agent42\src\file.cs").Value);
+            Assert.Equal("D:/src/file.cs", pathTransformer.Transform(@"C:\agent43\src\file.cs").Value);
+            Assert.Equal(@"C:\agent43\src\external\file.cs", pathTransformer.Transform(@"C:\agent43\src\external\file.cs").Value);
+
+            // Linux-style matching
+            Assert.Equal(@"src2/src/file.cs", pathTransformer.Transform(@"/agent/src/file.cs").Value);
+            Assert.Equal(@"src2/src/file.cs", pathTransformer.Transform(@"/agent42/src/file.cs").Value);
+            Assert.Equal(@"optsrc/file.cs", pathTransformer.Transform(@"/opt/src/file.cs").Value);
+        }
+    }
+}